Internet Protocol

RFC 1885 – Internet Control Message Protocol (ICMPv6) for IPv6 (OBSOLETE)

 
Network Working Group             A. Conta, Digital Equipment Corporation
Request for Comments: 1885 S. Deering, Xerox PARC
Category: Standards Track December 1995

Internet Control Message Protocol (ICMPv6)
for the Internet Protocol Version 6 (IPv6)
Specification

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document specifies a set of Internet Control Message Protocol
(ICMP) messages for use with version 6 of the Internet Protocol
(IPv6). The Internet Group Management Protocol (IGMP) messages
specified in STD 5, RFC 1112 have been merged into ICMP, for IPv6,
and are included in this document.

Table of Contents

1. Introduction........................................3

2. ICMPv6 (ICMP for IPv6)..............................3

2.1 Message General Format.......................3

2.2 Message Source Address Determination.........4

2.3 Message Checksum Calculation.................5

2.4 Message Processing Rules.....................5

3. ICMPv6 Error Messages...............................8

3.1 Destination Unreachable Message..............8

3.2 Packet Too Big Message......................10

3.3 Time Exceeded Message.......................11

3.4 Parameter Problem Message...................12

4. ICMPv6 Informational Messages......................14

4.1 Echo Request Message........................14

4.2 Echo Reply Message..........................15

4.3 Group Membership Messages...................17

5. References.........................................19

6. Acknowledgements...................................19

7. Security Considerations............................19

Authors' Addresses....................................20

1. Introduction

The Internet Protocol, version 6 (IPv6) is a new version of IP. IPv6
uses the Internet Control Message Protocol (ICMP) as defined for IPv4
[RFC-792], with a number of changes. The Internet Group Membership
Protocol (IGMP) specified for IPv4 [RFC-1112] has also been revised
and has been absorbed into ICMP for IPv6. The resulting protocol is
called ICMPv6, and has an IPv6 Next Header value of 58.

This document describes the format of a set of control messages used
in ICMPv6. It does not describe the procedures for using these
messages to achieve functions like Path MTU discovery or multicast
group membership maintenance; such procedures are described in other
documents (e.g., [RFC-1112, RFC-1191]). Other documents may also
introduce additional ICMPv6 message types, such as Neighbor Discovery
messages [IPv6-DISC], subject to the general rules for ICMPv6
messages given in section 2 of this document.

Terminology defined in the IPv6 specification [IPv6] and the IPv6
Routing and Addressing specification [IPv6-ADDR] applies to this
document as well.

2. ICMPv6 (ICMP for IPv6)

ICMPv6 is used by IPv6 nodes to report errors encountered in
processing packets, and to perform other internet-layer functions,
such as diagnostics (ICMPv6 "ping") and multicast membership
reporting. ICMPv6 is an integral part of IPv6 and MUST be fully
implemented by every IPv6 node.

2.1 Message General Format

ICMPv6 messages are grouped into two classes: error messages and
informational messages. Error messages are identified as such by
having a zero in the high-order bit of their message Type field
values. Thus, error messages have message Types from 0 to 127;
informational messages have message Types from 128 to 255.

This document defines the message formats for the following ICMPv6
messages:

ICMPv6 error messages:

1 Destination Unreachable (see section 3.1)
2 Packet Too Big (see section 3.2)
3 Time Exceeded (see section 3.3)
4 Parameter Problem (see section 3.4)

ICMPv6 informational messages:

128 Echo Request (see section 4.1)
129 Echo Reply (see section 4.2)
130 Group Membership Query (see section 4.3)
131 Group Membership Report (see section 4.3)
132 Group Membership Reduction (see section 4.3)

Every ICMPv6 message is preceded by an IPv6 header and zero or more
IPv6 extension headers. The ICMPv6 header is identified by a Next
Header value of 58 in the immediately preceding header. (NOTE: this
is different than the value used to identify ICMP for IPv4.)

The ICMPv6 messages have the following general format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Message Body +
| |

The type field indicates the type of the message. Its value
determines the format of the remaining data.

The code field depends on the message type. It is used to create an
additional level of message granularity.

The checksum field is used to detect data corruption in the ICMPv6
message and parts of the IPv6 header.

2.2 Message Source Address Determination

A node that sends an ICMPv6 message has to determine both the Source
and Destination IPv6 Addresses in the IPv6 header before calculating
the checksum. If the node has more than one unicast address, it must
choose the Source Address of the message as follows:

(a) If the message is a response to a message sent to one of the
node's unicast addresses, the Source Address of the reply must
be that same address.

(b) If the message is a response to a message sent to a multicast or
anycast group in which the node is a member, the Source Address
of the reply must be a unicast address belonging to the
interface on which the multicast or anycast packet was received.

(c) If the message is a response to a message sent to an address
that does not belong to the node, the Source Address should be
that unicast address belonging to the node that will be most
helpful in diagnosing the error. For example, if the message is
a response to a packet forwarding action that cannot complete
successfully, the Source Address should be a unicast address
belonging to the interface on which the packet forwarding
failed.

(d) Otherwise, the node's routing table must be examined to
determine which interface will be used to transmit the message
to its destination, and a unicast address belonging to that
interface must be used as the Source Address of the message.

2.3 Message Checksum Calculation

The checksum is the 16-bit one's complement of the one's complement
sum of the entire ICMPv6 message starting with the ICMPv6 message
type field, prepended with a "pseudo-header" of IPv6 header fields,
as specified in [IPv6, section 8.1]. The Next Header value used in
the pseudo-header is 58. (NOTE: the inclusion of a pseudo-header in
the ICMPv6 checksum is a change from IPv4; see [IPv6] for the
rationale for this change.)

For computing the checksum, the checksum field is set to zero.

2.4 Message Processing Rules

Implementations MUST observe the following rules when processing
ICMPv6 messages (from [RFC-1122]):

(a) If an ICMPv6 error message of unknown type is received, it MUST
be passed to the upper layer.

(b) If an ICMPv6 informational message of unknown type is received,
it MUST be silently discarded.

(c) Every ICMPv6 error message (type < 128) includes as much of the
IPv6 offending (invoking) packet (the packet that caused the
error) as will fit without making the error message packet
exceed 576 octets.

(d) In those cases where the internet-layer protocol is required to
pass an ICMPv6 error message to the upper-layer protocol, the
upper-layer protocol type is extracted from the original packet
(contained in the body of the ICMPv6 error message) and used to
select the appropriate upper-layer protocol entity to handle the
error.

If the original packet had an unusually large amount of
extension headers, it is possible that the upper-layer protocol
type may not be present in the ICMPv6 message, due to truncation
of the original packet to meet the 576-octet limit. In that
case, the error message is silently dropped after any IPv6-layer
processing.

(e) An ICMPv6 error message MUST NOT be sent as a result of
receiving:

(e.1) an ICMPv6 error message, or

(e.2) a packet destined to an IPv6 multicast address (there are
two exceptions to this rule: (1) the Packet Too Big
Message - Section 3.2 - to allow Path MTU discovery to
work for IPv6 multicast, and (2) the Parameter Problem
Message, Code 2 - Section 3.4 - reporting an unrecognized
IPv6 option that has the Option Type highest-order two
bits set to 10), or

(e.3) a packet sent as a link-layer multicast, (the exception
from e.2 applies to this case too), or

(e.4) a packet sent as a link-layer broadcast, (the exception
from e.2 applies to this case too), or

(e.5) a packet whose source address does not uniquely identify
a single node -- e.g., the IPv6 Unspecified Address, an
IPv6 multicast address, or an address known by the ICMP
message sender to be an IPv6 anycast address.

(f) Finally, to each sender of an erroneous data packet, an IPv6
node MUST limit the rate of ICMPv6 error messages sent, in order
to limit the bandwidth and forwarding costs incurred by the
error messages when a generator of erroneous packets does not
respond to those error messages by ceasing its transmissions.

There are a variety of ways of implementing the rate-limiting
function, for example:

(f.1) Timer-based - for example, limiting the rate of
transmission of error messages to a given source, or to
any source, to at most once every T milliseconds.

(f.2) Bandwidth-based - for example, limiting the rate at
which error messages are sent from a particular interface
to some fraction F of the attached link's bandwidth.

The limit parameters (e.g., T or F in the above examples) MUST
be configurable for the node, with a conservative default value
(e.g., T = 1 second, NOT 0 seconds, or F = 2 percent, NOT 100
percent).

The following sections describe the message formats for the above
ICMPv6 messages.

3. ICMPv6 Error Messages

3.1 Destination Unreachable Message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding 576 octets |

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 1

Code 0 - no route to destination
1 - communication with destination
administratively prohibited
2 - not a neighbor
3 - address unreachable
4 - port unreachable

Unused This field is unused for all code values.
It must be initialized to zero by the sender
and ignored by the receiver.
Description

A Destination Unreachable message SHOULD be generated by a router, or
by the IPv6 layer in the originating node, in response to a packet
that cannot be delivered to its destination address for reasons other
than congestion. (An ICMPv6 message MUST NOT be generated if a
packet is dropped due to congestion.)

If the reason for the failure to deliver is lack of a matching entry
in the forwarding node's routing table, the Code field is set to 0
(NOTE: this error can occur only in nodes that do not hold a "default
route" in their routing tables).

If the reason for the failure to deliver is administrative
prohibition, e.g., a "firewall filter", the Code field is set to 1.

If the reason for the failure to deliver is that the next destination
address in the Routing header is not a neighbor of the processing
node but the "strict" bit is set for that address, then the Code
field is set to 2.

If there is any other reason for the failure to deliver, e.g.,
inability to resolve the IPv6 destination address into a
corresponding link address, or a link-specific problem of some sort,
then the Code field is set to 3.

A destination node SHOULD send a Destination Unreachable message with
Code 4 in response to a packet for which the transport protocol
(e.g., UDP) has no listener, if that transport protocol has no
alternative means to inform the sender.

Upper layer notification

A node receiving the ICMPv6 Destination Unreachable message MUST
notify the upper-layer protocol.

3.2 Packet Too Big Message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding 576 octets |

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 2

Code 0

MTU The Maximum Transmission Unit of the next-hop link.

Description

A Packet Too Big MUST be sent by a router in response to a packet
that it cannot forward because the packet is larger than the MTU of
the outgoing link. The information in this message is used as part
of the Path MTU Discovery process [RFC-1191].

Sending a Packet Too Big Message makes an exception to one of the
rules of when to send an ICMPv6 error message, in that unlike other
messages, it is sent in response to a packet received with an IPv6
multicast destination address, or a link-layer multicast or link-
layer broadcast address.

Upper layer notification

An incoming Packet Too Big message MUST be passed to the upper-layer
protocol.

3.3 Time Exceeded Message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding 576 octets |

IPv6 Fields:

Destination Address
Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 3

Code 0 - hop limit exceeded in transit

1 - fragment reassembly time exceeded

Unused This field is unused for all code values.
It must be initialized to zero by the sender
and ignored by the receiver.

Description

If a router receives a packet with a Hop Limit of zero, or a router
decrements a packet's Hop Limit to zero, it MUST discard the packet
and send an ICMPv6 Time Exceeded message with Code 0 to the source of
the packet. This indicates either a routing loop or too small an
initial Hop Limit value.

The router sending an ICMPv6 Time Exceeded message with Code 0 SHOULD
consider the receiving interface of the packet as the interface on
which the packet forwarding failed in following rule (d) for
selecting the Source Address of the message.

Upper layer notification

An incoming Time Exceeded message MUST be passed to the upper-layer
protocol.

3.4 Parameter Problem Message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding 576 octets |

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 4

Code 0 - erroneous header field encountered

1 - unrecognized Next Header type encountered

2 - unrecognized IPv6 option encountered

Pointer Identifies the octet offset within the
invoking packet where the error was detected.

The pointer will point beyond the end of the ICMPv6
packet if the field in error is beyond what can fit
in the 576-byte limit of an ICMPv6 error message.

Description

If an IPv6 node processing a packet finds a problem with a field in
the IPv6 header or extension headers such that it cannot complete
processing the packet, it MUST discard the packet and SHOULD send an
ICMPv6 Parameter Problem message to the packet's source, indicating
the type and location of the problem.

The pointer identifies the octet of the original packet's header
where the error was detected. For example, an ICMPv6 message with
Type field = 4, Code field = 1, and Pointer field = 40 would indicate

that the IPv6 extension header following the IPv6 header of the
original packet holds an unrecognized Next Header field value.

Upper layer notification

A node receiving this ICMPv6 message MUST notify the upper-layer
protocol.

4. ICMPv6 Informational Messages

4.1 Echo Request Message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-

IPv6 Fields:

Destination Address

Any legal IPv6 address.

ICMPv6 Fields:

Type 128

Code 0

Identifier An identifier to aid in matching Echo Replies
to this Echo Request. May be zero.

Sequence Number

A sequence number to aid in matching Echo Replies
to this Echo Request. May be zero.

Data Zero or more octets of arbitrary data.

Description

Every node MUST implement an ICMPv6 Echo responder function that
receives Echo Requests and sends corresponding Echo Replies. A node
SHOULD also implement an application-layer interface for sending Echo
Requests and receiving Echo Replies, for diagnostic purposes.

Upper layer notification

A node receiving this ICMPv6 message MAY notify the upper-layer
protocol.

4.2 Echo Reply Message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
Echo Request packet.

ICMPv6 Fields:

Type 129

Code 0

Identifier The identifier from the invoking Echo Request message.

Sequence The sequence number from the invoking Echo Request
Number message.

Data The data from the invoking Echo Request message.

Description

Every node MUST implement an ICMPv6 Echo responder function that
receives Echo Requests and sends corresponding Echo Replies. A node
SHOULD also implement an application-layer interface for sending Echo
Requests and receiving Echo Replies, for diagnostic purposes.

The source address of an Echo Reply sent in response to a unicast
Echo Request message MUST be the same as the destination address of
that Echo Request message.

An Echo Reply SHOULD be sent in response to an Echo Request message
sent to an IPv6 multicast address. The source address of the reply
MUST be a unicast address belonging to the interface on which the
multicast Echo Request message was received.

The data received in the ICMPv6 Echo Request message MUST be returned
entirely and unmodified in the ICMPv6 Echo Reply message, unless the
Echo Reply would exceed the MTU of the path back to the Echo
requester, in which case the data is truncated to fit that path MTU.

Upper layer notification

Echo Reply messages MUST be passed to the ICMPv6 user interface,
unless the corresponding Echo Request originated in the IP layer.

4.3 Group Membership Messages

The ICMPv6 Group Membership Messages have the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Response Delay | Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Multicast |
+ +
| Address |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IPv6 Fields:

Destination Address

In a Group Membership Query message, the multicast
address of the group being queried, or the Link-Local
All-Nodes multicast address.

In a Group Membership Report or a Group Membership
Reduction message, the multicast address of the
group being reported or terminated.

Hop Limit 1

ICMPv6 Fields:

Type 130 - Group Membership Query
131 - Group Membership Report
132 - Group Membership Reduction

Code 0

Maximum Response Delay

In Query messages, the maximum time that responding
Report messages may be delayed, in milliseconds.

In Report and Reduction messages, this field is
is initialized to zero by the sender and ignored by
receivers.

Unused Initialized to zero by the sender; ignored by receivers.

Multicast Address

The address of the multicast group about which the
message is being sent. In Query messages, the Multicast
Address field may be zero, implying a query for all
groups.

Description

The ICMPv6 Group Membership messages are used to convey information
about multicast group membership from nodes to their neighboring
routers. The details of their usage is given in [RFC-1112].

5. References

[IPv6] Deering, S., and R. Hinden, "Internet Protocol, Version
6, Specification", RFC 1883, Xerox PARC, Ipsilon
Networks, December 1995.

[IPv6-ADDR] Hinden, R., and S. Deering, Editors, "IP Version 6
Addressing Architecture", RFC 1884, Ipsilon Networks,
Xerox PARC, December 1995.

[IPv6-DISC] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", Work in Progress.

[RFC-792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, USC/Information Sciences Institute, September
1981.

[RFC-1112] Deering, S., "Host Extensions for IP Multicasting", STD
5, RFC 1112, Stanford University, August 1989.

[RFC-1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, USC/Information
Sciences Institute, October 1989.

[RFC-1191] Mogul, J., and S. Deering, "Path MTU Discovery", RFC
1191, DECWRL, Stanford University, November 1990.

6. Acknowledgements

The document is derived from previous ICMP drafts of the SIPP and
IPng working group.

The IPng working group and particularly Robert Elz, Jim Bound, Bill
Simpson, Thomas Narten, Charlie Lynn, Bill Fink, and Scott Bradner
(in chronological order) provided extensive review information and
feedback.

7. Security Considerations

Security issues are not discussed in this memo.

Authors' Addresses:

Alex Conta Stephen Deering
Digital Equipment Corporation Xerox Palo Alto Research Center
110 Spitbrook Rd 3333 Coyote Hill Road
Nashua, NH 03062 Palo Alto, CA 94304

Phone: +1-603-881-0744 Phone: +1-415-812-4839
EMail: conta@zk3.dec.com EMail: deering@parc.xerox.com


RFC 792 – Internet Control Message Protocol


Network Working Group                                          J. Postel
Request for Comments:  792                                           ISI
                                                          September 1981
Updates:  RFCs 777, 760
Updates:  IENs 109, 128

                   INTERNET CONTROL MESSAGE PROTOCOL

                         DARPA INTERNET PROGRAM
                         PROTOCOL SPECIFICATION

Introduction

   The Internet Protocol (IP) [1] is used for host-to-host datagram
   service in a system of interconnected networks called the
   Catenet [2].  The network connecting devices are called Gateways.
   These gateways communicate between themselves for control purposes
   via a Gateway to Gateway Protocol (GGP) [3,4].  Occasionally a
   gateway or destination host will communicate with a source host, for
   example, to report an error in datagram processing.  For such
   purposes this protocol, the Internet Control Message Protocol (ICMP),
   is used.  ICMP, uses the basic support of IP as if it were a higher
   level protocol, however, ICMP is actually an integral part of IP, and
   must be implemented by every IP module.

   ICMP messages are sent in several situations:  for example, when a
   datagram cannot reach its destination, when the gateway does not have
   the buffering capacity to forward a datagram, and when the gateway
   can direct the host to send traffic on a shorter route.

   The Internet Protocol is not designed to be absolutely reliable.  The
   purpose of these control messages is to provide feedback about
   problems in the communication environment, not to make IP reliable.
   There are still no guarantees that a datagram will be delivered or a
   control message will be returned.  Some datagrams may still be
   undelivered without any report of their loss.  The higher level
   protocols that use IP must implement their own reliability procedures
   if reliable communication is required.

   The ICMP messages typically report errors in the processing of
   datagrams.  To avoid the infinite regress of messages about messages
   etc., no ICMP messages are sent about ICMP messages.  Also ICMP
   messages are only sent about errors in handling fragment zero of
   fragemented datagrams.  (Fragment zero has the fragment offeset equal
   zero).

                                                                [Page 1]

                                                          September 1981
RFC 792

Message Formats

   ICMP messages are sent using the basic IP header.  The first octet of
   the data portion of the datagram is a ICMP type field; the value of
   this field determines the format of the remaining data.  Any field
   labeled "unused" is reserved for later extensions and must be zero
   when sent, but receivers should not use these fields (except to
   include them in the checksum).  Unless otherwise noted under the
   individual format descriptions, the values of the internet header
   fields are as follows:

   Version

      4

   IHL

      Internet header length in 32-bit words.

   Type of Service

      0

   Total Length

      Length of internet header and data in octets.

   Identification, Flags, Fragment Offset

      Used in fragmentation, see [1].

   Time to Live

      Time to live in seconds; as this field is decremented at each
      machine in which the datagram is processed, the value in this
      field should be at least as great as the number of gateways which
      this datagram will traverse.

   Protocol

      ICMP = 1

   Header Checksum

      The 16 bit one's complement of the one's complement sum of all 16
      bit words in the header.  For computing the checksum, the checksum
      field should be zero.  This checksum may be replaced in the
      future.

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September 1981                                                          
RFC 792

   Source Address

      The address of the gateway or host that composes the ICMP message.
      Unless otherwise noted, this can be any of a gateway's addresses.

   Destination Address

      The address of the gateway or host to which the message should be
      sent.

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                                                          September 1981
RFC 792

Destination Unreachable Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address from the original datagram's data.

   ICMP Fields:

   Type

      3

   Code

      0 = net unreachable;

      1 = host unreachable;

      2 = protocol unreachable;

      3 = port unreachable;

      4 = fragmentation needed and DF set;

      5 = source route failed.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original

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September 1981                                                          
RFC 792

      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      If, according to the information in the gateway's routing tables,
      the network specified in the internet destination field of a
      datagram is unreachable, e.g., the distance to the network is
      infinity, the gateway may send a destination unreachable message
      to the internet source host of the datagram.  In addition, in some
      networks, the gateway may be able to determine if the internet
      destination host is unreachable.  Gateways in these networks may
      send destination unreachable messages to the source host when the
      destination host is unreachable.

      If, in the destination host, the IP module cannot deliver the
      datagram  because the indicated protocol module or process port is
      not active, the destination host may send a destination
      unreachable message to the source host.

      Another case is when a datagram must be fragmented to be forwarded
      by a gateway yet the Don't Fragment flag is on.  In this case the
      gateway must discard the datagram and may return a destination
      unreachable message.

      Codes 0, 1, 4, and 5 may be received from a gateway.  Codes 2 and
      3 may be received from a host.

                                                                [Page 5]

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Time Exceeded Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address from the original datagram's data.

   ICMP Fields:

   Type

      11

   Code

      0 = time to live exceeded in transit;

      1 = fragment reassembly time exceeded.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      If the gateway processing a datagram finds the time to live field

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      is zero it must discard the datagram.  The gateway may also notify
      the source host via the time exceeded message.

      If a host reassembling a fragmented datagram cannot complete the
      reassembly due to missing fragments within its time limit it
      discards the datagram, and it may send a time exceeded message.

      If fragment zero is not available then no time exceeded need be
      sent at all.

      Code 0 may be received from a gateway.  Code 1 may be received
      from a host.

                                                                [Page 7]

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Parameter Problem Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Pointer    |                   unused                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address from the original datagram's data.

   ICMP Fields:

   Type

      12

   Code

      0 = pointer indicates the error.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Pointer

      If code = 0, identifies the octet where an error was detected.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

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   Description

      If the gateway or host processing a datagram finds a problem with
      the header parameters such that it cannot complete processing the
      datagram it must discard the datagram.  One potential source of
      such a problem is with incorrect arguments in an option.  The
      gateway or host may also notify the source host via the parameter
      problem message.  This message is only sent if the error caused
      the datagram to be discarded.

      The pointer identifies the octet of the original datagram's header
      where the error was detected (it may be in the middle of an
      option).  For example, 1 indicates something is wrong with the
      Type of Service, and (if there are options present) 20 indicates
      something is wrong with the type code of the first option.

      Code 0 may be received from a gateway or a host.

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Source Quench Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address of the original datagram's data.

   ICMP Fields:

   Type

      4

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      A gateway may discard internet datagrams if it does not have the
      buffer space needed to queue the datagrams for output to the next
      network on the route to the destination network.  If a gateway

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      discards a datagram, it may send a source quench message to the
      internet source host of the datagram.  A destination host may also
      send a source quench message if datagrams arrive too fast to be
      processed.  The source quench message is a request to the host to
      cut back the rate at which it is sending traffic to the internet
      destination.  The gateway may send a source quench message for
      every message that it discards.  On receipt of a source quench
      message, the source host should cut back the rate at which it is
      sending traffic to the specified destination until it no longer
      receives source quench messages from the gateway.  The source host
      can then gradually increase the rate at which it sends traffic to
      the destination until it again receives source quench messages.

      The gateway or host may send the source quench message when it
      approaches its capacity limit rather than waiting until the
      capacity is exceeded.  This means that the data datagram which
      triggered the source quench message may be delivered.

      Code 0 may be received from a gateway or a host.

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Redirect Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Gateway Internet Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Internet Header + 64 bits of Original Data Datagram      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Destination Address

      The source network and address of the original datagram's data.

   ICMP Fields:

   Type

      5

   Code

      0 = Redirect datagrams for the Network.

      1 = Redirect datagrams for the Host.

      2 = Redirect datagrams for the Type of Service and Network.

      3 = Redirect datagrams for the Type of Service and Host.

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Gateway Internet Address

      Address of the gateway to which traffic for the network specified
      in the internet destination network field of the original
      datagram's data should be sent.

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   Internet Header + 64 bits of Data Datagram

      The internet header plus the first 64 bits of the original
      datagram's data.  This data is used by the host to match the
      message to the appropriate process.  If a higher level protocol
      uses port numbers, they are assumed to be in the first 64 data
      bits of the original datagram's data.

   Description

      The gateway sends a redirect message to a host in the following
      situation.  A gateway, G1, receives an internet datagram from a
      host on a network to which the gateway is attached.  The gateway,
      G1, checks its routing table and obtains the address of the next
      gateway, G2, on the route to the datagram's internet destination
      network, X.  If G2 and the host identified by the internet source
      address of the datagram are on the same network, a redirect
      message is sent to the host.  The redirect message advises the
      host to send its traffic for network X directly to gateway G2 as
      this is a shorter path to the destination.  The gateway forwards
      the original datagram's data to its internet destination.

      For datagrams with the IP source route options and the gateway
      address in the destination address field, a redirect message is
      not sent even if there is a better route to the ultimate
      destination than the next address in the source route.

      Codes 0, 1, 2, and 3 may be received from a gateway.

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Echo or Echo Reply Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Data ...
   +-+-+-+-+-

   IP Fields:

   Addresses

      The address of the source in an echo message will be the
      destination of the echo reply message.  To form an echo reply
      message, the source and destination addresses are simply reversed,
      the type code changed to 0, and the checksum recomputed.

   IP Fields:

   Type

      8 for echo message;

      0 for echo reply message.

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      If the total length is odd, the received data is padded with one
      octet of zeros for computing the checksum.  This checksum may be
      replaced in the future.

   Identifier

      If code = 0, an identifier to aid in matching echos and replies,
      may be zero.

   Sequence Number

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      If code = 0, a sequence number to aid in matching echos and
      replies, may be zero.

   Description

      The data received in the echo message must be returned in the echo
      reply message.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the echo requests.  For
      example, the identifier might be used like a port in TCP or UDP to
      identify a session, and the sequence number might be incremented
      on each echo request sent.  The echoer returns these same values
      in the echo reply.

      Code 0 may be received from a gateway or a host.

                                                               [Page 15]

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Timestamp or Timestamp Reply Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |      Code     |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Originate Timestamp                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Receive Timestamp                                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Transmit Timestamp                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Addresses

      The address of the source in a timestamp message will be the
      destination of the timestamp reply message.  To form a timestamp
      reply message, the source and destination addresses are simply
      reversed, the type code changed to 14, and the checksum
      recomputed.

   IP Fields:

   Type

      13 for timestamp message;

      14 for timestamp reply message.

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Identifier

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      If code = 0, an identifier to aid in matching timestamp and
      replies, may be zero.

   Sequence Number

      If code = 0, a sequence number to aid in matching timestamp and
      replies, may be zero.

   Description

      The data received (a timestamp) in the message is returned in the
      reply together with an additional timestamp.  The timestamp is 32
      bits of milliseconds since midnight UT.  One use of these
      timestamps is described by Mills [5].

      The Originate Timestamp is the time the sender last touched the
      message before sending it, the Receive Timestamp is the time the
      echoer first touched it on receipt, and the Transmit Timestamp is
      the time the echoer last touched the message on sending it.

      If the time is not available in miliseconds or cannot be provided
      with respect to midnight UT then any time can be inserted in a
      timestamp provided the high order bit of the timestamp is also set
      to indicate this non-standard value.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the requests.  For example,
      the identifier might be used like a port in TCP or UDP to identify
      a session, and the sequence number might be incremented on each
      request sent.  The destination returns these same values in the
      reply.

      Code 0 may be received from a gateway or a host.

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Information Request or Information Reply Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |      Code     |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP Fields:

   Addresses

      The address of the source in a information request message will be
      the destination of the information reply message.  To form a
      information reply message, the source and destination addresses
      are simply reversed, the type code changed to 16, and the checksum
      recomputed.

   IP Fields:

   Type

      15 for information request message;

      16 for information reply message.

   Code

      0

   Checksum

      The checksum is the 16-bit ones's complement of the one's
      complement sum of the ICMP message starting with the ICMP Type.
      For computing the checksum , the checksum field should be zero.
      This checksum may be replaced in the future.

   Identifier

      If code = 0, an identifier to aid in matching request and replies,
      may be zero.

   Sequence Number

      If code = 0, a sequence number to aid in matching request and
      replies, may be zero.

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   Description

      This message may be sent with the source network in the IP header
      source and destination address fields zero (which means "this"
      network).  The replying IP module should send the reply with the
      addresses fully specified.  This message is a way for a host to
      find out the number of the network it is on.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the requests.  For example,
      the identifier might be used like a port in TCP or UDP to identify
      a session, and the sequence number might be incremented on each
      request sent.  The destination returns these same values in the
      reply.

      Code 0 may be received from a gateway or a host.

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Summary of Message Types

    0  Echo Reply

    3  Destination Unreachable

    4  Source Quench

    5  Redirect

    8  Echo

   11  Time Exceeded

   12  Parameter Problem

   13  Timestamp

   14  Timestamp Reply

   15  Information Request

   16  Information Reply

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References

   [1]  Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
         Protocol Specification," RFC 791, USC/Information Sciences
         Institute, September 1981.

   [2]   Cerf, V., "The Catenet Model for Internetworking," IEN 48,
         Information Processing Techniques Office, Defense Advanced
         Research Projects Agency, July 1978.

   [3]   Strazisar, V., "Gateway Routing:  An Implementation
         Specification", IEN 30, Bolt Beranek and Newman, April 1979.

   [4]   Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek
         and Newman, August 1979.

   [5]   Mills, D., "DCNET Internet Clock Service," RFC 778, COMSAT
         Laboratories, April 1981.

RFC 2373 – IP Version 6 Addressing Architecture

 
Network Working Group                                        R. Hinden
Request for Comments: 2373 Nokia
Obsoletes: 1884 S. Deering
Category: Standards Track Cisco Systems
July 1998

IP Version 6 Addressing Architecture

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This specification defines the addressing architecture of the IP
Version 6 protocol [IPV6]. The document includes the IPv6 addressing
model, text representations of IPv6 addresses, definition of IPv6
unicast addresses, anycast addresses, and multicast addresses, and an
IPv6 node's required addresses.

Table of Contents

1. Introduction.................................................2
2. IPv6 Addressing..............................................2
2.1 Addressing Model.........................................3
2.2 Text Representation of Addresses.........................3
2.3 Text Representation of Address Prefixes..................5
2.4 Address Type Representation..............................6
2.5 Unicast Addresses........................................7
2.5.1 Interface Identifiers................................8
2.5.2 The Unspecified Address..............................9
2.5.3 The Loopback Address.................................9
2.5.4 IPv6 Addresses with Embedded IPv4 Addresses.........10
2.5.5 NSAP Addresses......................................10
2.5.6 IPX Addresses.......................................10
2.5.7 Aggregatable Global Unicast Addresses...............11
2.5.8 Local-use IPv6 Unicast Addresses....................11
2.6 Anycast Addresses.......................................12
2.6.1 Required Anycast Address............................13
2.7 Multicast Addresses.....................................14

2.7.1 Pre-Defined Multicast Addresses.....................15
2.7.2 Assignment of New IPv6 Multicast Addresses..........17
2.8 A Node's Required Addresses.............................17
3. Security Considerations.....................................18
APPENDIX A: Creating EUI-64 based Interface Identifiers........19
APPENDIX B: ABNF Description of Text Representations...........22
APPENDIX C: CHANGES FROM RFC-1884..............................23
REFERENCES.....................................................24
AUTHORS' ADDRESSES.............................................25
FULL COPYRIGHT STATEMENT.......................................26

1.0 INTRODUCTION

This specification defines the addressing architecture of the IP
Version 6 protocol. It includes a detailed description of the
currently defined address formats for IPv6 [IPV6].

The authors would like to acknowledge the contributions of Paul
Francis, Scott Bradner, Jim Bound, Brian Carpenter, Matt Crawford,
Deborah Estrin, Roger Fajman, Bob Fink, Peter Ford, Bob Gilligan,
Dimitry Haskin, Tom Harsch, Christian Huitema, Tony Li, Greg
Minshall, Thomas Narten, Erik Nordmark, Yakov Rekhter, Bill Simpson,
and Sue Thomson.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].

2.0 IPv6 ADDRESSING

IPv6 addresses are 128-bit identifiers for interfaces and sets of
interfaces. There are three types of addresses:

Unicast: An identifier for a single interface. A packet sent to
a unicast address is delivered to the interface
identified by that address.

Anycast: An identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to an
anycast address is delivered to one of the interfaces
identified by that address (the "nearest" one, according
to the routing protocols' measure of distance).

Multicast: An identifier for a set of interfaces (typically
belonging to different nodes). A packet sent to a
multicast address is delivered to all interfaces
identified by that address.

There are no broadcast addresses in IPv6, their function being
superseded by multicast addresses.

In this document, fields in addresses are given a specific name, for
example "subscriber". When this name is used with the term "ID" for
identifier after the name (e.g., "subscriber ID"), it refers to the
contents of the named field. When it is used with the term "prefix"
(e.g. "subscriber prefix") it refers to all of the address up to and
including this field.

In IPv6, all zeros and all ones are legal values for any field,
unless specifically excluded. Specifically, prefixes may contain
zero-valued fields or end in zeros.

2.1 Addressing Model

IPv6 addresses of all types are assigned to interfaces, not nodes.
An IPv6 unicast address refers to a single interface. Since each
interface belongs to a single node, any of that node's interfaces'
unicast addresses may be used as an identifier for the node.

All interfaces are required to have at least one link-local unicast
address (see section 2.8 for additional required addresses). A
single interface may also be assigned multiple IPv6 addresses of any
type (unicast, anycast, and multicast) or scope. Unicast addresses
with scope greater than link-scope are not needed for interfaces that
are not used as the origin or destination of any IPv6 packets to or
from non-neighbors. This is sometimes convenient for point-to-point
interfaces. There is one exception to this addressing model:

An unicast address or a set of unicast addresses may be assigned to
multiple physical interfaces if the implementation treats the
multiple physical interfaces as one interface when presenting it to
the internet layer. This is useful for load-sharing over multiple
physical interfaces.

Currently IPv6 continues the IPv4 model that a subnet prefix is
associated with one link. Multiple subnet prefixes may be assigned
to the same link.

2.2 Text Representation of Addresses

There are three conventional forms for representing IPv6 addresses as
text strings:

1. The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the
hexadecimal values of the eight 16-bit pieces of the address.
Examples:

FEDC:BA98:7654:3210:FEDC:BA98:7654:3210

1080:0:0:0:8:800:200C:417A

Note that it is not necessary to write the leading zeros in an
individual field, but there must be at least one numeral in every
field (except for the case described in 2.).

2. Due to some methods of allocating certain styles of IPv6
addresses, it will be common for addresses to contain long strings
of zero bits. In order to make writing addresses containing zero
bits easier a special syntax is available to compress the zeros.
The use of "::" indicates multiple groups of 16-bits of zeros.
The "::" can only appear once in an address. The "::" can also be
used to compress the leading and/or trailing zeros in an address.

For example the following addresses:

1080:0:0:0:8:800:200C:417A a unicast address
FF01:0:0:0:0:0:0:101 a multicast address
0:0:0:0:0:0:0:1 the loopback address
0:0:0:0:0:0:0:0 the unspecified addresses

may be represented as:

1080::8:800:200C:417A a unicast address
FF01::101 a multicast address
::1 the loopback address
:: the unspecified addresses

3. An alternative form that is sometimes more convenient when dealing
with a mixed environment of IPv4 and IPv6 nodes is
x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values of
the six high-order 16-bit pieces of the address, and the 'd's are
the decimal values of the four low-order 8-bit pieces of the
address (standard IPv4 representation). Examples:

0:0:0:0:0:0:13.1.68.3

0:0:0:0:0:FFFF:129.144.52.38

or in compressed form:

::13.1.68.3

::FFFF:129.144.52.38

2.3 Text Representation of Address Prefixes

The text representation of IPv6 address prefixes is similar to the
way IPv4 addresses prefixes are written in CIDR notation. An IPv6
address prefix is represented by the notation:

ipv6-address/prefix-length

where

ipv6-address is an IPv6 address in any of the notations listed
in section 2.2.

prefix-length is a decimal value specifying how many of the
leftmost contiguous bits of the address comprise
the prefix.

For example, the following are legal representations of the 60-bit
prefix 12AB00000000CD3 (hexadecimal):

12AB:0000:0000:CD30:0000:0000:0000:0000/60
12AB::CD30:0:0:0:0/60
12AB:0:0:CD30::/60

The following are NOT legal representations of the above prefix:

12AB:0:0:CD3/60 may drop leading zeros, but not trailing zeros,
within any 16-bit chunk of the address

12AB::CD30/60 address to left of "/" expands to
12AB:0000:0000:0000:0000:000:0000:CD30

12AB::CD3/60 address to left of "/" expands to
12AB:0000:0000:0000:0000:000:0000:0CD3

When writing both a node address and a prefix of that node address
(e.g., the node's subnet prefix), the two can combined as follows:

the node address 12AB:0:0:CD30:123:4567:89AB:CDEF
and its subnet number 12AB:0:0:CD30::/60

can be abbreviated as 12AB:0:0:CD30:123:4567:89AB:CDEF/60

2.4 Address Type Representation

The specific type of an IPv6 address is indicated by the leading bits
in the address. The variable-length field comprising these leading
bits is called the Format Prefix (FP). The initial allocation of
these prefixes is as follows:

Allocation Prefix Fraction of
(binary) Address Space
----------------------------------- -------- -------------
Reserved 0000 0000 1/256
Unassigned 0000 0001 1/256

Reserved for NSAP Allocation 0000 001 1/128
Reserved for IPX Allocation 0000 010 1/128

Unassigned 0000 011 1/128
Unassigned 0000 1 1/32
Unassigned 0001 1/16

Aggregatable Global Unicast Addresses 001 1/8
Unassigned 010 1/8
Unassigned 011 1/8
Unassigned 100 1/8
Unassigned 101 1/8
Unassigned 110 1/8

Unassigned 1110 1/16
Unassigned 1111 0 1/32
Unassigned 1111 10 1/64
Unassigned 1111 110 1/128
Unassigned 1111 1110 0 1/512

Link-Local Unicast Addresses 1111 1110 10 1/1024
Site-Local Unicast Addresses 1111 1110 11 1/1024

Multicast Addresses 1111 1111 1/256

Notes:

(1) The "unspecified address" (see section 2.5.2), the loopback
address (see section 2.5.3), and the IPv6 Addresses with
Embedded IPv4 Addresses (see section 2.5.4), are assigned out
of the 0000 0000 format prefix space.

(2) The format prefixes 001 through 111, except for Multicast
Addresses (1111 1111), are all required to have to have 64-bit
interface identifiers in EUI-64 format. See section 2.5.1 for
definitions.

This allocation supports the direct allocation of aggregation
addresses, local use addresses, and multicast addresses. Space is
reserved for NSAP addresses and IPX addresses. The remainder of the
address space is unassigned for future use. This can be used for
expansion of existing use (e.g., additional aggregatable addresses,
etc.) or new uses (e.g., separate locators and identifiers). Fifteen
percent of the address space is initially allocated. The remaining
85% is reserved for future use.

Unicast addresses are distinguished from multicast addresses by the
value of the high-order octet of the addresses: a value of FF
(11111111) identifies an address as a multicast address; any other
value identifies an address as a unicast address. Anycast addresses
are taken from the unicast address space, and are not syntactically
distinguishable from unicast addresses.

2.5 Unicast Addresses

IPv6 unicast addresses are aggregatable with contiguous bit-wise
masks similar to IPv4 addresses under Class-less Interdomain Routing
[CIDR].

There are several forms of unicast address assignment in IPv6,
including the global aggregatable global unicast address, the NSAP
address, the IPX hierarchical address, the site-local address, the
link-local address, and the IPv4-capable host address. Additional
address types can be defined in the future.

IPv6 nodes may have considerable or little knowledge of the internal
structure of the IPv6 address, depending on the role the node plays
(for instance, host versus router). At a minimum, a node may
consider that unicast addresses (including its own) have no internal
structure:

| 128 bits |
+-----------------------------------------------------------------+
| node address |
+-----------------------------------------------------------------+

A slightly sophisticated host (but still rather simple) may
additionally be aware of subnet prefix(es) for the link(s) it is
attached to, where different addresses may have different values for
n:

| n bits | 128-n bits |
+------------------------------------------------+----------------+
| subnet prefix | interface ID |
+------------------------------------------------+----------------+

Still more sophisticated hosts may be aware of other hierarchical
boundaries in the unicast address. Though a very simple router may
have no knowledge of the internal structure of IPv6 unicast
addresses, routers will more generally have knowledge of one or more
of the hierarchical boundaries for the operation of routing
protocols. The known boundaries will differ from router to router,
depending on what positions the router holds in the routing
hierarchy.

2.5.1 Interface Identifiers

Interface identifiers in IPv6 unicast addresses are used to identify
interfaces on a link. They are required to be unique on that link.
They may also be unique over a broader scope. In many cases an
interface's identifier will be the same as that interface's link-
layer address. The same interface identifier may be used on multiple
interfaces on a single node.

Note that the use of the same interface identifier on multiple
interfaces of a single node does not affect the interface
identifier's global uniqueness or each IPv6 addresses global
uniqueness created using that interface identifier.

In a number of the format prefixes (see section 2.4) Interface IDs
are required to be 64 bits long and to be constructed in IEEE EUI-64
format [EUI64]. EUI-64 based Interface identifiers may have global
scope when a global token is available (e.g., IEEE 48bit MAC) or may
have local scope where a global token is not available (e.g., serial
links, tunnel end-points, etc.). It is required that the "u" bit
(universal/local bit in IEEE EUI-64 terminology) be inverted when
forming the interface identifier from the EUI-64. The "u" bit is set
to one (1) to indicate global scope, and it is set to zero (0) to
indicate local scope. The first three octets in binary of an EUI-64
identifier are as follows:

0 0 0 1 1 2
|0 7 8 5 6 3|
+----+----+----+----+----+----+
|cccc|ccug|cccc|cccc|cccc|cccc|
+----+----+----+----+----+----+

written in Internet standard bit-order , where "u" is the
universal/local bit, "g" is the individual/group bit, and "c" are the
bits of the company_id. Appendix A: "Creating EUI-64 based Interface
Identifiers" provides examples on the creation of different EUI-64
based interface identifiers.

The motivation for inverting the "u" bit when forming the interface
identifier is to make it easy for system administrators to hand
configure local scope identifiers when hardware tokens are not
available. This is expected to be case for serial links, tunnel end-
points, etc. The alternative would have been for these to be of the
form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler ::1,
::2, etc.

The use of the universal/local bit in the IEEE EUI-64 identifier is
to allow development of future technology that can take advantage of
interface identifiers with global scope.

The details of forming interface identifiers are defined in the
appropriate "IPv6 over <link>" specification such as "IPv6 over
Ethernet" [ETHER], "IPv6 over FDDI" [FDDI], etc.

2.5.2 The Unspecified Address

The address 0:0:0:0:0:0:0:0 is called the unspecified address. It
must never be assigned to any node. It indicates the absence of an
address. One example of its use is in the Source Address field of
any IPv6 packets sent by an initializing host before it has learned
its own address.

The unspecified address must not be used as the destination address
of IPv6 packets or in IPv6 Routing Headers.

2.5.3 The Loopback Address

The unicast address 0:0:0:0:0:0:0:1 is called the loopback address.
It may be used by a node to send an IPv6 packet to itself. It may
never be assigned to any physical interface. It may be thought of as
being associated with a virtual interface (e.g., the loopback
interface).

The loopback address must not be used as the source address in IPv6
packets that are sent outside of a single node. An IPv6 packet with
a destination address of loopback must never be sent outside of a
single node and must never be forwarded by an IPv6 router.

2.5.4 IPv6 Addresses with Embedded IPv4 Addresses

The IPv6 transition mechanisms [TRAN] include a technique for hosts
and routers to dynamically tunnel IPv6 packets over IPv4 routing
infrastructure. IPv6 nodes that utilize this technique are assigned
special IPv6 unicast addresses that carry an IPv4 address in the low-
order 32-bits. This type of address is termed an "IPv4-compatible
IPv6 address" and has the format:

| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|0000| IPv4 address |
+--------------------------------------+----+---------------------+

A second type of IPv6 address which holds an embedded IPv4 address is
also defined. This address is used to represent the addresses of
IPv4-only nodes (those that *do not* support IPv6) as IPv6 addresses.
This type of address is termed an "IPv4-mapped IPv6 address" and has
the format:

| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|FFFF| IPv4 address |
+--------------------------------------+----+---------------------+

2.5.5 NSAP Addresses

This mapping of NSAP address into IPv6 addresses is defined in
[NSAP]. This document recommends that network implementors who have
planned or deployed an OSI NSAP addressing plan, and who wish to
deploy or transition to IPv6, should redesign a native IPv6
addressing plan to meet their needs. However, it also defines a set
of mechanisms for the support of OSI NSAP addressing in an IPv6
network. These mechanisms are the ones that must be used if such
support is required. This document also defines a mapping of IPv6
addresses within the OSI address format, should this be required.

2.5.6 IPX Addresses

This mapping of IPX address into IPv6 addresses is as follows:

| 7 | 121 bits |
+-------+---------------------------------------------------------+
|0000010| to be defined |
+-------+---------------------------------------------------------+

The draft definition, motivation, and usage are under study.

2.5.7 Aggregatable Global Unicast Addresses

The global aggregatable global unicast address is defined in [AGGR].
This address format is designed to support both the current provider
based aggregation and a new type of aggregation called exchanges.
The combination will allow efficient routing aggregation for both
sites which connect directly to providers and who connect to
exchanges. Sites will have the choice to connect to either type of
aggregation point.

The IPv6 aggregatable global unicast address format is as follows:

| 3| 13 | 8 | 24 | 16 | 64 bits |
+--+-----+---+--------+--------+--------------------------------+
|FP| TLA |RES| NLA | SLA | Interface ID |
| | ID | | ID | ID | |
+--+-----+---+--------+--------+--------------------------------+

Where

001 Format Prefix (3 bit) for Aggregatable Global
Unicast Addresses
TLA ID Top-Level Aggregation Identifier
RES Reserved for future use
NLA ID Next-Level Aggregation Identifier
SLA ID Site-Level Aggregation Identifier
INTERFACE ID Interface Identifier

The contents, field sizes, and assignment rules are defined in
[AGGR].

2.5.8 Local-Use IPv6 Unicast Addresses

There are two types of local-use unicast addresses defined. These
are Link-Local and Site-Local. The Link-Local is for use on a single
link and the Site-Local is for use in a single site. Link-Local
addresses have the following format:

| 10 |
| bits | 54 bits | 64 bits |
+----------+-------------------------+----------------------------+
|1111111010| 0 | interface ID |
+----------+-------------------------+----------------------------+

Link-Local addresses are designed to be used for addressing on a
single link for purposes such as auto-address configuration, neighbor
discovery, or when no routers are present.

Routers must not forward any packets with link-local source or
destination addresses to other links.

Site-Local addresses have the following format:

| 10 |
| bits | 38 bits | 16 bits | 64 bits |
+----------+-------------+-----------+----------------------------+
|1111111011| 0 | subnet ID | interface ID |
+----------+-------------+-----------+----------------------------+

Site-Local addresses are designed to be used for addressing inside of
a site without the need for a global prefix.

Routers must not forward any packets with site-local source or
destination addresses outside of the site.

2.6 Anycast Addresses

An IPv6 anycast address is an address that is assigned to more than
one interface (typically belonging to different nodes), with the
property that a packet sent to an anycast address is routed to the
"nearest" interface having that address, according to the routing
protocols' measure of distance.

Anycast addresses are allocated from the unicast address space, using
any of the defined unicast address formats. Thus, anycast addresses
are syntactically indistinguishable from unicast addresses. When a
unicast address is assigned to more than one interface, thus turning
it into an anycast address, the nodes to which the address is
assigned must be explicitly configured to know that it is an anycast
address.

For any assigned anycast address, there is a longest address prefix P
that identifies the topological region in which all interfaces
belonging to that anycast address reside. Within the region
identified by P, each member of the anycast set must be advertised as
a separate entry in the routing system (commonly referred to as a
"host route"); outside the region identified by P, the anycast
address may be aggregated into the routing advertisement for prefix
P.

Note that in, the worst case, the prefix P of an anycast set may be
the null prefix, i.e., the members of the set may have no topological
locality. In that case, the anycast address must be advertised as a
separate routing entry throughout the entire internet, which presents

a severe scaling limit on how many such "global" anycast sets may be
supported. Therefore, it is expected that support for global anycast
sets may be unavailable or very restricted.

One expected use of anycast addresses is to identify the set of
routers belonging to an organization providing internet service.
Such addresses could be used as intermediate addresses in an IPv6
Routing header, to cause a packet to be delivered via a particular
aggregation or sequence of aggregations. Some other possible uses
are to identify the set of routers attached to a particular subnet,
or the set of routers providing entry into a particular routing
domain.

There is little experience with widespread, arbitrary use of internet
anycast addresses, and some known complications and hazards when
using them in their full generality [ANYCST]. Until more experience
has been gained and solutions agreed upon for those problems, the
following restrictions are imposed on IPv6 anycast addresses:

o An anycast address must not be used as the source address of an
IPv6 packet.

o An anycast address must not be assigned to an IPv6 host, that
is, it may be assigned to an IPv6 router only.

2.6.1 Required Anycast Address

The Subnet-Router anycast address is predefined. Its format is as
follows:

| n bits | 128-n bits |
+------------------------------------------------+----------------+
| subnet prefix | 00000000000000 |
+------------------------------------------------+----------------+

The "subnet prefix" in an anycast address is the prefix which
identifies a specific link. This anycast address is syntactically
the same as a unicast address for an interface on the link with the
interface identifier set to zero.

Packets sent to the Subnet-Router anycast address will be delivered
to one router on the subnet. All routers are required to support the
Subnet-Router anycast addresses for the subnets which they have
interfaces.

The subnet-router anycast address is intended to be used for
applications where a node needs to communicate with one of a set of
routers on a remote subnet. For example when a mobile host needs to
communicate with one of the mobile agents on its "home" subnet.

2.7 Multicast Addresses

An IPv6 multicast address is an identifier for a group of nodes. A
node may belong to any number of multicast groups. Multicast
addresses have the following format:

| 8 | 4 | 4 | 112 bits |
+------ -+----+----+---------------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------------+

11111111 at the start of the address identifies the address as
being a multicast address.

+-+-+-+-+
flgs is a set of 4 flags: |0|0|0|T|
+-+-+-+-+

The high-order 3 flags are reserved, and must be initialized to
0.

T = 0 indicates a permanently-assigned ("well-known") multicast
address, assigned by the global internet numbering authority.

T = 1 indicates a non-permanently-assigned ("transient")
multicast address.

scop is a 4-bit multicast scope value used to limit the scope of
the multicast group. The values are:

0 reserved
1 node-local scope
2 link-local scope
3 (unassigned)
4 (unassigned)
5 site-local scope
6 (unassigned)
7 (unassigned)
8 organization-local scope
9 (unassigned)
A (unassigned)
B (unassigned)
C (unassigned)

D (unassigned)
E global scope
F reserved

group ID identifies the multicast group, either permanent or
transient, within the given scope.

The "meaning" of a permanently-assigned multicast address is
independent of the scope value. For example, if the "NTP servers
group" is assigned a permanent multicast address with a group ID of
101 (hex), then:

FF01:0:0:0:0:0:0:101 means all NTP servers on the same node as the
sender.

FF02:0:0:0:0:0:0:101 means all NTP servers on the same link as the
sender.

FF05:0:0:0:0:0:0:101 means all NTP servers at the same site as the
sender.

FF0E:0:0:0:0:0:0:101 means all NTP servers in the internet.

Non-permanently-assigned multicast addresses are meaningful only
within a given scope. For example, a group identified by the non-
permanent, site-local multicast address FF15:0:0:0:0:0:0:101 at one
site bears no relationship to a group using the same address at a
different site, nor to a non-permanent group using the same group ID
with different scope, nor to a permanent group with the same group
ID.

Multicast addresses must not be used as source addresses in IPv6
packets or appear in any routing header.

2.7.1 Pre-Defined Multicast Addresses

The following well-known multicast addresses are pre-defined:

Reserved Multicast Addresses: FF00:0:0:0:0:0:0:0
FF01:0:0:0:0:0:0:0
FF02:0:0:0:0:0:0:0
FF03:0:0:0:0:0:0:0
FF04:0:0:0:0:0:0:0
FF05:0:0:0:0:0:0:0
FF06:0:0:0:0:0:0:0
FF07:0:0:0:0:0:0:0
FF08:0:0:0:0:0:0:0
FF09:0:0:0:0:0:0:0

FF0A:0:0:0:0:0:0:0
FF0B:0:0:0:0:0:0:0
FF0C:0:0:0:0:0:0:0
FF0D:0:0:0:0:0:0:0
FF0E:0:0:0:0:0:0:0
FF0F:0:0:0:0:0:0:0

The above multicast addresses are reserved and shall never be
assigned to any multicast group.

All Nodes Addresses: FF01:0:0:0:0:0:0:1
FF02:0:0:0:0:0:0:1

The above multicast addresses identify the group of all IPv6 nodes,
within scope 1 (node-local) or 2 (link-local).

All Routers Addresses: FF01:0:0:0:0:0:0:2
FF02:0:0:0:0:0:0:2
FF05:0:0:0:0:0:0:2

The above multicast addresses identify the group of all IPv6 routers,
within scope 1 (node-local), 2 (link-local), or 5 (site-local).

Solicited-Node Address: FF02:0:0:0:0:1:FFXX:XXXX

The above multicast address is computed as a function of a node's
unicast and anycast addresses. The solicited-node multicast address
is formed by taking the low-order 24 bits of the address (unicast or
anycast) and appending those bits to the prefix
FF02:0:0:0:0:1:FF00::/104 resulting in a multicast address in the
range

FF02:0:0:0:0:1:FF00:0000

to

FF02:0:0:0:0:1:FFFF:FFFF

For example, the solicited node multicast address corresponding to
the IPv6 address 4037::01:800:200E:8C6C is FF02::1:FF0E:8C6C. IPv6
addresses that differ only in the high-order bits, e.g. due to
multiple high-order prefixes associated with different aggregations,
will map to the same solicited-node address thereby reducing the
number of multicast addresses a node must join.

A node is required to compute and join the associated Solicited-Node
multicast addresses for every unicast and anycast address it is
assigned.

2.7.2 Assignment of New IPv6 Multicast Addresses

The current approach [ETHER] to map IPv6 multicast addresses into
IEEE 802 MAC addresses takes the low order 32 bits of the IPv6
multicast address and uses it to create a MAC address. Note that
Token Ring networks are handled differently. This is defined in
[TOKEN]. Group ID's less than or equal to 32 bits will generate
unique MAC addresses. Due to this new IPv6 multicast addresses
should be assigned so that the group identifier is always in the low
order 32 bits as shown in the following:

| 8 | 4 | 4 | 80 bits | 32 bits |
+------ -+----+----+---------------------------+-----------------+
|11111111|flgs|scop| reserved must be zero | group ID |
+--------+----+----+---------------------------+-----------------+

While this limits the number of permanent IPv6 multicast groups to
2^32 this is unlikely to be a limitation in the future. If it
becomes necessary to exceed this limit in the future multicast will
still work but the processing will be sightly slower.

Additional IPv6 multicast addresses are defined and registered by the
IANA [MASGN].

2.8 A Node's Required Addresses

A host is required to recognize the following addresses as
identifying itself:

o Its Link-Local Address for each interface
o Assigned Unicast Addresses
o Loopback Address
o All-Nodes Multicast Addresses
o Solicited-Node Multicast Address for each of its assigned
unicast and anycast addresses
o Multicast Addresses of all other groups to which the host
belongs.

A router is required to recognize all addresses that a host is
required to recognize, plus the following addresses as identifying
itself:

o The Subnet-Router anycast addresses for the interfaces it is
configured to act as a router on.
o All other Anycast addresses with which the router has been
configured.
o All-Routers Multicast Addresses

o Multicast Addresses of all other groups to which the router
belongs.

The only address prefixes which should be predefined in an
implementation are the:

o Unspecified Address
o Loopback Address
o Multicast Prefix (FF)
o Local-Use Prefixes (Link-Local and Site-Local)
o Pre-Defined Multicast Addresses
o IPv4-Compatible Prefixes

Implementations should assume all other addresses are unicast unless
specifically configured (e.g., anycast addresses).

3. Security Considerations

IPv6 addressing documents do not have any direct impact on Internet
infrastructure security. Authentication of IPv6 packets is defined
in [AUTH].

APPENDIX A : Creating EUI-64 based Interface Identifiers
--------------------------------------------------------

Depending on the characteristics of a specific link or node there are
a number of approaches for creating EUI-64 based interface
identifiers. This appendix describes some of these approaches.

Links or Nodes with EUI-64 Identifiers

The only change needed to transform an EUI-64 identifier to an
interface identifier is to invert the "u" (universal/local) bit. For
example, a globally unique EUI-64 identifier of the form:

|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+----------------+

where "c" are the bits of the assigned company_id, "0" is the value
of the universal/local bit to indicate global scope, "g" is
individual/group bit, and "m" are the bits of the manufacturer-
selected extension identifier. The IPv6 interface identifier would
be of the form:

|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|cccccc1gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+----------------+

The only change is inverting the value of the universal/local bit.

Links or Nodes with IEEE 802 48 bit MAC's

[EUI64] defines a method to create a EUI-64 identifier from an IEEE
48bit MAC identifier. This is to insert two octets, with hexadecimal
values of 0xFF and 0xFE, in the middle of the 48 bit MAC (between the
company_id and vendor supplied id). For example the 48 bit MAC with
global scope:

|0 1|1 3|3 4|
|0 5|6 1|2 7|
+----------------+----------------+----------------+
|cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+

where "c" are the bits of the assigned company_id, "0" is the value
of the universal/local bit to indicate global scope, "g" is
individual/group bit, and "m" are the bits of the manufacturer-
selected extension identifier. The interface identifier would be of
the form:

|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|cccccc1gcccccccc|cccccccc11111111|11111110mmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+----------------+

When IEEE 802 48bit MAC addresses are available (on an interface or a
node), an implementation should use them to create interface
identifiers due to their availability and uniqueness properties.

Links with Non-Global Identifiers

There are a number of types of links that, while multi-access, do not
have globally unique link identifiers. Examples include LocalTalk
and Arcnet. The method to create an EUI-64 formatted identifier is
to take the link identifier (e.g., the LocalTalk 8 bit node
identifier) and zero fill it to the left. For example a LocalTalk 8
bit node identifier of hexadecimal value 0x4F results in the
following interface identifier:

|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|0000000000000000|0000000000000000|0000000000000000|0000000001001111|
+----------------+----------------+----------------+----------------+

Note that this results in the universal/local bit set to "0" to
indicate local scope.

Links without Identifiers

There are a number of links that do not have any type of built-in
identifier. The most common of these are serial links and configured
tunnels. Interface identifiers must be chosen that are unique for
the link.

When no built-in identifier is available on a link the preferred
approach is to use a global interface identifier from another
interface or one which is assigned to the node itself. To use this
approach no other interface connecting the same node to the same link
may use the same identifier.

If there is no global interface identifier available for use on the
link the implementation needs to create a local scope interface
identifier. The only requirement is that it be unique on the link.
There are many possible approaches to select a link-unique interface
identifier. They include:

Manual Configuration
Generated Random Number
Node Serial Number (or other node-specific token)

The link-unique interface identifier should be generated in a manner
that it does not change after a reboot of a node or if interfaces are
added or deleted from the node.

The selection of the appropriate algorithm is link and implementation
dependent. The details on forming interface identifiers are defined
in the appropriate "IPv6 over <link>" specification. It is strongly
recommended that a collision detection algorithm be implemented as
part of any automatic algorithm.

APPENDIX B: ABNF Description of Text Representations
----------------------------------------------------

This appendix defines the text representation of IPv6 addresses and
prefixes in Augmented BNF [ABNF] for reference purposes.

IPv6address = hexpart [ ":" IPv4address ]
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT

IPv6prefix = hexpart "/" 1*2DIGIT

hexpart = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG

APPENDIX C: CHANGES FROM RFC-1884
---------------------------------

The following changes were made from RFC-1884 "IP Version 6
Addressing Architecture":

- Added an appendix providing a ABNF description of text
representations.
- Clarification that link unique identifiers not change after
reboot or other interface reconfigurations.
- Clarification of Address Model based on comments.
- Changed aggregation format terminology to be consistent with
aggregation draft.
- Added text to allow interface identifier to be used on more than
one interface on same node.
- Added rules for defining new multicast addresses.
- Added appendix describing procedures for creating EUI-64 based
interface ID's.
- Added notation for defining IPv6 prefixes.
- Changed solicited node multicast definition to use a longer
prefix.
- Added site scope all routers multicast address.
- Defined Aggregatable Global Unicast Addresses to use "001" Format
Prefix.
- Changed "010" (Provider-Based Unicast) and "100" (Reserved for
Geographic) Format Prefixes to Unassigned.
- Added section on Interface ID definition for unicast addresses.
Requires use of EUI-64 in range of format prefixes and rules for
setting global/local scope bit in EUI-64.
- Updated NSAP text to reflect working in RFC1888.
- Removed protocol specific IPv6 multicast addresses (e.g., DHCP)
and referenced the IANA definitions.
- Removed section "Unicast Address Example". Had become OBE.
- Added new and updated references.
- Minor text clarifications and improvements.

REFERENCES

[ABNF] Crocker, D., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, November 1997.

[AGGR] Hinden, R., O'Dell, M., and S. Deering, "An
Aggregatable Global Unicast Address Format", RFC 2374, July
1998.

[AUTH] Atkinson, R., "IP Authentication Header", RFC 1826, August
1995.

[ANYCST] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.

[CIDR] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): An Address Assignment and
Aggregation Strategy", RFC 1519, September 1993.

[ETHER] Crawford, M., "Transmission of IPv6 Pacekts over Ethernet
Networks", Work in Progress.

[EUI64] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
Registration Authority",
http://standards.ieee.org/db/oui/tutorials/EUI64.html,
March 1997.

[FDDI] Crawford, M., "Transmission of IPv6 Packets over FDDI
Networks", Work in Progress.

[IPV6] Deering, S., and R. Hinden, Editors, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 1883, December 1995.

[MASGN] Hinden, R., and S. Deering, "IPv6 Multicast Address
Assignments", RFC 2375, July 1998.

[NSAP] Bound, J., Carpenter, B., Harrington, D., Houldsworth, J.,
and A. Lloyd, "OSI NSAPs and IPv6", RFC 1888, August 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[TOKEN] Thomas, S., "Transmission of IPv6 Packets over Token Ring
Networks", Work in Progress.

[TRAN] Gilligan, R., and E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC 1993, April 1996.

AUTHORS' ADDRESSES

Robert M. Hinden
Nokia
232 Java Drive
Sunnyvale, CA 94089
USA

Phone: +1 408 990-2004
Fax: +1 408 743-5677
EMail: hinden@iprg.nokia.com

Stephen E. Deering
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA

Phone: +1 408 527-8213
Fax: +1 408 527-8254
EMail: deering@cisco.com

Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

RFC 2463 – Internet Control Message Protocol (ICMPv6)

 
Network Working Group                                           A. Conta
Request for Comments: 2463 Lucent
Obsoletes: 1885 S. Deering
Category: Standards Track Cisco Systems
December 1998

Internet Control Message Protocol (ICMPv6)
for the Internet Protocol Version 6 (IPv6)
Specification

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This document specifies a set of Internet Control Message Protocol
(ICMP) messages for use with version 6 of the Internet Protocol
(IPv6).

Table of Contents

1. Introduction........................................2
2. ICMPv6 (ICMP for IPv6)..............................2
2.1 Message General Format.......................2
2.2 Message Source Address Determination.........3
2.3 Message Checksum Calculation.................4
2.4 Message Processing Rules.....................4
3. ICMPv6 Error Messages...............................6
3.1 Destination Unreachable Message..............6
3.2 Packet Too Big Message...................... 8
3.3 Time Exceeded Message....................... 9
3.4 Parameter Problem Message...................10
4. ICMPv6 Informational Messages......................11
4.1 Echo Request Message........................11
4.2 Echo Reply Message..........................12
5. Security Considerations............................13
6. References.........................................14
7. Acknowledgments....................................15
8. Authors' Addresses.................................16
Appendix A - Changes since RFC 1885...................17
Full Copyright Statement..............................18

1. Introduction


The Internet Protocol, version 6 (IPv6) is a new version of IP. IPv6
uses the Internet Control Message Protocol (ICMP) as defined for IPv4
[RFC-792], with a number of changes. The resulting protocol is
called ICMPv6, and has an IPv6 Next Header value of 58.

This document describes the format of a set of control messages used
in ICMPv6. It does not describe the procedures for using these
messages to achieve functions like Path MTU discovery; such
procedures are described in other documents (e.g., [PMTU]). Other
documents may also introduce additional ICMPv6 message types, such as
Neighbor Discovery messages [IPv6-DISC], subject to the general rules
for ICMPv6 messages given in section 2 of this document.

Terminology defined in the IPv6 specification [IPv6] and the IPv6
Routing and Addressing specification [IPv6-ADDR] applies to this
document as well.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-2119].

2. ICMPv6 (ICMP for IPv6)


ICMPv6 is used by IPv6 nodes to report errors encountered in
processing packets, and to perform other internet-layer functions,
such as diagnostics (ICMPv6 "ping"). ICMPv6 is an integral part of
IPv6 and MUST be fully implemented by every IPv6 node.

2.1 Message General Format


ICMPv6 messages are grouped into two classes: error messages and
informational messages. Error messages are identified as such by
having a zero in the high-order bit of their message Type field
values. Thus, error messages have message Types from 0 to 127;
informational messages have message Types from 128 to 255.

This document defines the message formats for the following ICMPv6
messages:

ICMPv6 error messages:

1 Destination Unreachable (see section 3.1)
2 Packet Too Big (see section 3.2)
3 Time Exceeded (see section 3.3)
4 Parameter Problem (see section 3.4)

ICMPv6 informational messages:

128 Echo Request (see section 4.1)
129 Echo Reply (see section 4.2)

Every ICMPv6 message is preceded by an IPv6 header and zero or more
IPv6 extension headers. The ICMPv6 header is identified by a Next
Header value of 58 in the immediately preceding header. (NOTE: this
is different than the value used to identify ICMP for IPv4.)

The ICMPv6 messages have the following general format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Message Body +
| |

The type field indicates the type of the message. Its value
determines the format of the remaining data.

The code field depends on the message type. It is used to create an
additional level of message granularity.

The checksum field is used to detect data corruption in the ICMPv6
message and parts of the IPv6 header.

2.2 Message Source Address Determination


A node that sends an ICMPv6 message has to determine both the Source
and Destination IPv6 Addresses in the IPv6 header before calculating
the checksum. If the node has more than one unicast address, it must
choose the Source Address of the message as follows:

(a) If the message is a response to a message sent to one of the
node's unicast addresses, the Source Address of the reply must
be that same address.

(b) If the message is a response to a message sent to a multicast or
anycast group in which the node is a member, the Source Address
of the reply must be a unicast address belonging to the
interface on which the multicast or anycast packet was received.

(c) If the message is a response to a message sent to an address
that does not belong to the node, the Source Address should be
that unicast address belonging to the node that will be most
helpful in diagnosing the error. For example, if the message is
a response to a packet forwarding action that cannot complete
successfully, the Source Address should be a unicast address
belonging to the interface on which the packet forwarding
failed.

(d) Otherwise, the node's routing table must be examined to
determine which interface will be used to transmit the message
to its destination, and a unicast address belonging to that
interface must be used as the Source Address of the message.

2.3 Message Checksum Calculation


The checksum is the 16-bit one's complement of the one's complement
sum of the entire ICMPv6 message starting with the ICMPv6 message
type field, prepended with a "pseudo-header" of IPv6 header fields,
as specified in [IPv6, section 8.1]. The Next Header value used in
the pseudo-header is 58. (NOTE: the inclusion of a pseudo-header in
the ICMPv6 checksum is a change from IPv4; see [IPv6] for the
rationale for this change.)

For computing the checksum, the checksum field is set to zero.

2.4 Message Processing Rules


Implementations MUST observe the following rules when processing
ICMPv6 messages (from [RFC-1122]):

(a) If an ICMPv6 error message of unknown type is received, it MUST
be passed to the upper layer.

(b) If an ICMPv6 informational message of unknown type is received,
it MUST be silently discarded.

(c) Every ICMPv6 error message (type < 128) includes as much of the
IPv6 offending (invoking) packet (the packet that caused the
error) as will fit without making the error message packet
exceed the minimum IPv6 MTU [IPv6].

(d) In those cases where the internet-layer protocol is required to
pass an ICMPv6 error message to the upper-layer process, the
upper-layer protocol type is extracted from the original packet
(contained in the body of the ICMPv6 error message) and used to
select the appropriate upper-layer process to handle the error.

If the original packet had an unusually large amount of
extension headers, it is possible that the upper-layer protocol
type may not be present in the ICMPv6 message, due to truncation
of the original packet to meet the minimum IPv6 MTU [IPv6]
limit. In that case, the error message is silently dropped
after any IPv6-layer processing.

(e) An ICMPv6 error message MUST NOT be sent as a result of
receiving:

(e.1) an ICMPv6 error message, or

(e.2) a packet destined to an IPv6 multicast address (there are
two exceptions to this rule: (1) the Packet Too Big
Message - Section 3.2 - to allow Path MTU discovery to
work for IPv6 multicast, and (2) the Parameter Problem
Message, Code 2 - Section 3.4 - reporting an unrecognized
IPv6 option that has the Option Type highest-order two
bits set to 10), or

(e.3) a packet sent as a link-layer multicast, (the exception
from e.2 applies to this case too), or

(e.4) a packet sent as a link-layer broadcast, (the exception
from e.2 applies to this case too), or

(e.5) a packet whose source address does not uniquely identify
a single node -- e.g., the IPv6 Unspecified Address, an
IPv6 multicast address, or an address known by the ICMP
message sender to be an IPv6 anycast address.

(f) Finally, in order to limit the bandwidth and forwarding costs
incurred sending ICMPv6 error messages, an IPv6 node MUST limit
the rate of ICMPv6 error messages it sends. This situation may
occur when a source sending a stream of erroneous packets fails
to heed the resulting ICMPv6 error messages. There are a
variety of ways of implementing the rate-limiting function, for
example:

(f.1) Timer-based - for example, limiting the rate of
transmission of error messages to a given source, or to
any source, to at most once every T milliseconds.

(f.2) Bandwidth-based - for example, limiting the rate at which
error messages are sent from a particular interface to
some fraction F of the attached link's bandwidth.

The limit parameters (e.g., T or F in the above examples) MUST
be configurable for the node, with a conservative default value
(e.g., T = 1 second, NOT 0 seconds, or F = 2 percent, NOT 100
percent).

The following sections describe the message formats for the above
ICMPv6 messages.

3. ICMPv6 Error Messages


3.1 Destination Unreachable Message


0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding the minimum IPv6 MTU [IPv6] |

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 1

Code 0 - no route to destination
1 - communication with destination
administratively prohibited
2 - (not assigned)
3 - address unreachable
4 - port unreachable

Unused This field is unused for all code values.
It must be initialized to zero by the sender
and ignored by the receiver.

Description

A Destination Unreachable message SHOULD be generated by a router, or
by the IPv6 layer in the originating node, in response to a packet
that cannot be delivered to its destination address for reasons other
than congestion. (An ICMPv6 message MUST NOT be generated if a
packet is dropped due to congestion.)

If the reason for the failure to deliver is lack of a matching entry
in the forwarding node's routing table, the Code field is set to 0
(NOTE: this error can occur only in nodes that do not hold a "default
route" in their routing tables).

If the reason for the failure to deliver is administrative
prohibition, e.g., a "firewall filter", the Code field is set to 1.

If there is any other reason for the failure to deliver, e.g.,
inability to resolve the IPv6 destination address into a
corresponding link address, or a link-specific problem of some sort,
then the Code field is set to 3.

A destination node SHOULD send a Destination Unreachable message with
Code 4 in response to a packet for which the transport protocol
(e.g., UDP) has no listener, if that transport protocol has no
alternative means to inform the sender.

Upper layer notification

A node receiving the ICMPv6 Destination Unreachable message MUST
notify the upper-layer process.

3.2 Packet Too Big Message


0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding the minimum IPv6 MTU [IPv6] |

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 2

Code Set to 0 (zero) by the sender and ignored by the
receiver

MTU The Maximum Transmission Unit of the next-hop link.

Description

A Packet Too Big MUST be sent by a router in response to a packet
that it cannot forward because the packet is larger than the MTU of
the outgoing link. The information in this message is used as part
of the Path MTU Discovery process [PMTU].

Sending a Packet Too Big Message makes an exception to one of the
rules of when to send an ICMPv6 error message, in that unlike other
messages, it is sent in response to a packet received with an IPv6
multicast destination address, or a link-layer multicast or link-
layer broadcast address.

Upper layer notification

An incoming Packet Too Big message MUST be passed to the upper-layer
process.

3.3 Time Exceeded Message


0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding the minimum IPv6 MTU [IPv6] |

IPv6 Fields:

Destination Address
Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 3

Code 0 - hop limit exceeded in transit

1 - fragment reassembly time exceeded

Unused This field is unused for all code values.
It must be initialized to zero by the sender
and ignored by the receiver.

Description

If a router receives a packet with a Hop Limit of zero, or a router
decrements a packet's Hop Limit to zero, it MUST discard the packet
and send an ICMPv6 Time Exceeded message with Code 0 to the source of
the packet. This indicates either a routing loop or too small an
initial Hop Limit value.

The rules for selecting the Source Address of this message are
defined in section 2.2.

Upper layer notification

An incoming Time Exceeded message MUST be passed to the upper-layer
process.

3.4 Parameter Problem Message


0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as will fit without the ICMPv6 packet +
| exceeding the minimum IPv6 MTU [IPv6] |

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
packet.

ICMPv6 Fields:

Type 4

Code 0 - erroneous header field encountered

1 - unrecognized Next Header type encountered

2 - unrecognized IPv6 option encountered

Pointer Identifies the octet offset within the
invoking packet where the error was detected.

The pointer will point beyond the end of the ICMPv6
packet if the field in error is beyond what can fit
in the maximum size of an ICMPv6 error message.

Description

If an IPv6 node processing a packet finds a problem with a field in
the IPv6 header or extension headers such that it cannot complete
processing the packet, it MUST discard the packet and SHOULD send an
ICMPv6 Parameter Problem message to the packet's source, indicating
the type and location of the problem.

The pointer identifies the octet of the original packet's header
where the error was detected. For example, an ICMPv6 message with
Type field = 4, Code field = 1, and Pointer field = 40 would indicate

that the IPv6 extension header following the IPv6 header of the
original packet holds an unrecognized Next Header field value.

Upper layer notification

A node receiving this ICMPv6 message MUST notify the upper-layer
process.

4. ICMPv6 Informational Messages


4.1 Echo Request Message


0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-

IPv6 Fields:

Destination Address

Any legal IPv6 address.

ICMPv6 Fields:

Type 128

Code 0

Identifier An identifier to aid in matching Echo Replies
to this Echo Request. May be zero.

Sequence Number

A sequence number to aid in matching Echo Replies
to this Echo Request. May be zero.

Data Zero or more octets of arbitrary data.

Description

Every node MUST implement an ICMPv6 Echo responder function that
receives Echo Requests and sends corresponding Echo Replies. A node
SHOULD also implement an application-layer interface for sending Echo
Requests and receiving Echo Replies, for diagnostic purposes.

Upper layer notification

Echo Request messages MAY be passed to processes receiving ICMP
messages.

4.2 Echo Reply Message


0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-

IPv6 Fields:

Destination Address

Copied from the Source Address field of the invoking
Echo Request packet.

ICMPv6 Fields:

Type 129

Code 0

Identifier The identifier from the invoking Echo Request message.

Sequence The sequence number from the invoking Echo Request
Number message.

Data The data from the invoking Echo Request message.

Description

Every node MUST implement an ICMPv6 Echo responder function that
receives Echo Requests and sends corresponding Echo Replies. A node
SHOULD also implement an application-layer interface for sending Echo
Requests and receiving Echo Replies, for diagnostic purposes.

The source address of an Echo Reply sent in response to a unicast
Echo Request message MUST be the same as the destination address of
that Echo Request message.

An Echo Reply SHOULD be sent in response to an Echo Request message
sent to an IPv6 multicast address. The source address of the reply
MUST be a unicast address belonging to the interface on which the
multicast Echo Request message was received.

The data received in the ICMPv6 Echo Request message MUST be returned
entirely and unmodified in the ICMPv6 Echo Reply message.

Upper layer notification

Echo Reply messages MUST be passed to the process that originated an
Echo Request message. It may be passed to processes that did not
originate the Echo Request message.

5. Security Considerations


5.1 Authentication and Encryption of ICMP messages


ICMP protocol packet exchanges can be authenticated using the IP
Authentication Header [IPv6-AUTH]. A node SHOULD include an
Authentication Header when sending ICMP messages if a security
association for use with the IP Authentication Header exists for the
destination address. The security associations may have been created
through manual configuration or through the operation of some key
management protocol.

Received Authentication Headers in ICMP packets MUST be verified for
correctness and packets with incorrect authentication MUST be ignored
and discarded.

It SHOULD be possible for the system administrator to configure a
node to ignore any ICMP messages that are not authenticated using
either the Authentication Header or Encapsulating Security Payload.
Such a switch SHOULD default to allowing unauthenticated messages.

Confidentiality issues are addressed by the IP Security Architecture
and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-
ESP].

5.2 ICMP Attacks


ICMP messages may be subject to various attacks. A complete
discussion can be found in the IP Security Architecture [IPv6-SA]. A
brief discussion of such attacks and their prevention is as follows:

1. ICMP messages may be subject to actions intended to cause the
receiver believe the message came from a different source than the
message originator. The protection against this attack can be
achieved by applying the IPv6 Authentication mechanism [IPv6-Auth]
to the ICMP message.

2. ICMP messages may be subject to actions intended to cause the
message or the reply to it go to a destination different than the
message originator's intention. The ICMP checksum calculation
provides a protection mechanism against changes by a malicious
interceptor in the destination and source address of the IP packet
carrying that message, provided the ICMP checksum field is
protected against change by authentication [IPv6-Auth] or
encryption [IPv6-ESP] of the ICMP message.

3. ICMP messages may be subject to changes in the message fields, or
payload. The authentication [IPv6-Auth] or encryption [IPv6-ESP]
of the ICMP message is a protection against such actions.

4. ICMP messages may be used as attempts to perform denial of service
attacks by sending back to back erroneous IP packets. An
implementation that correctly followed section 2.4, paragraph (f)
of this specifications, would be protected by the ICMP error rate
limiting mechanism.

6. References


[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version
6, (IPv6) Specification", RFC 2460, December 1998.

[IPv6-ADDR] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.

[IPv6-DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.

[RFC-792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.

[RFC-1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 5, RFC 1122, August 1989.

[PMTU] McCann, J., Deering, S. and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.

[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[IPv6-SA] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[IPv6-Auth] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.

[IPv6-ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Protocol (ESP)", RFC 2406, November 1998.

7. Acknowledgments


The document is derived from previous ICMP drafts of the SIPP and
IPng working group.

The IPng working group and particularly Robert Elz, Jim Bound, Bill
Simpson, Thomas Narten, Charlie Lynn, Bill Fink, Scott Bradner,
Dimitri Haskin, and Bob Hinden (in chronological order) provided
extensive review information and feedback.

8. Authors' Addresses


Alex Conta
Lucent Technologies Inc.
300 Baker Ave, Suite 100
Concord, MA 01742
USA

Phone: +1 978 287-2842
EMail: aconta@lucent.com

Stephen Deering
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA

Phone: +1 408 527-8213
EMail: deering@cisco.com

Appendix A - Changes from RFC 1885


Version 2-02

- Excluded mentioning informational replies from paragraph (f.2) of
section 2.4.
- In "Upper layer notification" sections changed "upper-layer
protocol" and "User Interface" to "process".
- Changed section 5.2, item 2 and 3 to also refer to AH
authentication.
- Removed item 5. from section 5.2 on denial of service attacks.
- Updated phone numbers and Email addresses in the "Authors'
Addresses" section.

Version 2-01

- Replaced all references to "576 octets" as the maximum for an ICMP
message size with "minimum IPv6 MTU" as defined by the base IPv6
specification.
- Removed rate control from informational messages.
- Added requirement that receivers ignore Code value in Packet Too
Big message.
- Removed "Not a Neighbor" (code 2) from destination unreachable
message.
- Fixed typos and update references.

Version 2-00

- Applied rate control to informational messages
- Removed section 2.4 on Group Management ICMP messages
- Removed references to IGMP in Abstract and Section 1.
- Updated references to other IPv6 documents
- Removed references to RFC-1112 in Abstract, and Section 1, and to
RFC-1191 in section 1, and section 3.2
- Added security section
- Added Appendix A - changes

Full Copyright Statement


Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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RFC 2401 – Security Architecture for the Internet Protocol

 
Network Working Group                                            S. Kent
Request for Comments: 2401 BBN Corp
Obsoletes: 1825 R. Atkinson
Category: Standards Track @Home Network
November 1998

Security Architecture for the Internet Protocol

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Table of Contents

1. Introduction........................................................3
1.1 Summary of Contents of Document..................................3
1.2 Audience.........................................................3
1.3 Related Documents................................................4
2. Design Objectives...................................................4
2.1 Goals/Objectives/Requirements/Problem Description................4
2.2 Caveats and Assumptions..........................................5
3. System Overview.....................................................5
3.1 What IPsec Does..................................................6
3.2 How IPsec Works..................................................6
3.3 Where IPsec May Be Implemented...................................7
4. Security Associations...............................................8
4.1 Definition and Scope.............................................8
4.2 Security Association Functionality..............................10
4.3 Combining Security Associations.................................11
4.4 Security Association Databases..................................13
4.4.1 The Security Policy Database (SPD).........................14
4.4.2 Selectors..................................................17
4.4.3 Security Association Database (SAD)........................21
4.5 Basic Combinations of Security Associations.....................24
4.6 SA and Key Management...........................................26
4.6.1 Manual Techniques..........................................27
4.6.2 Automated SA and Key Management............................27
4.6.3 Locating a Security Gateway................................28
4.7 Security Associations and Multicast.............................29

5. IP Traffic Processing..............................................30
5.1 Outbound IP Traffic Processing..................................30
5.1.1 Selecting and Using an SA or SA Bundle.....................30
5.1.2 Header Construction for Tunnel Mode........................31
5.1.2.1 IPv4 -- Header Construction for Tunnel Mode...........31
5.1.2.2 IPv6 -- Header Construction for Tunnel Mode...........32
5.2 Processing Inbound IP Traffic...................................33
5.2.1 Selecting and Using an SA or SA Bundle.....................33
5.2.2 Handling of AH and ESP tunnels.............................34
6. ICMP Processing (relevant to IPsec)................................35
6.1 PMTU/DF Processing..............................................36
6.1.1 DF Bit.....................................................36
6.1.2 Path MTU Discovery (PMTU)..................................36
6.1.2.1 Propagation of PMTU...................................36
6.1.2.2 Calculation of PMTU...................................37
6.1.2.3 Granularity of PMTU Processing........................37
6.1.2.4 PMTU Aging............................................38
7. Auditing...........................................................39
8. Use in Systems Supporting Information Flow Security................39
8.1 Relationship Between Security Associations and Data Sensitivity.40
8.2 Sensitivity Consistency Checking................................40
8.3 Additional MLS Attributes for Security Association Databases....41
8.4 Additional Inbound Processing Steps for MLS Networking..........41
8.5 Additional Outbound Processing Steps for MLS Networking.........41
8.6 Additional MLS Processing for Security Gateways.................42
9. Performance Issues.................................................42
10. Conformance Requirements..........................................43
11. Security Considerations...........................................43
12. Differences from RFC 1825.........................................43
Acknowledgements......................................................44
Appendix A -- Glossary................................................45
Appendix B -- Analysis/Discussion of PMTU/DF/Fragmentation Issues.....48
B.1 DF bit..........................................................48
B.2 Fragmentation...................................................48
B.3 Path MTU Discovery..............................................52
B.3.1 Identifying the Originating Host(s)........................53
B.3.2 Calculation of PMTU........................................55
B.3.3 Granularity of Maintaining PMTU Data.......................56
B.3.4 Per Socket Maintenance of PMTU Data........................57
B.3.5 Delivery of PMTU Data to the Transport Layer...............57
B.3.6 Aging of PMTU Data.........................................57
Appendix C -- Sequence Space Window Code Example......................58
Appendix D -- Categorization of ICMP messages.........................60
References............................................................63
Disclaimer............................................................64
Author Information....................................................65
Full Copyright Statement..............................................66

1. Introduction

1.1 Summary of Contents of Document

This memo specifies the base architecture for IPsec compliant
systems. The goal of the architecture is to provide various security
services for traffic at the IP layer, in both the IPv4 and IPv6
environments. This document describes the goals of such systems,
their components and how they fit together with each other and into
the IP environment. It also describes the security services offered
by the IPsec protocols, and how these services can be employed in the
IP environment. This document does not address all aspects of IPsec
architecture. Subsequent documents will address additional
architectural details of a more advanced nature, e.g., use of IPsec
in NAT environments and more complete support for IP multicast. The
following fundamental components of the IPsec security architecture
are discussed in terms of their underlying, required functionality.
Additional RFCs (see Section 1.3 for pointers to other documents)
define the protocols in (a), (c), and (d).

a. Security Protocols -- Authentication Header (AH) and
Encapsulating Security Payload (ESP)
b. Security Associations -- what they are and how they work,
how they are managed, associated processing
c. Key Management -- manual and automatic (The Internet Key
Exchange (IKE))
d. Algorithms for authentication and encryption

This document is not an overall Security Architecture for the
Internet; it addresses security only at the IP layer, provided
through the use of a combination of cryptographic and protocol
security mechanisms.

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [Bra97].

1.2 Audience

The target audience for this document includes implementers of this
IP security technology and others interested in gaining a general
background understanding of this system. In particular, prospective
users of this technology (end users or system administrators) are
part of the target audience. A glossary is provided as an appendix

to help fill in gaps in background/vocabulary. This document assumes
that the reader is familiar with the Internet Protocol, related
networking technology, and general security terms and concepts.

1.3 Related Documents

As mentioned above, other documents provide detailed definitions of
some of the components of IPsec and of their inter-relationship.
They include RFCs on the following topics:

a. "IP Security Document Roadmap" [TDG97] -- a document
providing guidelines for specifications describing encryption
and authentication algorithms used in this system.
b. security protocols -- RFCs describing the Authentication
Header (AH) [KA98a] and Encapsulating Security Payload (ESP)
[KA98b] protocols.
c. algorithms for authentication and encryption -- a separate
RFC for each algorithm.
d. automatic key management -- RFCs on "The Internet Key
Exchange (IKE)" [HC98], "Internet Security Association and
Key Management Protocol (ISAKMP)" [MSST97],"The OAKLEY Key
Determination Protocol" [Orm97], and "The Internet IP
Security Domain of Interpretation for ISAKMP" [Pip98].

2. Design Objectives

2.1 Goals/Objectives/Requirements/Problem Description

IPsec is designed to provide interoperable, high quality,
cryptographically-based security for IPv4 and IPv6. The set of
security services offered includes access control, connectionless
integrity, data origin authentication, protection against replays (a
form of partial sequence integrity), confidentiality (encryption),
and limited traffic flow confidentiality. These services are
provided at the IP layer, offering protection for IP and/or upper
layer protocols.

These objectives are met through the use of two traffic security
protocols, the Authentication Header (AH) and the Encapsulating
Security Payload (ESP), and through the use of cryptographic key
management procedures and protocols. The set of IPsec protocols
employed in any context, and the ways in which they are employed,
will be determined by the security and system requirements of users,
applications, and/or sites/organizations.

When these mechanisms are correctly implemented and deployed, they
ought not to adversely affect users, hosts, and other Internet
components that do not employ these security mechanisms for

protection of their traffic. These mechanisms also are designed to
be algorithm-independent. This modularity permits selection of
different sets of algorithms without affecting the other parts of the
implementation. For example, different user communities may select
different sets of algorithms (creating cliques) if required.

A standard set of default algorithms is specified to facilitate
interoperability in the global Internet. The use of these
algorithms, in conjunction with IPsec traffic protection and key
management protocols, is intended to permit system and application
developers to deploy high quality, Internet layer, cryptographic
security technology.

2.2 Caveats and Assumptions

The suite of IPsec protocols and associated default algorithms are
designed to provide high quality security for Internet traffic.
However, the security offered by use of these protocols ultimately
depends on the quality of the their implementation, which is outside
the scope of this set of standards. Moreover, the security of a
computer system or network is a function of many factors, including
personnel, physical, procedural, compromising emanations, and
computer security practices. Thus IPsec is only one part of an
overall system security architecture.

Finally, the security afforded by the use of IPsec is critically
dependent on many aspects of the operating environment in which the
IPsec implementation executes. For example, defects in OS security,
poor quality of random number sources, sloppy system management
protocols and practices, etc. can all degrade the security provided
by IPsec. As above, none of these environmental attributes are
within the scope of this or other IPsec standards.

3. System Overview

This section provides a high level description of how IPsec works,
the components of the system, and how they fit together to provide
the security services noted above. The goal of this description is
to enable the reader to "picture" the overall process/system, see how
it fits into the IP environment, and to provide context for later
sections of this document, which describe each of the components in
more detail.

An IPsec implementation operates in a host or a security gateway
environment, affording protection to IP traffic. The protection
offered is based on requirements defined by a Security Policy
Database (SPD) established and maintained by a user or system
administrator, or by an application operating within constraints

established by either of the above. In general, packets are selected
for one of three processing modes based on IP and transport layer
header information (Selectors, Section 4.4.2) matched against entries
in the database (SPD). Each packet is either afforded IPsec security
services, discarded, or allowed to bypass IPsec, based on the
applicable database policies identified by the Selectors.

3.1 What IPsec Does

IPsec provides security services at the IP layer by enabling a system
to select required security protocols, determine the algorithm(s) to
use for the service(s), and put in place any cryptographic keys
required to provide the requested services. IPsec can be used to
protect one or more "paths" between a pair of hosts, between a pair
of security gateways, or between a security gateway and a host. (The
term "security gateway" is used throughout the IPsec documents to
refer to an intermediate system that implements IPsec protocols. For
example, a router or a firewall implementing IPsec is a security
gateway.)

The set of security services that IPsec can provide includes access
control, connectionless integrity, data origin authentication,
rejection of replayed packets (a form of partial sequence integrity),
confidentiality (encryption), and limited traffic flow
confidentiality. Because these services are provided at the IP
layer, they can be used by any higher layer protocol, e.g., TCP, UDP,
ICMP, BGP, etc.

The IPsec DOI also supports negotiation of IP compression [SMPT98],
motivated in part by the observation that when encryption is employed
within IPsec, it prevents effective compression by lower protocol
layers.

3.2 How IPsec Works

IPsec uses two protocols to provide traffic security --
Authentication Header (AH) and Encapsulating Security Payload (ESP).
Both protocols are described in more detail in their respective RFCs
[KA98a, KA98b].

o The IP Authentication Header (AH) [KA98a] provides
connectionless integrity, data origin authentication, and an
optional anti-replay service.
o The Encapsulating Security Payload (ESP) protocol [KA98b] may
provide confidentiality (encryption), and limited traffic flow
confidentiality. It also may provide connectionless

integrity, data origin authentication, and an anti-replay
service. (One or the other set of these security services
must be applied whenever ESP is invoked.)
o Both AH and ESP are vehicles for access control, based on the
distribution of cryptographic keys and the management of
traffic flows relative to these security protocols.

These protocols may be applied alone or in combination with each
other to provide a desired set of security services in IPv4 and IPv6.
Each protocol supports two modes of use: transport mode and tunnel
mode. In transport mode the protocols provide protection primarily
for upper layer protocols; in tunnel mode, the protocols are applied
to tunneled IP packets. The differences between the two modes are
discussed in Section 4.

IPsec allows the user (or system administrator) to control the
granularity at which a security service is offered. For example, one
can create a single encrypted tunnel to carry all the traffic between
two security gateways or a separate encrypted tunnel can be created
for each TCP connection between each pair of hosts communicating
across these gateways. IPsec management must incorporate facilities
for specifying:

o which security services to use and in what combinations
o the granularity at which a given security protection should be
applied
o the algorithms used to effect cryptographic-based security

Because these security services use shared secret values
(cryptographic keys), IPsec relies on a separate set of mechanisms
for putting these keys in place. (The keys are used for
authentication/integrity and encryption services.) This document
requires support for both manual and automatic distribution of keys.
It specifies a specific public-key based approach (IKE -- [MSST97,
Orm97, HC98]) for automatic key management, but other automated key
distribution techniques MAY be used. For example, KDC-based systems
such as Kerberos and other public-key systems such as SKIP could be
employed.

3.3 Where IPsec May Be Implemented

There are several ways in which IPsec may be implemented in a host or
in conjunction with a router or firewall (to create a security
gateway). Several common examples are provided below:

a. Integration of IPsec into the native IP implementation. This
requires access to the IP source code and is applicable to
both hosts and security gateways.

b. "Bump-in-the-stack" (BITS) implementations, where IPsec is
implemented "underneath" an existing implementation of an IP
protocol stack, between the native IP and the local network
drivers. Source code access for the IP stack is not required
in this context, making this implementation approach
appropriate for use with legacy systems. This approach, when
it is adopted, is usually employed in hosts.

c. The use of an outboard crypto processor is a common design
feature of network security systems used by the military, and
of some commercial systems as well. It is sometimes referred
to as a "Bump-in-the-wire" (BITW) implementation. Such
implementations may be designed to serve either a host or a
gateway (or both). Usually the BITW device is IP
addressable. When supporting a single host, it may be quite
analogous to a BITS implementation, but in supporting a
router or firewall, it must operate like a security gateway.

4. Security Associations

This section defines Security Association management requirements for
all IPv6 implementations and for those IPv4 implementations that
implement AH, ESP, or both. The concept of a "Security Association"
(SA) is fundamental to IPsec. Both AH and ESP make use of SAs and a
major function of IKE is the establishment and maintenance of
Security Associations. All implementations of AH or ESP MUST support
the concept of a Security Association as described below. The
remainder of this section describes various aspects of Security
Association management, defining required characteristics for SA
policy management, traffic processing, and SA management techniques.

4.1 Definition and Scope

A Security Association (SA) is a simplex "connection" that affords
security services to the traffic carried by it. Security services
are afforded to an SA by the use of AH, or ESP, but not both. If
both AH and ESP protection is applied to a traffic stream, then two
(or more) SAs are created to afford protection to the traffic stream.
To secure typical, bi-directional communication between two hosts, or
between two security gateways, two Security Associations (one in each
direction) are required.

A security association is uniquely identified by a triple consisting
of a Security Parameter Index (SPI), an IP Destination Address, and a
security protocol (AH or ESP) identifier. In principle, the
Destination Address may be a unicast address, an IP broadcast
address, or a multicast group address. However, IPsec SA management
mechanisms currently are defined only for unicast SAs. Hence, in the

discussions that follow, SAs will be described in the context of
point-to-point communication, even though the concept is applicable
in the point-to-multipoint case as well.

As noted above, two types of SAs are defined: transport mode and
tunnel mode. A transport mode SA is a security association between
two hosts. In IPv4, a transport mode security protocol header
appears immediately after the IP header and any options, and before
any higher layer protocols (e.g., TCP or UDP). In IPv6, the security
protocol header appears after the base IP header and extensions, but
may appear before or after destination options, and before higher
layer protocols. In the case of ESP, a transport mode SA provides
security services only for these higher layer protocols, not for the
IP header or any extension headers preceding the ESP header. In the
case of AH, the protection is also extended to selected portions of
the IP header, selected portions of extension headers, and selected
options (contained in the IPv4 header, IPv6 Hop-by-Hop extension
header, or IPv6 Destination extension headers). For more details on
the coverage afforded by AH, see the AH specification [KA98a].

A tunnel mode SA is essentially an SA applied to an IP tunnel.
Whenever either end of a security association is a security gateway,
the SA MUST be tunnel mode. Thus an SA between two security gateways
is always a tunnel mode SA, as is an SA between a host and a security
gateway. Note that for the case where traffic is destined for a
security gateway, e.g., SNMP commands, the security gateway is acting
as a host and transport mode is allowed. But in that case, the
security gateway is not acting as a gateway, i.e., not transiting
traffic. Two hosts MAY establish a tunnel mode SA between
themselves. The requirement for any (transit traffic) SA involving a
security gateway to be a tunnel SA arises due to the need to avoid
potential problems with regard to fragmentation and reassembly of
IPsec packets, and in circumstances where multiple paths (e.g., via
different security gateways) exist to the same destination behind the
security gateways.

For a tunnel mode SA, there is an "outer" IP header that specifies
the IPsec processing destination, plus an "inner" IP header that
specifies the (apparently) ultimate destination for the packet. The
security protocol header appears after the outer IP header, and
before the inner IP header. If AH is employed in tunnel mode,
portions of the outer IP header are afforded protection (as above),
as well as all of the tunneled IP packet (i.e., all of the inner IP
header is protected, as well as higher layer protocols). If ESP is
employed, the protection is afforded only to the tunneled packet, not
to the outer header.

In summary,
a) A host MUST support both transport and tunnel mode.
b) A security gateway is required to support only tunnel
mode. If it supports transport mode, that should be used
only when the security gateway is acting as a host, e.g.,
for network management.

4.2 Security Association Functionality

The set of security services offered by an SA depends on the security
protocol selected, the SA mode, the endpoints of the SA, and on the
election of optional services within the protocol. For example, AH
provides data origin authentication and connectionless integrity for
IP datagrams (hereafter referred to as just "authentication"). The
"precision" of the authentication service is a function of the
granularity of the security association with which AH is employed, as
discussed in Section 4.4.2, "Selectors".

AH also offers an anti-replay (partial sequence integrity) service at
the discretion of the receiver, to help counter denial of service
attacks. AH is an appropriate protocol to employ when
confidentiality is not required (or is not permitted, e.g , due to
government restrictions on use of encryption). AH also provides
authentication for selected portions of the IP header, which may be
necessary in some contexts. For example, if the integrity of an IPv4
option or IPv6 extension header must be protected en route between
sender and receiver, AH can provide this service (except for the
non-predictable but mutable parts of the IP header.)

ESP optionally provides confidentiality for traffic. (The strength
of the confidentiality service depends in part, on the encryption
algorithm employed.) ESP also may optionally provide authentication
(as defined above). If authentication is negotiated for an ESP SA,
the receiver also may elect to enforce an anti-replay service with
the same features as the AH anti-replay service. The scope of the
authentication offered by ESP is narrower than for AH, i.e., the IP
header(s) "outside" the ESP header is(are) not protected. If only
the upper layer protocols need to be authenticated, then ESP
authentication is an appropriate choice and is more space efficient
than use of AH encapsulating ESP. Note that although both
confidentiality and authentication are optional, they cannot both be
omitted. At least one of them MUST be selected.

If confidentiality service is selected, then an ESP (tunnel mode) SA
between two security gateways can offer partial traffic flow
confidentiality. The use of tunnel mode allows the inner IP headers
to be encrypted, concealing the identities of the (ultimate) traffic
source and destination. Moreover, ESP payload padding also can be

invoked to hide the size of the packets, further concealing the
external characteristics of the traffic. Similar traffic flow
confidentiality services may be offered when a mobile user is
assigned a dynamic IP address in a dialup context, and establishes a
(tunnel mode) ESP SA to a corporate firewall (acting as a security
gateway). Note that fine granularity SAs generally are more
vulnerable to traffic analysis than coarse granularity ones which are
carrying traffic from many subscribers.

4.3 Combining Security Associations

The IP datagrams transmitted over an individual SA are afforded
protection by exactly one security protocol, either AH or ESP, but
not both. Sometimes a security policy may call for a combination of
services for a particular traffic flow that is not achievable with a
single SA. In such instances it will be necessary to employ multiple
SAs to implement the required security policy. The term "security
association bundle" or "SA bundle" is applied to a sequence of SAs
through which traffic must be processed to satisfy a security policy.
The order of the sequence is defined by the policy. (Note that the
SAs that comprise a bundle may terminate at different endpoints. For
example, one SA may extend between a mobile host and a security
gateway and a second, nested SA may extend to a host behind the
gateway.)

Security associations may be combined into bundles in two ways:
transport adjacency and iterated tunneling.

o Transport adjacency refers to applying more than one
security protocol to the same IP datagram, without invoking
tunneling. This approach to combining AH and ESP allows
for only one level of combination; further nesting yields
no added benefit (assuming use of adequately strong
algorithms in each protocol) since the processing is
performed at one IPsec instance at the (ultimate)
destination.

Host 1 --- Security ---- Internet -- Security --- Host 2
| | Gwy 1 Gwy 2 | |
| | | |
| -----Security Association 1 (ESP transport)------- |
| |
-------Security Association 2 (AH transport)----------

o Iterated tunneling refers to the application of multiple
layers of security protocols effected through IP tunneling.
This approach allows for multiple levels of nesting, since
each tunnel can originate or terminate at a different IPsec

site along the path. No special treatment is expected for
ISAKMP traffic at intermediate security gateways other than
what can be specified through appropriate SPD entries (See
Case 3 in Section 4.5)

There are 3 basic cases of iterated tunneling -- support is
required only for cases 2 and 3.:

1. both endpoints for the SAs are the same -- The inner and
outer tunnels could each be either AH or ESP, though it
is unlikely that Host 1 would specify both to be the
same, i.e., AH inside of AH or ESP inside of ESP.

Host 1 --- Security ---- Internet -- Security --- Host 2
| | Gwy 1 Gwy 2 | |
| | | |
| -------Security Association 1 (tunnel)---------- | |
| |
---------Security Association 2 (tunnel)--------------

2. one endpoint of the SAs is the same -- The inner and
uter tunnels could each be either AH or ESP.

Host 1 --- Security ---- Internet -- Security --- Host 2
| | Gwy 1 Gwy 2 |
| | | |
| ----Security Association 1 (tunnel)---- |
| |
---------Security Association 2 (tunnel)-------------

3. neither endpoint is the same -- The inner and outer
tunnels could each be either AH or ESP.

Host 1 --- Security ---- Internet -- Security --- Host 2
| Gwy 1 Gwy 2 |
| | | |
| --Security Assoc 1 (tunnel)- |
| |
-----------Security Association 2 (tunnel)-----------

These two approaches also can be combined, e.g., an SA bundle could
be constructed from one tunnel mode SA and one or two transport mode
SAs, applied in sequence. (See Section 4.5 "Basic Combinations of
Security Associations.") Note that nested tunnels can also occur
where neither the source nor the destination endpoints of any of the
tunnels are the same. In that case, there would be no host or
security gateway with a bundle corresponding to the nested tunnels.

For transport mode SAs, only one ordering of security protocols seems
appropriate. AH is applied to both the upper layer protocols and
(parts of) the IP header. Thus if AH is used in a transport mode, in
conjunction with ESP, AH SHOULD appear as the first header after IP,
prior to the appearance of ESP. In that context, AH is applied to
the ciphertext output of ESP. In contrast, for tunnel mode SAs, one
can imagine uses for various orderings of AH and ESP. The required
set of SA bundle types that MUST be supported by a compliant IPsec
implementation is described in Section 4.5.

4.4 Security Association Databases

Many of the details associated with processing IP traffic in an IPsec
implementation are largely a local matter, not subject to
standardization. However, some external aspects of the processing
must be standardized, to ensure interoperability and to provide a
minimum management capability that is essential for productive use of
IPsec. This section describes a general model for processing IP
traffic relative to security associations, in support of these
interoperability and functionality goals. The model described below
is nominal; compliant implementations need not match details of this
model as presented, but the external behavior of such implementations
must be mappable to the externally observable characteristics of this
model.

There are two nominal databases in this model: the Security Policy
Database and the Security Association Database. The former specifies
the policies that determine the disposition of all IP traffic inbound
or outbound from a host, security gateway, or BITS or BITW IPsec
implementation. The latter database contains parameters that are
associated with each (active) security association. This section
also defines the concept of a Selector, a set of IP and upper layer
protocol field values that is used by the Security Policy Database to
map traffic to a policy, i.e., an SA (or SA bundle).

Each interface for which IPsec is enabled requires nominally separate
inbound vs. outbound databases (SAD and SPD), because of the
directionality of many of the fields that are used as selectors.
Typically there is just one such interface, for a host or security
gateway (SG). Note that an SG would always have at least 2
interfaces, but the "internal" one to the corporate net, usually
would not have IPsec enabled and so only one pair of SADs and one
pair of SPDs would be needed. On the other hand, if a host had
multiple interfaces or an SG had multiple external interfaces, it
might be necessary to have separate SAD and SPD pairs for each
interface.

4.4.1 The Security Policy Database (SPD)

Ultimately, a security association is a management construct used to
enforce a security policy in the IPsec environment. Thus an
essential element of SA processing is an underlying Security Policy
Database (SPD) that specifies what services are to be offered to IP
datagrams and in what fashion. The form of the database and its
interface are outside the scope of this specification. However, this
section does specify certain minimum management functionality that
must be provided, to allow a user or system administrator to control
how IPsec is applied to traffic transmitted or received by a host or
transiting a security gateway.

The SPD must be consulted during the processing of all traffic
(INBOUND and OUTBOUND), including non-IPsec traffic. In order to
support this, the SPD requires distinct entries for inbound and
outbound traffic. One can think of this as separate SPDs (inbound
vs. outbound). In addition, a nominally separate SPD must be
provided for each IPsec-enabled interface.

An SPD must discriminate among traffic that is afforded IPsec
protection and traffic that is allowed to bypass IPsec. This applies
to the IPsec protection to be applied by a sender and to the IPsec
protection that must be present at the receiver. For any outbound or
inbound datagram, three processing choices are possible: discard,
bypass IPsec, or apply IPsec. The first choice refers to traffic
that is not allowed to exit the host, traverse the security gateway,
or be delivered to an application at all. The second choice refers
to traffic that is allowed to pass without additional IPsec
protection. The third choice refers to traffic that is afforded
IPsec protection, and for such traffic the SPD must specify the
security services to be provided, protocols to be employed,
algorithms to be used, etc.

For every IPsec implementation, there MUST be an administrative
interface that allows a user or system administrator to manage the
SPD. Specifically, every inbound or outbound packet is subject to
processing by IPsec and the SPD must specify what action will be
taken in each case. Thus the administrative interface must allow the
user (or system administrator) to specify the security processing to
be applied to any packet entering or exiting the system, on a packet
by packet basis. (In a host IPsec implementation making use of a
socket interface, the SPD may not need to be consulted on a per
packet basis, but the effect is still the same.) The management
interface for the SPD MUST allow creation of entries consistent with
the selectors defined in Section 4.4.2, and MUST support (total)
ordering of these entries. It is expected that through the use of
wildcards in various selector fields, and because all packets on a

single UDP or TCP connection will tend to match a single SPD entry,
this requirement will not impose an unreasonably detailed level of
SPD specification. The selectors are analogous to what are found in
a stateless firewall or filtering router and which are currently
manageable this way.

In host systems, applications MAY be allowed to select what security
processing is to be applied to the traffic they generate and consume.
(Means of signalling such requests to the IPsec implementation are
outside the scope of this standard.) However, the system
administrator MUST be able to specify whether or not a user or
application can override (default) system policies. Note that
application specified policies may satisfy system requirements, so
that the system may not need to do additional IPsec processing beyond
that needed to meet an application's requirements. The form of the
management interface is not specified by this document and may differ
for hosts vs. security gateways, and within hosts the interface may
differ for socket-based vs. BITS implementations. However, this
document does specify a standard set of SPD elements that all IPsec
implementations MUST support.

The SPD contains an ordered list of policy entries. Each policy
entry is keyed by one or more selectors that define the set of IP
traffic encompassed by this policy entry. (The required selector
types are defined in Section 4.4.2.) These define the granularity of
policies or SAs. Each entry includes an indication of whether
traffic matching this policy will be bypassed, discarded, or subject
to IPsec processing. If IPsec processing is to be applied, the entry
includes an SA (or SA bundle) specification, listing the IPsec
protocols, modes, and algorithms to be employed, including any
nesting requirements. For example, an entry may call for all
matching traffic to be protected by ESP in transport mode using
3DES-CBC with an explicit IV, nested inside of AH in tunnel mode
using HMAC/SHA-1. For each selector, the policy entry specifies how
to derive the corresponding values for a new Security Association
Database (SAD, see Section 4.4.3) entry from those in the SPD and the
packet (Note that at present, ranges are only supported for IP
addresses; but wildcarding can be expressed for all selectors):

a. use the value in the packet itself -- This will limit use
of the SA to those packets which have this packet's value
for the selector even if the selector for the policy entry
has a range of allowed values or a wildcard for this
selector.
b. use the value associated with the policy entry -- If this
were to be just a single value, then there would be no
difference between (b) and (a). However, if the allowed
values for the selector are a range (for IP addresses) or

wildcard, then in the case of a range,(b) would enable use
of the SA by any packet with a selector value within the
range not just by packets with the selector value of the
packet that triggered the creation of the SA. In the case
of a wildcard, (b) would allow use of the SA by packets
with any value for this selector.

For example, suppose there is an SPD entry where the allowed value
for source address is any of a range of hosts (192.168.2.1 to
192.168.2.10). And suppose that a packet is to be sent that has a
source address of 192.168.2.3. The value to be used for the SA could
be any of the sample values below depending on what the policy entry
for this selector says is the source of the selector value:

source for the example of
value to be new SAD
used in the SA selector value
--------------- ------------
a. packet 192.168.2.3 (one host)
b. SPD entry 192.168.2.1 to 192.168.2.10 (range of hosts)

Note that if the SPD entry had an allowed value of wildcard for the
source address, then the SAD selector value could be wildcard (any
host). Case (a) can be used to prohibit sharing, even among packets
that match the same SPD entry.

As described below in Section 4.4.3, selectors may include "wildcard"
entries and hence the selectors for two entries may overlap. (This
is analogous to the overlap that arises with ACLs or filter entries
in routers or packet filtering firewalls.) Thus, to ensure
consistent, predictable processing, SPD entries MUST be ordered and
the SPD MUST always be searched in the same order, so that the first
matching entry is consistently selected. (This requirement is
necessary as the effect of processing traffic against SPD entries
must be deterministic, but there is no way to canonicalize SPD
entries given the use of wildcards for some selectors.) More detail
on matching of packets against SPD entries is provided in Section 5.

Note that if ESP is specified, either (but not both) authentication
or encryption can be omitted. So it MUST be possible to configure
the SPD value for the authentication or encryption algorithms to be
"NULL". However, at least one of these services MUST be selected,
i.e., it MUST NOT be possible to configure both of them as "NULL".

The SPD can be used to map traffic to specific SAs or SA bundles.
Thus it can function both as the reference database for security
policy and as the map to existing SAs (or SA bundles). (To
accommodate the bypass and discard policies cited above, the SPD also

MUST provide a means of mapping traffic to these functions, even
though they are not, per se, IPsec processing.) The way in which the
SPD operates is different for inbound vs. outbound traffic and it
also may differ for host vs. security gateway, BITS, and BITW
implementations. Sections 5.1 and 5.2 describe the use of the SPD
for outbound and inbound processing, respectively.

Because a security policy may require that more than one SA be
applied to a specified set of traffic, in a specific order, the
policy entry in the SPD must preserve these ordering requirements,
when present. Thus, it must be possible for an IPsec implementation
to determine that an outbound or inbound packet must be processed
thorough a sequence of SAs. Conceptually, for outbound processing,
one might imagine links (to the SAD) from an SPD entry for which
there are active SAs, and each entry would consist of either a single
SA or an ordered list of SAs that comprise an SA bundle. When a
packet is matched against an SPD entry and there is an existing SA or
SA bundle that can be used to carry the traffic, the processing of
the packet is controlled by the SA or SA bundle entry on the list.
For an inbound IPsec packet for which multiple IPsec SAs are to be
applied, the lookup based on destination address, IPsec protocol, and
SPI should identify a single SA.

The SPD is used to control the flow of ALL traffic through an IPsec
system, including security and key management traffic (e.g., ISAKMP)
from/to entities behind a security gateway. This means that ISAKMP
traffic must be explicitly accounted for in the SPD, else it will be
discarded. Note that a security gateway could prohibit traversal of
encrypted packets in various ways, e.g., having a DISCARD entry in
the SPD for ESP packets or providing proxy key exchange. In the
latter case, the traffic would be internally routed to the key
management module in the security gateway.

4.4.2 Selectors

An SA (or SA bundle) may be fine-grained or coarse-grained, depending
on the selectors used to define the set of traffic for the SA. For
example, all traffic between two hosts may be carried via a single
SA, and afforded a uniform set of security services. Alternatively,
traffic between a pair of hosts might be spread over multiple SAs,
depending on the applications being used (as defined by the Next
Protocol and Port fields), with different security services offered
by different SAs. Similarly, all traffic between a pair of security
gateways could be carried on a single SA, or one SA could be assigned
for each communicating host pair. The following selector parameters
MUST be supported for SA management to facilitate control of SA
granularity. Note that in the case of receipt of a packet with an
ESP header, e.g., at an encapsulating security gateway or BITW

implementation, the transport layer protocol, source/destination
ports, and Name (if present) may be "OPAQUE", i.e., inaccessible
because of encryption or fragmentation. Note also that both Source
and Destination addresses should either be IPv4 or IPv6.

- Destination IP Address (IPv4 or IPv6): this may be a single IP
address (unicast, anycast, broadcast (IPv4 only), or multicast
group), a range of addresses (high and low values (inclusive),
address + mask, or a wildcard address. The last three are used
to support more than one destination system sharing the same SA
(e.g., behind a security gateway). Note that this selector is
conceptually different from the "Destination IP Address" field
in the <Destination IP Address, IPsec Protocol, SPI> tuple used
to uniquely identify an SA. When a tunneled packet arrives at
the tunnel endpoint, its SPI/Destination address/Protocol are
used to look up the SA for this packet in the SAD. This
destination address comes from the encapsulating IP header.
Once the packet has been processed according to the tunnel SA
and has come out of the tunnel, its selectors are "looked up" in
the Inbound SPD. The Inbound SPD has a selector called
destination address. This IP destination address is the one in
the inner (encapsulated) IP header. In the case of a
transport'd packet, there will be only one IP header and this
ambiguity does not exist. [REQUIRED for all implementations]

- Source IP Address(es) (IPv4 or IPv6): this may be a single IP
address (unicast, anycast, broadcast (IPv4 only), or multicast
group), range of addresses (high and low values inclusive),
address + mask, or a wildcard address. The last three are used
to support more than one source system sharing the same SA
(e.g., behind a security gateway or in a multihomed host).
[REQUIRED for all implementations]

- Name: There are 2 cases (Note that these name forms are
supported in the IPsec DOI.)
1. User ID
a. a fully qualified user name string (DNS), e.g.,
mozart@foo.bar.com
b. X.500 distinguished name, e.g., C = US, SP = MA,
O = GTE Internetworking, CN = Stephen T. Kent.
2. System name (host, security gateway, etc.)
a. a fully qualified DNS name, e.g., foo.bar.com
b. X.500 distinguished name
c. X.500 general name

NOTE: One of the possible values of this selector is "OPAQUE".

[REQUIRED for the following cases. Note that support for name
forms other than addresses is not required for manually keyed
SAs.
o User ID
- native host implementations
- BITW and BITS implementations acting as HOSTS
with only one user
- security gateway implementations for INBOUND
processing.
o System names -- all implementations]

- Data sensitivity level: (IPSO/CIPSO labels)
[REQUIRED for all systems providing information flow security as
per Section 8, OPTIONAL for all other systems.]

- Transport Layer Protocol: Obtained from the IPv4 "Protocol" or
the IPv6 "Next Header" fields. This may be an individual
protocol number. These packet fields may not contain the
Transport Protocol due to the presence of IP extension headers,
e.g., a Routing Header, AH, ESP, Fragmentation Header,
Destination Options, Hop-by-hop options, etc. Note that the
Transport Protocol may not be available in the case of receipt
of a packet with an ESP header, thus a value of "OPAQUE" SHOULD
be supported.
[REQUIRED for all implementations]

NOTE: To locate the transport protocol, a system has to chain
through the packet headers checking the "Protocol" or "Next
Header" field until it encounters either one it recognizes as a
transport protocol, or until it reaches one that isn't on its
list of extension headers, or until it encounters an ESP header
that renders the transport protocol opaque.

- Source and Destination (e.g., TCP/UDP) Ports: These may be
individual UDP or TCP port values or a wildcard port. (The use
of the Next Protocol field and the Source and/or Destination
Port fields (in conjunction with the Source and/or Destination
Address fields), as an SA selector is sometimes referred to as
"session-oriented keying."). Note that the source and
destination ports may not be available in the case of receipt of
a packet with an ESP header, thus a value of "OPAQUE" SHOULD be
supported.

The following table summarizes the relationship between the
"Next Header" value in the packet and SPD and the derived Port
Selector value for the SPD and SAD.

Next Hdr Transport Layer Derived Port Selector Field
in Packet Protocol in SPD Value in SPD and SAD
-------- --------------- ---------------------------
ESP ESP or ANY ANY (i.e., don't look at it)
-don't care- ANY ANY (i.e., don't look at it)
specific value specific value NOT ANY (i.e., drop packet)
fragment
specific value specific value actual port selector field
not fragment

If the packet has been fragmented, then the port information may
not be available in the current fragment. If so, discard the
fragment. An ICMP PMTU should be sent for the first fragment,
which will have the port information. [MAY be supported]

The IPsec implementation context determines how selectors are used.
For example, a host implementation integrated into the stack may make
use of a socket interface. When a new connection is established the
SPD can be consulted and an SA (or SA bundle) bound to the socket.
Thus traffic sent via that socket need not result in additional
lookups to the SPD/SAD. In contrast, a BITS, BITW, or security
gateway implementation needs to look at each packet and perform an
SPD/SAD lookup based on the selectors. The allowable values for the
selector fields differ between the traffic flow, the security
association, and the security policy.

The following table summarizes the kinds of entries that one needs to
be able to express in the SPD and SAD. It shows how they relate to
the fields in data traffic being subjected to IPsec screening.
(Note: the "wild" or "wildcard" entry for src and dst addresses
includes a mask, range, etc.)

Field Traffic Value SAD Entry SPD Entry
-------- ------------- ---------------- --------------------
src addr single IP addr single,range,wild single,range,wildcard
dst addr single IP addr single,range,wild single,range,wildcard
xpt protocol* xpt protocol single,wildcard single,wildcard
src port* single src port single,wildcard single,wildcard
dst port* single dst port single,wildcard single,wildcard
user id* single user id single,wildcard single,wildcard
sec. labels single value single,wildcard single,wildcard

* The SAD and SPD entries for these fields could be "OPAQUE"
because the traffic value is encrypted.

NOTE: In principle, one could have selectors and/or selector values
in the SPD which cannot be negotiated for an SA or SA bundle.
Examples might include selector values used to select traffic for

discarding or enumerated lists which cause a separate SA to be
created for each item on the list. For now, this is left for future
versions of this document and the list of required selectors and
selector values is the same for the SPD and the SAD. However, it is
acceptable to have an administrative interface that supports use of
selector values which cannot be negotiated provided that it does not
mislead the user into believing it is creating an SA with these
selector values. For example, the interface may allow the user to
specify an enumerated list of values but would result in the creation
of a separate policy and SA for each item on the list. A vendor
might support such an interface to make it easier for its customers
to specify clear and concise policy specifications.

4.4.3 Security Association Database (SAD)

In each IPsec implementation there is a nominal Security Association
Database, in which each entry defines the parameters associated with
one SA. Each SA has an entry in the SAD. For outbound processing,
entries are pointed to by entries in the SPD. Note that if an SPD
entry does not currently point to an SA that is appropriate for the
packet, the implementation creates an appropriate SA (or SA Bundle)
and links the SPD entry to the SAD entry (see Section 5.1.1). For
inbound processing, each entry in the SAD is indexed by a destination
IP address, IPsec protocol type, and SPI. The following parameters
are associated with each entry in the SAD. This description does not
purport to be a MIB, but only a specification of the minimal data
items required to support an SA in an IPsec implementation.

For inbound processing: The following packet fields are used to look
up the SA in the SAD:

o Outer Header's Destination IP address: the IPv4 or IPv6
Destination address.
[REQUIRED for all implementations]
o IPsec Protocol: AH or ESP, used as an index for SA lookup
in this database. Specifies the IPsec protocol to be
applied to the traffic on this SA.
[REQUIRED for all implementations]
o SPI: the 32-bit value used to distinguish among different
SAs terminating at the same destination and using the same
IPsec protocol.
[REQUIRED for all implementations]

For each of the selectors defined in Section 4.4.2, the SA entry in
the SAD MUST contain the value or values which were negotiated at the
time the SA was created. For the sender, these values are used to
decide whether a given SA is appropriate for use with an outbound
packet. This is part of checking to see if there is an existing SA

that can be used. For the receiver, these values are used to check
that the selector values in an inbound packet match those for the SA
(and thus indirectly those for the matching policy). For the
receiver, this is part of verifying that the SA was appropriate for
this packet. (See Section 6 for rules for ICMP messages.) These
fields can have the form of specific values, ranges, wildcards, or
"OPAQUE" as described in section 4.4.2, "Selectors". Note that for
an ESP SA, the encryption algorithm or the authentication algorithm
could be "NULL". However they MUST not both be "NULL".

The following SAD fields are used in doing IPsec processing:

o Sequence Number Counter: a 32-bit value used to generate the
Sequence Number field in AH or ESP headers.
[REQUIRED for all implementations, but used only for outbound
traffic.]
o Sequence Counter Overflow: a flag indicating whether overflow
of the Sequence Number Counter should generate an auditable
event and prevent transmission of additional packets on the
SA.
[REQUIRED for all implementations, but used only for outbound
traffic.]
o Anti-Replay Window: a 32-bit counter and a bit-map (or
equivalent) used to determine whether an inbound AH or ESP
packet is a replay.
[REQUIRED for all implementations but used only for inbound
traffic. NOTE: If anti-replay has been disabled by the
receiver, e.g., in the case of a manually keyed SA, then the
Anti-Replay Window is not used.]
o AH Authentication algorithm, keys, etc.
[REQUIRED for AH implementations]
o ESP Encryption algorithm, keys, IV mode, IV, etc.
[REQUIRED for ESP implementations]
o ESP authentication algorithm, keys, etc. If the
authentication service is not selected, this field will be
null.
[REQUIRED for ESP implementations]
o Lifetime of this Security Association: a time interval after
which an SA must be replaced with a new SA (and new SPI) or
terminated, plus an indication of which of these actions
should occur. This may be expressed as a time or byte count,
or a simultaneous use of both, the first lifetime to expire
taking precedence. A compliant implementation MUST support
both types of lifetimes, and must support a simultaneous use
of both. If time is employed, and if IKE employs X.509
certificates for SA establishment, the SA lifetime must be
constrained by the validity intervals of the certificates,
and the NextIssueDate of the CRLs used in the IKE exchange

for the SA. Both initiator and responder are responsible for
constraining SA lifetime in this fashion.
[REQUIRED for all implementations]

NOTE: The details of how to handle the refreshing of keys
when SAs expire is a local matter. However, one reasonable
approach is:
(a) If byte count is used, then the implementation
SHOULD count the number of bytes to which the IPsec
algorithm is applied. For ESP, this is the encryption
algorithm (including Null encryption) and for AH,
this is the authentication algorithm. This includes
pad bytes, etc. Note that implementations SHOULD be
able to handle having the counters at the ends of an
SA get out of synch, e.g., because of packet loss or
because the implementations at each end of the SA
aren't doing things the same way.
(b) There SHOULD be two kinds of lifetime -- a soft
lifetime which warns the implementation to initiate
action such as setting up a replacement SA and a
hard lifetime when the current SA ends.
(c) If the entire packet does not get delivered during
the SAs lifetime, the packet SHOULD be discarded.

o IPsec protocol mode: tunnel, transport or wildcard.
Indicates which mode of AH or ESP is applied to traffic on
this SA. Note that if this field is "wildcard" at the
sending end of the SA, then the application has to specify
the mode to the IPsec implementation. This use of wildcard
allows the same SA to be used for either tunnel or transport
mode traffic on a per packet basis, e.g., by different
sockets. The receiver does not need to know the mode in
order to properly process the packet's IPsec headers.

[REQUIRED as follows, unless implicitly defined by context:
- host implementations must support all modes
- gateway implementations must support tunnel mode]

NOTE: The use of wildcard for the protocol mode of an inbound
SA may add complexity to the situation in the receiver (host
only). Since the packets on such an SA could be delivered in
either tunnel or transport mode, the security of an incoming
packet could depend in part on which mode had been used to
deliver it. If, as a result, an application cared about the
SA mode of a given packet, then the application would need a
mechanism to obtain this mode information.

o Path MTU: any observed path MTU and aging variables. See
Section 6.1.2.4
[REQUIRED for all implementations but used only for outbound
traffic]

4.5 Basic Combinations of Security Associations

This section describes four examples of combinations of security
associations that MUST be supported by compliant IPsec hosts or
security gateways. Additional combinations of AH and/or ESP in
tunnel and/or transport modes MAY be supported at the discretion of
the implementor. Compliant implementations MUST be capable of
generating these four combinations and on receipt, of processing
them, but SHOULD be able to receive and process any combination. The
diagrams and text below describe the basic cases. The legend for the
diagrams is:

==== = one or more security associations (AH or ESP, transport
or tunnel)
---- = connectivity (or if so labelled, administrative boundary)
Hx = host x
SGx = security gateway x
X* = X supports IPsec

NOTE: The security associations below can be either AH or ESP. The
mode (tunnel vs transport) is determined by the nature of the
endpoints. For host-to-host SAs, the mode can be either transport or
tunnel.

Case 1. The case of providing end-to-end security between 2 hosts
across the Internet (or an Intranet).

====================================
| |
H1* ------ (Inter/Intranet) ------ H2*

Note that either transport or tunnel mode can be selected by the
hosts. So the headers in a packet between H1 and H2 could look
like any of the following:

Transport Tunnel
----------------- ---------------------
1. [IP1][AH][upper] 4. [IP2][AH][IP1][upper]
2. [IP1][ESP][upper] 5. [IP2][ESP][IP1][upper]
3. [IP1][AH][ESP][upper]

Note that there is no requirement to support general nesting,
but in transport mode, both AH and ESP can be applied to the
packet. In this event, the SA establishment procedure MUST
ensure that first ESP, then AH are applied to the packet.

Case 2. This case illustrates simple virtual private networks
support.

===========================
| |
---------------------|---- ---|-----------------------
| | | | | |
| H1 -- (Local --- SG1* |--- (Internet) ---| SG2* --- (Local --- H2 |
| Intranet) | | Intranet) |
-------------------------- ---------------------------
admin. boundary admin. boundary

Only tunnel mode is required here. So the headers in a packet
between SG1 and SG2 could look like either of the following:

Tunnel
---------------------
4. [IP2][AH][IP1][upper]
5. [IP2][ESP][IP1][upper]

Case 3. This case combines cases 1 and 2, adding end-to-end security
between the sending and receiving hosts. It imposes no new
requirements on the hosts or security gateways, other than a
requirement for a security gateway to be configurable to pass
IPsec traffic (including ISAKMP traffic) for hosts behind it.

===============================================================
| |
| ========================= |
| | | |
---|-----------------|---- ---|-------------------|---
| | | | | | | |
| H1* -- (Local --- SG1* |-- (Internet) --| SG2* --- (Local --- H2* |
| Intranet) | | Intranet) |
-------------------------- ---------------------------
admin. boundary admin. boundary

Case 4. This covers the situation where a remote host (H1) uses the
Internet to reach an organization's firewall (SG2) and to then
gain access to some server or other machine (H2). The remote
host could be a mobile host (H1) dialing up to a local PPP/ARA
server (not shown) on the Internet and then crossing the
Internet to the home organization's firewall (SG2), etc. The

details of support for this case, (how H1 locates SG2,
authenticates it, and verifies its authorization to represent
H2) are discussed in Section 4.6.3, "Locating a Security
Gateway".

======================================================
| |
|============================== |
|| | |
|| ---|----------------------|---
|| | | | |
H1* ----- (Internet) ------| SG2* ---- (Local ----- H2* |
^ | Intranet) |
| ------------------------------
could be dialup admin. boundary (optional)
to PPP/ARA server

Only tunnel mode is required between H1 and SG2. So the choices
for the SA between H1 and SG2 would be one of the ones in case
2. The choices for the SA between H1 and H2 would be one of the
ones in case 1.

Note that in this case, the sender MUST apply the transport
header before the tunnel header. Therefore the management
interface to the IPsec implementation MUST support configuration
of the SPD and SAD to ensure this ordering of IPsec header
application.

As noted above, support for additional combinations of AH and ESP is
optional. Use of other, optional combinations may adversely affect
interoperability.

4.6 SA and Key Management

IPsec mandates support for both manual and automated SA and
cryptographic key management. The IPsec protocols, AH and ESP, are
largely independent of the associated SA management techniques,
although the techniques involved do affect some of the security
services offered by the protocols. For example, the optional anti-
replay services available for AH and ESP require automated SA
management. Moreover, the granularity of key distribution employed
with IPsec determines the granularity of authentication provided.
(See also a discussion of this issue in Section 4.7.) In general,
data origin authentication in AH and ESP is limited by the extent to
which secrets used with the authentication algorithm (or with a key
management protocol that creates such secrets) are shared among
multiple possible sources.

The following text describes the minimum requirements for both types
of SA management.

4.6.1 Manual Techniques

The simplest form of management is manual management, in which a
person manually configures each system with keying material and
security association management data relevant to secure communication
with other systems. Manual techniques are practical in small, static
environments but they do not scale well. For example, a company
could create a Virtual Private Network (VPN) using IPsec in security
gateways at several sites. If the number of sites is small, and
since all the sites come under the purview of a single administrative
domain, this is likely to be a feasible context for manual management
techniques. In this case, the security gateway might selectively
protect traffic to and from other sites within the organization using
a manually configured key, while not protecting traffic for other
destinations. It also might be appropriate when only selected
communications need to be secured. A similar argument might apply to
use of IPsec entirely within an organization for a small number of
hosts and/or gateways. Manual management techniques often employ
statically configured, symmetric keys, though other options also
exist.

4.6.2 Automated SA and Key Management

Widespread deployment and use of IPsec requires an Internet-standard,
scalable, automated, SA management protocol. Such support is
required to facilitate use of the anti-replay features of AH and ESP,
and to accommodate on-demand creation of SAs, e.g., for user- and
session-oriented keying. (Note that the notion of "rekeying" an SA
actually implies creation of a new SA with a new SPI, a process that
generally implies use of an automated SA/key management protocol.)

The default automated key management protocol selected for use with
IPsec is IKE [MSST97, Orm97, HC98] under the IPsec domain of
interpretation [Pip98]. Other automated SA management protocols MAY
be employed.

When an automated SA/key management protocol is employed, the output
from this protocol may be used to generate multiple keys, e.g., for a
single ESP SA. This may arise because:

o the encryption algorithm uses multiple keys (e.g., triple DES)
o the authentication algorithm uses multiple keys
o both encryption and authentication algorithms are employed

The Key Management System may provide a separate string of bits for
each key or it may generate one string of bits from which all of them
are extracted. If a single string of bits is provided, care needs to
be taken to ensure that the parts of the system that map the string
of bits to the required keys do so in the same fashion at both ends
of the SA. To ensure that the IPsec implementations at each end of
the SA use the same bits for the same keys, and irrespective of which
part of the system divides the string of bits into individual keys,
the encryption key(s) MUST be taken from the first (left-most, high-
order) bits and the authentication key(s) MUST be taken from the
remaining bits. The number of bits for each key is defined in the
relevant algorithm specification RFC. In the case of multiple
encryption keys or multiple authentication keys, the specification
for the algorithm must specify the order in which they are to be
selected from a single string of bits provided to the algorithm.

4.6.3 Locating a Security Gateway

This section discusses issues relating to how a host learns about the
existence of relevant security gateways and once a host has contacted
these security gateways, how it knows that these are the correct
security gateways. The details of where the required information is
stored is a local matter.

Consider a situation in which a remote host (H1) is using the
Internet to gain access to a server or other machine (H2) and there
is a security gateway (SG2), e.g., a firewall, through which H1's
traffic must pass. An example of this situation would be a mobile
host (Road Warrior) crossing the Internet to the home organization's
firewall (SG2). (See Case 4 in the section 4.5 Basic Combinations of
Security Associations.) This situation raises several issues:

1. How does H1 know/learn about the existence of the security
gateway SG2?
2. How does it authenticate SG2, and once it has authenticated
SG2, how does it confirm that SG2 has been authorized to
represent H2?
3. How does SG2 authenticate H1 and verify that H1 is authorized
to contact H2?
4. How does H1 know/learn about backup gateways which provide
alternate paths to H2?

To address these problems, a host or security gateway MUST have an
administrative interface that allows the user/administrator to
configure the address of a security gateway for any sets of
destination addresses that require its use. This includes the ability
to configure:

o the requisite information for locating and authenticating the
security gateway and verifying its authorization to represent
the destination host.
o the requisite information for locating and authenticating any
backup gateways and verifying their authorization to represent
the destination host.

It is assumed that the SPD is also configured with policy information
that covers any other IPsec requirements for the path to the security
gateway and the destination host.

This document does not address the issue of how to automate the
discovery/verification of security gateways.

4.7 Security Associations and Multicast

The receiver-orientation of the Security Association implies that, in
the case of unicast traffic, the destination system will normally
select the SPI value. By having the destination select the SPI
value, there is no potential for manually configured Security
Associations to conflict with automatically configured (e.g., via a
key management protocol) Security Associations or for Security
Associations from multiple sources to conflict with each other. For
multicast traffic, there are multiple destination systems per
multicast group. So some system or person will need to coordinate
among all multicast groups to select an SPI or SPIs on behalf of each
multicast group and then communicate the group's IPsec information to
all of the legitimate members of that multicast group via mechanisms
not defined here.

Multiple senders to a multicast group SHOULD use a single Security
Association (and hence Security Parameter Index) for all traffic to
that group when a symmetric key encryption or authentication
algorithm is employed. In such circumstances, the receiver knows only
that the message came from a system possessing the key for that
multicast group. In such circumstances, a receiver generally will
not be able to authenticate which system sent the multicast traffic.
Specifications for other, more general multicast cases are deferred
to later IPsec documents.

At the time this specification was published, automated protocols for
multicast key distribution were not considered adequately mature for
standardization. For multicast groups having relatively few members,
manual key distribution or multiple use of existing unicast key
distribution algorithms such as modified Diffie-Hellman appears
feasible. For very large groups, new scalable techniques will be
needed. An example of current work in this area is the Group Key
Management Protocol (GKMP) [HM97].

5. IP Traffic Processing

As mentioned in Section 4.4.1 "The Security Policy Database (SPD)",
the SPD must be consulted during the processing of all traffic
(INBOUND and OUTBOUND), including non-IPsec traffic. If no policy is
found in the SPD that matches the packet (for either inbound or
outbound traffic), the packet MUST be discarded.

NOTE: All of the cryptographic algorithms used in IPsec expect their
input in canonical network byte order (see Appendix in RFC 791) and
generate their output in canonical network byte order. IP packets
are also transmitted in network byte order.

5.1 Outbound IP Traffic Processing

5.1.1 Selecting and Using an SA or SA Bundle

In a security gateway or BITW implementation (and in many BITS
implementations), each outbound packet is compared against the SPD to
determine what processing is required for the packet. If the packet
is to be discarded, this is an auditable event. If the traffic is
allowed to bypass IPsec processing, the packet continues through
"normal" processing for the environment in which the IPsec processing
is taking place. If IPsec processing is required, the packet is
either mapped to an existing SA (or SA bundle), or a new SA (or SA
bundle) is created for the packet. Since a packet's selectors might
match multiple policies or multiple extant SAs and since the SPD is
ordered, but the SAD is not, IPsec MUST:

1. Match the packet's selector fields against the outbound
policies in the SPD to locate the first appropriate
policy, which will point to zero or more SA bundles in the
SAD.

2. Match the packet's selector fields against those in the SA
bundles found in (1) to locate the first SA bundle that
matches. If no SAs were found or none match, create an
appropriate SA bundle and link the SPD entry to the SAD
entry. If no key management entity is found, drop the
packet.

3. Use the SA bundle found/created in (2) to do the required
IPsec processing, e.g., authenticate and encrypt.

In a host IPsec implementation based on sockets, the SPD will be
consulted whenever a new socket is created, to determine what, if
any, IPsec processing will be applied to the traffic that will flow
on that socket.

NOTE: A compliant implementation MUST not allow instantiation of an
ESP SA that employs both a NULL encryption and a NULL authentication
algorithm. An attempt to negotiate such an SA is an auditable event.

5.1.2 Header Construction for Tunnel Mode

This section describes the handling of the inner and outer IP
headers, extension headers, and options for AH and ESP tunnels. This
includes how to construct the encapsulating (outer) IP header, how to
handle fields in the inner IP header, and what other actions should
be taken. The general idea is modeled after the one used in RFC
2003, "IP Encapsulation with IP":

o The outer IP header Source Address and Destination Address
identify the "endpoints" of the tunnel (the encapsulator and
decapsulator). The inner IP header Source Address and
Destination Addresses identify the original sender and
recipient of the datagram, (from the perspective of this
tunnel), respectively. (see footnote 3 after the table in
5.1.2.1 for more details on the encapsulating source IP
address.)
o The inner IP header is not changed except to decrement the TTL
as noted below, and remains unchanged during its delivery to
the tunnel exit point.
o No change to IP options or extension headers in the inner
header occurs during delivery of the encapsulated datagram
through the tunnel.
o If need be, other protocol headers such as the IP
Authentication header may be inserted between the outer IP
header and the inner IP header.

The tables in the following sub-sections show the handling for the
different header/option fields (constructed = the value in the outer
field is constructed independently of the value in the inner).

5.1.2.1 IPv4 -- Header Construction for Tunnel Mode

<-- How Outer Hdr Relates to Inner Hdr -->
Outer Hdr at Inner Hdr at
IPv4 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 4 (1) no change
header length constructed no change
TOS copied from inner hdr (5) no change
total length constructed no change
ID constructed no change
flags (DF,MF) constructed, DF (4) no change
fragmt offset constructed no change

TTL constructed (2) decrement (2)
protocol AH, ESP, routing hdr no change
checksum constructed constructed (2)
src address constructed (3) no change
dest address constructed (3) no change
Options never copied no change

1. The IP version in the encapsulating header can be different
from the value in the inner header.

2. The TTL in the inner header is decremented by the
encapsulator prior to forwarding and by the decapsulator if
it forwards the packet. (The checksum changes when the TTL
changes.)

Note: The decrementing of the TTL is one of the usual actions
that takes place when forwarding a packet. Packets
originating from the same node as the encapsulator do not
have their TTL's decremented, as the sending node is
originating the packet rather than forwarding it.

3. src and dest addresses depend on the SA, which is used to
determine the dest address which in turn determines which src
address (net interface) is used to forward the packet.

NOTE: In principle, the encapsulating IP source address can
be any of the encapsulator's interface addresses or even an
address different from any of the encapsulator's IP
addresses, (e.g., if it's acting as a NAT box) so long as the
address is reachable through the encapsulator from the
environment into which the packet is sent. This does not
cause a problem because IPsec does not currently have any
INBOUND processing requirement that involves the Source
Address of the encapsulating IP header. So while the
receiving tunnel endpoint looks at the Destination Address in
the encapsulating IP header, it only looks at the Source
Address in the inner (encapsulated) IP header.

4. configuration determines whether to copy from the inner
header (IPv4 only), clear or set the DF.

5. If Inner Hdr is IPv4 (Protocol = 4), copy the TOS. If Inner
Hdr is IPv6 (Protocol = 41), map the Class to TOS.

5.1.2.2 IPv6 -- Header Construction for Tunnel Mode

See previous section 5.1.2 for notes 1-5 indicated by (footnote
number).

<-- How Outer Hdr Relates Inner Hdr --->
Outer Hdr at Inner Hdr at
IPv6 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 6 (1) no change
class copied or configured (6) no change
flow id copied or configured no change
len constructed no change
next header AH,ESP,routing hdr no change
hop limit constructed (2) decrement (2)
src address constructed (3) no change
dest address constructed (3) no change
Extension headers never copied no change

6. If Inner Hdr is IPv6 (Next Header = 41), copy the Class. If
Inner Hdr is IPv4 (Next Header = 4), map the TOS to Class.

5.2 Processing Inbound IP Traffic

Prior to performing AH or ESP processing, any IP fragments are
reassembled. Each inbound IP datagram to which IPsec processing will
be applied is identified by the appearance of the AH or ESP values in
the IP Next Protocol field (or of AH or ESP as an extension header in
the IPv6 context).

Note: Appendix C contains sample code for a bitmask check for a 32
packet window that can be used for implementing anti-replay service.

5.2.1 Selecting and Using an SA or SA Bundle

Mapping the IP datagram to the appropriate SA is simplified because
of the presence of the SPI in the AH or ESP header. Note that the
selector checks are made on the inner headers not the outer (tunnel)
headers. The steps followed are:

1. Use the packet's destination address (outer IP header),
IPsec protocol, and SPI to look up the SA in the SAD. If
the SA lookup fails, drop the packet and log/report the
error.

2. Use the SA found in (1) to do the IPsec processing, e.g.,
authenticate and decrypt. This step includes matching the
packet's (Inner Header if tunneled) selectors to the
selectors in the SA. Local policy determines the
specificity of the SA selectors (single value, list,
range, wildcard). In general, a packet's source address
MUST match the SA selector value. However, an ICMP packet
received on a tunnel mode SA may have a source address

other than that bound to the SA and thus such packets
should be permitted as exceptions to this check. For an
ICMP packet, the selectors from the enclosed problem
packet (the source and destination addresses and ports
should be swapped) should be checked against the selectors
for the SA. Note that some or all of these selectors may
be inaccessible because of limitations on how many bits of
the problem packet the ICMP packet is allowed to carry or
due to encryption. See Section 6.

Do (1) and (2) for every IPsec header until a Transport
Protocol Header or an IP header that is NOT for this
system is encountered. Keep track of what SAs have been
used and their order of application.

3. Find an incoming policy in the SPD that matches the
packet. This could be done, for example, by use of
backpointers from the SAs to the SPD or by matching the
packet's selectors (Inner Header if tunneled) against
those of the policy entries in the SPD.

4. Check whether the required IPsec processing has been
applied, i.e., verify that the SA's found in (1) and (2)
match the kind and order of SAs required by the policy
found in (3).

NOTE: The correct "matching" policy will not necessarily
be the first inbound policy found. If the check in (4)
fails, steps (3) and (4) are repeated until all policy
entries have been checked or until the check succeeds.

At the end of these steps, pass the resulting packet to the Transport
Layer or forward the packet. Note that any IPsec headers processed
in these steps may have been removed, but that this information,
i.e., what SAs were used and the order of their application, may be
needed for subsequent IPsec or firewall processing.

Note that in the case of a security gateway, if forwarding causes a
packet to exit via an IPsec-enabled interface, then additional IPsec
processing may be applied.

5.2.2 Handling of AH and ESP tunnels

The handling of the inner and outer IP headers, extension headers,
and options for AH and ESP tunnels should be performed as described
in the tables in Section 5.1.

6. ICMP Processing (relevant to IPsec)

The focus of this section is on the handling of ICMP error messages.
Other ICMP traffic, e.g., Echo/Reply, should be treated like other
traffic and can be protected on an end-to-end basis using SAs in the
usual fashion.

An ICMP error message protected by AH or ESP and generated by a
router SHOULD be processed and forwarded in a tunnel mode SA. Local
policy determines whether or not it is subjected to source address
checks by the router at the destination end of the tunnel. Note that
if the router at the originating end of the tunnel is forwarding an
ICMP error message from another router, the source address check
would fail. An ICMP message protected by AH or ESP and generated by
a router MUST NOT be forwarded on a transport mode SA (unless the SA
has been established to the router acting as a host, e.g., a Telnet
connection used to manage a router). An ICMP message generated by a
host SHOULD be checked against the source IP address selectors bound
to the SA in which the message arrives. Note that even if the source
of an ICMP error message is authenticated, the returned IP header
could be invalid. Accordingly, the selector values in the IP header
SHOULD also be checked to be sure that they are consistent with the
selectors for the SA over which the ICMP message was received.

The table in Appendix D characterize ICMP messages as being either
host generated, router generated, both, unknown/unassigned. ICMP
messages falling into the last two categories should be handled as
determined by the receiver's policy.

An ICMP message not protected by AH or ESP is unauthenticated and its
processing and/or forwarding may result in denial of service. This
suggests that, in general, it would be desirable to ignore such
messages. However, it is expected that many routers (vs. security
gateways) will not implement IPsec for transit traffic and thus
strict adherence to this rule would cause many ICMP messages to be
discarded. The result is that some critical IP functions would be
lost, e.g., redirection and PMTU processing. Thus it MUST be
possible to configure an IPsec implementation to accept or reject
(router) ICMP traffic as per local security policy.

The remainder of this section addresses how PMTU processing MUST be
performed at hosts and security gateways. It addresses processing of
both authenticated and unauthenticated ICMP PMTU messages. However,
as noted above, unauthenticated ICMP messages MAY be discarded based
on local policy.

6.1 PMTU/DF Processing

6.1.1 DF Bit

In cases where a system (host or gateway) adds an encapsulating
header (ESP tunnel or AH tunnel), it MUST support the option of
copying the DF bit from the original packet to the encapsulating
header (and processing ICMP PMTU messages). This means that it MUST
be possible to configure the system's treatment of the DF bit (set,
clear, copy from encapsulated header) for each interface. (See
Appendix B for rationale.)

6.1.2 Path MTU Discovery (PMTU)

This section discusses IPsec handling for Path MTU Discovery
messages. ICMP PMTU is used here to refer to an ICMP message for:

IPv4 (RFC 792):
- Type = 3 (Destination Unreachable)
- Code = 4 (Fragmentation needed and DF set)
- Next-Hop MTU in the low-order 16 bits of the second
word of the ICMP header (labelled "unused" in RFC
792), with high-order 16 bits set to zero

IPv6 (RFC 1885):
- Type = 2 (Packet Too Big)
- Code = 0 (Fragmentation needed)
- Next-Hop MTU in the 32 bit MTU field of the ICMP6
message

6.1.2.1 Propagation of PMTU

The amount of information returned with the ICMP PMTU message (IPv4
or IPv6) is limited and this affects what selectors are available for
use in further propagating the PMTU information. (See Appendix B for
more detailed discussion of this topic.)

o PMTU message with 64 bits of IPsec header -- If the ICMP PMTU
message contains only 64 bits of the IPsec header (minimum for
IPv4), then a security gateway MUST support the following options
on a per SPI/SA basis:

a. if the originating host can be determined (or the possible
sources narrowed down to a manageable number), send the PM
information to all the possible originating hosts.
b. if the originating host cannot be determined, store the PMTU
with the SA and wait until the next packet(s) arrive from the
originating host for the relevant security association. If

the packet(s) are bigger than the PMTU, drop the packet(s),
and compose ICMP PMTU message(s) with the new packet(s) and
the updated PMTU, and send the ICMP message(s) about the
problem to the originating host. Retain the PMTU information
for any message that might arrive subsequently (see Section
6.1.2.4, "PMTU Aging").

o PMTU message with >64 bits of IPsec header -- If the ICMP message
contains more information from the original packet then there may
be enough non-opaque information to immediately determine to which
host to propagate the ICMP/PMTU message and to provide that system
with the 5 fields (source address, destination address, source
port, destination port, transport protocol) needed to determine
where to store/update the PMTU. Under such circumstances, a
security gateway MUST generate an ICMP PMTU message immediately
upon receipt of an ICMP PMTU from further down the path.

o Distributing the PMTU to the Transport Layer -- The host mechanism
for getting the updated PMTU to the transport layer is unchanged,
as specified in RFC 1191 (Path MTU Discovery).

6.1.2.2 Calculation of PMTU

The calculation of PMTU from an ICMP PMTU MUST take into account the
addition of any IPsec header -- AH transport, ESP transport, AH/ESP
transport, ESP tunnel, AH tunnel. (See Appendix B for discussion of
implementation issues.)

Note: In some situations the addition of IPsec headers could result
in an effective PMTU (as seen by the host or application) that is
unacceptably small. To avoid this problem, the implementation may
establish a threshold below which it will not report a reduced PMTU.
In such cases, the implementation would apply IPsec and then fragment
the resulting packet according to the PMTU. This would result in a
more efficient use of the available bandwidth.

6.1.2.3 Granularity of PMTU Processing

In hosts, the granularity with which ICMP PMTU processing can be done
differs depending on the implementation situation. Looking at a
host, there are 3 situations that are of interest with respect to
PMTU issues (See Appendix B for additional details on this topic.):

a. Integration of IPsec into the native IP implementation
b. Bump-in-the-stack implementations, where IPsec is implemented
"underneath" an existing implementation of a TCP/IP protocol
stack, between the native IP and the local network drivers

c. No IPsec implementation -- This case is included because it
is relevant in cases where a security gateway is sending PMTU
information back to a host.

Only in case (a) can the PMTU data be maintained at the same
granularity as communication associations. In (b) and (c), the IP
layer will only be able to maintain PMTU data at the granularity of
source and destination IP addresses (and optionally TOS), as
described in RFC 1191. This is an important difference, because more
than one communication association may map to the same source and
destination IP addresses, and each communication association may have
a different amount of IPsec header overhead (e.g., due to use of
different transforms or different algorithms).

Implementation of the calculation of PMTU and support for PMTUs at
the granularity of individual communication associations is a local
matter. However, a socket-based implementation of IPsec in a host
SHOULD maintain the information on a per socket basis. Bump in the
stack systems MUST pass an ICMP PMTU to the host IP implementation,
after adjusting it for any IPsec header overhead added by these
systems. The calculation of the overhead SHOULD be determined by
analysis of the SPI and any other selector information present in a
returned ICMP PMTU message.

6.1.2.4 PMTU Aging

In all systems (host or gateway) implementing IPsec and maintaining
PMTU information, the PMTU associated with a security association
(transport or tunnel) MUST be "aged" and some mechanism put in place
for updating the PMTU in a timely manner, especially for discovering
if the PMTU is smaller than it needs to be. A given PMTU has to
remain in place long enough for a packet to get from the source end
of the security association to the system at the other end of the
security association and propagate back an ICMP error message if the
current PMTU is too big. Note that if there are nested tunnels,
multiple packets and round trip times might be required to get an
ICMP message back to an encapsulator or originating host.

Systems SHOULD use the approach described in the Path MTU Discovery
document (RFC 1191, Section 6.3), which suggests periodically
resetting the PMTU to the first-hop data-link MTU and then letting
the normal PMTU Discovery processes update the PMTU as necessary.
The period SHOULD be configurable.

7. Auditing

Not all systems that implement IPsec will implement auditing. For
the most part, the granularity of auditing is a local matter.
However, several auditable events are identified in the AH and ESP
specifications and for each of these events a minimum set of
information that SHOULD be included in an audit log is defined.
Additional information also MAY be included in the audit log for each
of these events, and additional events, not explicitly called out in
this specification, also MAY result in audit log entries. There is
no requirement for the receiver to transmit any message to the
purported transmitter in response to the detection of an auditable
event, because of the potential to induce denial of service via such
action.

8. Use in Systems Supporting Information Flow Security

Information of various sensitivity levels may be carried over a
single network. Information labels (e.g., Unclassified, Company
Proprietary, Secret) [DoD85, DoD87] are often employed to distinguish
such information. The use of labels facilitates segregation of
information, in support of information flow security models, e.g.,
the Bell-LaPadula model [BL73]. Such models, and corresponding
supporting technology, are designed to prevent the unauthorized flow
of sensitive information, even in the face of Trojan Horse attacks.
Conventional, discretionary access control (DAC) mechanisms, e.g.,
based on access control lists, generally are not sufficient to
support such policies, and thus facilities such as the SPD do not
suffice in such environments.

In the military context, technology that supports such models is
often referred to as multi-level security (MLS). Computers and
networks often are designated "multi-level secure" if they support
the separation of labelled data in conjunction with information flow
security policies. Although such technology is more broadly
applicable than just military applications, this document uses the
acronym "MLS" to designate the technology, consistent with much
extant literature.

IPsec mechanisms can easily support MLS networking. MLS networking
requires the use of strong Mandatory Access Controls (MAC), which
unprivileged users or unprivileged processes are incapable of
controlling or violating. This section pertains only to the use of
these IP security mechanisms in MLS (information flow security
policy) environments. Nothing in this section applies to systems not
claiming to provide MLS.

As used in this section, "sensitivity information" might include
implementation-defined hierarchic levels, categories, and/or
releasability information.

AH can be used to provide strong authentication in support of
mandatory access control decisions in MLS environments. If explicit
IP sensitivity information (e.g., IPSO [Ken91]) is used and
confidentiality is not considered necessary within the particular
operational environment, AH can be used to authenticate the binding
between sensitivity labels in the IP header and the IP payload
(including user data). This is a significant improvement over
labeled IPv4 networks where the sensitivity information is trusted
even though there is no authentication or cryptographic binding of
the information to the IP header and user data. IPv4 networks might
or might not use explicit labelling. IPv6 will normally use implicit
sensitivity information that is part of the IPsec Security
Association but not transmitted with each packet instead of using
explicit sensitivity information. All explicit IP sensitivity
information MUST be authenticated using either ESP, AH, or both.

Encryption is useful and can be desirable even when all of the hosts
are within a protected environment, for example, behind a firewall or
disjoint from any external connectivity. ESP can be used, in
conjunction with appropriate key management and encryption
algorithms, in support of both DAC and MAC. (The choice of
encryption and authentication algorithms, and the assurance level of
an IPsec implementation will determine the environments in which an
implementation may be deemed sufficient to satisfy MLS requirements.)
Key management can make use of sensitivity information to provide
MAC. IPsec implementations on systems claiming to provide MLS SHOULD
be capable of using IPsec to provide MAC for IP-based communications.

8.1 Relationship Between Security Associations and Data Sensitivity

Both the Encapsulating Security Payload and the Authentication Header
can be combined with appropriate Security Association policies to
provide multi-level secure networking. In this case each SA (or SA
bundle) is normally used for only a single instance of sensitivity
information. For example, "PROPRIETARY - Internet Engineering" must
be associated with a different SA (or SA bundle) from "PROPRIETARY -
Finance".

8.2 Sensitivity Consistency Checking

An MLS implementation (both host and router) MAY associate
sensitivity information, or a range of sensitivity information with
an interface, or a configured IP address with its associated prefix
(the latter is sometimes referred to as a logical interface, or an

interface alias). If such properties exist, an implementation SHOULD
compare the sensitivity information associated with the packet
against the sensitivity information associated with the interface or
address/prefix from which the packet arrived, or through which the
packet will depart. This check will either verify that the
sensitivities match, or that the packet's sensitivity falls within
the range of the interface or address/prefix.

The checking SHOULD be done on both inbound and outbound processing.

8.3 Additional MLS Attributes for Security Association Databases

Section 4.4 discussed two Security Association databases (the
Security Policy Database (SPD) and the Security Association Database
(SAD)) and the associated policy selectors and SA attributes. MLS
networking introduces an additional selector/attribute:

- Sensitivity information.

The Sensitivity information aids in selecting the appropriate
algorithms and key strength, so that the traffic gets a level of
protection appropriate to its importance or sensitivity as described
in section 8.1. The exact syntax of the sensitivity information is
implementation defined.

8.4 Additional Inbound Processing Steps for MLS Networking

After an inbound packet has passed through IPsec processing, an MLS
implementation SHOULD first check the packet's sensitivity (as
defined by the SA (or SA bundle) used for the packet) with the
interface or address/prefix as described in section 8.2 before
delivering the datagram to an upper-layer protocol or forwarding it.

The MLS system MUST retain the binding between the data received in
an IPsec protected packet and the sensitivity information in the SA
or SAs used for processing, so appropriate policy decisions can be
made when delivering the datagram to an application or forwarding
engine. The means for maintaining this binding are implementation
specific.

8.5 Additional Outbound Processing Steps for MLS Networking

An MLS implementation of IPsec MUST perform two additional checks
besides the normal steps detailed in section 5.1.1. When consulting
the SPD or the SAD to find an outbound security association, the MLS
implementation MUST use the sensitivity of the data to select an

appropriate outbound SA or SA bundle. The second check comes before
forwarding the packet out to its destination, and is the sensitivity
consistency checking described in section 8.2.

8.6 Additional MLS Processing for Security Gateways

An MLS security gateway MUST follow the previously mentioned inbound
and outbound processing rules as well as perform some additional
processing specific to the intermediate protection of packets in an
MLS environment.

A security gateway MAY act as an outbound proxy, creating SAs for MLS
systems that originate packets forwarded by the gateway. These MLS
systems may explicitly label the packets to be forwarded, or the
whole originating network may have sensitivity characteristics
associated with it. The security gateway MUST create and use
appropriate SAs for AH, ESP, or both, to protect such traffic it
forwards.

Similarly such a gateway SHOULD accept and process inbound AH and/or
ESP packets and forward appropriately, using explicit packet
labeling, or relying on the sensitivity characteristics of the
destination network.

9. Performance Issues

The use of IPsec imposes computational performance costs on the hosts
or security gateways that implement these protocols. These costs are
associated with the memory needed for IPsec code and data structures,
and the computation of integrity check values, encryption and
decryption, and added per-packet handling. The per-packet
computational costs will be manifested by increased latency and,
possibly, reduced throughout. Use of SA/key management protocols,
especially ones that employ public key cryptography, also adds
computational performance costs to use of IPsec. These per-
association computational costs will be manifested in terms of
increased latency in association establishment. For many hosts, it
is anticipated that software-based cryptography will not appreciably
reduce throughput, but hardware may be required for security gateways
(since they represent aggregation points), and for some hosts.

The use of IPsec also imposes bandwidth utilization costs on
transmission, switching, and routing components of the Internet
infrastructure, components not implementing IPsec. This is due to
the increase in the packet size resulting from the addition of AH
and/or ESP headers, AH and ESP tunneling (which adds a second IP
header), and the increased packet traffic associated with key
management protocols. It is anticipated that, in most instances,

this increased bandwidth demand will not noticeably affect the
Internet infrastructure. However, in some instances, the effects may
be significant, e.g., transmission of ESP encrypted traffic over a
dialup link that otherwise would have compressed the traffic.

Note: The initial SA establishment overhead will be felt in the first
packet. This delay could impact the transport layer and application.
For example, it could cause TCP to retransmit the SYN before the
ISAKMP exchange is done. The effect of the delay would be different
on UDP than TCP because TCP shouldn't transmit anything other than
the SYN until the connection is set up whereas UDP will go ahead and
transmit data beyond the first packet.

Note: As discussed earlier, compression can still be employed at
layers above IP. There is an IETF working group (IP Payload
Compression Protocol (ippcp)) working on "protocol specifications
that make it possible to perform lossless compression on individual
payloads before the payload is processed by a protocol that encrypts
it. These specifications will allow for compression operations to be
performed prior to the encryption of a payload by IPsec protocols."

10. Conformance Requirements

All IPv4 systems that claim to implement IPsec MUST comply with all
requirements of the Security Architecture document. All IPv6 systems
MUST comply with all requirements of the Security Architecture
document.

11. Security Considerations

The focus of this document is security; hence security considerations
permeate this specification.

12. Differences from RFC 1825

This architecture document differs substantially from RFC 1825 in
detail and in organization, but the fundamental notions are
unchanged. This document provides considerable additional detail in
terms of compliance specifications. It introduces the SPD and SAD,
and the notion of SA selectors. It is aligned with the new versions
of AH and ESP, which also differ from their predecessors. Specific
requirements for supported combinations of AH and ESP are newly
added, as are details of PMTU management.

Acknowledgements

Many of the concepts embodied in this specification were derived from
or influenced by the US Government's SP3 security protocol, ISO/IEC's
NLSP, the proposed swIPe security protocol [SDNS, ISO, IB93, IBK93],
and the work done for SNMP Security and SNMPv2 Security.

For over 3 years (although it sometimes seems *much* longer), this
document has evolved through multiple versions and iterations.
During this time, many people have contributed significant ideas and
energy to the process and the documents themselves. The authors
would like to thank Karen Seo for providing extensive help in the
review, editing, background research, and coordination for this
version of the specification. The authors would also like to thank
the members of the IPsec and IPng working groups, with special
mention of the efforts of (in alphabetic order): Steve Bellovin,
Steve Deering, James Hughes, Phil Karn, Frank Kastenholz, Perry
Metzger, David Mihelcic, Hilarie Orman, Norman Shulman, William
Simpson, Harry Varnis, and Nina Yuan.

Appendix A -- Glossary

This section provides definitions for several key terms that are
employed in this document. Other documents provide additional
definitions and background information relevant to this technology,
e.g., [VK83, HA94]. Included in this glossary are generic security
service and security mechanism terms, plus IPsec-specific terms.

Access Control
Access control is a security service that prevents unauthorized
use of a resource, including the prevention of use of a resource
in an unauthorized manner. In the IPsec context, the resource
to which access is being controlled is often:
o for a host, computing cycles or data
o for a security gateway, a network behind the gateway
or
bandwidth on that network.

Anti-replay
[See "Integrity" below]

Authentication
This term is used informally to refer to the combination of two
nominally distinct security services, data origin authentication
and connectionless integrity. See the definitions below for
each of these services.

Availability
Availability, when viewed as a security service, addresses the
security concerns engendered by attacks against networks that
deny or degrade service. For example, in the IPsec context, the
use of anti-replay mechanisms in AH and ESP support
availability.

Confidentiality
Confidentiality is the security service that protects data from
unauthorized disclosure. The primary confidentiality concern in
most instances is unauthorized disclosure of application level
data, but disclosure of the external characteristics of
communication also can be a concern in some circumstances.
Traffic flow confidentiality is the service that addresses this
latter concern by concealing source and destination addresses,
message length, or frequency of communication. In the IPsec
context, using ESP in tunnel mode, especially at a security
gateway, can provide some level of traffic flow confidentiality.
(See also traffic analysis, below.)

Encryption
Encryption is a security mechanism used to transform data from
an intelligible form (plaintext) into an unintelligible form
(ciphertext), to provide confidentiality. The inverse
transformation process is designated "decryption". Oftimes the
term "encryption" is used to generically refer to both
processes.

Data Origin Authentication
Data origin authentication is a security service that verifies
the identity of the claimed source of data. This service is
usually bundled with connectionless integrity service.

Integrity
Integrity is a security service that ensures that modifications
to data are detectable. Integrity comes in various flavors to
match application requirements. IPsec supports two forms of
integrity: connectionless and a form of partial sequence
integrity. Connectionless integrity is a service that detects
modification of an individual IP datagram, without regard to the
ordering of the datagram in a stream of traffic. The form of
partial sequence integrity offered in IPsec is referred to as
anti-replay integrity, and it detects arrival of duplicate IP
datagrams (within a constrained window). This is in contrast to
connection-oriented integrity, which imposes more stringent
sequencing requirements on traffic, e.g., to be able to detect
lost or re-ordered messages. Although authentication and
integrity services often are cited separately, in practice they
are intimately connected and almost always offered in tandem.

Security Association (SA)
A simplex (uni-directional) logical connection, created for
security purposes. All traffic traversing an SA is provided the
same security processing. In IPsec, an SA is an internet layer
abstraction implemented through the use of AH or ESP.

Security Gateway
A security gateway is an intermediate system that acts as the
communications interface between two networks. The set of hosts
(and networks) on the external side of the security gateway is
viewed as untrusted (or less trusted), while the networks and
hosts and on the internal side are viewed as trusted (or more
trusted). The internal subnets and hosts served by a security
gateway are presumed to be trusted by virtue of sharing a
common, local, security administration. (See "Trusted
Subnetwork" below.) In the IPsec context, a security gateway is
a point at which AH and/or ESP is implemented in order to serve

a set of internal hosts, providing security services for these
hosts when they communicate with external hosts also employing
IPsec (either directly or via another security gateway).

SPI
Acronym for "Security Parameters Index". The combination of a
destination address, a security protocol, and an SPI uniquely
identifies a security association (SA, see above). The SPI is
carried in AH and ESP protocols to enable the receiving system
to select the SA under which a received packet will be
processed. An SPI has only local significance, as defined by
the creator of the SA (usually the receiver of the packet
carrying the SPI); thus an SPI is generally viewed as an opaque
bit string. However, the creator of an SA may choose to
interpret the bits in an SPI to facilitate local processing.

Traffic Analysis
The analysis of network traffic flow for the purpose of deducing
information that is useful to an adversary. Examples of such
information are frequency of transmission, the identities of the
conversing parties, sizes of packets, flow identifiers, etc.
[Sch94]

Trusted Subnetwork
A subnetwork containing hosts and routers that trust each other
not to engage in active or passive attacks. There also is an
assumption that the underlying communications channel (e.g., a
LAN or CAN) isn't being attacked by other means.

Appendix B -- Analysis/Discussion of PMTU/DF/Fragmentation Issues

B.1 DF bit

In cases where a system (host or gateway) adds an encapsulating
header (e.g., ESP tunnel), should/must the DF bit in the original
packet be copied to the encapsulating header?

Fragmenting seems correct for some situations, e.g., it might be
appropriate to fragment packets over a network with a very small MTU,
e.g., a packet radio network, or a cellular phone hop to mobile node,
rather than propagate back a very small PMTU for use over the rest of
the path. In other situations, it might be appropriate to set the DF
bit in order to get feedback from later routers about PMTU
constraints which require fragmentation. The existence of both of
these situations argues for enabling a system to decide whether or
not to fragment over a particular network "link", i.e., for requiring
an implementation to be able to copy the DF bit (and to process ICMP
PMTU messages), but making it an option to be selected on a per
interface basis. In other words, an administrator should be able to
configure the router's treatment of the DF bit (set, clear, copy from
encapsulated header) for each interface.

Note: If a bump-in-the-stack implementation of IPsec attempts to
apply different IPsec algorithms based on source/destination ports,
it will be difficult to apply Path MTU adjustments.

B.2 Fragmentation

If required, IP fragmentation occurs after IPsec processing within an
IPsec implementation. Thus, transport mode AH or ESP is applied only
to whole IP datagrams (not to IP fragments). An IP packet to which
AH or ESP has been applied may itself be fragmented by routers en
route, and such fragments MUST be reassembled prior to IPsec
processing at a receiver. In tunnel mode, AH or ESP is applied to an
IP packet, the payload of which may be a fragmented IP packet. For
example, a security gateway, "bump-in-the-stack" (BITS), or "bump-
in-the-wire" (BITW) IPsec implementation may apply tunnel mode AH to
such fragments. Note that BITS or BITW implementations are examples
of where a host IPsec implementation might receive fragments to which
tunnel mode is to be applied. However, if transport mode is to be
applied, then these implementations MUST reassemble the fragments
prior to applying IPsec.

NOTE: IPsec always has to figure out what the encapsulating IP header
fields are. This is independent of where you insert IPsec and is
intrinsic to the definition of IPsec. Therefore any IPsec
implementation that is not integrated into an IP implementation must
include code to construct the necessary IP headers (e.g., IP2):

o AH-tunnel --> IP2-AH-IP1-Transport-Data
o ESP-tunnel --> IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer

*********************************************************************

Overall, the fragmentation/reassembly approach described above works
for all cases examined.

AH Xport AH Tunnel ESP Xport ESP Tunnel
Implementation approach IPv4 IPv6 IPv4 IPv6 IPv4 IPv6 IPv4 IPv6
----------------------- ---- ---- ---- ---- ---- ---- ---- ----
Hosts (integr w/ IP stack) Y Y Y Y Y Y Y Y
Hosts (betw/ IP and drivers) Y Y Y Y Y Y Y Y
S. Gwy (integr w/ IP stack) Y Y Y Y
Outboard crypto processor *

* If the crypto processor system has its own IP address, then it
is covered by the security gateway case. This box receives
the packet from the host and performs IPsec processing. It
has to be able to handle the same AH, ESP, and related
IPv4/IPv6 tunnel processing that a security gateway would have
to handle. If it doesn't have it's own address, then it is
similar to the bump-in-the stack implementation between IP and
the network drivers.

The following analysis assumes that:

1. There is only one IPsec module in a given system's stack.
There isn't an IPsec module A (adding ESP/encryption and
thus) hiding the transport protocol, SRC port, and DEST port
from IPsec module B.
2. There are several places where IPsec could be implemented (as
shown in the table above).
a. Hosts with integration of IPsec into the native IP
implementation. Implementer has access to the source
for the stack.
b. Hosts with bump-in-the-stack implementations, where
IPsec is implemented between IP and the local network
drivers. Source access for stack is not available;
but there are well-defined interfaces that allows the
IPsec code to be incorporated into the system.

c. Security gateways and outboard crypto processors with
integration of IPsec into the stack.
3. Not all of the above approaches are feasible in all hosts.
But it was assumed that for each approach, there are some
hosts for whom the approach is feasible.

For each of the above 3 categories, there are IPv4 and IPv6, AH
transport and tunnel modes, and ESP transport and tunnel modes -- for
a total of 24 cases (3 x 2 x 4).

Some header fields and interface fields are listed here for ease of
reference -- they're not in the header order, but instead listed to
allow comparison between the columns. (* = not covered by AH
authentication. ESP authentication doesn't cover any headers that
precede it.)

IP/Transport Interface
IPv4 IPv6 (RFC 1122 -- Sec 3.4)
---- ---- ----------------------
Version = 4 Version = 6
Header Len
*TOS Class,Flow Lbl TOS
Packet Len Payload Len Len
ID ID (optional)
*Flags DF
*Offset
*TTL *Hop Limit TTL
Protocol Next Header
*Checksum
Src Address Src Address Src Address
Dst Address Dst Address Dst Address
Options? Options? Opt

? = AH covers Option-Type and Option-Length, but
might not cover Option-Data.

The results for each of the 20 cases is shown below ("works" = will
work if system fragments after outbound IPsec processing, reassembles
before inbound IPsec processing). Notes indicate implementation
issues.

a. Hosts (integrated into IP stack)
o AH-transport --> (IP1-AH-Transport-Data)
- IPv4 -- works
- IPv6 -- works
o AH-tunnel --> (IP2-AH-IP1-Transport-Data)
- IPv4 -- works
- IPv6 -- works

o ESP-transport --> (IP1-ESP_hdr-Transport-Data-ESP_trailer)
- IPv4 -- works
- IPv6 -- works
o ESP-tunnel --> (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer)
- IPv4 -- works
- IPv6 -- works

b. Hosts (Bump-in-the-stack) -- put IPsec between IP layer and
network drivers. In this case, the IPsec module would have to do
something like one of the following for fragmentation and
reassembly.
- do the fragmentation/reassembly work itself and
send/receive the packet directly to/from the network
layer. In AH or ESP transport mode, this is fine. In AH
or ESP tunnel mode where the tunnel end is at the ultimate
destination, this is fine. But in AH or ESP tunnel modes
where the tunnel end is different from the ultimate
destination and where the source host is multi-homed, this
approach could result in sub-optimal routing because the
IPsec module may be unable to obtain the information
needed (LAN interface and next-hop gateway) to direct the
packet to the appropriate network interface. This is not
a problem if the interface and next-hop gateway are the
same for the ultimate destination and for the tunnel end.
But if they are different, then IPsec would need to know
the LAN interface and the next-hop gateway for the tunnel
end. (Note: The tunnel end (security gateway) is highly
likely to be on the regular path to the ultimate
destination. But there could also be more than one path
to the destination, e.g., the host could be at an
organization with 2 firewalls. And the path being used
could involve the less commonly chosen firewall.) OR
- pass the IPsec'd packet back to the IP layer where an
extra IP header would end up being pre-pended and the
IPsec module would have to check and let IPsec'd fragments
go by.
OR
- pass the packet contents to the IP layer in a form such
that the IP layer recreates an appropriate IP header

At the network layer, the IPsec module will have access to the
following selectors from the packet -- SRC address, DST address,
Next Protocol, and if there's a transport layer header --> SRC
port and DST port. One cannot assume IPsec has access to the
Name. It is assumed that the available selector information is
sufficient to figure out the relevant Security Policy entry and
Security Association(s).

o AH-transport --> (IP1-AH-Transport-Data)
- IPv4 -- works
- IPv6 -- works
o AH-tunnel --> (IP2-AH-IP1-Transport-Data)
- IPv4 -- works
- IPv6 -- works
o ESP-transport --> (IP1-ESP_hdr-Transport-Data-ESP_trailer)
- IPv4 -- works
- IPv6 -- works
o ESP-tunnel --> (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer)
- IPv4 -- works
- IPv6 -- works

c. Security gateways -- integrate IPsec into the IP stack

NOTE: The IPsec module will have access to the following
selectors from the packet -- SRC address, DST address, Next
Protocol, and if there's a transport layer header --> SRC port
and DST port. It won't have access to the User ID (only Hosts
have access to User ID information.) Unlike some Bump-in-the-
stack implementations, security gateways may be able to look up
the Source Address in the DNS to provide a System Name, e.g., in
situations involving use of dynamically assigned IP addresses in
conjunction with dynamically updated DNS entries. It also won't
have access to the transport layer information if there is an ESP
header, or if it's not the first fragment of a fragmented
message. It is assumed that the available selector information
is sufficient to figure out the relevant Security Policy entry
and Security Association(s).

o AH-tunnel --> (IP2-AH-IP1-Transport-Data)
- IPv4 -- works
- IPv6 -- works
o ESP-tunnel --> (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer)
- IPv4 -- works
- IPv6 -- works

**********************************************************************

B.3 Path MTU Discovery

As mentioned earlier, "ICMP PMTU" refers to an ICMP message used for
Path MTU Discovery.

The legend for the diagrams below in B.3.1 and B.3.3 (but not B.3.2)
is:

==== = security association (AH or ESP, transport or tunnel)

---- = connectivity (or if so labelled, administrative boundary)
.... = ICMP message (hereafter referred to as ICMP PMTU) for

IPv4:
- Type = 3 (Destination Unreachable)
- Code = 4 (Fragmentation needed and DF set)
- Next-Hop MTU in the low-order 16 bits of the second
word of the ICMP header (labelled unused in RFC 792),
with high-order 16 bits set to zero

IPv6 (RFC 1885):
- Type = 2 (Packet Too Big)
- Code = 0 (Fragmentation needed and DF set)
- Next-Hop MTU in the 32 bit MTU field of the ICMP6

Hx = host x
Rx = router x
SGx = security gateway x
X* = X supports IPsec

B.3.1 Identifying the Originating Host(s)

The amount of information returned with the ICMP message is limited
and this affects what selectors are available to identify security
associations, originating hosts, etc. for use in further propagating
the PMTU information.

In brief... An ICMP message must contain the following information
from the "offending" packet:
- IPv4 (RFC 792) -- IP header plus a minimum of 64 bits

Accordingly, in the IPv4 context, an ICMP PMTU may identify only the
first (outermost) security association. This is because the ICMP
PMTU may contain only 64 bits of the "offending" packet beyond the IP
header, which would capture only the first SPI from AH or ESP. In
the IPv6 context, an ICMP PMTU will probably provide all the SPIs and
the selectors in the IP header, but maybe not the SRC/DST ports (in
the transport header) or the encapsulated (TCP, UDP, etc.) protocol.
Moreover, if ESP is used, the transport ports and protocol selectors
may be encrypted.

Looking at the diagram below of a security gateway tunnel (as
mentioned elsewhere, security gateways do not use transport mode)...

H1 =================== H3
\ | | /
H0 -- SG1* ---- R1 ---- SG2* ---- R2 -- H5
/ ^ | \
H2 |........| H4

Suppose that the security policy for SG1 is to use a single SA to SG2
for all the traffic between hosts H0, H1, and H2 and hosts H3, H4,
and H5. And suppose H0 sends a data packet to H5 which causes R1 to
send an ICMP PMTU message to SG1. If the PMTU message has only the
SPI, SG1 will be able to look up the SA and find the list of possible
hosts (H0, H1, H2, wildcard); but SG1 will have no way to figure out
that H0 sent the traffic that triggered the ICMP PMTU message.

original after IPsec ICMP
packet processing packet
-------- ----------- ------
IP-3 header (S = R1, D = SG1)
ICMP header (includes PMTU)
IP-2 header IP-2 header (S = SG1, D = SG2)
ESP header minimum of 64 bits of ESP hdr (*)
IP-1 header IP-1 header
TCP header TCP header
TCP data TCP data
ESP trailer

(*) The 64 bits will include enough of the ESP (or AH) header to
include the SPI.
- ESP -- SPI (32 bits), Seq number (32 bits)
- AH -- Next header (8 bits), Payload Len (8 bits),
Reserved (16 bits), SPI (32 bits)

This limitation on the amount of information returned with an ICMP
message creates a problem in identifying the originating hosts for
the packet (so as to know where to further propagate the ICMP PMTU
information). If the ICMP message contains only 64 bits of the IPsec
header (minimum for IPv4), then the IPsec selectors (e.g., Source and
Destination addresses, Next Protocol, Source and Destination ports,
etc.) will have been lost. But the ICMP error message will still
provide SG1 with the SPI, the PMTU information and the source and
destination gateways for the relevant security association.

The destination security gateway and SPI uniquely define a security
association which in turn defines a set of possible originating
hosts. At this point, SG1 could:

a. send the PMTU information to all the possible originating hosts.
This would not work well if the host list is a wild card or if
many/most of the hosts weren't sending to SG1; but it might work
if the SPI/destination/etc mapped to just one or a small number of
hosts.
b. store the PMTU with the SPI/etc and wait until the next packet(s)
arrive from the originating host(s) for the relevant security
association. If it/they are bigger than the PMTU, drop the
packet(s), and compose ICMP PMTU message(s) with the new packet(s)
and the updated PMTU, and send the originating host(s) the ICMP
message(s) about the problem. This involves a delay in notifying
the originating host(s), but avoids the problems of (a).

Since only the latter approach is feasible in all instances, a
security gateway MUST provide such support, as an option. However,
if the ICMP message contains more information from the original
packet, then there may be enough information to immediately determine
to which host to propagate the ICMP/PMTU message and to provide that
system with the 5 fields (source address, destination address, source
port, destination port, and transport protocol) needed to determine
where to store/update the PMTU. Under such circumstances, a security
gateway MUST generate an ICMP PMTU message immediately upon receipt
of an ICMP PMTU from further down the path. NOTE: The Next Protocol
field may not be contained in the ICMP message and the use of ESP
encryption may hide the selector fields that have been encrypted.

B.3.2 Calculation of PMTU

The calculation of PMTU from an ICMP PMTU has to take into account
the addition of any IPsec header by H1 -- AH and/or ESP transport, or
ESP or AH tunnel. Within a single host, multiple applications may
share an SPI and nesting of security associations may occur. (See
Section 4.5 Basic Combinations of Security Associations for
description of the combinations that MUST be supported). The diagram
below illustrates an example of security associations between a pair
of hosts (as viewed from the perspective of one of the hosts.) (ESPx
or AHx = transport mode)

Socket 1 -------------------------|
|
Socket 2 (ESPx/SPI-A) ---------- AHx (SPI-B) -- Internet

In order to figure out the PMTU for each socket that maps to SPI-B,
it will be necessary to have backpointers from SPI-B to each of the 2
paths that lead to it -- Socket 1 and Socket 2/SPI-A.

B.3.3 Granularity of Maintaining PMTU Data

In hosts, the granularity with which PMTU ICMP processing can be done
differs depending on the implementation situation. Looking at a
host, there are three situations that are of interest with respect to
PMTU issues:

a. Integration of IPsec into the native IP implementation
b. Bump-in-the-stack implementations, where IPsec is implemented
"underneath" an existing implementation of a TCP/IP protocol
stack, between the native IP and the local network drivers
c. No IPsec implementation -- This case is included because it is
relevant in cases where a security gateway is sending PMTU
information back to a host.

Only in case (a) can the PMTU data be maintained at the same
granularity as communication associations. In the other cases, the
IP layer will maintain PMTU data at the granularity of Source and
Destination IP addresses (and optionally TOS/Class), as described in
RFC 1191. This is an important difference, because more than one
communication association may map to the same source and destination
IP addresses, and each communication association may have a different
amount of IPsec header overhead (e.g., due to use of different
transforms or different algorithms). The examples below illustrate
this.

In cases (a) and (b)... Suppose you have the following situation.
H1 is sending to H2 and the packet to be sent from R1 to R2 exceeds
the PMTU of the network hop between them.

==================================
| |
H1* --- R1 ----- R2 ---- R3 ---- H2*
^ |
|.......|

If R1 is configured to not fragment subscriber traffic, then R1 sends
an ICMP PMTU message with the appropriate PMTU to H1. H1's
processing would vary with the nature of the implementation. In case
(a) (native IP), the security services are bound to sockets or the
equivalent. Here the IP/IPsec implementation in H1 can store/update
the PMTU for the associated socket. In case (b), the IP layer in H1
can store/update the PMTU but only at the granularity of Source and
Destination addresses and possibly TOS/Class, as noted above. So the
result may be sub-optimal, since the PMTU for a given
SRC/DST/TOS/Class will be the subtraction of the largest amount of
IPsec header used for any communication association between a given
source and destination.

In case (c), there has to be a security gateway to have any IPsec
processing. So suppose you have the following situation. H1 is
sending to H2 and the packet to be sent from SG1 to R exceeds the
PMTU of the network hop between them.

================
| |
H1 ---- SG1* --- R --- SG2* ---- H2
^ |
|.......|

As described above for case (b), the IP layer in H1 can store/update
the PMTU but only at the granularity of Source and Destination
addresses, and possibly TOS/Class. So the result may be sub-optimal,
since the PMTU for a given SRC/DST/TOS/Class will be the subtraction
of the largest amount of IPsec header used for any communication
association between a given source and destination.

B.3.4 Per Socket Maintenance of PMTU Data

Implementation of the calculation of PMTU (Section B.3.2) and support
for PMTUs at the granularity of individual "communication
associations" (Section B.3.3) is a local matter. However, a socket-
based implementation of IPsec in a host SHOULD maintain the
information on a per socket basis. Bump in the stack systems MUST
pass an ICMP PMTU to the host IP implementation, after adjusting it
for any IPsec header overhead added by these systems. The
determination of the overhead SHOULD be determined by analysis of the
SPI and any other selector information present in a returned ICMP
PMTU message.

B.3.5 Delivery of PMTU Data to the Transport Layer

The host mechanism for getting the updated PMTU to the transport
layer is unchanged, as specified in RFC 1191 (Path MTU Discovery).

B.3.6 Aging of PMTU Data

This topic is covered in Section 6.1.2.4.

Appendix C -- Sequence Space Window Code Example

This appendix contains a routine that implements a bitmask check for
a 32 packet window. It was provided by James Hughes
(jim_hughes@stortek.com) and Harry Varnis (hgv@anubis.network.com)
and is intended as an implementation example. Note that this code
both checks for a replay and updates the window. Thus the algorithm,
as shown, should only be called AFTER the packet has been
authenticated. Implementers might wish to consider splitting the
code to do the check for replays before computing the ICV. If the
packet is not a replay, the code would then compute the ICV, (discard
any bad packets), and if the packet is OK, update the window.

#include <stdio.h>
#include <stdlib.h>
typedef unsigned long u_long;

enum {
ReplayWindowSize = 32
};

u_long bitmap = 0; /* session state - must be 32 bits */
u_long lastSeq = 0; /* session state */

/* Returns 0 if packet disallowed, 1 if packet permitted */
int ChkReplayWindow(u_long seq);

int ChkReplayWindow(u_long seq) {
u_long diff;

if (seq == 0) return 0; /* first == 0 or wrapped */
if (seq > lastSeq) { /* new larger sequence number */
diff = seq - lastSeq;
if (diff < ReplayWindowSize) { /* In window */
bitmap <<= diff;
bitmap |= 1; /* set bit for this packet */
} else bitmap = 1; /* This packet has a "way larger" */
lastSeq = seq;
return 1; /* larger is good */
}
diff = lastSeq - seq;
if (diff >= ReplayWindowSize) return 0; /* too old or wrapped */
if (bitmap & ((u_long)1 << diff)) return 0; /* already seen */
bitmap |= ((u_long)1 << diff); /* mark as seen */
return 1; /* out of order but good */
}

char string_buffer[512];

#define STRING_BUFFER_SIZE sizeof(string_buffer)

int main() {
int result;
u_long last, current, bits;

printf("Input initial state (bits in hex, last msgnum):\n");
if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) exit(0);
sscanf(string_buffer, "%lx %lu", &bits, &last);
if (last != 0)
bits |= 1;
bitmap = bits;
lastSeq = last;
printf("bits:%08lx last:%lu\n", bitmap, lastSeq);
printf("Input value to test (current):\n");

while (1) {
if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) break;
sscanf(string_buffer, "%lu", ¤t);
result = ChkReplayWindow(current);
printf("%-3s", result ? "OK" : "BAD");
printf(" bits:%08lx last:%lu\n", bitmap, lastSeq);
}
return 0;
}

Appendix D -- Categorization of ICMP messages

The tables below characterize ICMP messages as being either host
generated, router generated, both, unassigned/unknown. The first set
are IPv4. The second set are IPv6.

IPv4

Type Name/Codes Reference
========================================================================
HOST GENERATED:
3 Destination Unreachable
2 Protocol Unreachable [RFC792]
3 Port Unreachable [RFC792]
8 Source Host Isolated [RFC792]
14 Host Precedence Violation [RFC1812]
10 Router Selection [RFC1256]

Type Name/Codes Reference
========================================================================
ROUTER GENERATED:
3 Destination Unreachable
0 Net Unreachable [RFC792]
4 Fragmentation Needed, Don't Fragment was Set [RFC792]
5 Source Route Failed [RFC792]
6 Destination Network Unknown [RFC792]
7 Destination Host Unknown [RFC792]
9 Comm. w/Dest. Net. is Administratively Prohibited [RFC792]
11 Destination Network Unreachable for Type of Service[RFC792]
5 Redirect
0 Redirect Datagram for the Network (or subnet) [RFC792]
2 Redirect Datagram for the Type of Service & Network[RFC792]
9 Router Advertisement [RFC1256]
18 Address Mask Reply [RFC950]

IPv4
Type Name/Codes Reference
========================================================================
BOTH ROUTER AND HOST GENERATED:
0 Echo Reply [RFC792]
3 Destination Unreachable
1 Host Unreachable [RFC792]
10 Comm. w/Dest. Host is Administratively Prohibited [RFC792]
12 Destination Host Unreachable for Type of Service [RFC792]
13 Communication Administratively Prohibited [RFC1812]
15 Precedence cutoff in effect [RFC1812]
4 Source Quench [RFC792]
5 Redirect
1 Redirect Datagram for the Host [RFC792]
3 Redirect Datagram for the Type of Service and Host [RFC792]
6 Alternate Host Address [JBP]
8 Echo [RFC792]
11 Time Exceeded [RFC792]
12 Parameter Problem [RFC792,RFC1108]
13 Timestamp [RFC792]
14 Timestamp Reply [RFC792]
15 Information Request [RFC792]
16 Information Reply [RFC792]
17 Address Mask Request [RFC950]
30 Traceroute [RFC1393]
31 Datagram Conversion Error [RFC1475]
32 Mobile Host Redirect [Johnson]
39 SKIP [Markson]
40 Photuris [Simpson]

Type Name/Codes Reference
========================================================================
UNASSIGNED TYPE OR UNKNOWN GENERATOR:
1 Unassigned [JBP]
2 Unassigned [JBP]
7 Unassigned [JBP]
19 Reserved (for Security) [Solo]
20-29 Reserved (for Robustness Experiment) [ZSu]
33 IPv6 Where-Are-You [Simpson]
34 IPv6 I-Am-Here [Simpson]
35 Mobile Registration Request [Simpson]
36 Mobile Registration Reply [Simpson]
37 Domain Name Request [Simpson]
38 Domain Name Reply [Simpson]
41-255 Reserved [JBP]

IPv6

Type Name/Codes Reference
========================================================================
HOST GENERATED:
1 Destination Unreachable [RFC 1885]
4 Port Unreachable

Type Name/Codes Reference
========================================================================
ROUTER GENERATED:
1 Destination Unreachable [RFC1885]
0 No Route to Destination
1 Comm. w/Destination is Administratively Prohibited
2 Not a Neighbor
3 Address Unreachable
2 Packet Too Big [RFC1885]
0
3 Time Exceeded [RFC1885]
0 Hop Limit Exceeded in Transit
1 Fragment reassembly time exceeded

Type Name/Codes Reference
========================================================================
BOTH ROUTER AND HOST GENERATED:
4 Parameter Problem [RFC1885]
0 Erroneous Header Field Encountered
1 Unrecognized Next Header Type Encountered
2 Unrecognized IPv6 Option Encountered

References

[BL73] Bell, D.E. & LaPadula, L.J., "Secure Computer Systems:
Mathematical Foundations and Model", Technical Report M74-
244, The MITRE Corporation, Bedford, MA, May 1973.

[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.

[DoD85] US National Computer Security Center, "Department of
Defense Trusted Computer System Evaluation Criteria", DoD
5200.28-STD, US Department of Defense, Ft. Meade, MD.,
December 1985.

[DoD87] US National Computer Security Center, "Trusted Network
Interpretation of the Trusted Computer System Evaluation
Criteria", NCSC-TG-005, Version 1, US Department of
Defense, Ft. Meade, MD., 31 July 1987.

[HA94] Haller, N., and R. Atkinson, "On Internet Authentication",
RFC 1704, October 1994.

[HC98] Harkins, D., and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.

[HM97] Harney, H., and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Architecture", RFC 2094, July 1997.

[ISO] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
DIS 11577, International Standards Organisation, Geneva,
Switzerland, 29 November 1992.

[IB93] John Ioannidis and Matt Blaze, "Architecture and
Implementation of Network-layer Security Under Unix",
Proceedings of USENIX Security Symposium, Santa Clara, CA,
October 1993.

[IBK93] John Ioannidis, Matt Blaze, & Phil Karn, "swIPe: Network-
Layer Security for IP", presentation at the Spring 1993
IETF Meeting, Columbus, Ohio

[KA98a] Kent, S., and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.

[KA98b] Kent, S., and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.

[Ken91] Kent, S., "US DoD Security Options for the Internet
Protocol", RFC 1108, November 1991.

[MSST97] Maughan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.

[Orm97] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998.

[Pip98] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.

[Sch94] Bruce Schneier, Applied Cryptography, Section 8.6, John
Wiley & Sons, New York, NY, 1994.

[SDNS] SDNS Secure Data Network System, Security Protocol 3, SP3,
Document SDN.301, Revision 1.5, 15 May 1989, published in
NIST Publication NIST-IR-90-4250, February 1990.

[SMPT98] Shacham, A., Monsour, R., Pereira, R., and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 2393, August
1998.

[TDG97] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.

[VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High-
level Networks", ACM Computing Surveys, Vol. 15, No. 2,
June 1983.

Disclaimer

The views and specification expressed in this document are those of
the authors and are not necessarily those of their employers. The
authors and their employers specifically disclaim responsibility for
any problems arising from correct or incorrect implementation or use
of this design.

Author Information

Stephen Kent
BBN Corporation
70 Fawcett Street
Cambridge, MA 02140
USA

Phone: +1 (617) 873-3988
EMail: kent@bbn.com

Randall Atkinson
@Home Network
425 Broadway
Redwood City, CA 94063
USA

Phone: +1 (415) 569-5000
EMail: rja@corp.home.net

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