The beginning of the end
Today is 6/6/2012, World IPv6 Launch Day. The day the Internet community permanently enables the IPv6 Internet protocol on their infrastructure. Some refer to this protocol as ‘The New Internet Protocol’. But is it new? No. Not at all.
To deal with the anticipated IPv4 address exhaustion, the Internet Engineering Task Force (IETF) developed IPv6 and described it in Internet standard document RFC 2460. This was published in December 1998. Due to the incompatibilty with the current IPv4 protocol, it was never widely adopted. Now that address exhaustion is imminent, the world is in a hurry to set things straight.
I am the proud owner of what is arguably the coolest IPv6 Internet domain name in the world: ipv6.net. I have owned it for a long time. Not too long ago I realized that 6 days after 6/6/2012, it has been exactly 15 years since the domain name was registered. Apparently, back in 1997, I envisioned that IPv6 was going to be big. I just didn’t know it would take such a long time. But are we there yet? No. Not even close.
Back then the community thought we would run out of IP addresses in just a couple of years. With some tricks we managed to stretch things out until now. We even back-ported some cool stuff from the new protocol into the old. It wasn’t until mid 2011 that we saw some serious global industry initiatives to promote adoption of IPv6: World IPv6 Day on June 8th. On that day some of the smaller as well as larger members of the global Internet community temporarily enabled IPv6 on their infrastructure. For some, just to see what would happen. For others a good test of their transition plan or chosen technology. Some ‘forgot’ to switch it off again. For most it was a big success; a final rehearsal for the big step: a global transition from IPv4 towards IPv6.
Today is the start of that transition. Content providers around the globe will provide access to their services over IPv6. Access providers will provide IPv6 access to their end-users. Hard- and software manufacturers will bring out IPv6 support for their products. This broad involvement will certainly help to solve the chicken and egg, content versus access, problem.
So what will happen after today? If all goes well, and I certainly expect so, we will have marked the beginning of the end of IPv4. It will take many years before IPv6 has become the dominant protocol and IPv4 is marked ‘legacy’. But I expect that after today more and more companies will make a start with their transition. For many it will be hard to make a good business case for it as there is not always a clear added business value. Just don’t wait too long as the landscape is rapidly changing.
Some advice for those about to take the plunge: take ample time to gather knowledge, create awareness among those involved, decide on a sound transition scenario, test and start planning.
And for me? Well, as an IT professional I will be helping out customers doing just that. Personally, I will continue to blog and tweet about IPv6 for a long time to come…
Cheers,
IPv6 Can No Longer Be Ignored
RFC 2464 – Transmission of IPv6 Packets over Ethernet Networks
Network Working Group M. Crawford
Request for Comments: 2464 Fermilab
Obsoletes: 1972 December 1998
Category: Standards Track
Transmission of IPv6 Packets over Ethernet Networks
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.
1. Introduction
This document specifies the frame format for transmission of IPv6
packets and the method of forming IPv6 link-local addresses and
statelessly autoconfigured addresses on Ethernet networks. It also
specifies the content of the Source/Target Link-layer Address option
used in Router Solicitation, Router Advertisement, Neighbor
Solicitation, Neighbor Advertisement and Redirect messages when those
messages are transmitted on an Ethernet.
This document replaces RFC 1972, "A Method for the Transmission of
IPv6 Packets over Ethernet Networks", which will become historic.
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. Maximum Transmission Unit
The default MTU size for IPv6 [IPV6] packets on an Ethernet is 1500
octets. This size may be reduced by a Router Advertisement [DISC]
containing an MTU option which specifies a smaller MTU, or by manual
configuration of each node. If a Router Advertisement received on an
Ethernet interface has an MTU option specifying an MTU larger than
1500, or larger than a manually configured value, that MTU option may
be logged to system management but must be otherwise ignored.
For purposes of this document, information received from DHCP is
considered "manually configured" and the term Ethernet includes
CSMA/CD and full-duplex subnetworks based on ISO/IEC 8802-3, with
various data rates.
3. Frame Format
IPv6 packets are transmitted in standard Ethernet frames. The
Ethernet header contains the Destination and Source Ethernet
addresses and the Ethernet type code, which must contain the value
86DD hexadecimal. The data field contains the IPv6 header followed
immediately by the payload, and possibly padding octets to meet the
minimum frame size for the Ethernet link.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination |
+- -+
| Ethernet |
+- -+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source |
+- -+
| Ethernet |
+- -+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0 0 0 0 1 1 0 1 1 0 1 1 1 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 |
+- -+
| header |
+- -+
| and |
+- -+
/ payload ... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(Each tic mark represents one bit.)
4. Stateless Autoconfiguration
The Interface Identifier [AARCH] for an Ethernet interface is based
on the EUI-64 identifier [EUI64] derived from the interface's built-
in 48-bit IEEE 802 address. The EUI-64 is formed as follows.
(Canonical bit order is assumed throughout.)
The OUI of the Ethernet address (the first three octets) becomes the
company_id of the EUI-64 (the first three octets). The fourth and
fifth octets of the EUI are set to the fixed value FFFE hexadecimal.
The last three octets of the Ethernet address become the last three
octets of the EUI-64.
The Interface Identifier is then formed from the EUI-64 by
complementing the "Universal/Local" (U/L) bit, which is the next-to-
lowest order bit of the first octet of the EUI-64. Complementing
this bit will generally change a 0 value to a 1, since an interface's
built-in address is expected to be from a universally administered
address space and hence have a globally unique value. A universally
administered IEEE 802 address or an EUI-64 is signified by a 0 in the
U/L bit position, while a globally unique IPv6 Interface Identifier
is signified by a 1 in the corresponding position. For further
discussion on this point, see [AARCH].
For example, the Interface Identifier for an Ethernet interface whose
built-in address is, in hexadecimal,
34-56-78-9A-BC-DE
would be
36-56-78-FF-FE-9A-BC-DE.
A different MAC address set manually or by software should not be
used to derive the Interface Identifier. If such a MAC address must
be used, its global uniqueness property should be reflected in the
value of the U/L bit.
An IPv6 address prefix used for stateless autoconfiguration [ACONF]
of an Ethernet interface must have a length of 64 bits.
5. Link-Local Addresses
The IPv6 link-local address [AARCH] for an Ethernet interface is
formed by appending the Interface Identifier, as defined above, to
the prefix FE80::/64.
10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+
6. Address Mapping -- Unicast
The procedure for mapping IPv6 unicast addresses into Ethernet link-
layer addresses is described in [DISC]. The Source/Target Link-layer
Address option has the following form when the link layer is
Ethernet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Ethernet -+
| |
+- Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option fields:
Type 1 for Source Link-layer address.
2 for Target Link-layer address.
Length 1 (in units of 8 octets).
Ethernet Address
The 48 bit Ethernet IEEE 802 address, in canonical bit
order. This is the address the interface currently
responds to, and may be different from the built-in
address used to derive the Interface Identifier.
7. Address Mapping -- Multicast
An IPv6 packet with a multicast destination address DST, consisting
of the sixteen octets DST[1] through DST[16], is transmitted to the
Ethernet multicast address whose first two octets are the value 3333
hexadecimal and whose last four octets are the last four octets of
DST.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 1 1 0 0 1 1|0 0 1 1 0 0 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DST[13] | DST[14] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DST[15] | DST[16] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8. Differences From RFC 1972
The following are the functional differences between this
specification and RFC 1972.
The Address Token, which was a node's 48-bit MAC address, is
replaced with the Interface Identifier, which is 64 bits in
length and based on the EUI-64 format [EUI64]. An IEEE-defined
mapping exists from 48-bit MAC addresses to EUI-64 form.
A prefix used for stateless autoconfiguration must now be 64 bits
long rather than 80. The link-local prefix is also shortened to
64 bits.
9. Security Considerations
The method of derivation of Interface Identifiers from MAC addresses
is intended to preserve global uniqueness when possible. However,
there is no protection from duplication through accident or forgery.
10. References
[AARCH] Hinden, R. and S. Deering "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[ACONF] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[EUI64] "Guidelines For 64-bit Global Identifier (EUI-64)",
http://standards.ieee.org/db/oui/tutorials/EUI64.html
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11. Author's Address
Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA
Phone: +1 630 840-3461
EMail: crawdad@fnal.gov
12. 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
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
RFC 2460 – Internet Protocol, Version 6 (IPv6) Specification
Network Working Group S. Deering Request for Comments: 2460 Cisco Obsoletes: 1883 R. Hinden Category: Standards Track Nokia December 1998 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 version 6 of the Internet Protocol (IPv6), also sometimes referred to as IP Next Generation or IPng. Table of Contents 1. Introduction..................................................2 2. Terminology...................................................3 3. IPv6 Header Format............................................4 4. IPv6 Extension Headers........................................6 4.1 Extension Header Order...................................7 4.2 Options..................................................9 4.3 Hop-by-Hop Options Header...............................11 4.4 Routing Header..........................................12 4.5 Fragment Header.........................................18 4.6 Destination Options Header..............................23 4.7 No Next Header..........................................24 5. Packet Size Issues...........................................24 6. Flow Labels..................................................25 7. Traffic Classes..............................................25 8. Upper-Layer Protocol Issues..................................27 8.1 Upper-Layer Checksums...................................27 8.2 Maximum Packet Lifetime.................................28 8.3 Maximum Upper-Layer Payload Size........................28 8.4 Responding to Packets Carrying Routing Headers..........29 Deering & Hinden Standards Track [Page 1] RFC 2460 IPv6 Specification December 1998 Appendix A. Semantics and Usage of the Flow Label Field.........30 Appendix B. Formatting Guidelines for Options...................32 Security Considerations.........................................35 Acknowledgments.................................................35 Authors' Addresses..............................................35 References......................................................35 Changes Since RFC-1883..........................................36 Full Copyright Statement........................................39 1. Introduction IP version 6 (IPv6) is a new version of the Internet Protocol, designed as the successor to IP version 4 (IPv4) [RFC-791]. The changes from IPv4 to IPv6 fall primarily into the following categories: o Expanded Addressing Capabilities IPv6 increases the IP address size from 32 bits to 128 bits, to support more levels of addressing hierarchy, a much greater number of addressable nodes, and simpler auto-configuration of addresses. The scalability of multicast routing is improved by adding a "scope" field to multicast addresses. And a new type of address called an "anycast address" is defined, used to send a packet to any one of a group of nodes. o Header Format Simplification Some IPv4 header fields have been dropped or made optional, to reduce the common-case processing cost of packet handling and to limit the bandwidth cost of the IPv6 header. o Improved Support for Extensions and Options Changes in the way IP header options are encoded allows for more efficient forwarding, less stringent limits on the length of options, and greater flexibility for introducing new options in the future. o Flow Labeling Capability A new capability is added to enable the labeling of packets belonging to particular traffic "flows" for which the sender requests special handling, such as non-default quality of service or "real-time" service. Deering & Hinden Standards Track [Page 2] RFC 2460 IPv6 Specification December 1998 o Authentication and Privacy Capabilities Extensions to support authentication, data integrity, and (optional) data confidentiality are specified for IPv6. This document specifies the basic IPv6 header and the initially- defined IPv6 extension headers and options. It also discusses packet size issues, the semantics of flow labels and traffic classes, and the effects of IPv6 on upper-layer protocols. The format and semantics of IPv6 addresses are specified separately in [ADDRARCH]. The IPv6 version of ICMP, which all IPv6 implementations are required to include, is specified in [ICMPv6]. 2. Terminology node - a device that implements IPv6. router - a node that forwards IPv6 packets not explicitly addressed to itself. [See Note below]. host - any node that is not a router. [See Note below]. upper layer - a protocol layer immediately above IPv6. Examples are transport protocols such as TCP and UDP, control protocols such as ICMP, routing protocols such as OSPF, and internet or lower-layer protocols being "tunneled" over (i.e., encapsulated in) IPv6 such as IPX, AppleTalk, or IPv6 itself. link - a communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IPv6. Examples are Ethernets (simple or bridged); PPP links; X.25, Frame Relay, or ATM networks; and internet (or higher) layer "tunnels", such as tunnels over IPv4 or IPv6 itself. neighbors - nodes attached to the same link. interface - a node's attachment to a link. address - an IPv6-layer identifier for an interface or a set of interfaces. packet - an IPv6 header plus payload. link MTU - the maximum transmission unit, i.e., maximum packet size in octets, that can be conveyed over a link. Deering & Hinden Standards Track [Page 3] RFC 2460 IPv6 Specification December 1998 path MTU - the minimum link MTU of all the links in a path between a source node and a destination node. Note: it is possible, though unusual, for a device with multiple interfaces to be configured to forward non-self-destined packets arriving from some set (fewer than all) of its interfaces, and to discard non-self-destined packets arriving from its other interfaces. Such a device must obey the protocol requirements for routers when receiving packets from, and interacting with neighbors over, the former (forwarding) interfaces. It must obey the protocol requirements for hosts when receiving packets from, and interacting with neighbors over, the latter (non-forwarding) interfaces. 3. IPv6 Header Format +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| Traffic Class | Flow Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Length | Next Header | Hop Limit | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Source Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Destination Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version 4-bit Internet Protocol version number = 6. Traffic Class 8-bit traffic class field. See section 7. Flow Label 20-bit flow label. See section 6. Payload Length 16-bit unsigned integer. Length of the IPv6 payload, i.e., the rest of the packet following this IPv6 header, in octets. (Note that any Deering & Hinden Standards Track [Page 4] RFC 2460 IPv6 Specification December 1998 extension headers [section 4] present are considered part of the payload, i.e., included in the length count.) Next Header 8-bit selector. Identifies the type of header immediately following the IPv6 header. Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.]. Hop Limit 8-bit unsigned integer. Decremented by 1 by each node that forwards the packet. The packet is discarded if Hop Limit is decremented to zero. Source Address 128-bit address of the originator of the packet. See [ADDRARCH]. Destination Address 128-bit address of the intended recipient of the packet (possibly not the ultimate recipient, if a Routing header is present). See [ADDRARCH] and section 4.4. Deering & Hinden Standards Track [Page 5] RFC 2460 IPv6 Specification December 1998 4. IPv6 Extension Headers In IPv6, optional internet-layer information is encoded in separate headers that may be placed between the IPv6 header and the upper- layer header in a packet. There are a small number of such extension headers, each identified by a distinct Next Header value. As illustrated in these examples, an IPv6 packet may carry zero, one, or more extension headers, each identified by the Next Header field of the preceding header: +---------------+------------------------ | IPv6 header | TCP header + data | | | Next Header = | | TCP | +---------------+------------------------ +---------------+----------------+------------------------ | IPv6 header | Routing header | TCP header + data | | | | Next Header = | Next Header = | | Routing | TCP | +---------------+----------------+------------------------ +---------------+----------------+-----------------+----------------- | IPv6 header | Routing header | Fragment header | fragment of TCP | | | | header + data | Next Header = | Next Header = | Next Header = | | Routing | Fragment | TCP | +---------------+----------------+-----------------+----------------- With one exception, extension headers are not examined or processed by any node along a packet's delivery path, until the packet reaches the node (or each of the set of nodes, in the case of multicast) identified in the Destination Address field of the IPv6 header. There, normal demultiplexing on the Next Header field of the IPv6 header invokes the module to process the first extension header, or the upper-layer header if no extension header is present. The contents and semantics of each extension header determine whether or not to proceed to the next header. Therefore, extension headers must be processed strictly in the order they appear in the packet; a receiver must not, for example, scan through a packet looking for a particular kind of extension header and process that header prior to processing all preceding ones. Deering & Hinden Standards Track [Page 6] RFC 2460 IPv6 Specification December 1998 The exception referred to in the preceding paragraph is the Hop-by- Hop Options header, which carries information that must be examined and processed by every node along a packet's delivery path, including the source and destination nodes. The Hop-by-Hop Options header, when present, must immediately follow the IPv6 header. Its presence is indicated by the value zero in the Next Header field of the IPv6 header. If, as a result of processing a header, a node is required to proceed to the next header but the Next Header value in the current header is unrecognized by the node, it should discard the packet and send an ICMP Parameter Problem message to the source of the packet, with an ICMP Code value of 1 ("unrecognized Next Header type encountered") and the ICMP Pointer field containing the offset of the unrecognized value within the original packet. The same action should be taken if a node encounters a Next Header value of zero in any header other than an IPv6 header. Each extension header is an integer multiple of 8 octets long, in order to retain 8-octet alignment for subsequent headers. Multi- octet fields within each extension header are aligned on their natural boundaries, i.e., fields of width n octets are placed at an integer multiple of n octets from the start of the header, for n = 1, 2, 4, or 8. A full implementation of IPv6 includes implementation of the following extension headers: Hop-by-Hop Options Routing (Type 0) Fragment Destination Options Authentication Encapsulating Security Payload The first four are specified in this document; the last two are specified in [RFC-2402] and [RFC-2406], respectively. 4.1 Extension Header Order When more than one extension header is used in the same packet, it is recommended that those headers appear in the following order: IPv6 header Hop-by-Hop Options header Destination Options header (note 1) Routing header Fragment header Deering & Hinden Standards Track [Page 7] RFC 2460 IPv6 Specification December 1998 Authentication header (note 2) Encapsulating Security Payload header (note 2) Destination Options header (note 3) upper-layer header note 1: for options to be processed by the first destination that appears in the IPv6 Destination Address field plus subsequent destinations listed in the Routing header. note 2: additional recommendations regarding the relative order of the Authentication and Encapsulating Security Payload headers are given in [RFC-2406]. note 3: for options to be processed only by the final destination of the packet. Each extension header should occur at most once, except for the Destination Options header which should occur at most twice (once before a Routing header and once before the upper-layer header). If the upper-layer header is another IPv6 header (in the case of IPv6 being tunneled over or encapsulated in IPv6), it may be followed by its own extension headers, which are separately subject to the same ordering recommendations. If and when other extension headers are defined, their ordering constraints relative to the above listed headers must be specified. IPv6 nodes must accept and attempt to process extension headers in any order and occurring any number of times in the same packet, except for the Hop-by-Hop Options header which is restricted to appear immediately after an IPv6 header only. Nonetheless, it is strongly advised that sources of IPv6 packets adhere to the above recommended order until and unless subsequent specifications revise that recommendation. Deering & Hinden Standards Track [Page 8] RFC 2460 IPv6 Specification December 1998 4.2 Options Two of the currently-defined extension headers -- the Hop-by-Hop Options header and the Destination Options header -- carry a variable number of type-length-value (TLV) encoded "options", of the following format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - | Option Type | Opt Data Len | Option Data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - Option Type 8-bit identifier of the type of option. Opt Data Len 8-bit unsigned integer. Length of the Option Data field of this option, in octets. Option Data Variable-length field. Option-Type-specific data. The sequence of options within a header must be processed strictly in the order they appear in the header; a receiver must not, for example, scan through the header looking for a particular kind of option and process that option prior to processing all preceding ones. The Option Type identifiers are internally encoded such that their highest-order two bits specify the action that must be taken if the processing IPv6 node does not recognize the Option Type: 00 - skip over this option and continue processing the header. 01 - discard the packet. 10 - discard the packet and, regardless of whether or not the packet's Destination Address was a multicast address, send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type. 11 - discard the packet and, only if the packet's Destination Address was not a multicast address, send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type. The third-highest-order bit of the Option Type specifies whether or not the Option Data of that option can change en-route to the packet's final destination. When an Authentication header is present Deering & Hinden Standards Track [Page 9] RFC 2460 IPv6 Specification December 1998 in the packet, for any option whose data may change en-route, its entire Option Data field must be treated as zero-valued octets when computing or verifying the packet's authenticating value. 0 - Option Data does not change en-route 1 - Option Data may change en-route The three high-order bits described above are to be treated as part of the Option Type, not independent of the Option Type. That is, a particular option is identified by a full 8-bit Option Type, not just the low-order 5 bits of an Option Type. The same Option Type numbering space is used for both the Hop-by-Hop Options header and the Destination Options header. However, the specification of a particular option may restrict its use to only one of those two headers. Individual options may have specific alignment requirements, to ensure that multi-octet values within Option Data fields fall on natural boundaries. The alignment requirement of an option is specified using the notation xn+y, meaning the Option Type must appear at an integer multiple of x octets from the start of the header, plus y octets. For example: 2n means any 2-octet offset from the start of the header. 8n+2 means any 8-octet offset from the start of the header, plus 2 octets. There are two padding options which are used when necessary to align subsequent options and to pad out the containing header to a multiple of 8 octets in length. These padding options must be recognized by all IPv6 implementations: Pad1 option (alignment requirement: none) +-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ NOTE! the format of the Pad1 option is a special case -- it does not have length and value fields. The Pad1 option is used to insert one octet of padding into the Options area of a header. If more than one octet of padding is required, the PadN option, described next, should be used, rather than multiple Pad1 options. Deering & Hinden Standards Track [Page 10] RFC 2460 IPv6 Specification December 1998 PadN option (alignment requirement: none) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - | 1 | Opt Data Len | Option Data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - The PadN option is used to insert two or more octets of padding into the Options area of a header. For N octets of padding, the Opt Data Len field contains the value N-2, and the Option Data consists of N-2 zero-valued octets. Appendix B contains formatting guidelines for designing new options. 4.3 Hop-by-Hop Options Header The Hop-by-Hop Options header is used to carry optional information that must be examined by every node along a packet's delivery path. The Hop-by-Hop Options header is identified by a Next Header value of 0 in the IPv6 header, and has the following format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | . . . Options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the type of header immediately following the Hop-by-Hop Options header. Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.]. Hdr Ext Len 8-bit unsigned integer. Length of the Hop-by- Hop Options header in 8-octet units, not including the first 8 octets. Options Variable-length field, of length such that the complete Hop-by-Hop Options header is an integer multiple of 8 octets long. Contains one or more TLV-encoded options, as described in section 4.2. The only hop-by-hop options defined in this document are the Pad1 and PadN options specified in section 4.2. Deering & Hinden Standards Track [Page 11] RFC 2460 IPv6 Specification December 1998 4.4 Routing Header The Routing header is used by an IPv6 source to list one or more intermediate nodes to be "visited" on the way to a packet's destination. This function is very similar to IPv4's Loose Source and Record Route option. The Routing header is identified by a Next Header value of 43 in the immediately preceding header, and has the following format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | Routing Type | Segments Left | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . type-specific data . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the type of header immediately following the Routing header. Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.]. Hdr Ext Len 8-bit unsigned integer. Length of the Routing header in 8-octet units, not including the first 8 octets. Routing Type 8-bit identifier of a particular Routing header variant. Segments Left 8-bit unsigned integer. Number of route segments remaining, i.e., number of explicitly listed intermediate nodes still to be visited before reaching the final destination. type-specific data Variable-length field, of format determined by the Routing Type, and of length such that the complete Routing header is an integer multiple of 8 octets long. If, while processing a received packet, a node encounters a Routing header with an unrecognized Routing Type value, the required behavior of the node depends on the value of the Segments Left field, as follows: Deering & Hinden Standards Track [Page 12] RFC 2460 IPv6 Specification December 1998 If Segments Left is zero, the node must ignore the Routing header and proceed to process the next header in the packet, whose type is identified by the Next Header field in the Routing header. If Segments Left is non-zero, the node must discard the packet and send an ICMP Parameter Problem, Code 0, message to the packet's Source Address, pointing to the unrecognized Routing Type. If, after processing a Routing header of a received packet, an intermediate node determines that the packet is to be forwarded onto a link whose link MTU is less than the size of the packet, the node must discard the packet and send an ICMP Packet Too Big message to the packet's Source Address. Deering & Hinden Standards Track [Page 13] RFC 2460 IPv6 Specification December 1998 The Type 0 Routing header has the following format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | Routing Type=0| Segments Left | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Address[1] + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Address[2] + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Address[n] + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the type of header immediately following the Routing header. Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.]. Hdr Ext Len 8-bit unsigned integer. Length of the Routing header in 8-octet units, not including the first 8 octets. For the Type 0 Routing header, Hdr Ext Len is equal to two times the number of addresses in the header. Routing Type 0. Deering & Hinden Standards Track [Page 14] RFC 2460 IPv6 Specification December 1998 Segments Left 8-bit unsigned integer. Number of route segments remaining, i.e., number of explicitly listed intermediate nodes still to be visited before reaching the final destination. Reserved 32-bit reserved field. Initialized to zero for transmission; ignored on reception. Address[1..n] Vector of 128-bit addresses, numbered 1 to n. Multicast addresses must not appear in a Routing header of Type 0, or in the IPv6 Destination Address field of a packet carrying a Routing header of Type 0. A Routing header is not examined or processed until it reaches the node identified in the Destination Address field of the IPv6 header. In that node, dispatching on the Next Header field of the immediately preceding header causes the Routing header module to be invoked, which, in the case of Routing Type 0, performs the following algorithm: Deering & Hinden Standards Track [Page 15] RFC 2460 IPv6 Specification December 1998 if Segments Left = 0 { proceed to process the next header in the packet, whose type is identified by the Next Header field in the Routing header } else if Hdr Ext Len is odd { send an ICMP Parameter Problem, Code 0, message to the Source Address, pointing to the Hdr Ext Len field, and discard the packet } else { compute n, the number of addresses in the Routing header, by dividing Hdr Ext Len by 2 if Segments Left is greater than n { send an ICMP Parameter Problem, Code 0, message to the Source Address, pointing to the Segments Left field, and discard the packet } else { decrement Segments Left by 1; compute i, the index of the next address to be visited in the address vector, by subtracting Segments Left from n if Address [i] or the IPv6 Destination Address is multicast { discard the packet } else { swap the IPv6 Destination Address and Address[i] if the IPv6 Hop Limit is less than or equal to 1 { send an ICMP Time Exceeded -- Hop Limit Exceeded in Transit message to the Source Address and discard the packet } else { decrement the Hop Limit by 1 resubmit the packet to the IPv6 module for transmission to the new destination } } } } Deering & Hinden Standards Track [Page 16] RFC 2460 IPv6 Specification December 1998 As an example of the effects of the above algorithm, consider the case of a source node S sending a packet to destination node D, using a Routing header to cause the packet to be routed via intermediate nodes I1, I2, and I3. The values of the relevant IPv6 header and Routing header fields on each segment of the delivery path would be as follows: As the packet travels from S to I1: Source Address = S Hdr Ext Len = 6 Destination Address = I1 Segments Left = 3 Address[1] = I2 Address[2] = I3 Address[3] = D As the packet travels from I1 to I2: Source Address = S Hdr Ext Len = 6 Destination Address = I2 Segments Left = 2 Address[1] = I1 Address[2] = I3 Address[3] = D As the packet travels from I2 to I3: Source Address = S Hdr Ext Len = 6 Destination Address = I3 Segments Left = 1 Address[1] = I1 Address[2] = I2 Address[3] = D As the packet travels from I3 to D: Source Address = S Hdr Ext Len = 6 Destination Address = D Segments Left = 0 Address[1] = I1 Address[2] = I2 Address[3] = I3 Deering & Hinden Standards Track [Page 17] RFC 2460 IPv6 Specification December 1998 4.5 Fragment Header The Fragment header is used by an IPv6 source to send a packet larger than would fit in the path MTU to its destination. (Note: unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path -- see section 5.) The Fragment header is identified by a Next Header value of 44 in the immediately preceding header, and has the following format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Reserved | Fragment Offset |Res|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the initial header type of the Fragmentable Part of the original packet (defined below). Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.]. Reserved 8-bit reserved field. Initialized to zero for transmission; ignored on reception. Fragment Offset 13-bit unsigned integer. The offset, in 8-octet units, of the data following this header, relative to the start of the Fragmentable Part of the original packet. Res 2-bit reserved field. Initialized to zero for transmission; ignored on reception. M flag 1 = more fragments; 0 = last fragment. Identification 32 bits. See description below. In order to send a packet that is too large to fit in the MTU of the path to its destination, a source node may divide the packet into fragments and send each fragment as a separate packet, to be reassembled at the receiver. For every packet that is to be fragmented, the source node generates an Identification value. The Identification must be different than that of any other fragmented packet sent recently* with the same Source Address and Destination Address. If a Routing header is present, the Destination Address of concern is that of the final destination. Deering & Hinden Standards Track [Page 18] RFC 2460 IPv6 Specification December 1998 * "recently" means within the maximum likely lifetime of a packet, including transit time from source to destination and time spent awaiting reassembly with other fragments of the same packet. However, it is not required that a source node know the maximum packet lifetime. Rather, it is assumed that the requirement can be met by maintaining the Identification value as a simple, 32- bit, "wrap-around" counter, incremented each time a packet must be fragmented. It is an implementation choice whether to maintain a single counter for the node or multiple counters, e.g., one for each of the node's possible source addresses, or one for each active (source address, destination address) combination. The initial, large, unfragmented packet is referred to as the "original packet", and it is considered to consist of two parts, as illustrated: original packet: +------------------+----------------------//-----------------------+ | Unfragmentable | Fragmentable | | Part | Part | +------------------+----------------------//-----------------------+ The Unfragmentable Part consists of the IPv6 header plus any extension headers that must be processed by nodes en route to the destination, that is, all headers up to and including the Routing header if present, else the Hop-by-Hop Options header if present, else no extension headers. The Fragmentable Part consists of the rest of the packet, that is, any extension headers that need be processed only by the final destination node(s), plus the upper-layer header and data. The Fragmentable Part of the original packet is divided into fragments, each, except possibly the last ("rightmost") one, being an integer multiple of 8 octets long. The fragments are transmitted in separate "fragment packets" as illustrated: original packet: +------------------+--------------+--------------+--//--+----------+ | Unfragmentable | first | second | | last | | Part | fragment | fragment | .... | fragment | +------------------+--------------+--------------+--//--+----------+ Deering & Hinden Standards Track [Page 19] RFC 2460 IPv6 Specification December 1998 fragment packets: +------------------+--------+--------------+ | Unfragmentable |Fragment| first | | Part | Header | fragment | +------------------+--------+--------------+ +------------------+--------+--------------+ | Unfragmentable |Fragment| second | | Part | Header | fragment | +------------------+--------+--------------+ o o o +------------------+--------+----------+ | Unfragmentable |Fragment| last | | Part | Header | fragment | +------------------+--------+----------+ Each fragment packet is composed of: (1) The Unfragmentable Part of the original packet, with the Payload Length of the original IPv6 header changed to contain the length of this fragment packet only (excluding the length of the IPv6 header itself), and the Next Header field of the last header of the Unfragmentable Part changed to 44. (2) A Fragment header containing: The Next Header value that identifies the first header of the Fragmentable Part of the original packet. A Fragment Offset containing the offset of the fragment, in 8-octet units, relative to the start of the Fragmentable Part of the original packet. The Fragment Offset of the first ("leftmost") fragment is 0. An M flag value of 0 if the fragment is the last ("rightmost") one, else an M flag value of 1. The Identification value generated for the original packet. (3) The fragment itself. The lengths of the fragments must be chosen such that the resulting fragment packets fit within the MTU of the path to the packets' destination(s). Deering & Hinden Standards Track [Page 20] RFC 2460 IPv6 Specification December 1998 At the destination, fragment packets are reassembled into their original, unfragmented form, as illustrated: reassembled original packet: +------------------+----------------------//------------------------+ | Unfragmentable | Fragmentable | | Part | Part | +------------------+----------------------//------------------------+ The following rules govern reassembly: An original packet is reassembled only from fragment packets that have the same Source Address, Destination Address, and Fragment Identification. The Unfragmentable Part of the reassembled packet consists of all headers up to, but not including, the Fragment header of the first fragment packet (that is, the packet whose Fragment Offset is zero), with the following two changes: The Next Header field of the last header of the Unfragmentable Part is obtained from the Next Header field of the first fragment's Fragment header. The Payload Length of the reassembled packet is computed from the length of the Unfragmentable Part and the length and offset of the last fragment. For example, a formula for computing the Payload Length of the reassembled original packet is: PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.last where PL.orig = Payload Length field of reassembled packet. PL.first = Payload Length field of first fragment packet. FL.first = length of fragment following Fragment header of first fragment packet. FO.last = Fragment Offset field of Fragment header of last fragment packet. FL.last = length of fragment following Fragment header of last fragment packet. The Fragmentable Part of the reassembled packet is constructed from the fragments following the Fragment headers in each of the fragment packets. The length of each fragment is computed by subtracting from the packet's Payload Length the length of the Deering & Hinden Standards Track [Page 21] RFC 2460 IPv6 Specification December 1998 headers between the IPv6 header and fragment itself; its relative position in Fragmentable Part is computed from its Fragment Offset value. The Fragment header is not present in the final, reassembled packet. The following error conditions may arise when reassembling fragmented packets: If insufficient fragments are received to complete reassembly of a packet within 60 seconds of the reception of the first-arriving fragment of that packet, reassembly of that packet must be abandoned and all the fragments that have been received for that packet must be discarded. If the first fragment (i.e., the one with a Fragment Offset of zero) has been received, an ICMP Time Exceeded -- Fragment Reassembly Time Exceeded message should be sent to the source of that fragment. If the length of a fragment, as derived from the fragment packet's Payload Length field, is not a multiple of 8 octets and the M flag of that fragment is 1, then that fragment must be discarded and an ICMP Parameter Problem, Code 0, message should be sent to the source of the fragment, pointing to the Payload Length field of the fragment packet. If the length and offset of a fragment are such that the Payload Length of the packet reassembled from that fragment would exceed 65,535 octets, then that fragment must be discarded and an ICMP Parameter Problem, Code 0, message should be sent to the source of the fragment, pointing to the Fragment Offset field of the fragment packet. The following conditions are not expected to occur, but are not considered errors if they do: The number and content of the headers preceding the Fragment header of different fragments of the same original packet may differ. Whatever headers are present, preceding the Fragment header in each fragment packet, are processed when the packets arrive, prior to queueing the fragments for reassembly. Only those headers in the Offset zero fragment packet are retained in the reassembled packet. The Next Header values in the Fragment headers of different fragments of the same original packet may differ. Only the value from the Offset zero fragment packet is used for reassembly. Deering & Hinden Standards Track [Page 22] RFC 2460 IPv6 Specification December 1998 4.6 Destination Options Header The Destination Options header is used to carry optional information that need be examined only by a packet's destination node(s). The Destination Options header is identified by a Next Header value of 60 in the immediately preceding header, and has the following format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | . . . Options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Next Header 8-bit selector. Identifies the type of header immediately following the Destination Options header. Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.]. Hdr Ext Len 8-bit unsigned integer. Length of the Destination Options header in 8-octet units, not including the first 8 octets. Options Variable-length field, of length such that the complete Destination Options header is an integer multiple of 8 octets long. Contains one or more TLV-encoded options, as described in section 4.2. The only destination options defined in this document are the Pad1 and PadN options specified in section 4.2. Note that there are two possible ways to encode optional destination information in an IPv6 packet: either as an option in the Destination Options header, or as a separate extension header. The Fragment header and the Authentication header are examples of the latter approach. Which approach can be used depends on what action is desired of a destination node that does not understand the optional information: o If the desired action is for the destination node to discard the packet and, only if the packet's Destination Address is not a multicast address, send an ICMP Unrecognized Type message to the packet's Source Address, then the information may be encoded either as a separate header or as an option in the Deering & Hinden Standards Track [Page 23] RFC 2460 IPv6 Specification December 1998 Destination Options header whose Option Type has the value 11 in its highest-order two bits. The choice may depend on such factors as which takes fewer octets, or which yields better alignment or more efficient parsing. o If any other action is desired, the information must be encoded as an option in the Destination Options header whose Option Type has the value 00, 01, or 10 in its highest-order two bits, specifying the desired action (see section 4.2). 4.7 No Next Header The value 59 in the Next Header field of an IPv6 header or any extension header indicates that there is nothing following that header. If the Payload Length field of the IPv6 header indicates the presence of octets past the end of a header whose Next Header field contains 59, those octets must be ignored, and passed on unchanged if the packet is forwarded. 5. Packet Size Issues IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6. Links that have a configurable MTU (for example, PPP links [RFC- 1661]) must be configured to have an MTU of at least 1280 octets; it is recommended that they be configured with an MTU of 1500 octets or greater, to accommodate possible encapsulations (i.e., tunneling) without incurring IPv6-layer fragmentation. From each link to which a node is directly attached, the node must be able to accept packets as large as that link's MTU. It is strongly recommended that IPv6 nodes implement Path MTU Discovery [RFC-1981], in order to discover and take advantage of path MTUs greater than 1280 octets. However, a minimal IPv6 implementation (e.g., in a boot ROM) may simply restrict itself to sending packets no larger than 1280 octets, and omit implementation of Path MTU Discovery. In order to send a packet larger than a path's MTU, a node may use the IPv6 Fragment header to fragment the packet at the source and have it reassembled at the destination(s). However, the use of such fragmentation is discouraged in any application that is able to adjust its packets to fit the measured path MTU (i.e., down to 1280 octets). Deering & Hinden Standards Track [Page 24] RFC 2460 IPv6 Specification December 1998 A node must be able to accept a fragmented packet that, after reassembly, is as large as 1500 octets. A node is permitted to accept fragmented packets that reassemble to more than 1500 octets. An upper-layer protocol or application that depends on IPv6 fragmentation to send packets larger than the MTU of a path should not send packets larger than 1500 octets unless it has assurance that the destination is capable of reassembling packets of that larger size. In response to an IPv6 packet that is sent to an IPv4 destination (i.e., a packet that undergoes translation from IPv6 to IPv4), the originating IPv6 node may receive an ICMP Packet Too Big message reporting a Next-Hop MTU less than 1280. In that case, the IPv6 node is not required to reduce the size of subsequent packets to less than 1280, but must include a Fragment header in those packets so that the IPv6-to-IPv4 translating router can obtain a suitable Identification value to use in resulting IPv4 fragments. Note that this means the payload may have to be reduced to 1232 octets (1280 minus 40 for the IPv6 header and 8 for the Fragment header), and smaller still if additional extension headers are used. 6. Flow Labels The 20-bit Flow Label field in the IPv6 header may be used by a source to label sequences of packets for which it requests special handling by the IPv6 routers, such as non-default quality of service or "real-time" service. This aspect of IPv6 is, at the time of writing, still experimental and subject to change as the requirements for flow support in the Internet become clearer. Hosts or routers that do not support the functions of the Flow Label field are required to set the field to zero when originating a packet, pass the field on unchanged when forwarding a packet, and ignore the field when receiving a packet. Appendix A describes the current intended semantics and usage of the Flow Label field. 7. Traffic Classes The 8-bit Traffic Class field in the IPv6 header is available for use by originating nodes and/or forwarding routers to identify and distinguish between different classes or priorities of IPv6 packets. At the point in time at which this specification is being written, there are a number of experiments underway in the use of the IPv4 Type of Service and/or Precedence bits to provide various forms of "differentiated service" for IP packets, other than through the use of explicit flow set-up. The Traffic Class field in the IPv6 header is intended to allow similar functionality to be supported in IPv6. Deering & Hinden Standards Track [Page 25] RFC 2460 IPv6 Specification December 1998 It is hoped that those experiments will eventually lead to agreement on what sorts of traffic classifications are most useful for IP packets. Detailed definitions of the syntax and semantics of all or some of the IPv6 Traffic Class bits, whether experimental or intended for eventual standardization, are to be provided in separate documents. The following general requirements apply to the Traffic Class field: o The service interface to the IPv6 service within a node must provide a means for an upper-layer protocol to supply the value of the Traffic Class bits in packets originated by that upper- layer protocol. The default value must be zero for all 8 bits. o Nodes that support a specific (experimental or eventual standard) use of some or all of the Traffic Class bits are permitted to change the value of those bits in packets that they originate, forward, or receive, as required for that specific use. Nodes should ignore and leave unchanged any bits of the Traffic Class field for which they do not support a specific use. o An upper-layer protocol must not assume that the value of the Traffic Class bits in a received packet are the same as the value sent by the packet's source. Deering & Hinden Standards Track [Page 26] RFC 2460 IPv6 Specification December 1998 8. Upper-Layer Protocol Issues 8.1 Upper-Layer Checksums Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses. In particular, the following illustration shows the TCP and UDP "pseudo-header" for IPv6: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Source Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Destination Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Upper-Layer Packet Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | zero | Next Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o If the IPv6 packet contains a Routing header, the Destination Address used in the pseudo-header is that of the final destination. At the originating node, that address will be in the last element of the Routing header; at the recipient(s), that address will be in the Destination Address field of the IPv6 header. o The Next Header value in the pseudo-header identifies the upper-layer protocol (e.g., 6 for TCP, or 17 for UDP). It will differ from the Next Header value in the IPv6 header if there are extension headers between the IPv6 header and the upper- layer header. o The Upper-Layer Packet Length in the pseudo-header is the length of the upper-layer header and data (e.g., TCP header plus TCP data). Some upper-layer protocols carry their own Deering & Hinden Standards Track [Page 27] RFC 2460 IPv6 Specification December 1998 length information (e.g., the Length field in the UDP header); for such protocols, that is the length used in the pseudo- header. Other protocols (such as TCP) do not carry their own length information, in which case the length used in the pseudo-header is the Payload Length from the IPv6 header, minus the length of any extension headers present between the IPv6 header and the upper-layer header. o Unlike IPv4, when UDP packets are originated by an IPv6 node, the UDP checksum is not optional. That is, whenever originating a UDP packet, an IPv6 node must compute a UDP checksum over the packet and the pseudo-header, and, if that computation yields a result of zero, it must be changed to hex FFFF for placement in the UDP header. IPv6 receivers must discard UDP packets containing a zero checksum, and should log the error. The IPv6 version of ICMP [ICMPv6] includes the above pseudo-header in its checksum computation; this is a change from the IPv4 version of ICMP, which does not include a pseudo-header in its checksum. The reason for the change is to protect ICMP from misdelivery or corruption of those fields of the IPv6 header on which it depends, which, unlike IPv4, are not covered by an internet-layer checksum. The Next Header field in the pseudo-header for ICMP contains the value 58, which identifies the IPv6 version of ICMP. 8.2 Maximum Packet Lifetime Unlike IPv4, IPv6 nodes are not required to enforce maximum packet lifetime. That is the reason the IPv4 "Time to Live" field was renamed "Hop Limit" in IPv6. In practice, very few, if any, IPv4 implementations conform to the requirement that they limit packet lifetime, so this is not a change in practice. Any upper-layer protocol that relies on the internet layer (whether IPv4 or IPv6) to limit packet lifetime ought to be upgraded to provide its own mechanisms for detecting and discarding obsolete packets. 8.3 Maximum Upper-Layer Payload Size When computing the maximum payload size available for upper-layer data, an upper-layer protocol must take into account the larger size of the IPv6 header relative to the IPv4 header. For example, in IPv4, TCP's MSS option is computed as the maximum packet size (a default value or a value learned through Path MTU Discovery) minus 40 octets (20 octets for the minimum-length IPv4 header and 20 octets for the minimum-length TCP header). When using TCP over IPv6, the MSS must be computed as the maximum packet size minus 60 octets, Deering & Hinden Standards Track [Page 28] RFC 2460 IPv6 Specification December 1998 because the minimum-length IPv6 header (i.e., an IPv6 header with no extension headers) is 20 octets longer than a minimum-length IPv4 header. 8.4 Responding to Packets Carrying Routing Headers When an upper-layer protocol sends one or more packets in response to a received packet that included a Routing header, the response packet(s) must not include a Routing header that was automatically derived by "reversing" the received Routing header UNLESS the integrity and authenticity of the received Source Address and Routing header have been verified (e.g., via the use of an Authentication header in the received packet). In other words, only the following kinds of packets are permitted in response to a received packet bearing a Routing header: o Response packets that do not carry Routing headers. o Response packets that carry Routing headers that were NOT derived by reversing the Routing header of the received packet (for example, a Routing header supplied by local configuration). o Response packets that carry Routing headers that were derived by reversing the Routing header of the received packet IF AND ONLY IF the integrity and authenticity of the Source Address and Routing header from the received packet have been verified by the responder. Deering & Hinden Standards Track [Page 29] RFC 2460 IPv6 Specification December 1998 Appendix A. Semantics and Usage of the Flow Label Field A flow is a sequence of packets sent from a particular source to a particular (unicast or multicast) destination for which the source desires special handling by the intervening routers. The nature of that special handling might be conveyed to the routers by a control protocol, such as a resource reservation protocol, or by information within the flow's packets themselves, e.g., in a hop-by-hop option. The details of such control protocols or options are beyond the scope of this document. There may be multiple active flows from a source to a destination, as well as traffic that is not associated with any flow. A flow is uniquely identified by the combination of a source address and a non-zero flow label. Packets that do not belong to a flow carry a flow label of zero. A flow label is assigned to a flow by the flow's source node. New flow labels must be chosen (pseudo-)randomly and uniformly from the range 1 to FFFFF hex. The purpose of the random allocation is to make any set of bits within the Flow Label field suitable for use as a hash key by routers, for looking up the state associated with the flow. All packets belonging to the same flow must be sent with the same source address, destination address, and flow label. If any of those packets includes a Hop-by-Hop Options header, then they all must be originated with the same Hop-by-Hop Options header contents (excluding the Next Header field of the Hop-by-Hop Options header). If any of those packets includes a Routing header, then they all must be originated with the same contents in all extension headers up to and including the Routing header (excluding the Next Header field in the Routing header). The routers or destinations are permitted, but not required, to verify that these conditions are satisfied. If a violation is detected, it should be reported to the source by an ICMP Parameter Problem message, Code 0, pointing to the high-order octet of the Flow Label field (i.e., offset 1 within the IPv6 packet). The maximum lifetime of any flow-handling state established along a flow's path must be specified as part of the description of the state-establishment mechanism, e.g., the resource reservation protocol or the flow-setup hop-by-hop option. A source must not re- use a flow label for a new flow within the maximum lifetime of any flow-handling state that might have been established for the prior use of that flow label. Deering & Hinden Standards Track [Page 30] RFC 2460 IPv6 Specification December 1998 When a node stops and restarts (e.g., as a result of a "crash"), it must be careful not to use a flow label that it might have used for an earlier flow whose lifetime may not have expired yet. This may be accomplished by recording flow label usage on stable storage so that it can be remembered across crashes, or by refraining from using any flow labels until the maximum lifetime of any possible previously established flows has expired. If the minimum time for rebooting the node is known, that time can be deducted from the necessary waiting period before starting to allocate flow labels. There is no requirement that all, or even most, packets belong to flows, i.e., carry non-zero flow labels. This observation is placed here to remind protocol designers and implementors not to assume otherwise. For example, it would be unwise to design a router whose performance would be adequate only if most packets belonged to flows, or to design a header compression scheme that only worked on packets that belonged to flows. Deering & Hinden Standards Track [Page 31] RFC 2460 IPv6 Specification December 1998 Appendix B. Formatting Guidelines for Options This appendix gives some advice on how to lay out the fields when designing new options to be used in the Hop-by-Hop Options header or the Destination Options header, as described in section 4.2. These guidelines are based on the following assumptions: o One desirable feature is that any multi-octet fields within the Option Data area of an option be aligned on their natural boundaries, i.e., fields of width n octets should be placed at an integer multiple of n octets from the start of the Hop-by- Hop or Destination Options header, for n = 1, 2, 4, or 8. o Another desirable feature is that the Hop-by-Hop or Destination Options header take up as little space as possible, subject to the requirement that the header be an integer multiple of 8 octets long. o It may be assumed that, when either of the option-bearing headers are present, they carry a very small number of options, usually only one. These assumptions suggest the following approach to laying out the fields of an option: order the fields from smallest to largest, with no interior padding, then derive the alignment requirement for the entire option based on the alignment requirement of the largest field (up to a maximum alignment of 8 octets). This approach is illustrated in the following examples: Example 1 If an option X required two data fields, one of length 8 octets and one of length 4 octets, it would be laid out as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type=X |Opt Data Len=12| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + 8-octet field + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Deering & Hinden Standards Track [Page 32] RFC 2460 IPv6 Specification December 1998 Its alignment requirement is 8n+2, to ensure that the 8-octet field starts at a multiple-of-8 offset from the start of the enclosing header. A complete Hop-by-Hop or Destination Options header containing this one option would look as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + 8-octet field + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Example 2 If an option Y required three data fields, one of length 4 octets, one of length 2 octets, and one of length 1 octet, it would be laid out as follows: +-+-+-+-+-+-+-+-+ | Option Type=Y | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opt Data Len=7 | 1-octet field | 2-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Its alignment requirement is 4n+3, to ensure that the 4-octet field starts at a multiple-of-4 offset from the start of the enclosing header. A complete Hop-by-Hop or Destination Options header containing this one option would look as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len=1 | Pad1 Option=0 | Option Type=Y | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opt Data Len=7 | 1-octet field | 2-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PadN Option=1 |Opt Data Len=2 | 0 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Deering & Hinden Standards Track [Page 33] RFC 2460 IPv6 Specification December 1998 Example 3 A Hop-by-Hop or Destination Options header containing both options X and Y from Examples 1 and 2 would have one of the two following formats, depending on which option appeared first: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len=3 | Option Type=X |Opt Data Len=12| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + 8-octet field + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PadN Option=1 |Opt Data Len=1 | 0 | Option Type=Y | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opt Data Len=7 | 1-octet field | 2-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PadN Option=1 |Opt Data Len=2 | 0 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len=3 | Pad1 Option=0 | Option Type=Y | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opt Data Len=7 | 1-octet field | 2-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PadN Option=1 |Opt Data Len=4 | 0 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 0 | Option Type=X |Opt Data Len=12| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4-octet field | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + 8-octet field + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Deering & Hinden Standards Track [Page 34] RFC 2460 IPv6 Specification December 1998 Security Considerations The security features of IPv6 are described in the Security Architecture for the Internet Protocol [RFC-2401]. Acknowledgments The authors gratefully acknowledge the many helpful suggestions of the members of the IPng working group, the End-to-End Protocols research group, and the Internet Community At Large. Authors' Addresses 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 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 References [RFC-2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC-2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998. [RFC-2406] Kent, S. and R. Atkinson, "IP Encapsulating Security Protocol (ESP)", RFC 2406, November 1998. [ICMPv6] Conta, A. and S. Deering, "ICMP for the Internet Protocol Version 6 (IPv6)", RFC 2463, December 1998. Deering & Hinden Standards Track [Page 35] RFC 2460 IPv6 Specification December 1998 [ADDRARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998. [RFC-1981] McCann, J., Mogul, J. and S. Deering, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [RFC-791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC-1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. See also: http://www.iana.org/numbers.html [RFC-1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. CHANGES SINCE RFC-1883 This memo has the following changes from RFC-1883. Numbers identify the Internet-Draft version in which the change was made. 02) Removed all references to jumbograms and the Jumbo Payload option (moved to a separate document). 02) Moved most of Flow Label description from section 6 to (new) Appendix A. 02) In Flow Label description, now in Appendix A, corrected maximum Flow Label value from FFFFFF to FFFFF (i.e., one less "F") due to reduction of size of Flow Label field from 24 bits to 20 bits. 02) Renumbered (relettered?) the previous Appendix A to be Appendix B. 02) Changed the wording of the Security Considerations section to avoid dependency loop between this spec and the IPsec specs. 02) Updated R. Hinden's email address and company affiliation. -------------------------------------------------------- 01) In section 3, changed field name "Class" to "Traffic Class" and increased its size from 4 to 8 bits. Decreased size of Flow Label field from 24 to 20 bits to compensate for increase in Traffic Class field. Deering & Hinden Standards Track [Page 36] RFC 2460 IPv6 Specification December 1998 01) In section 4.1, restored the order of the Authentication Header and the ESP header, which were mistakenly swapped in the 00 version of this memo. 01) In section 4.4, deleted the Strict/Loose Bit Map field and the strict routing functionality from the Type 0 Routing header, and removed the restriction on number of addresses that may be carried in the Type 0 Routing header (was limited to 23 addresses, because of the size of the strict/loose bit map). 01) In section 5, changed the minimum IPv6 MTU from 576 to 1280 octets, and added a recommendation that links with configurable MTU (e.g., PPP links) be configured to have an MTU of at least 1500 octets. 01) In section 5, deleted the requirement that a node must not send fragmented packets that reassemble to more than 1500 octets without knowledge of the destination reassembly buffer size, and replaced it with a recommendation that upper-layer protocols or applications should not do that. 01) Replaced reference to the IPv4 Path MTU Discovery spec (RFC- 1191) with reference to the IPv6 Path MTU Discovery spec (RFC- 1981), and deleted the Notes at the end of section 5 regarding Path MTU Discovery, since those details are now covered by RFC- 1981. 01) In section 6, deleted specification of "opportunistic" flow set-up, and removed all references to the 6-second maximum lifetime for opportunistically established flow state. 01) In section 7, deleted the provisional description of the internal structure and semantics of the Traffic Class field, and specified that such descriptions be provided in separate documents. -------------------------------------------------------- 00) In section 4, corrected the Code value to indicate "unrecognized Next Header type encountered" in an ICMP Parameter Problem message (changed from 2 to 1). 00) In the description of the Payload Length field in section 3, and of the Jumbo Payload Length field in section 4.3, made it clearer that extension headers are included in the payload length count. Deering & Hinden Standards Track [Page 37] RFC 2460 IPv6 Specification December 1998 00) In section 4.1, swapped the order of the Authentication header and the ESP header. (NOTE: this was a mistake, and the change was undone in version 01.) 00) In section 4.2, made it clearer that options are identified by the full 8-bit Option Type, not by the low-order 5 bits of an Option Type. Also specified that the same Option Type numbering space is used for both Hop-by-Hop Options and Destination Options headers. 00) In section 4.4, added a sentence requiring that nodes processing a Routing header must send an ICMP Packet Too Big message in response to a packet that is too big to fit in the next hop link (rather than, say, performing fragmentation). 00) Changed the name of the IPv6 Priority field to "Class", and replaced the previous description of Priority in section 7 with a description of the Class field. Also, excluded this field from the set of fields that must remain the same for all packets in the same flow, as specified in section 6. 00) In the pseudo-header in section 8.1, changed the name of the "Payload Length" field to "Upper-Layer Packet Length". Also clarified that, in the case of protocols that carry their own length info (like non-jumbogram UDP), it is the upper-layer- derived length, not the IP-layer-derived length, that is used in the pseudo-header. 00) Added section 8.4, specifying that upper-layer protocols, when responding to a received packet that carried a Routing header, must not include the reverse of the Routing header in the response packet(s) unless the received Routing header was authenticated. 00) Fixed some typos and grammatical errors. 00) Authors' contact info updated. -------------------------------------------------------- Deering & Hinden Standards Track [Page 38] RFC 2460 IPv6 Specification December 1998 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. Deering & Hinden Standards Track [Page 39]