By RFC org•September 5, 2025•

rfc

rfc2460

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

Introduction..................................................2
Terminology...................................................3
IPv6 Header Format............................................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
Packet Size Issues...........................................24
Flow Labels..................................................25
Traffic Classes..............................................25
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

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].

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.

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

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]

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.

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.

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.

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

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

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.

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.

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

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]

specified that such descriptions be provided in separatedocuments.

response packet(s) unless the received Routing header wasauthenticated. 00) Fixed some typos and grammatical errors. 00) Authors’ contact info updated.

specified that such descriptions be provided in separate
documents.

response packet(s) unless the received Routing header was
authenticated. 00) Fixed some typos and grammatical errors. 00) Authors’ contact info updated.