Internet Engineering Task Force (IETF) S. Deering

Internet Engineering Task Force (IETF) S. Deering
Request for Comments: 8200 Retired
STD: 86 R. Hinden
Obsoletes: 2460 Check Point Software
Category: Standards Track July 2017
ISSN: 2070-1721

Internet Protocol, Version 6 (IPv6) Specification

Abstract

This document specifies version 6 of the Internet Protocol (IPv6).
It obsoletes RFC 2460.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8200.

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RFC 8200 IPv6 Specification July 2017

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

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RFC 8200 IPv6 Specification July 2017

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. IPv6 Header Format . . . . . . . . . . . . . . . . . . . . . 6
4. IPv6 Extension Headers . . . . . . . . . . . . . . . . . . . 7
4.1. Extension Header Order . . . . . . . . . . . . . . . . . 10
4.2. Options . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Hop-by-Hop Options Header . . . . . . . . . . . . . . . . 13
4.4. Routing Header . . . . . . . . . . . . . . . . . . . . . 14
4.5. Fragment Header . . . . . . . . . . . . . . . . . . . . . 15
4.6. Destination Options Header . . . . . . . . . . . . . . . 23
4.7. No Next Header . . . . . . . . . . . . . . . . . . . . . 24
4.8. Defining New Extension Headers and Options . . . . . . . 24
5. Packet Size Issues . . . . . . . . . . . . . . . . . . . . . 25
6. Flow Labels . . . . . . . . . . . . . . . . . . . . . . . . . 26
7. Traffic Classes . . . . . . . . . . . . . . . . . . . . . . . 26
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 . . . . . . . . . . . . 29
8.4. Responding to Packets Carrying Routing Headers . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.1. Normative References . . . . . . . . . . . . . . . . . . 32
11.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Formatting Guidelines for Options . . . . . . . . . 36
Appendix B. Changes Since RFC 2460 . . . . . . . . . . . . . . . 39
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42

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RFC 8200 IPv6 Specification July 2017

1. Introduction

IP version 6 (IPv6) is a new version of the Internet Protocol (IP),
designed as the successor to IP version 4 (IPv4) [RFC791]. 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 autoconfiguration 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; it is 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 sequences
of packets that the sender requests to be treated in the
network as a single flow.

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 [RFC4291].
The IPv6 version of ICMP, which all IPv6 implementations are required
to include, is specified in [RFC4443].

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The data transmission order for IPv6 is the same as for IPv4 as
defined in Appendix B of [RFC791].

Note: As this document obsoletes [RFC2460], any document referenced
in this document that includes pointers to RFC 2460 should be
interpreted as referencing this document.

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-layer or lower-layer protocols being
"tunneled" over (i.e., encapsulated in) IPv6 such as
Internetwork Packet Exchange (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-layer 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.

path MTU the minimum link MTU of all the links in a path between
a source node and a destination node.

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Note: it is possible 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 extension headers (see Section 4)
present are considered part of the payload,
i.e., included in the length count.)

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Next Header 8-bit selector. Identifies the type of header
immediately following the IPv6 header. Uses
the same values as the IPv4 Protocol field
[IANA-PN].

Hop Limit 8-bit unsigned integer. Decremented by 1 by
each node that forwards the packet. When
forwarding, the packet is discarded if Hop
Limit was zero when received or is decremented
to zero. A node that is the destination of a
packet should not discard a packet with Hop
Limit equal to zero; it should process the
packet normally.

Source Address 128-bit address of the originator of the
packet. See [RFC4291].

Destination Address 128-bit address of the intended recipient of
the packet (possibly not the ultimate
recipient, if a Routing header is present).
See [RFC4291] and Section 4.4.

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 is a small number of such extension
headers, each one identified by a distinct Next Header value.

Extension headers are numbered from IANA IP Protocol Numbers
[IANA-PN], the same values used for IPv4 and IPv6. When processing a
sequence of Next Header values in a packet, the first one that is not
an extension header [IANA-EH] indicates that the next item in the
packet is the corresponding upper-layer header. A special "No Next
Header" value is used if there is no upper-layer header.

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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 |
+---------------+----------------+-----------------+-----------------

Extension headers (except for the Hop-by-Hop Options header) are not
processed, inserted, or deleted 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.

The Hop-by-Hop Options header is not inserted or deleted, but may be
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. 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.

NOTE: While [RFC2460] required that all nodes must examine and
process the Hop-by-Hop Options header, it is now expected that nodes
along a packet's delivery path only examine and process the
Hop-by-Hop Options header if explicitly configured to do so.

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At the destination node, 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.

If, as a result of processing a header, the destination 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
Fragment
Destination Options
Routing
Authentication
Encapsulating Security Payload

The first four are specified in this document; the last two are
specified in [RFC4302] and [RFC4303], respectively. The current list
of IPv6 extension headers can be found at [IANA-EH].

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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
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 [RFC4303].

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.

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4.2. Options

Two of the currently defined extension headers specified in this
document -- the Hop-by-Hop Options header and the Destination Options
header -- carry a variable number of "options" that are type-length-
value (TLV) encoded in 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 2 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

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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 that 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 1 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.

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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 A 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 may be examined and processed 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 [IANA-PN].

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.

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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 [IANA-PN].

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.

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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:

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.

The currently defined IPv6 Routing Headers and their status can be
found at [IANA-RH]. Allocation guidelines for IPv6 Routing Headers
can be found in [RFC5871].

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 [IANA-PN].

Reserved 8-bit reserved field. Initialized to zero for
transmission; ignored on reception.

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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.

* "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 knows
the maximum packet lifetime. Rather, it is assumed that the
requirement can be met by implementing an algorithm that
results in a low identification reuse frequency. Examples of
algorithms that can meet this requirement are described in
[RFC7739].

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The initial, large, unfragmented packet is referred to as the
"original packet", and it is considered to consist of three parts, as
illustrated:

original packet:

+------------------+-------------------------+---//----------------+
| Per-Fragment | Extension & Upper-Layer | Fragmentable |
| Headers | Headers | Part |
+------------------+-------------------------+---//----------------+

The Per-Fragment headers must consist 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 Extension headers are all other extension headers that are not
included in the Per-Fragment headers part of the packet. For this
purpose, the Encapsulating Security Payload (ESP) is not
considered an extension header. The Upper-Layer header is the
first upper-layer header that is not an IPv6 extension header.
Examples of upper-layer headers include TCP, UDP, IPv4, IPv6,
ICMPv6, and as noted ESP.

The Fragmentable Part consists of the rest of the packet after the
upper-layer header or after any header (i.e., initial IPv6 header
or extension header) that contains a Next Header value of No Next
Header.

The Fragmentable Part of the original packet is divided into
fragments. The lengths of the fragments must be chosen such that the
resulting fragment packets fit within the MTU of the path to the
packet's destination(s). Each complete fragment, except possibly the
last ("rightmost") one, is an integer multiple of 8 octets long.

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The fragments are transmitted in separate "fragment packets" as
illustrated:

original packet:

+-----------------+-----------------+--------+--------+-//-+--------+
| Per-Fragment |Ext & Upper-Layer| first | second | | last |
| Headers | Headers |fragment|fragment|....|fragment|
+-----------------+-----------------+--------+--------+-//-+--------+

fragment packets:

+------------------+---------+-------------------+----------+
| Per-Fragment |Fragment | Ext & Upper-Layer | first |
| Headers | Header | Headers | fragment |
+------------------+---------+-------------------+----------+

+------------------+--------+-------------------------------+
| Per-Fragment |Fragment| second |
| Headers | Header | fragment |
+------------------+--------+-------------------------------+
o
o
o
+------------------+--------+----------+
| Per-Fragment |Fragment| last |
| Headers | Header | fragment |
+------------------+--------+----------+

The first fragment packet is composed of:

(1) The Per-Fragment headers 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 Per-Fragment headers changed to 44.

(2) A Fragment header containing:

The Next Header value that identifies the first header
after the Per-Fragment headers 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 1 as this is the first fragment.

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The Identification value generated for the original
packet.

(3) Extension headers, if any, and the Upper-Layer header. These
headers must be in the first fragment. Note: This restricts
the size of the headers through the Upper-Layer header to the
MTU of the path to the packet's destinations(s).

(4) The first fragment.

The subsequent fragment packets are composed of:

(1) The Per-Fragment headers 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 Per-Fragment headers changed to 44.

(2) A Fragment header containing:

The Next Header value that identifies the first header
after the Per-Fragment headers 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.

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.

Fragments must not be created that overlap with any other fragments
created from the original packet.

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At the destination, fragment packets are reassembled into their
original, unfragmented form, as illustrated:

reassembled original packet:

+---------------+-----------------+---------+--------+-//--+--------+
| Per-Fragment |Ext & Upper-Layer| first | second | | last |
| Headers | Headers |frag data|fragment|.....|fragment|
+---------------+-----------------+---------+--------+-//--+--------+

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 Per-Fragment headers 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 Per-Fragment
headers 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 Per-Fragment headers 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 headers between the IPv6 header and fragment itself; its

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relative position in Fragmentable Part is computed from its
Fragment Offset value.

The Fragment header is not present in the final, reassembled
packet.

If the fragment is a whole datagram (that is, both the Fragment
Offset field and the M flag are zero), then it does not need
any further reassembly and should be processed as a fully
reassembled packet (i.e., updating Next Header, adjust Payload
Length, removing the Fragment header, etc.). Any other
fragments that match this packet (i.e., the same IPv6 Source
Address, IPv6 Destination Address, and Fragment Identification)
should be processed independently.

The following error conditions may arise when reassembling fragmented
packets:

o 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.

o 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.

o 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.

o If the first fragment does not include all headers through an
Upper-Layer header, then that fragment should be discarded and
an ICMP Parameter Problem, Code 3, message should be sent to
the source of the fragment, with the Pointer field set to zero.

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o If any of the fragments being reassembled overlap with any
other fragments being reassembled for the same packet,
reassembly of that packet must be abandoned and all the
fragments that have been received for that packet must be
discarded, and no ICMP error messages should be sent.

It should be noted that fragments may be duplicated in the
network. Instead of treating these exact duplicate fragments
as overlapping fragments, an implementation may choose to
detect this case and drop exact duplicate fragments while
keeping the other fragments belonging to the same packet.

The following conditions are not expected to occur frequently 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.

Other fields in the IPv6 header may also vary across the fragments
being reassembled. Specifications that use these fields may
provide additional instructions if the basic mechanism of using
the values from the Offset zero fragment is not sufficient. For
example, Section 5.3 of [RFC3168] describes how to combine the
Explicit Congestion Notification (ECN) bits from different
fragments to derive the ECN bits of the reassembled packet.

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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 [IANA-PN].

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

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Destination Options header whose Option Type has the value 11
in its highest-order 2 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 2 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.

4.8. Defining New Extension Headers and Options

Defining new IPv6 extension headers is not recommended, unless there
are no existing IPv6 extension headers that can be used by specifying
a new option for that IPv6 extension header. A proposal to specify a
new IPv6 extension header must include a detailed technical
explanation of why an existing IPv6 extension header can not be used
for the desired new function. See [RFC6564] for additional
background information.

Note: New extension headers that require hop-by-hop behavior must not
be defined because, as specified in Section 4 of this document, the
only extension header that has hop-by-hop behavior is the Hop-by-Hop
Options header.

New hop-by-hop options are not recommended because nodes may be
configured to ignore the Hop-by-Hop Options header, drop packets
containing a Hop-by-Hop Options header, or assign packets containing
a Hop-by-Hop Options header to a slow processing path. Designers
considering defining new hop-by-hop options need to be aware of this
likely behavior. There has to be a very clear justification why any
new hop-by-hop option is needed before it is standardized.

Instead of defining new extension headers, it is recommended that the
Destination Options header is used to carry optional information that
must be examined only by a packet's destination node(s), because they
provide better handling and backward compatibility.

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If new extension headers are defined, they need to use the following
format:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Header-Specific Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Next Header 8-bit selector. Identifies the type of
header immediately following the extension
header. Uses the same values as the IPv4
Protocol field [IANA-PN].

Hdr Ext Len 8-bit unsigned integer. Length of the
Destination Options header in 8-octet units,
not including the first 8 octets.

Header Specific Data Variable-length field. Fields specific to
the extension header.

5. Packet Size Issues

IPv6 requires that every link in the Internet have an MTU of 1280
octets or greater. This is known as the IPv6 minimum link MTU. 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 [RFC1661])
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 [RFC8201], 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.

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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).

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.

6. Flow Labels

The 20-bit Flow Label field in the IPv6 header is used by a source to
label sequences of packets to be treated in the network as a single
flow.

The current definition of the IPv6 Flow Label can be found in
[RFC6437].

7. Traffic Classes

The 8-bit Traffic Class field in the IPv6 header is used by the
network for traffic management. The value of the Traffic Class bits
in a received packet or fragment might be different from the value
sent by the packet's source.

The current use of the Traffic Class field for Differentiated
Services and Explicit Congestion Notification is specified in
[RFC2474] and [RFC3168].

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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.

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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
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, the default behavior when UDP packets are
originated by an IPv6 node is that 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.

o As an exception to the default behavior, protocols that use UDP
as a tunnel encapsulation may enable zero-checksum mode for a
specific port (or set of ports) for sending and/or receiving.
Any node implementing zero-checksum mode must follow the
requirements specified in "Applicability Statement for the Use
of IPv6 UDP Datagrams with Zero Checksums" [RFC6936].

The IPv6 version of ICMP [RFC4443] 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.

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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 Maximum Segment Size (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, 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.

9. IANA Considerations

RFC 2460 is referenced in a number of IANA registries. These
include:

o Internet Protocol Version 6 (IPv6) Parameters [IANA-6P]

o Assigned Internet Protocol Numbers [IANA-PN]

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RFC 8200 IPv6 Specification July 2017

o ONC RPC Network Identifiers (netids) [IANA-NI]

o Network Layer Protocol Identifiers (NLPIDs) of Interest
[IANA-NL]

o Protocol Registries [IANA-PR]

The IANA has updated these references to point to this document.

10. Security Considerations

IPv6, from the viewpoint of the basic format and transmission of
packets, has security properties that are similar to IPv4. These
security issues include:

o Eavesdropping, where on-path elements can observe the whole
packet (including both contents and metadata) of each IPv6
datagram.
o Replay, where the attacker records a sequence of packets off of
the wire and plays them back to the party that originally
received them.
o Packet insertion, where the attacker forges a packet with some
chosen set of properties and injects it into the network.
o Packet deletion, where the attacker removes a packet from the
wire.
o Packet modification, where the attacker removes a packet from
the wire, modifies it, and reinjects it into the network.
o Man-in-the-middle (MITM) attacks, where the attacker subverts
the communication stream in order to pose as the sender to
receiver and the receiver to the sender.
o Denial-of-service (DoS) attacks, where the attacker sends large
amounts of legitimate traffic to a destination to overwhelm it.

IPv6 packets can be protected from eavesdropping, replay, packet
insertion, packet modification, and MITM attacks by use of the
"Security Architecture for the Internet Protocol" [RFC4301]. In
addition, upper-layer protocols such as Transport Layer Security
(TLS) or Secure Shell (SSH) can be used to protect the application-
layer traffic running on top of IPv6.

There is not any mechanism to protect against DoS attacks. Defending
against these type of attacks is outside the scope of this
specification.

IPv6 addresses are significantly larger than IPv4 addresses making it
much harder to scan the address space across the Internet and even on
a single network link (e.g., Local Area Network). See [RFC7707] for
more information.

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IPv6 addresses of nodes are expected to be more visible on the
Internet as compared with IPv4 since the use of address translation
technology is reduced. This creates some additional privacy issues
such as making it easier to distinguish endpoints. See [RFC7721] for
more information.

The design of IPv6 extension header architecture, while adding a lot
of flexibility, also creates new security challenges. As noted
below, issues relating to the Fragment extension header have been
resolved, but it's clear that for any new extension header designed
in the future, the security implications need to be examined
thoroughly, and this needs to include how the new extension header
works with existing extension headers. See [RFC7045] for more
information.

This version of the IPv6 specification resolves a number of security
issues that were found with the previous version [RFC2460] of the
IPv6 specification. These include:

o Revised the text to handle the case of fragments that are whole
datagrams (i.e., both the Fragment Offset field and the M flag
are zero). If received, they should be processed as a
reassembled packet. Any other fragments that match should be
processed independently. The Fragment creation process was
modified to not create whole datagram fragments (Fragment
Offset field and the M flag are zero). See [RFC6946] and
[RFC8021] for more information.

o Removed the paragraph in Section 5 that required including a
Fragment header to outgoing packets if an ICMP Packet Too Big
message reporting a Next-Hop MTU is less than 1280. See
[RFC6946] for more information.

o Changed the text to require that IPv6 nodes must not create
overlapping fragments. Also, when reassembling an IPv6
datagram, if one or more of its constituent fragments is
determined to be an overlapping fragment, the entire datagram
(and any constituent fragments) must be silently discarded.
Includes clarification that no ICMP error message should be
sent if overlapping fragments are received. See [RFC5722] for
more information.

o Revised the text to require that all headers through the first
upper-layer header are in the first fragment. See [RFC7112]
for more information.

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RFC 8200 IPv6 Specification July 2017

o Incorporated the updates from [RFC5095] and [RFC5871] to remove
the description of the Routing Header type 0 (RH0), that the
allocations guidelines for Routing headers are specified in RFC
5871, and removed RH0 from the list of required extension
headers.

Security issues relating to other parts of IPv6 including addressing,
ICMPv6, Path MTU Discovery, etc., are discussed in the appropriate
specifications.

11. References

11.1. Normative References

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<http://www.rfc-editor.org/info/rfc4443>.

[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<http://www.rfc-editor.org/info/rfc6437>.

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11.2. Informative References

[Err2541] RFC Errata, Erratum ID 2541, RFC 2460.

[Err4279] RFC Errata, Erratum ID 4279, RFC 2460.

[Err4657] RFC Errata, Erratum ID 4657, RFC 2460.

[Err4662] RFC Errata, Erratum ID 4662, RFC 2460.

[IANA-6P] IANA, "Internet Protocol Version 6 (IPv6) Parameters",
<https://www.iana.org/assignments/ipv6-parameters>.

[IANA-EH] IANA, "IPv6 Extension Header Types",
<https://www.iana.org/assignments/ipv6-parameters>.

[IANA-NI] IANA, "ONC RPC Network Identifiers (netids)",
<https://www.iana.org/assignments/rpc-netids>.

[IANA-NL] IANA, "Network Layer Protocol Identifiers (NLPIDs) of
Interest", <https://www.iana.org/assignments/nlpids>.

[IANA-PN] IANA, "Protocol Numbers",
<https://www.iana.org/assignments/protocol-numbers>.

[IANA-PR] IANA, "Protocol Registries", <https://www.iana.org/
protocols>.

[IANA-RH] IANA, "Routing Types", <https://www.iana.org/assignments/
ipv6-parameters>.

[RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, DOI 10.17487/RFC1661, July 1994,
<http://www.rfc-editor.org/info/rfc1661>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.

[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<http://www.rfc-editor.org/info/rfc4302>.

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[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<http://www.rfc-editor.org/info/rfc4303>.

[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
DOI 10.17487/RFC5095, December 2007,
<http://www.rfc-editor.org/info/rfc5095>.

[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, DOI 10.17487/RFC5722, December 2009,
<http://www.rfc-editor.org/info/rfc5722>.

[RFC5871] Arkko, J. and S. Bradner, "IANA Allocation Guidelines for
the IPv6 Routing Header", RFC 5871, DOI 10.17487/RFC5871,
May 2010, <http://www.rfc-editor.org/info/rfc5871>.

[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, DOI 10.17487/RFC6564, April 2012,
<http://www.rfc-editor.org/info/rfc6564>.

[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
<http://www.rfc-editor.org/info/rfc6936>.

[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments",
RFC 6946, DOI 10.17487/RFC6946, May 2013,
<http://www.rfc-editor.org/info/rfc6946>.

[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045,
DOI 10.17487/RFC7045, December 2013,
<http://www.rfc-editor.org/info/rfc7045>.

[RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of
Oversized IPv6 Header Chains", RFC 7112,
DOI 10.17487/RFC7112, January 2014,
<http://www.rfc-editor.org/info/rfc7112>.

[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<http://www.rfc-editor.org/info/rfc7707>.

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RFC 8200 IPv6 Specification July 2017

[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>.

[RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2016, <http://www.rfc-editor.org/info/rfc7739>.

[RFC8021] Gont, F., Liu, W., and T. Anderson, "Generation of IPv6
Atomic Fragments Considered Harmful", RFC 8021,
DOI 10.17487/RFC8021, January 2017,
<http://www.rfc-editor.org/info/rfc8021>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, "Path
MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<http://www.rfc-editor.org/info/rfc8201>.

Deering & Hinden Standards Track [Page 35]

RFC 8200 IPv6 Specification July 2017

Appendix A. 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 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 8200 IPv6 Specification July 2017

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 8200 IPv6 Specification July 2017

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 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 8200 IPv6 Specification July 2017

Appendix B. Changes Since RFC 2460

This memo has the following changes from RFC 2460.

o Removed IP Next Generation from the Abstract.

o Added text in Section 1 that the data transmission order is the
same as IPv4 as defined in RFC 791.

o Clarified the text in Section 3 about decrementing the Hop Limit.

o Clarified that extension headers (except for the Hop-by-Hop
Options header) are not processed, inserted, or deleted by any
node along a packet's delivery path.

o Changed requirement for the Hop-by-Hop Options header to a "may",
and added a note to indicate what is expected regarding the
Hop-by-Hop Options header.

o Added a paragraph to Section 4 to clarify how extension headers
are numbered and which are upper-layer headers.

o Added a reference to the end of Section 4 to the "IPv6 Extension
Header Types" IANA registry.

o Incorporated the updates from RFCs 5095 and 5871 to remove the
description of RH0, that the allocations guidelines for routing
headers are specified in RFC 5871, and removed RH0 from the list
of required extension headers.

o Revised Section 4.5 on IPv6 fragmentation based on updates from
RFCs 5722, 6946, 7112, and 8021. This includes:

- Revised the text to handle the case of fragments that are whole
datagrams (i.e., both the Fragment Offset field and the M flag
are zero). If received, they should be processed as a
reassembled packet. Any other fragments that match should be
processed independently. The revised Fragment creation process
was modified to not create whole datagram fragments (Fragment
Offset field and the M flag are zero).

- Changed the text to require that IPv6 nodes must not create
overlapping fragments. Also, when reassembling an IPv6
datagram, if one or more its constituent fragments is
determined to be an overlapping fragment, the entire datagram
(and any constituent fragments) must be silently discarded.
Includes a clarification that no ICMP error message should be
sent if overlapping fragments are received.

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RFC 8200 IPv6 Specification July 2017

- Revised the text to require that all headers through the first
Upper-Layer header are in the first fragment. This changed the
text describing how packets are fragmented and reassembled and
added a new error case.

- Added text to the Fragment header process on handling exact
duplicate fragments.

- Updated the Fragmentation header text to correct the inclusion
of an Authentication Header (AH) and noted No Next Header case.

- Changed terminology in the Fragment header section from
"Unfragmentable Headers" to "Per-Fragment headers".

- Removed the paragraph in Section 5 that required including a
Fragment header to outgoing packets if an ICMP Packet Too Big
message reports a Next-Hop MTU less than 1280.

- Changed the text to clarify MTU restriction and 8-byte
restrictions, and noted the restriction on headers in the first
fragment.

o In Section 4.5, added clarification noting that some fields in the
IPv6 header may also vary across the fragments being reassembled,
and that other specifications may provide additional instructions
for how they should be reassembled. See, for example, Section 5.3
of [RFC3168].

o Incorporated the update from RFC 6564 to add a new Section 4.8
that describes recommendations for defining new extension headers
and options.

o Added text to Section 5 to define "IPv6 minimum link MTU".

o Simplified the text in Section 6 about Flow Labels and removed
what was Appendix A ("Semantics and Usage of the Flow Label
Field"); instead, pointed to the current specifications of the
IPv6 Flow Label field in [RFC6437] and the Traffic Class field in
[RFC2474] and [RFC3168].

o Incorporated the update made by RFC 6935 ("IPv6 and UDP Checksums
for Tunneled Packets") in Section 8. Added an exception to the
default behavior for the handling of UDP packets with zero
checksums for tunnels.

o Added instruction to Section 9, "IANA Considerations", to change
references to RFC 2460 to this document.

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RFC 8200 IPv6 Specification July 2017

o Revised and expanded Section 10, "Security Considerations".

o Added a paragraph to the Acknowledgments section acknowledging the
authors of the updating documents.

o Updated references to current versions and assigned references to
normative and informative.

o Made changes to resolve the errata on RFC 2460. These are:

Erratum ID 2541 [Err2541]: This erratum notes that RFC 2460
didn't update RFC 2205 when the length of the flow label was
changed from 24 to 20 bits from RFC 1883. This issue was
resolved in RFC 6437 where the flow label is defined. This
specification now references RFC 6437. No change is required.

Erratum ID 4279 [Err4279]: This erratum noted that the
specification doesn't handle the case of a forwarding node
receiving a packet with a zero Hop Limit. This is fixed in
Section 3 of this specification.

Erratum ID 4657 [Err4657]: This erratum proposed text that
extension headers must never be inserted by any node other than
the source of the packet. This was resolved in Section 4,
"IPv6 Extension Headers".

Erratum ID 4662 [Err4662]: This erratum proposed text that
extension headers, with one exception, are not examined,
processed, modified, inserted, or deleted by any node along a
packet's delivery path. This was resolved in Section 4, "IPv6
Extension Headers".

Erratum ID 2843: This erratum is marked "Rejected". No change
was made.

Deering & Hinden Standards Track [Page 41]

RFC 8200 IPv6 Specification July 2017

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.

The authors would also like to acknowledge the authors of the
updating RFCs that were incorporated in this document to move the
IPv6 specification to Internet Standard. They are Joe Abley, Shane
Amante, Jari Arkko, Manav Bhatia, Ronald P. Bonica, Scott Bradner,
Brian Carpenter, P.F. Chimento, Marshall Eubanks, Fernando Gont,
James Hoagland, Sheng Jiang, Erik Kline, Suresh Krishnan, Vishwas
Manral, George Neville-Neil, Jarno Rajahalme, Pekka Savola, Magnus
Westerlund, and James Woodyatt.

Authors' Addresses

Stephen E. Deering
Retired
Vancouver, British Columbia
Canada

Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, CA 94070
United States of America

Email: [email protected]

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