RFC 8200
Internet Protocol, Version 6 (IPv6) Specification
Pages: 42
Internet Standard: 86
→ Errata
Obsoletes: 2460
Updated by: 9673
Internet Standard: 86
→ Errata
Obsoletes: 2460
Updated by: 9673
Part 1 of 2 – Pages 1 to 25
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) SpecificationAbstract
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.
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.
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
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].
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.
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.)
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.
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.
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].
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.
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
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.
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.
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.
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.
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].
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.
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.
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.
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
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.
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.
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
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.
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.
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