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RFC 8200

STD 86
Pages: 42
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Internet Protocol, Version 6 (IPv6) Specification

Part 1 of 2, p. 1 to 25
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Obsoletes:    2460


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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.

[Page 2] 
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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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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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].

Top      ToC       Page 17 
   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.

Top      ToC       Page 19 
              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

Top      ToC       Page 21 
         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

Top      ToC       Page 24 
         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.



(page 25 continued on part 2)

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