    Network Working Group                                         S. Deering
    Request for Comments: 2460                                         Cisco
    Obsoletes: 1883                                                R. Hinden
    Category: Standards Track                                          Nokia
                                                               December 1998

                      Internet Protocol, Version 6 (IPv6)
                                 Specification

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1998).  All Rights Reserved.

    Abstract

       This document specifies version 6 of the Internet Protocol (IPv6),
       also sometimes referred to as IP Next Generation or IPng.

    Table of Contents

       1. Introduction..................................................2
       2. Terminology...................................................3
       3. IPv6 Header Format............................................4
       4. IPv6 Extension Headers........................................6
           4.1 Extension Header Order...................................7
           4.2 Options..................................................9
           4.3 Hop-by-Hop Options Header...............................11
           4.4 Routing Header..........................................12
           4.5 Fragment Header.........................................18
           4.6 Destination Options Header..............................23
           4.7 No Next Header..........................................24
       5. Packet Size Issues...........................................24
       6. Flow Labels..................................................25
       7. Traffic Classes..............................................25
       8. Upper-Layer Protocol Issues..................................27
           8.1 Upper-Layer Checksums...................................27
           8.2 Maximum Packet Lifetime.................................28
           8.3 Maximum Upper-Layer Payload Size........................28
           8.4 Responding to Packets Carrying Routing Headers..........29

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       Appendix A. Semantics and Usage of the Flow Label Field.........30
       Appendix B. Formatting Guidelines for Options...................32
       Security Considerations.........................................35
       Acknowledgments.................................................35
       Authors' Addresses..............................................35
       References......................................................35
       Changes Since RFC-1883..........................................36
       Full Copyright Statement........................................39

    1.  Introduction

       IP version 6 (IPv6) is a new version of the Internet Protocol,
       designed as the successor to IP version 4 (IPv4) [RFC-791].  The
       changes from IPv4 to IPv6 fall primarily into the following
       categories:

          o  Expanded Addressing Capabilities

             IPv6 increases the IP address size from 32 bits to 128 bits, to
             support more levels of addressing hierarchy, a much greater
             number of addressable nodes, and simpler auto-configuration of
             addresses.  The scalability of multicast routing is improved by
             adding a "scope" field to multicast addresses.  And a new type
             of address called an "anycast address" is defined, used to send
             a packet to any one of a group of nodes.

          o  Header Format Simplification

             Some IPv4 header fields have been dropped or made optional, to
             reduce the common-case processing cost of packet handling and
             to limit the bandwidth cost of the IPv6 header.

          o  Improved Support for Extensions and Options

             Changes in the way IP header options are encoded allows for
             more efficient forwarding, less stringent limits on the length
             of options, and greater flexibility for introducing new options
             in the future.

          o  Flow Labeling Capability

             A new capability is added to enable the labeling of packets
             belonging to particular traffic "flows" for which the sender
             requests special handling, such as non-default quality of
             service or "real-time" service.

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          o  Authentication and Privacy Capabilities

             Extensions to support authentication, data integrity, and
             (optional) data confidentiality are specified for IPv6.

       This document specifies the basic IPv6 header and the initially-
       defined IPv6 extension headers and options.  It also discusses packet
       size issues, the semantics of flow labels and traffic classes, and
       the effects of IPv6 on upper-layer protocols.  The format and
       semantics of IPv6 addresses are specified separately in [ADDRARCH].
       The IPv6 version of ICMP, which all IPv6 implementations are required
       to include, is specified in [ICMPv6].

    2.  Terminology

       node        - a device that implements IPv6.

       router      - a node that forwards IPv6 packets not explicitly
                     addressed to itself.  [See Note below].

       host        - any node that is not a router.  [See Note below].

       upper layer - a protocol layer immediately above IPv6.  Examples are
                     transport protocols such as TCP and UDP, control
                     protocols such as ICMP, routing protocols such as OSPF,
                     and internet or lower-layer protocols being "tunneled"
                     over (i.e., encapsulated in) IPv6 such as IPX,
                     AppleTalk, or IPv6 itself.

       link        - a communication facility or medium over which nodes can
                     communicate at the link layer, i.e., the layer
                     immediately below IPv6.  Examples are Ethernets (simple
                     or bridged); PPP links; X.25, Frame Relay, or ATM
                     networks; and internet (or higher) layer "tunnels",
                     such as tunnels over IPv4 or IPv6 itself.

       neighbors   - nodes attached to the same link.

       interface   - a node's attachment to a link.

       address     - an IPv6-layer identifier for an interface or a set of
                     interfaces.

       packet      - an IPv6 header plus payload.

       link MTU    - the maximum transmission unit, i.e., maximum packet
                     size in octets, that can be conveyed over a link.

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

       Note: it is possible, though unusual, for a device with multiple
       interfaces to be configured to forward non-self-destined packets
       arriving from some set (fewer than all) of its interfaces, and to
       discard non-self-destined packets arriving from its other interfaces.
       Such a device must obey the protocol requirements for routers when
       receiving packets from, and interacting with neighbors over, the
       former (forwarding) interfaces.  It must obey the protocol
       requirements for hosts when receiving packets from, and interacting
       with neighbors over, the latter (non-forwarding) interfaces.

    3.  IPv6 Header Format

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version| Traffic Class |           Flow Label                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Payload Length        |  Next Header  |   Hop Limit   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                         Source Address                        +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                      Destination Address                      +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Version              4-bit Internet Protocol version number = 6.

       Traffic Class        8-bit traffic class field.  See section 7.

       Flow Label           20-bit flow label.  See section 6.

       Payload Length       16-bit unsigned integer.  Length of the IPv6
                            payload, i.e., the rest of the packet following
                            this IPv6 header, in octets.  (Note that any

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                            extension headers [section 4] present are
                            considered part of the payload, i.e., included
                            in the length count.)

       Next Header          8-bit selector.  Identifies the type of header
                            immediately following the IPv6 header.  Uses the
                            same values as the IPv4 Protocol field [RFC-1700
                            et seq.].

       Hop Limit            8-bit unsigned integer.  Decremented by 1 by
                            each node that forwards the packet. The packet
                            is discarded if Hop Limit is decremented to
                            zero.

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

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

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    4.  IPv6 Extension Headers

       In IPv6, optional internet-layer information is encoded in separate
       headers that may be placed between the IPv6 header and the upper-
       layer header in a packet.  There are a small number of such extension
       headers, each identified by a distinct Next Header value.  As
       illustrated in these examples, an IPv6 packet may carry zero, one, or
       more extension headers, each identified by the Next Header field of
       the preceding header:

       +---------------+------------------------
       |  IPv6 header  | TCP header + data
       |               |
       | Next Header = |
       |      TCP      |
       +---------------+------------------------

       +---------------+----------------+------------------------
       |  IPv6 header  | Routing header | TCP header + data
       |               |                |
       | Next Header = |  Next Header = |
       |    Routing    |      TCP       |
       +---------------+----------------+------------------------

       +---------------+----------------+-----------------+-----------------
       |  IPv6 header  | Routing header | Fragment header | fragment of TCP
       |               |                |                 |  header + data
       | Next Header = |  Next Header = |  Next Header =  |
       |    Routing    |    Fragment    |       TCP       |
       +---------------+----------------+-----------------+-----------------

       With one exception, extension headers are not examined or processed
       by any node along a packet's delivery path, until the packet reaches
       the node (or each of the set of nodes, in the case of multicast)
       identified in the Destination Address field of the IPv6 header.
       There, normal demultiplexing on the Next Header field of the IPv6
       header invokes the module to process the first extension header, or
       the upper-layer header if no extension header is present.  The
       contents and semantics of each extension header determine whether or
       not to proceed to the next header.  Therefore, extension headers must
       be processed strictly in the order they appear in the packet; a
       receiver must not, for example, scan through a packet looking for a
       particular kind of extension header and process that header prior to
       processing all preceding ones.

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       The exception referred to in the preceding paragraph is the Hop-by-
       Hop Options header, which carries information that must be examined
       and processed by every node along a packet's delivery path, including
       the source and destination nodes.  The Hop-by-Hop Options header,
       when present, must immediately follow the IPv6 header.  Its presence
       is indicated by the value zero in the Next Header field of the IPv6
       header.

       If, as a result of processing a header, a node is required to proceed
       to the next header but the Next Header value in the current header is
       unrecognized by the node, it should discard the packet and send an
       ICMP Parameter Problem message to the source of the packet, with an
       ICMP Code value of 1 ("unrecognized Next Header type encountered")
       and the ICMP Pointer field containing the offset of the unrecognized
       value within the original packet.  The same action should be taken if
       a node encounters a Next Header value of zero in any header other
       than an IPv6 header.

       Each extension header is an integer multiple of 8 octets long, in
       order to retain 8-octet alignment for subsequent headers.  Multi-
       octet fields within each extension header are aligned on their
       natural boundaries, i.e., fields of width n octets are placed at an
       integer multiple of n octets from the start of the header, for n = 1,
       2, 4, or 8.

       A full implementation of IPv6 includes implementation of the
       following extension headers:

               Hop-by-Hop Options
               Routing (Type 0)
               Fragment
               Destination Options
               Authentication
               Encapsulating Security Payload

       The first four are specified in this document; the last two are
       specified in [RFC-2402] and [RFC-2406], respectively.

    4.1  Extension Header Order

       When more than one extension header is used in the same packet, it is
       recommended that those headers appear in the following order:

               IPv6 header
               Hop-by-Hop Options header
               Destination Options header (note 1)
               Routing header
               Fragment header

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               Authentication header (note 2)
               Encapsulating Security Payload header (note 2)
               Destination Options header (note 3)
               upper-layer header

               note 1: for options to be processed by the first destination
                       that appears in the IPv6 Destination Address field
                       plus subsequent destinations listed in the Routing
                       header.

               note 2: additional recommendations regarding the relative
                       order of the Authentication and Encapsulating
                       Security Payload headers are given in [RFC-2406].

               note 3: for options to be processed only by the final
                       destination of the packet.

       Each extension header should occur at most once, except for the
       Destination Options header which should occur at most twice (once
       before a Routing header and once before the upper-layer header).

       If the upper-layer header is another IPv6 header (in the case of IPv6
       being tunneled over or encapsulated in IPv6), it may be followed by
       its own extension headers, which are separately subject to the same
       ordering recommendations.

       If and when other extension headers are defined, their ordering
       constraints relative to the above listed headers must be specified.

       IPv6 nodes must accept and attempt to process extension headers in
       any order and occurring any number of times in the same packet,
       except for the Hop-by-Hop Options header which is restricted to
       appear immediately after an IPv6 header only.  Nonetheless, it is
       strongly advised that sources of IPv6 packets adhere to the above
       recommended order until and unless subsequent specifications revise
       that recommendation.

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    4.2  Options

       Two of the currently-defined extension headers -- the Hop-by-Hop
       Options header and the Destination Options header -- carry a variable
       number of type-length-value (TLV) encoded "options", of the following
       format:

          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
          |  Option Type  |  Opt Data Len |  Option Data
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

          Option Type          8-bit identifier of the type of option.

          Opt Data Len         8-bit unsigned integer.  Length of the Option
                               Data field of this option, in octets.

          Option Data          Variable-length field.  Option-Type-specific
                               data.

       The sequence of options within a header must be processed strictly in
       the order they appear in the header; a receiver must not, for
       example, scan through the header looking for a particular kind of
       option and process that option prior to processing all preceding
       ones.

       The Option Type identifiers are internally encoded such that their
       highest-order two bits specify the action that must be taken if the
       processing IPv6 node does not recognize the Option Type:

          00 - skip over this option and continue processing the header.

          01 - discard the packet.

          10 - discard the packet and, regardless of whether or not the
               packet's Destination Address was a multicast address, send an
               ICMP Parameter Problem, Code 2, message to the packet's
               Source Address, pointing to the unrecognized Option Type.

          11 - discard the packet and, only if the packet's Destination
               Address was not a multicast address, send an ICMP Parameter
               Problem, Code 2, message to the packet's Source Address,
               pointing to the unrecognized Option Type.

       The third-highest-order bit of the Option Type specifies whether or
       not the Option Data of that option can change en-route to the
       packet's final destination.  When an Authentication header is present

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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 which are used when necessary to align
       subsequent options and to pad out the containing header to a multiple
       of 8 octets in length.  These padding options must be recognized by
       all IPv6 implementations:

       Pad1 option  (alignment requirement: none)

          +-+-+-+-+-+-+-+-+
          |       0       |
          +-+-+-+-+-+-+-+-+

          NOTE! the format of the Pad1 option is a special case -- it does
                not have length and value fields.

          The Pad1 option is used to insert one octet of padding into the
          Options area of a header.  If more than one octet of padding is
          required, the PadN option, described next, should be used, rather
          than multiple Pad1 options.

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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 B contains formatting guidelines for designing new options.

    4.3  Hop-by-Hop Options Header

       The Hop-by-Hop Options header is used to carry optional information
       that must be examined by every node along a packet's delivery path.
       The Hop-by-Hop Options header is identified by a Next Header value of
       0 in the IPv6 header, and has the following format:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |  Next Header  |  Hdr Ext Len  |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
        |                                                               |
        .                                                               .
        .                            Options                            .
        .                                                               .
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Next Header          8-bit selector.  Identifies the type of header
                            immediately following the Hop-by-Hop Options
                            header.  Uses the same values as the IPv4
                            Protocol field [RFC-1700 et seq.].

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

       Options              Variable-length field, of length such that the
                            complete Hop-by-Hop Options header is an integer
                            multiple of 8 octets long.  Contains one or more
                            TLV-encoded options, as described in section
                            4.2.

       The only hop-by-hop options defined in this document are the Pad1 and
       PadN options specified in section 4.2.

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    RFC 2460                   IPv6 Specification              December 1998

    4.4  Routing Header

       The Routing header is used by an IPv6 source to list one or more
       intermediate nodes to be "visited" on the way to a packet's
       destination.  This function is very similar to IPv4's Loose Source
       and Record Route option.  The Routing header is identified by a Next
       Header value of 43 in the immediately preceding header, and has the
       following format:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |  Next Header  |  Hdr Ext Len  |  Routing Type | Segments Left |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        .                                                               .
        .                       type-specific data                      .
        .                                                               .
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Next Header          8-bit selector.  Identifies the type of header
                            immediately following the Routing header.  Uses
                            the same values as the IPv4 Protocol field
                            [RFC-1700 et seq.].

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

       Routing Type         8-bit identifier of a particular Routing header
                            variant.

       Segments Left        8-bit unsigned integer.  Number of route
                            segments remaining, i.e., number of explicitly
                            listed intermediate nodes still to be visited
                            before reaching the final destination.

       type-specific data   Variable-length field, of format determined by
                            the Routing Type, and of length such that the
                            complete Routing header is an integer multiple
                            of 8 octets long.

       If, while processing a received packet, a node encounters a Routing
       header with an unrecognized Routing Type value, the required behavior
       of the node depends on the value of the Segments Left field, as
       follows:

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          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.

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       The Type 0 Routing header has the following format:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |  Next Header  |  Hdr Ext Len  | Routing Type=0| Segments Left |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                            Reserved                           |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        +                                                               +
        |                                                               |
        +                           Address[1]                          +
        |                                                               |
        +                                                               +
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        +                                                               +
        |                                                               |
        +                           Address[2]                          +
        |                                                               |
        +                                                               +
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        .                               .                               .
        .                               .                               .
        .                               .                               .
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        +                                                               +
        |                                                               |
        +                           Address[n]                          +
        |                                                               |
        +                                                               +
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Next Header          8-bit selector.  Identifies the type of header
                            immediately following the Routing header.  Uses
                            the same values as the IPv4 Protocol field
                            [RFC-1700 et seq.].

       Hdr Ext Len          8-bit unsigned integer.  Length of the Routing
                            header in 8-octet units, not including the first
                            8 octets.  For the Type 0 Routing header, Hdr
                            Ext Len is equal to two times the number of
                            addresses in the header.

       Routing Type         0.

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       Segments Left        8-bit unsigned integer.  Number of route
                            segments remaining, i.e., number of explicitly
                            listed intermediate nodes still to be visited
                            before reaching the final destination.

       Reserved             32-bit reserved field.  Initialized to zero for
                            transmission; ignored on reception.

       Address[1..n]        Vector of 128-bit addresses, numbered 1 to n.

       Multicast addresses must not appear in a Routing header of Type 0, or
       in the IPv6 Destination Address field of a packet carrying a Routing
       header of Type 0.

       A Routing header is not examined or processed until it reaches the
       node identified in the Destination Address field of the IPv6 header.
       In that node, dispatching on the Next Header field of the immediately
       preceding header causes the Routing header module to be invoked,
       which, in the case of Routing Type 0, performs the following
       algorithm:

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       if Segments Left = 0 {
          proceed to process the next header in the packet, whose type is
          identified by the Next Header field in the Routing header
       }
       else if Hdr Ext Len is odd {
             send an ICMP Parameter Problem, Code 0, message to the Source
             Address, pointing to the Hdr Ext Len field, and discard the
             packet
       }
       else {
          compute n, the number of addresses in the Routing header, by
          dividing Hdr Ext Len by 2

          if Segments Left is greater than n {
             send an ICMP Parameter Problem, Code 0, message to the Source
             Address, pointing to the Segments Left field, and discard the
             packet
          }
          else {
             decrement Segments Left by 1;
             compute i, the index of the next address to be visited in
             the address vector, by subtracting Segments Left from n

             if Address [i] or the IPv6 Destination Address is multicast {
                discard the packet
             }
             else {
                swap the IPv6 Destination Address and Address[i]

                if the IPv6 Hop Limit is less than or equal to 1 {
                   send an ICMP Time Exceeded -- Hop Limit Exceeded in
                   Transit message to the Source Address and discard the
                   packet
                }
                else {
                   decrement the Hop Limit by 1

                   resubmit the packet to the IPv6 module for transmission
                   to the new destination
                }
             }
          }
       }

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       As an example of the effects of the above algorithm, consider the
       case of a source node S sending a packet to destination node D, using
       a Routing header to cause the packet to be routed via intermediate
       nodes I1, I2, and I3.  The values of the relevant IPv6 header and
       Routing header fields on each segment of the delivery path would be
       as follows:

       As the packet travels from S to I1:

            Source Address = S                  Hdr Ext Len = 6
            Destination Address = I1            Segments Left = 3
                                                Address[1] = I2
                                                Address[2] = I3
                                                Address[3] = D

       As the packet travels from I1 to I2:

            Source Address = S                  Hdr Ext Len = 6
            Destination Address = I2            Segments Left = 2
                                                Address[1] = I1
                                                Address[2] = I3
                                                Address[3] = D

       As the packet travels from I2 to I3:

            Source Address = S                  Hdr Ext Len = 6
            Destination Address = I3            Segments Left = 1
                                                Address[1] = I1
                                                Address[2] = I2
                                                Address[3] = D

       As the packet travels from I3 to D:

            Source Address = S                  Hdr Ext Len = 6
            Destination Address = D             Segments Left = 0
                                                Address[1] = I1
                                                Address[2] = I2
                                                Address[3] = I3

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    RFC 2460                   IPv6 Specification              December 1998

    4.5  Fragment Header

       The Fragment header is used by an IPv6 source to send a packet larger
       than would fit in the path MTU to its destination.  (Note: unlike
       IPv4, fragmentation in IPv6 is performed only by source nodes, not by
       routers along a packet's delivery path -- see section 5.)  The
       Fragment header is identified by a Next Header value of 44 in the
       immediately preceding header, and has the following format:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Next Header  |   Reserved    |      Fragment Offset    |Res|M|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Identification                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Next Header          8-bit selector.  Identifies the initial header
                            type of the Fragmentable Part of the original
                            packet (defined below).  Uses the same values as
                            the IPv4 Protocol field [RFC-1700 et seq.].

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

       Fragment Offset      13-bit unsigned integer.  The offset, in 8-octet
                            units, of the data following this header,
                            relative to the start of the Fragmentable Part
                            of the original packet.

       Res                  2-bit reserved field.  Initialized to zero for
                            transmission; ignored on reception.

       M flag               1 = more fragments; 0 = last fragment.

       Identification       32 bits.  See description below.

       In order to send a packet that is too large to fit in the MTU of the
       path to its destination, a source node may divide the packet into
       fragments and send each fragment as a separate packet, to be
       reassembled at the receiver.

       For every packet that is to be fragmented, the source node generates
       an Identification value. The Identification must be different than
       that of any other fragmented packet sent recently* with the same
       Source Address and Destination Address.  If a Routing header is
       present, the Destination Address of concern is that of the final
       destination.

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          * "recently" means within the maximum likely lifetime of a packet,
            including transit time from source to destination and time spent
            awaiting reassembly with other fragments of the same packet.
            However, it is not required that a source node know the maximum
            packet lifetime.  Rather, it is assumed that the requirement can
            be met by maintaining the Identification value as a simple, 32-
            bit, "wrap-around" counter, incremented each time a packet must
            be fragmented.  It is an implementation choice whether to
            maintain a single counter for the node or multiple counters,
            e.g., one for each of the node's possible source addresses, or
            one for each active (source address, destination address)
            combination.

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

       original packet:

       +------------------+----------------------//-----------------------+
       |  Unfragmentable  |                 Fragmentable                  |
       |       Part       |                     Part                      |
       +------------------+----------------------//-----------------------+

          The Unfragmentable Part consists of the IPv6 header plus any
          extension headers that must be processed by nodes en route to the
          destination, that is, all headers up to and including the Routing
          header if present, else the Hop-by-Hop Options header if present,
          else no extension headers.

          The Fragmentable Part consists of the rest of the packet, that is,
          any extension headers that need be processed only by the final
          destination node(s), plus the upper-layer header and data.

       The Fragmentable Part of the original packet is divided into
       fragments, each, except possibly the last ("rightmost") one, being an
       integer multiple of 8 octets long.  The fragments are transmitted in
       separate "fragment packets" as illustrated:

       original packet:

       +------------------+--------------+--------------+--//--+----------+
       |  Unfragmentable  |    first     |    second    |      |   last   |
       |       Part       |   fragment   |   fragment   | .... | fragment |
       +------------------+--------------+--------------+--//--+----------+

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       fragment packets:

       +------------------+--------+--------------+
       |  Unfragmentable  |Fragment|    first     |
       |       Part       | Header |   fragment   |
       +------------------+--------+--------------+

       +------------------+--------+--------------+
       |  Unfragmentable  |Fragment|    second    |
       |       Part       | Header |   fragment   |
       +------------------+--------+--------------+
                             o
                             o
                             o
       +------------------+--------+----------+
       |  Unfragmentable  |Fragment|   last   |
       |       Part       | Header | fragment |
       +------------------+--------+----------+

       Each fragment packet is composed of:

          (1) The Unfragmentable Part of the original packet, with the
              Payload Length of the original IPv6 header changed to contain
              the length of this fragment packet only (excluding the length
              of the IPv6 header itself), and the Next Header field of the
              last header of the Unfragmentable Part changed to 44.

          (2) A Fragment header containing:

                   The Next Header value that identifies the first header of
                   the Fragmentable Part of the original packet.

                   A Fragment Offset containing the offset of the fragment,
                   in 8-octet units, relative to the start of the
                   Fragmentable Part of the original packet.  The Fragment
                   Offset of the first ("leftmost") fragment is 0.

                   An M flag value of 0 if the fragment is the last
                   ("rightmost") one, else an M flag value of 1.

                   The Identification value generated for the original
                   packet.

          (3) The fragment itself.

       The lengths of the fragments must be chosen such that the resulting
       fragment packets fit within the MTU of the path to the packets'
       destination(s).

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

       reassembled original packet:

       +------------------+----------------------//------------------------+
       |  Unfragmentable  |                 Fragmentable                   |
       |       Part       |                     Part                       |
       +------------------+----------------------//------------------------+

       The following rules govern reassembly:

          An original packet is reassembled only from fragment packets that
          have the same Source Address, Destination Address, and Fragment
          Identification.

          The Unfragmentable Part of the reassembled packet consists of all
          headers up to, but not including, the Fragment header of the first
          fragment packet (that is, the packet whose Fragment Offset is
          zero), with the following two changes:

             The Next Header field of the last header of the Unfragmentable
             Part is obtained from the Next Header field of the first
             fragment's Fragment header.

             The Payload Length of the reassembled packet is computed from
             the length of the Unfragmentable Part and the length and offset
             of the last fragment.  For example, a formula for computing the
             Payload Length of the reassembled original packet is:

               PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.last

               where
               PL.orig  = Payload Length field of reassembled packet.
               PL.first = Payload Length field of first fragment packet.
               FL.first = length of fragment following Fragment header of
                          first fragment packet.
               FO.last  = Fragment Offset field of Fragment header of
                          last fragment packet.
               FL.last  = length of fragment following Fragment header of
                          last fragment packet.

          The Fragmentable Part of the reassembled packet is constructed
          from the fragments following the Fragment headers in each of the
          fragment packets.  The length of each fragment is computed by
          subtracting from the packet's Payload Length the length of the

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          headers between the IPv6 header and fragment itself; its relative
          position in Fragmentable Part is computed from its Fragment Offset
          value.

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

       The following error conditions may arise when reassembling fragmented
       packets:

          If insufficient fragments are received to complete reassembly of a
          packet within 60 seconds of the reception of the first-arriving
          fragment of that packet, reassembly of that packet must be
          abandoned and all the fragments that have been received for that
          packet must be discarded.  If the first fragment (i.e., the one
          with a Fragment Offset of zero) has been received, an ICMP Time
          Exceeded -- Fragment Reassembly Time Exceeded message should be
          sent to the source of that fragment.

          If the length of a fragment, as derived from the fragment packet's
          Payload Length field, is not a multiple of 8 octets and the M flag
          of that fragment is 1, then that fragment must be discarded and an
          ICMP Parameter Problem, Code 0, message should be sent to the
          source of the fragment, pointing to the Payload Length field of
          the fragment packet.

          If the length and offset of a fragment are such that the Payload
          Length of the packet reassembled from that fragment would exceed
          65,535 octets, then that fragment must be discarded and an ICMP
          Parameter Problem, Code 0, message should be sent to the source of
          the fragment, pointing to the Fragment Offset field of the
          fragment packet.

       The following conditions are not expected to occur, but are not
       considered errors if they do:

          The number and content of the headers preceding the Fragment
          header of different fragments of the same original packet may
          differ.  Whatever headers are present, preceding the Fragment
          header in each fragment packet, are processed when the packets
          arrive, prior to queueing the fragments for reassembly.  Only
          those headers in the Offset zero fragment packet are retained in
          the reassembled packet.

          The Next Header values in the Fragment headers of different
          fragments of the same original packet may differ.  Only the value
          from the Offset zero fragment packet is used for reassembly.

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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 [RFC-1700 et seq.].

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

       Options              Variable-length field, of length such that the
                            complete Destination Options header is an
                            integer multiple of 8 octets long.  Contains one
                            or  more TLV-encoded options, as described in
                            section 4.2.

       The only destination options defined in this document are the Pad1
       and PadN options specified in section 4.2.

       Note that there are two possible ways to encode optional destination
       information in an IPv6 packet: either as an option in the Destination
       Options header, or as a separate extension header.  The Fragment
       header and the Authentication header are examples of the latter
       approach.  Which approach can be used depends on what action is
       desired of a destination node that does not understand the optional
       information:

          o  If the desired action is for the destination node to discard
             the packet and, only if the packet's Destination Address is not
             a multicast address, send an ICMP Unrecognized Type message to
             the packet's Source Address, then the information may be
             encoded either as a separate header or as an option in the

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             Destination Options header whose Option Type has the value 11
             in its highest-order two bits.  The choice may depend on such
             factors as which takes fewer octets, or which yields better
             alignment or more efficient parsing.

          o  If any other action is desired, the information must be encoded
             as an option in the Destination Options header whose Option
             Type has the value 00, 01, or 10 in its highest-order two bits,
             specifying the desired action (see section 4.2).

    4.7 No Next Header

       The value 59 in the Next Header field of an IPv6 header or any
       extension header indicates that there is nothing following that
       header.  If the Payload Length field of the IPv6 header indicates the
       presence of octets past the end of a header whose Next Header field
       contains 59, those octets must be ignored, and passed on unchanged if
       the packet is forwarded.

    5. Packet Size Issues

       IPv6 requires that every link in the internet have an MTU of 1280
       octets or greater.  On any link that cannot convey a 1280-octet
       packet in one piece, link-specific fragmentation and reassembly must
       be provided at a layer below IPv6.

       Links that have a configurable MTU (for example, PPP links [RFC-
       1661]) must be configured to have an MTU of at least 1280 octets; it
       is recommended that they be configured with an MTU of 1500 octets or
       greater, to accommodate possible encapsulations (i.e., tunneling)
       without incurring IPv6-layer fragmentation.

       From each link to which a node is directly attached, the node must be
       able to accept packets as large as that link's MTU.

       It is strongly recommended that IPv6 nodes implement Path MTU
       Discovery [RFC-1981], in order to discover and take advantage of path
       MTUs greater than 1280 octets.  However, a minimal IPv6
       implementation (e.g., in a boot ROM) may simply restrict itself to
       sending packets no larger than 1280 octets, and omit implementation
       of Path MTU Discovery.

       In order to send a packet larger than a path's MTU, a node may use
       the IPv6 Fragment header to fragment the packet at the source and
       have it reassembled at the destination(s).  However, the use of such
       fragmentation is discouraged in any application that is able to
       adjust its packets to fit the measured path MTU (i.e., down to 1280
       octets).

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       A node must be able to accept a fragmented packet that, after
       reassembly, is as large as 1500 octets.  A node is permitted to
       accept fragmented packets that reassemble to more than 1500 octets.
       An upper-layer protocol or application that depends on IPv6
       fragmentation to send packets larger than the MTU of a path should
       not send packets larger than 1500 octets unless it has assurance that
       the destination is capable of reassembling packets of that larger
       size.

       In response to an IPv6 packet that is sent to an IPv4 destination
       (i.e., a packet that undergoes translation from IPv6 to IPv4), the
       originating IPv6 node may receive an ICMP Packet Too Big message
       reporting a Next-Hop MTU less than 1280.  In that case, the IPv6 node
       is not required to reduce the size of subsequent packets to less than
       1280, but must include a Fragment header in those packets so that the
       IPv6-to-IPv4 translating router can obtain a suitable Identification
       value to use in resulting IPv4 fragments.  Note that this means the
       payload may have to be reduced to 1232 octets (1280 minus 40 for the
       IPv6 header and 8 for the Fragment header), and smaller still if
       additional extension headers are used.

    6.  Flow Labels

       The 20-bit Flow Label field in the IPv6 header may be used by a
       source to label sequences of packets for which it requests special
       handling by the IPv6 routers, such as non-default quality of service
       or "real-time" service.  This aspect of IPv6 is, at the time of
       writing, still experimental and subject to change as the requirements
       for flow support in the Internet become clearer.  Hosts or routers
       that do not support the functions of the Flow Label field are
       required to set the field to zero when originating a packet, pass the
       field on unchanged when forwarding a packet, and ignore the field
       when receiving a packet.

       Appendix A describes the current intended semantics and usage of the
       Flow Label field.

    7.  Traffic Classes

       The 8-bit Traffic Class field in the IPv6 header is available for use
       by originating nodes and/or forwarding routers to identify and
       distinguish between different classes or priorities of IPv6 packets.
       At the point in time at which this specification is being written,
       there are a number of experiments underway in the use of the IPv4
       Type of Service and/or Precedence bits to provide various forms of
       "differentiated service" for IP packets, other than through the use
       of explicit flow set-up.  The Traffic Class field in the IPv6 header
       is intended to allow similar functionality to be supported in IPv6.

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       It is hoped that those experiments will eventually lead to agreement
       on what sorts of traffic classifications are most useful for IP
       packets.  Detailed definitions of the syntax and semantics of all or
       some of the IPv6 Traffic Class bits, whether experimental or intended
       for eventual standardization, are to be provided in separate
       documents.

       The following general requirements apply to the Traffic Class field:

          o  The service interface to the IPv6 service within a node must
             provide a means for an upper-layer protocol to supply the value
             of the Traffic Class bits in packets originated by that upper-
             layer protocol.  The default value must be zero for all 8 bits.

          o  Nodes that support a specific (experimental or eventual
             standard) use of some or all of the Traffic Class bits are
             permitted to change the value of those bits in packets that
             they originate, forward, or receive, as required for that
             specific use.  Nodes should ignore and leave unchanged any bits
             of the Traffic Class field for which they do not support a
             specific use.

          o  An upper-layer protocol must not assume that the value of the
             Traffic Class bits in a received packet are the same as the
             value sent by the packet's source.

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    8. Upper-Layer Protocol Issues

    8.1 Upper-Layer Checksums

       Any transport or other upper-layer protocol that includes the
       addresses from the IP header in its checksum computation must be
       modified for use over IPv6, to include the 128-bit IPv6 addresses
       instead of 32-bit IPv4 addresses.  In particular, the following
       illustration shows the TCP and UDP "pseudo-header" for IPv6:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                         Source Address                        +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                      Destination Address                      +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Upper-Layer Packet Length                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      zero                     |  Next Header  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          o  If the IPv6 packet contains a Routing header, the Destination
             Address used in the pseudo-header is that of the final
             destination.  At the originating node, that address will be in
             the last element of the Routing header; at the recipient(s),
             that address will be in the Destination Address field of the
             IPv6 header.

          o  The Next Header value in the pseudo-header identifies the
             upper-layer protocol (e.g., 6 for TCP, or 17 for UDP).  It will
             differ from the Next Header value in the IPv6 header if there
             are extension headers between the IPv6 header and the upper-
             layer header.

          o  The Upper-Layer Packet Length in the pseudo-header is the
             length of the upper-layer header and data (e.g., TCP header
             plus TCP data).  Some upper-layer protocols carry their own

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             length information (e.g., the Length field in the UDP header);
             for such protocols, that is the length used in the pseudo-
             header.  Other protocols (such as TCP) do not carry their own
             length information, in which case the length used in the
             pseudo-header is the Payload Length from the IPv6 header, minus
             the length of any extension headers present between the IPv6
             header and the upper-layer header.

          o  Unlike IPv4, when UDP packets are originated by an IPv6 node,
             the UDP checksum is not optional.  That is, whenever
             originating a UDP packet, an IPv6 node must compute a UDP
             checksum over the packet and the pseudo-header, and, if that
             computation yields a result of zero, it must be changed to hex
             FFFF for placement in the UDP header.  IPv6 receivers must
             discard UDP packets containing a zero checksum, and should log
             the error.

       The IPv6 version of ICMP [ICMPv6] includes the above pseudo-header in
       its checksum computation; this is a change from the IPv4 version of
       ICMP, which does not include a pseudo-header in its checksum.  The
       reason for the change is to protect ICMP from misdelivery or
       corruption of those fields of the IPv6 header on which it depends,
       which, unlike IPv4, are not covered by an internet-layer checksum.
       The Next Header field in the pseudo-header for ICMP contains the
       value 58, which identifies the IPv6 version of ICMP.

    8.2 Maximum Packet Lifetime

       Unlike IPv4, IPv6 nodes are not required to enforce maximum packet
       lifetime.  That is the reason the IPv4 "Time to Live" field was
       renamed "Hop Limit" in IPv6.  In practice, very few, if any, IPv4
       implementations conform to the requirement that they limit packet
       lifetime, so this is not a change in practice.  Any upper-layer
       protocol that relies on the internet layer (whether IPv4 or IPv6) to
       limit packet lifetime ought to be upgraded to provide its own
       mechanisms for detecting and discarding obsolete packets.

    8.3 Maximum Upper-Layer Payload Size

       When computing the maximum payload size available for upper-layer
       data, an upper-layer protocol must take into account the larger size
       of the IPv6 header relative to the IPv4 header.  For example, in
       IPv4, TCP's MSS option is computed as the maximum packet size (a
       default value or a value learned through Path MTU Discovery) minus 40
       octets (20 octets for the minimum-length IPv4 header and 20 octets
       for the minimum-length TCP header).  When using TCP over IPv6, the
       MSS must be computed as the maximum packet size minus 60 octets,

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       because the minimum-length IPv6 header (i.e., an IPv6 header with no
       extension headers) is 20 octets longer than a minimum-length IPv4
       header.

    8.4 Responding to Packets Carrying Routing Headers

       When an upper-layer protocol sends one or more packets in response to
       a received packet that included a Routing header, the response
       packet(s) must not include a Routing header that was automatically
       derived by "reversing" the received Routing header UNLESS the
       integrity and authenticity of the received Source Address and Routing
       header have been verified (e.g., via the use of an Authentication
       header in the received packet).  In other words, only the following
       kinds of packets are permitted in response to a received packet
       bearing a Routing header:

          o  Response packets that do not carry Routing headers.

          o  Response packets that carry Routing headers that were NOT
             derived by reversing the Routing header of the received packet
             (for example, a Routing header supplied by local
             configuration).

          o  Response packets that carry Routing headers that were derived
             by reversing the Routing header of the received packet IF AND
             ONLY IF the integrity and authenticity of the Source Address
             and Routing header from the received packet have been verified
             by the responder.

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    Appendix A. Semantics and Usage of the Flow Label Field

       A flow is a sequence of packets sent from a particular source to a
       particular (unicast or multicast) destination for which the source
       desires special handling by the intervening routers.  The nature of
       that special handling might be conveyed to the routers by a control
       protocol, such as a resource reservation protocol, or by information
       within the flow's packets themselves, e.g., in a hop-by-hop option.
       The details of such control protocols or options are beyond the scope
       of this document.

       There may be multiple active flows from a source to a destination, as
       well as traffic that is not associated with any flow.  A flow is
       uniquely identified by the combination of a source address and a
       non-zero flow label.  Packets that do not belong to a flow carry a
       flow label of zero.

       A flow label is assigned to a flow by the flow's source node.  New
       flow labels must be chosen (pseudo-)randomly and uniformly from the
       range 1 to FFFFF hex.  The purpose of the random allocation is to
       make any set of bits within the Flow Label field suitable for use as
       a hash key by routers, for looking up the state associated with the
       flow.

       All packets belonging to the same flow must be sent with the same
       source address, destination address, and flow label.  If any of those
       packets includes a Hop-by-Hop Options header, then they all must be
       originated with the same Hop-by-Hop Options header contents
       (excluding the Next Header field of the Hop-by-Hop Options header).
       If any of those packets includes a Routing header, then they all must
       be originated with the same contents in all extension headers up to
       and including the Routing header (excluding the Next Header field in
       the Routing header).  The routers or destinations are permitted, but
       not required, to verify that these conditions are satisfied.  If a
       violation is detected, it should be reported to the source by an ICMP
       Parameter Problem message, Code 0, pointing to the high-order octet
       of the Flow Label field (i.e., offset 1 within the IPv6 packet).

       The maximum lifetime of any flow-handling state established along a
       flow's path must be specified as part of the description of the
       state-establishment mechanism, e.g., the resource reservation
       protocol or the flow-setup hop-by-hop option.  A source must not re-
       use a flow label for a new flow within the maximum lifetime of any
       flow-handling state that might have been established for the prior
       use of that flow label.

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       When a node stops and restarts (e.g., as a result of a "crash"), it
       must be careful not to use a flow label that it might have used for
       an earlier flow whose lifetime may not have expired yet.  This may be
       accomplished by recording flow label usage on stable storage so that
       it can be remembered across crashes, or by refraining from using any
       flow labels until the maximum lifetime of any possible previously
       established flows has expired.  If the minimum time for rebooting the
       node is known, that time can be deducted from the necessary waiting
       period before starting to allocate flow labels.

       There is no requirement that all, or even most, packets belong to
       flows, i.e., carry non-zero flow labels.  This observation is placed
       here to remind protocol designers and implementors not to assume
       otherwise.  For example, it would be unwise to design a router whose
       performance would be adequate only if most packets belonged to flows,
       or to design a header compression scheme that only worked on packets
       that belonged to flows.

    Deering & Hinden            Standards Track                    [Page 31]

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    Appendix B. Formatting Guidelines for Options

       This appendix gives some advice on how to lay out the fields when
       designing new options to be used in the Hop-by-Hop Options header or
       the Destination Options header, as described in section 4.2.  These
       guidelines are based on the following assumptions:

          o  One desirable feature is that any multi-octet fields within the
             Option Data area of an option be aligned on their natural
             boundaries, i.e., fields of width n octets should be placed at
             an integer multiple of n octets from the start of the Hop-by-
             Hop or Destination Options header, for n = 1, 2, 4, or 8.

          o  Another desirable feature is that the Hop-by-Hop or Destination
             Options header take up as little space as possible, subject to
             the requirement that the header be an integer multiple of 8
             octets long.

          o  It may be assumed that, when either of the option-bearing
             headers are present, they carry a very small number of options,
             usually only one.

       These assumptions suggest the following approach to laying out the
       fields of an option: order the fields from smallest to largest, with
       no interior padding, then derive the alignment requirement for the
       entire option based on the alignment requirement of the largest field
       (up to a maximum alignment of 8 octets).  This approach is
       illustrated in the following examples:

       Example 1

       If an option X required two data fields, one of length 8 octets and
       one of length 4 octets, it would be laid out as follows:

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       | Option Type=X |Opt Data Len=12|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         8-octet field                         +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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    RFC 2460                   IPv6 Specification              December 1998

       Its alignment requirement is 8n+2, to ensure that the 8-octet field
       starts at a multiple-of-8 offset from the start of the enclosing
       header.  A complete Hop-by-Hop or Destination Options header
       containing this one option would look as follows:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Next Header  | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         8-octet field                         +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Example 2

       If an option Y required three data fields, one of length 4 octets,
       one of length 2 octets, and one of length 1 octet, it would be laid
       out as follows:

                                                       +-+-+-+-+-+-+-+-+
                                                       | Option Type=Y |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Opt Data Len=7 | 1-octet field |         2-octet field         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Its alignment requirement is 4n+3, to ensure that the 4-octet field
       starts at a multiple-of-4 offset from the start of the enclosing
       header.  A complete Hop-by-Hop or Destination Options header
       containing this one option would look as follows:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Next Header  | Hdr Ext Len=1 | Pad1 Option=0 | Option Type=Y |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Opt Data Len=7 | 1-octet field |         2-octet field         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | PadN Option=1 |Opt Data Len=2 |       0       |       0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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    RFC 2460                   IPv6 Specification              December 1998

       Example 3

       A Hop-by-Hop or Destination Options header containing both options X
       and Y from Examples 1 and 2 would have one of the two following
       formats, depending on which option appeared first:

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Next Header  | Hdr Ext Len=3 | Option Type=X |Opt Data Len=12|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         8-octet field                         +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | PadN Option=1 |Opt Data Len=1 |       0       | Option Type=Y |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Opt Data Len=7 | 1-octet field |         2-octet field         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | PadN Option=1 |Opt Data Len=2 |       0       |       0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Next Header  | Hdr Ext Len=3 | Pad1 Option=0 | Option Type=Y |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Opt Data Len=7 | 1-octet field |         2-octet field         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | PadN Option=1 |Opt Data Len=4 |       0       |       0       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       0       |       0       | Option Type=X |Opt Data Len=12|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         4-octet field                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         8-octet field                         +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Deering & Hinden            Standards Track                    [Page 34]

    RFC 2460                   IPv6 Specification              December 1998

    Security Considerations

       The security features of IPv6 are described in the Security
       Architecture for the Internet Protocol [RFC-2401].

    Acknowledgments

       The authors gratefully acknowledge the many helpful suggestions of
       the members of the IPng working group, the End-to-End Protocols
       research group, and the Internet Community At Large.

    Authors' Addresses

       Stephen E. Deering
       Cisco Systems, Inc.
       170 West Tasman Drive
       San Jose, CA 95134-1706
       USA

       Phone: +1 408 527 8213
       Fax:   +1 408 527 8254
       EMail: deering@cisco.com

       Robert M. Hinden
       Nokia
       232 Java Drive
       Sunnyvale, CA 94089
       USA

       Phone: +1 408 990-2004
       Fax:   +1 408 743-5677
       EMail: hinden@iprg.nokia.com

    References

       [RFC-2401]   Kent, S. and R. Atkinson, "Security Architecture for the
                    Internet Protocol", RFC 2401, November 1998.

       [RFC-2402]   Kent, S. and R. Atkinson, "IP Authentication Header",
                    RFC 2402, November 1998.

       [RFC-2406]   Kent, S. and R. Atkinson, "IP Encapsulating Security
                    Protocol (ESP)", RFC 2406, November 1998.

       [ICMPv6]     Conta, A. and S. Deering, "ICMP for the Internet
                    Protocol Version 6 (IPv6)", RFC 2463, December 1998.

    Deering & Hinden            Standards Track                    [Page 35]

    RFC 2460                   IPv6 Specification              December 1998

       [ADDRARCH]   Hinden, R. and S. Deering, "IP Version 6 Addressing
                    Architecture", RFC 2373, July 1998.

       [RFC-1981]   McCann, J., Mogul, J. and S. Deering, "Path MTU
                    Discovery for IP version 6", RFC 1981, August 1996.

       [RFC-791]    Postel, J., "Internet Protocol", STD 5, RFC 791,
                    September 1981.

       [RFC-1700]   Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
                    RFC 1700, October 1994.  See also:
                    http://www.iana.org/numbers.html

       [RFC-1661]   Simpson, W., "The Point-to-Point Protocol (PPP)", STD
                    51, RFC 1661, July 1994.

    CHANGES SINCE RFC-1883

       This memo has the following changes from RFC-1883.  Numbers identify
       the Internet-Draft version in which the change was made.

        02) Removed all references to jumbograms and the Jumbo Payload
            option (moved to a separate document).

        02) Moved most of Flow Label description from section 6 to (new)
            Appendix A.

        02) In Flow Label description, now in Appendix A, corrected maximum
            Flow Label value from FFFFFF to FFFFF (i.e., one less "F") due
            to reduction of size of Flow Label field from 24 bits to 20
            bits.

        02) Renumbered (relettered?) the previous Appendix A to be Appendix
            B.

        02) Changed the wording of the Security Considerations section to
            avoid dependency loop between this spec and the IPsec specs.

        02) Updated R. Hinden's email address and company affiliation.

            --------------------------------------------------------

        01) In section 3, changed field name "Class" to "Traffic Class" and
            increased its size from 4 to 8 bits.  Decreased size of Flow
            Label field from 24 to 20 bits to compensate for increase in
            Traffic Class field.

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    RFC 2460                   IPv6 Specification              December 1998

        01) In section 4.1, restored the order of the Authentication Header
            and the ESP header, which were mistakenly swapped in the 00
            version of this memo.

        01) In section 4.4, deleted the Strict/Loose Bit Map field and the
            strict routing functionality from the Type 0 Routing header, and
            removed the restriction on number of addresses that may be
            carried in the Type 0 Routing header (was limited to 23
            addresses, because of the size of the strict/loose bit map).

        01) In section 5, changed the minimum IPv6 MTU from 576 to 1280
            octets, and added a recommendation that links with configurable
            MTU (e.g., PPP links) be configured to have an MTU of at least
            1500 octets.

        01) In section 5, deleted the requirement that a node must not send
            fragmented packets that reassemble to more than 1500 octets
            without knowledge of the destination reassembly buffer size, and
            replaced it with a recommendation that upper-layer protocols or
            applications should not do that.

        01) Replaced reference to the IPv4 Path MTU Discovery spec (RFC-
            1191) with reference to the IPv6 Path MTU Discovery spec (RFC-
            1981), and deleted the Notes at the end of section 5 regarding
            Path MTU Discovery, since those details are now covered by RFC-
            1981.

        01) In section 6, deleted specification of "opportunistic" flow
            set-up, and removed all references to the 6-second maximum
            lifetime for opportunistically established flow state.

        01) In section 7, deleted the provisional description of the
            internal structure and semantics of the Traffic Class field, and
            specified that such descriptions be provided in separate
            documents.

            --------------------------------------------------------

        00) In section 4, corrected the Code value to indicate "unrecognized
            Next Header type encountered" in an ICMP Parameter Problem
            message (changed from 2 to 1).

        00) In the description of the Payload Length field in section 3, and
            of the Jumbo Payload Length field in section 4.3, made it
            clearer that extension headers are included in the payload
            length count.

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    RFC 2460                   IPv6 Specification              December 1998

        00) In section 4.1, swapped the order of the Authentication header
            and the ESP header.  (NOTE: this was a mistake, and the change
            was undone in version 01.)

        00) In section 4.2, made it clearer that options are identified by
            the full 8-bit Option Type, not by the low-order 5 bits of an
            Option Type.  Also specified that the same Option Type numbering
            space is used for both Hop-by-Hop Options and Destination
            Options headers.

        00) In section 4.4, added a sentence requiring that nodes processing
            a Routing header must send an ICMP Packet Too Big message in
            response to a packet that is too big to fit in the next hop link
            (rather than, say, performing fragmentation).

        00) Changed the name of the IPv6 Priority field to "Class", and
            replaced the previous description of Priority in section 7 with
            a description of the Class field.  Also, excluded this field
            from the set of fields that must remain the same for all packets
            in the same flow, as specified in section 6.

        00) In the pseudo-header in section 8.1, changed the name of the
            "Payload Length" field to "Upper-Layer Packet Length".  Also
            clarified that, in the case of protocols that carry their own
            length info (like non-jumbogram UDP), it is the upper-layer-
            derived length, not the IP-layer-derived length, that is used in
            the pseudo-header.

        00) Added section 8.4, specifying that upper-layer protocols, when
            responding to a received packet that carried a Routing header,
            must not include the reverse of the Routing header in the
            response packet(s) unless the received Routing header was
            authenticated.

        00) Fixed some typos and grammatical errors.

        00) Authors' contact info updated.

            --------------------------------------------------------

    Deering & Hinden            Standards Track                    [Page 38]

    RFC 2460                   IPv6 Specification              December 1998

    Full Copyright Statement

       Copyright (C) The Internet Society (1998).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Deering & Hinden            Standards Track                    [Page 39]