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INTERNET DRAFT





Network Working Group                             S. Deering, Xerox PARC
March 17, 1995
Request for Comments: 1883                  R.  Hinden, Ipsilon
Obsoletes: draft-hinden-ipng-ipv6-spec-00.txt                    Editors Networks
Category: Standards Track                                  December 1995




                  Internet Protocol, Version 6 (IPv6)
                             Specification

                  <draft-ietf-ipngwg-ipv6-spec-01.txt>



                                Abstract

This document specifies version 6 of the Internet Protocol, a proposed
successor to IP version 4.  Changes from the previous draft are listed
in Appendix B.





Status of this Memo

   This document is specifies an Internet-Draft.  Internet-Drafts are working
documents of Internet standards track protocol for the
   Internet Engineering Task Force (IETF), its areas, community, and
its working groups.  Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months requests discussion and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or suggestions for
   improvements.  Please refer to cite them other than as ``work in progress.''

To learn the current status edition of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.














draft-ietf-ipngwg-ipv6-spec-01.txt


Abstract


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























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Contents

   Status


Table of this Memo..............................................1 Contents

   1. Introduction..................................................3

   2. Terminology...................................................4

   3. IPv6 Header Format............................................5

   4. IPv6 Extension Headers........................................6
       4.1 Extension Header Order...................................8
       4.2 Options..................................................9
       4.3 Hop-by-Hop Options Header...............................11
       4.4 Routing Header..........................................13
       4.5 Fragment Header.........................................16 Header.........................................19
       4.6 Authentication Header...................................18
       4.7 Destination Options Header..............................19
       4.8 Header..............................24
       4.7 No Next Header..........................................20 Header..........................................25

   5. Packet Size Issues...........................................21 Issues...........................................26

   6. Flow Labels..................................................23 Labels..................................................28

   7. Priority.....................................................25 Priority.....................................................30

   8. Upper-Layer Protocol Issues..................................26 Issues..................................31
       8.1 Upper-Layer Checksums...................................26 Checksums...................................31
       8.2 Maximum Packet Lifetime.................................27 Lifetime.................................32
       8.3 Maximum Upper-Layer Payload Size........................27 Size........................32

   Appendix A. Formatting Guidelines for Options...................28

   Appendix B. Changes from Previous Draft.........................31 Options...................33

   Security Considerations.........................................33

   Acknowledgments.................................................33

   Document Editors' Addresses.....................................33

   References......................................................34









draft-ietf-ipngwg-ipv6-spec-01.txt Considerations.........................................36

   Acknowledgments.................................................36

   Authors' Addresses..............................................36

   References......................................................37














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1.  Introduction

   IP version 6 (IPv6) is a new version of the Internet Protocol,
   designed as a 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 a "region an "anycast address" is defined, used to identify topological
      regions rather than individual 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.

      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 initially-
   defined IPv6 extension headers and options.  It also discusses packet
   size issues, the semantics of flow labels and priority, and the
   effects of IPv6 on upper-layer protocols.  Other aspects  The format and semantics
   of IPv6 addresses are specified separately in
separate documents, including the following:





draft-ietf-ipngwg-ipv6-spec-01.txt [RFC-1884].  The IPv6
   version of ICMP, which all IPv6 implementations are required to
   include, is specified in [RFC-1885].


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   o  IP Version 6 Addressing Architecture [IPV6-ADDR]

   o  ICMP for the Internet Protocol Version 6 [IPV6-ICMP]

   o  Transition Mechanisms for IPv6 Hosts and Routers[IPV6-TRAN]


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 in one piece over
                 a link.

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





draft-ietf-ipngwg-ipv6-spec-01.txt

   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.



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3.  IPv6 Header Format

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| Prio. |                   Flow Label                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Payload Length        |  Next Header  |   Hop Limit   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                         Source Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version              4-bit Internet Protocol version number = 6.

   Prio.                4-bit priority value.  See section 7.

   Flow Label           24-bit flow label.  See section 6.

   Payload Length       16-bit unsigned integer.  Length of payload,
                        i.e., the rest of the packet following the
                        IPv6 header, in octets.  If zero, indicates that
                        the payload length is carried in a Jumbo Payload
                        hop-by-hop option.

   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].
                        [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 [IPV6-ADDR].



draft-ietf-ipngwg-ipv6-spec-01.txt [RFC-1884].



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   Destination Address  128-bit address of the intended recipient
                        of the packet (possibly not the ultimate
                        recipient, if a Routing header is present).
                        See [IPV6-ADDR] [RFC-1884] 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 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



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   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.

   The exception referred to in the preceding paragraph is the Hop-by-Hop 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, while 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 2 ("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  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.




























draft-ietf-ipngwg-ipv6-spec-01.txt

   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-1826] and [RFC-1827], respectively.








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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 (1) (note 1)
           Routing header
           Fragment header
           Authentication header (note 2)
           Encapsulating Security Payload header (note 2)
           Destination Options header (2) (note 3)
           upper-layer header

         (1)

           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.
         (2)

           note 2: additional recommendations regarding the relative
                   order of the Authentication and Encapsulating
                   Security Payload headers are given in [RFC-1827].

           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 extensions 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.













draft-ietf-ipngwg-ipv6-spec-01.txt





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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 -- may carry a variable
   number of Type-Length-Value 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 and, regardless of whether or not the
           packets'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
        is 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.  Data that can  When an Authentication header is present
   in the packet, for any option whose data may change en-route en-route, its
   entire Option Data field must be excluded from the
integrity assurance computation performed treated as zero-valued octets when
   computing or verifying the Authentication header
is present. packet's authenticating value.


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      0 - Option Data does not change en-route

      1 - Option Data may change en-route

   Individual options may have specific alignment requirements, to
   ensure that multi-octet values within Option Data fields fall on
   natural



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   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.


   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.








draft-ietf-ipngwg-ipv6-spec-01.txt


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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]. [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.

   In addition to the Pad1 and PadN options specified in section 4.2,
   the following hop-by-hop option is defined:

   Jumbo Payload option  (alignment requirement: 4n + 2)

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |      194      |Opt Data Len=4 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Jumbo Payload Length                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The Jumbo Payload option is used to send IPv6 packets with
       payloads longer than 65,535 octets.  The Jumbo Payload Length is



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       the length of the packet in octets, excluding the IPv6 header.
       It has a maximum value of 4,294,967,295, that is, 2^32-1.
       It has a minimum legal value of 8, which is header but
       including the length of a Hop-by-Hop Options header containing only this option, header; it must be greater than
       65,535.  If a packet is received with no
       additional headers or data; however, use of this a Jumbo Payload option for
       packets with payloads
       containing a Jumbo Payload Length less than 65,535 octets is not recommended. or equal to 65,535,


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       an ICMP Parameter Problem message, Code 0, should be sent to the
       packet's source, pointing to the high-order octet of the invalid
       Jumbo Payload Length field.

       The Payload Length field in the IPv6 header must be set to zero
       in every packet that carries the Jumbo Payload option.  If a
       packet is received with a valid Jumbo Payload option present and
       a non-zero IPv6 Payload Length field, an ICMP Parameter Problem
       message, Code 0, should be sent to the packet's source, pointing
       to the Option Type field of the Jumbo Payload option.

       The Jumbo Payload option must not be used in a packet that
       carries a Fragment header.  If a Fragment Header header is encountered
       in a packet that contains a valid Jumbo Payload option, an ICMP
       Parameter Problem message, Code 0, should be sent to the packet's
       source, pointing to the first octet of the Fragment header.
































draft-ietf-ipngwg-ipv6-spec-01.txt

       An implementation that does not support the Jumbo Payload option
       cannot have interfaces to links whose link MTU is greater than
       65,575 (40 octets of IPv6 header plus 65,535 octets of payload).































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4.4  Routing Header

   The Routing header is used by an IPv6 source to list one or more
   intermediate nodes (or topological regions) to be "visited" on the way to a packet's
   destination.  This function is very similar to IPv4's Source Route
   options.  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].
                        [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 the












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   If, while processing a received packet, a node encounters a Routing
   header does not recognize
the with an unrecognized Routing Type value, it 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 discard ignore the packet and, only if Routing header
      and proceed to process the
packet's Destination Address next header in the packet, whose type
      is not a multicast address, 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.







































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   The Type 0 Routing header has the following format:










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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |Routing Type=0  |   Num Addrs  Hdr Ext Len  |   Next Addr Routing Type=0| Segments Left |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Reserved    |             Strict/Loose Bit Mask Map              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                           Address[0]                           Address[1]                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                           Address[1]                           Address[2]                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                               .                               .
   .                               .                               .
   .                               .                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Address[Num Addrs - 1]                           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].

   Routing Type         0.

   Num Addrs
                        [RFC-1700 et seq.].

   Hdr Ext Len          8-bit unsigned integer.  Number  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, and must
                        be an even number less than or equal to 46.

   Routing header.  Maximum legal value = 24.

   Next Addr Type         0.


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RFC 1883                   IPv6 Specification              December 1995


   Segments Left        8-bit unsigned integer.  Index  Number of next address route
                        segments remaining, i.e., number of explicitly
                        listed intermediate nodes still to be processed; initialized to 0 by visited
                        before reaching the
                        originating node.




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                        Maximum legal value = 23.

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

   Strict/Loose Bit Mask Map
                        24-bit bit-mask, bit-map, numbered 0 to 23, left-to-right.
                        If bit n is 1, then
                        Indicates, for each segment of the packet may route, whether
                        or not the next destination address must be forwarded
                        to Address[n] by a
                        neighbor of the node that places Address[n] preceding address: 1 means strict
                        (must be a neighbor), 0 means loose (need not be
                        a neighbor).

   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 Field only if Address field of a packet carrying a Routing
   header of Type 0.

   If bit number 0 of the
                        interface identified by Address[n] is Strict/Loose Bit Map has value 1, the
   Destination Address field of the IPv6 header in the original packet
   must identify a neighbor of the forwarding originating node.  If bit number 0
   has value 0, the originator may use any legal, non-multicast address
   as the initial Destination Address.

   Bits numbered greater than n, where n is 0, then
                        Address[n] need not be a neighbor the number of addresses in
   the
                        forwarding node. Routing header, must be set to 0 by the originator and ignored by
   receivers.

   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:

   o  If Next Addr < Num Addrs, swap the IPv6 Destination Address and
      Address[Next Addr].  If Bit Mask[Next Addr]











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   if Segments Left = 0 or if {
      proceed to process the new
      destination address next header in the packet, whose type is known to be a neighbor of this node,
      increment Next Addr
      identified by one and re-submit the packet to the IPv6
      module for forwarding to Next Header field in the new destination, Routing header
   }
   else if Hdr Ext Len is odd or greater than 46 {
         send an ICMP
      Destination Unreachable, Not a Neighbor Parameter Problem, Code 0, message to the Source
      Address
         Address, pointing to the Hdr Ext Len field, and discard the packet.

   o  If Next Addr = Num Addrs, dispatch to the next header processing
      module, as identified by
         packet
   }
   else {
      compute n, the Next Header field number of addresses in the Routing
      header.

   o  If Next Addr > Num Addrs, 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 Num Addrs Segments Left field, and discard the packet.

Multicast addresses must not appear in a Routing header
         packet
      }
      else {
         decrement Segments Left by 1;
         compute i, the index of Type 0, or the next address to be visited in
         the address vector, by subtracting Segments Left from n

         if Address [i] or the IPv6 Destination Address field of a is multicast {
            discard the packet carrying a Routing header
of Type 0.












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         }
         else {
            swap the IPv6 Specification             March 17, 1995


4.5  Fragment Header

The Fragment header Destination Address and Address[i]

            if bit i of the Strict/Loose Bit map has value 1 and the
            new Destination Address is used by not the address of a neighbor
            of this node {
               send an ICMP Destination Unreachable -- Not a Neighbor
               message to the Source Address and discard the packet
            }
            else if the IPv6 source Hop Limit is less than or equal to 1 {
               send payloads larger
than would fit an ICMP Time Exceeded -- Hop Limit Exceeded in the path MTU
               Transit message to their destinations.  (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, Source Address and has the following format:


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

   Next Header          8-bit selector.  Identifies the type of header
                        immediately following discard the Fragment header.
                        Uses
               packet
            }
            else {
               decrement the same values as Hop Limit by 1

               resubmit the IPv4 Protocol field
                        [RFC-1700].

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

   Fragment Offset      13-bit unsigned integer.  The offset, in 8-octet
                        units, of the following payload, relative IPv6 module for transmission
               to the
                        start new destination
            }
         }
      }
   }


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   As an example of the original, unfragmented payload.

   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.


The fragmentation algorithm is as follows:  The payload (including any
extension headers that need be processed only by effects of the destination
node(s)) is divided into fragments, each, except possibly above algorithm, consider the last,
being an integer multiple
   case of 8 octets long.  Each fragment is prepended
with a Fragment header and sent in source node S sending a separate IPv6 packet.  The M
("more") flag is set packet to 1 on all fragments of the same payload except destination node D, using
   a Routing header to cause the last. packet to be routed via intermediate
   nodes I1, I2, and I3.  The original payload is assigned an Identification value that
is different than that values of any other fragmented payload sent recently*
with the same relevant IPv6 Source Address, IPv6 Destination Address, header and
Fragment Next Header value.  (If a
   Routing header is present, fields on each segment of the IPv6 delivery path would be
   as follows:

   As the packet travels from S to I1:

        Source Address = S                  Hdr Ext Len = 6
        Destination Address is that = I1            Segments Left = 3
                                            Address[1] = I2
        (if bit 0 of the final destination.)  The
Identification value Bit Map is carried in the Fragment header of all of the



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original payload's fragments, 1,      Address[2] = I3
         S and I1 must be neighbors;        Address[3] = D
         this is used checked by S)

   As the destination to identify
all fragments belonging to the same original payload.

   * "recently" means within the maximum likely lifetime of a packet,
     including transit time packet travels from source I1 to destination and time spent
     awaiting reassembly with other fragments I2:

        Source Address = S                  Hdr Ext Len = 6
        Destination Address = I2            Segments Left = 2
                                            Address[1] = I1
        (if bit 1 of the same payload.
     However, it Bit Map is not required that a source node know 1,      Address[2] = I3
         I1 and I2 must be neighbors;       Address[3] = D
         this is checked by I1)

   As the maximum packet lifetime.  Rather, it is assumed that travels from I2 to I3:

        Source Address = S                  Hdr Ext Len = 6
        Destination Address = I3            Segments Left = 1
                                            Address[1] = I1
        (if bit 2 of the requirement can Bit Map is 1,      Address[2] = I2
         I2 and I3 must be
     met neighbors;       Address[3] = D
         this is checked by maintaining I2)

   As the Identification value as a simple, 32-bit,
     "wrap-around" counter, incremented each time a payload packet travels from I3 to D:

        Source Address = S                  Hdr Ext Len = 6
        Destination Address = D             Segments Left = 0
                                            Address[1] = I1
        (if bit 3 of the Bit Map is 1,      Address[2] = I2
         I3 and D must be
     fragmented.  It neighbors;        Address[3] = I3
         this is checked by I3)









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4.5  Fragment Header

   The Fragment header is used by an implementation choice whether IPv6 source 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, next header type)
     combination.

In a packet with a Fragment header, the Payload Length field of the IPv6
header contains the length of that packet only (excluding the IPv6
header itself), not the length of send packets larger
   than would fit in the original, unfragmented payload.
































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4.6  Authentication Header

The Authentication header is used path MTU to provide authentication and
integrity assurance for their destinations.  (Note: unlike
   IPv4, fragmentation in IPv6 packets.  Non-repudiation may be provided
by an authentication algorithm used with the Authentication header, but
it is performed only by source nodes, not provided with all authentication algorithms that might be used
with this header. by
   routers along a packet's delivery path -- see section 5.)  The Authentication
   Fragment header is identified by a Next Header value of 51 44 in the
   immediately preceding header, and has the following format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Payload Type | Auth Data Len  Next Header  |   Reserved    |      Fragment Offset    |Res|M|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Security Association ID                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                      Authentication Data                      .
   .                                                               .
   |                         Identification                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Payload Type

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

   Auth Data Len        8-bit unsigned integer.  Length of the
                        Authentication Data field in 8-octet units. [RFC-1700 et seq.].

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

   Security Assoc. ID   32 bits.  When combined with

   Fragment Offset      13-bit unsigned integer.  The offset, in 8-octet
                        units, of the IPv6 Destination
                        Address, identifies data following this header,
                        relative to the receiver(s) the
                        pre-established security association to which
                        this packet belongs.

   Authentication Data  Variable-length field, an integer multiple start of
                        8 octets long.  Algorithm-specific information
                        required authenticate the source Fragmentable Part
                        of the packet
                        and assure its integrity, as specified original packet.

   Res                  2-bit reserved field.  Initialized to zero for the
                        pre-established security association.


Use of the Authentication header
                        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 specified in [IPV6-AUTH].  All IPv6
nodes are required too large to support fit in the keyed MD5 algorithm used with MTU of the
Authentication header
   path to its destination, a source node may divide the packet into
   fragments and send each fragment as described in a separate packet, to be
   reassembled at the receiver.

   For every packet that document.



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4.7  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). fragmented, the source node generates
   an Identification value. The
Destination Options header is identified by a Next Header value Identification must be different than
   that of TBD
in any other fragmented packet sent recently* with the immediately preceding header, same
   Source Address and has the following format:


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

   Next Header          8-bit selector.  Identifies the type of Destination Address.  If a Routing header
                        immediately following is
   present, the Destination Options
                        header.  Uses Address of concern is that of the same values as final
   destination.

      * "recently" means within the IPv4
                        Protocol field [RFC-1700].

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

   Options              Variable-length field, transit time from source to destination and time spent


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        awaiting reassembly with other fragments of length such that the
                        complete Destination Options header is 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
                        integer implementation choice whether to
        maintain a single counter for the node or multiple of 8 octets long.  Contains counters,
        e.g., one or  more TLV-encoded options, as described
                        in section 4.2.


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

Note that there are two node's possible ways to encode optional source addresses, or
        one for each active (source address, destination
information in an IPv6 packet: either address)
        combination.

   The initial, large, unfragmented packet is referred to as an option in the Destination
Options header, or
   "original packet", and it is considered to consist of two parts, as a separate extension header.
   illustrated:

   original packet:

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

      The Fragment 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 Authentication Routing
      header are examples of if present, else the latter approach.
Which approach can be used depends on what action is desired Hop-by-Hop Options header if present,
      else no extension headers.

      The Fragmentable Part consists of a
destination node that does not understand the optional information:

   o  if rest of the desired action is for packet, that is,
      any extension headers that need be processed only by the final
      destination node to discard node(s), plus the
      packet and, only if upper-layer header and data.

   The Fragmentable Part of the packet's Destination Address original packet is not a
      multicast address, send an ICMP Unrecognized Type message to the
      packet's Source Address, then divided into
   fragments, each, except possibly the information may be encoded



draft-ietf-ipngwg-ipv6-spec-01.txt 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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      either as a separate header or as an option in the Destination
      Options header whose Option Type has the value 11 in its highest-
      order two bits.  The choice may depend on such factors as which
      takes fewer octets, or which yields better alignment or more
      efficient parsing.


   fragment packets:

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

   +------------------+--------+--------------+
   |  Unfragmentable  |Fragment|    second    |
   |       Part       | Header |   fragment   |
   +------------------+--------+--------------+
                         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).



4.8 No Next
                         o
                         o
   +------------------+--------+----------+
   |  Unfragmentable  |Fragment|   last   |
   |       Part       | Header | fragment |
   +------------------+--------+----------+

   Each fragment packet is composed of:

      (1) The value 59 in the Next Header field Unfragmentable Part of an IPv6 header or any extension
header indicates that there is nothing following that header.  If the original packet, with the
          Payload Length field of the original IPv6 header indicates changed to contain
          the presence length of octets
past this fragment packet only (excluding the end length
          of a the IPv6 header whose itself), and the Next Header field contains 59, those
octets must be ignored, and passed on unchanged if of the packet is
forwarded.






























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5. Packet Size Issues

IPv6 requires
          last header of the Unfragmentable Part changed to 44.

      (2) A Fragment header containing:

               The Next Header value that every link in identifies the internet have an MTU first header of 576 octets
or greater.  On any link that cannot convey a 576-octet packet in one
piece, link-specific fragmentation and reassembly must be provided at a
layer below IPv6.

     Note: this minimum link MTU is NOT
               the same as Fragmentable Part of the one original packet.

               A Fragment Offset containing the offset of the fragment,
               in IPv4.  In
     IPv4, 8-octet units, relative to the minimum link MTU start of the
               Fragmentable Part of the original packet.  The Fragment
               Offset of the first ("leftmost") fragment is 68 octets [RFC-791, page 25]; 576
     octets 0.

               An M flag value of 0 if the fragment is the minimum reassembly buffer size required in last
               ("rightmost") one, else an IPv4
     node, which has nothing to do with link MTUs.

 From each link to which a node is directly attached, M flag value of 1.

               The Identification value generated for the node original
               packet.

      (3) The fragment itself.

   The lengths of the fragments must be
able to accept packets as large as that link's MTU.  Links chosen such that have a
configurable MTU (for example, PPP links [RFC-1548]) must be configured
to have an the resulting
   fragment packets fit within the MTU of at least 576 octets; it is recommended that a larger
MTU be configured, the path to accommodate possible encapsulations (i.e.,
tunneling) without incurring fragmentation. the packets'
   destination(s).



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   At the destination, fragment packets are expected to implement Path MTU Discovery [RFC-1191], in
order to discover and take advantage of paths with MTU greater than 576
octets.  However, a minimal IPv6 implementation (e.g., in a boot ROM)
may simply restrict itself to sending 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 no larger than 576 octets, that
      have the same Source Address, Destination Address, and omit implementation Fragment
      Identification.

      The Unfragmentable Part of Path MTU Discovery.

In order to send a the reassembled packet larger than a path's MTU, a node may use consists of all
      headers up to, but not including, the
IPv6 Fragment header to fragment of the first
      fragment packet at the source and have it
reassembled at (that is, the destination(s).  However, packet whose Fragment Offset is
      zero), with the use following two changes:

         The Next Header field of such
fragmentation is discouraged in any application that is able to adapt
its packets to fit the measured path MTU (i.e., down to 576 octets).  A
node must not send a packet larger than last header of the path MTU (i.e., fragments
that reassemble to a size larger than Unfragmentable
         Part is obtained from the path MTU) unless it has
explicit knowledge that Next Header field of the destination(s) can reassemble a packet first
         fragment's Fragment header.

         The Payload Length of
that size.

In response to an IPv6 the reassembled packet that is sent to an IPv4 destination (i.e.,
a packet that undergoes translation computed from IPv6 to IPv4),
         the originating
IPv6 node may receive an ICMP Packet Too Big message reporting a Next-
Hop MTU less than 576.  In that case, length of the IPv6 node is not required to
reduce Unfragmentable Part and the size length and offset
         of subsequent packets to less than 576, but must include
a Fragment header in those packets so that the IPv6-to-IPv4 translating
router can obtain last fragment.  For example, a suitable Identification value to use in resulting
IPv4 fragments.  Note that this means the payload may have to be reduced
to 528 octets (576 minus 40 formula for computing the IPv6 header and
         Payload Length of the reassembled original packet is:

           PL.orig = PL.first - FL.first - 8 for + (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
header), and smaller still if additional extension headers are used.

     Note: Path MTU Discovery must be performed even in cases where a
     host "thinks" a destination each of the
      fragment packets.  The length of each fragment is attached to computed by
      subtracting from the same link as itself.



draft-ietf-ipngwg-ipv6-spec-01.txt packet's Payload Length the length of the
      headers between the IPv6 header and fragment itself; its relative
      position in Fragmentable Part is computed from its Fragment Offset
      value.


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     Note: Unlike IPv4, it


      The Fragment header is unnecessary not present in IPv6 the final, reassembled
      packet.

   The following error conditions may arise when reassembling fragmented
   packets:

      If insufficient fragments are received to set complete reassembly of a "Don't
     Fragment" flag in the
      packet header in order to perform Path MTU
     Discovery; that is an implicit attribute within 60 seconds of every IPv6 packet.
     Also, those parts the reception of the RFC-1191 procedures that involve use first-arriving
      fragment of a
     table that packet, reassembly of MTU "plateaus" do not apply to IPv6, because the IPv6
     version of the "Datagram Too Big" message always identifies the
     exact MTU to that packet must be used.












































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6.  Flow Labels

The 24-bit Flow Label field in
      abandoned and all the IPv6 header may be used by a source
to label those packets fragments that have been received for which it requests special handling by that
      packet must be discarded.  If the
IPv6 routers, such as non-default quality of service or "real-time"
service.  This aspect of IPv6 is, at first fragment (i.e., the time one
      with a Fragment Offset of writing, still
experimental and subject zero) has been received, an ICMP Time
      Exceeded -- Fragment Reassembly Time Exceeded message should be
      sent to change as the requirements for flow support
in the Internet become clearer.  Hosts or routers source of that do not support fragment.

      If the functions length of the Flow Label field are required to set the field to
zero when originating a packet, pass fragment, as derived from the field on unchanged when
forwarding fragment packet's
      Payload Length field, is not a packet, multiple of 8 octets and ignore the field when receiving a packet.

A flow is a sequence M flag
      of packets that fragment is 1, then that fragment must be discarded and an
      ICMP Parameter Problem, Code 0, message should be 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 the fragment, pointing to the routers by Payload Length field of
      the fragment packet.

      If the length and offset of a control protocol, fragment are such as a resource reservation protocol, or by information within that the
flow's packets themselves, e.g., in a hop-by-hop option.  The details Payload
      Length of
such control protocols or options are beyond the scope of this document.

There may be multiple active flows packet reassembled 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 fragment would exceed
      65,535 octets, then 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 fragment must be chosen (pseudo-)randomly discarded and uniformly from the range 1 an ICMP
      Parameter Problem, Code 0, message should be sent to FFFFFF hex.  The purpose the source of
      the random allocation is fragment, pointing to make any set
of bits within the Flow Label Fragment Offset field suitable for use as a hash key by
routers, for looking up the state associated with of the flow.

All packets belonging
      fragment packet.

   The following conditions are not expected to the same flow must be sent with the same source
address, same destination address, occur, but are not
   considered errors if they do:

      The number and same non-zero flow label.  If any content 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 headers preceding the Next Header field Fragment
      header of the Hop-by-Hop Options header).  If
any different fragments of those packets includes a Routing header, then they all must be
originated with the same contents in all extension original packet may
      differ.  Whatever headers up to and
including are present, preceding the Routing Fragment
      header (excluding the Next Header field in the
Routing header).  The routers or destinations each fragment packet, are permitted, but not
required, processed when the packets
      arrive, prior to verify that these conditions queueing the fragments for reassembly.  Only
      those headers in the Offset zero fragment packet are satisfied.  If a violation
is detected, it should be reported to retained in
      the source by an ICMP Parameter
Problem message, Code 0, pointing to reassembled packet.

      The Next Header values in the high-order octet Fragment headers of different
      fragments of the Flow
Label field (i.e., offset 1 within same original packet may differ.  Only the IPv6 packet).

Routers are free to "opportunistically" set up flow-handling state value
      from the Offset zero fragment packet is used for
any flow, even when no explicit flow establishment information has been



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provided


4.6  Destination Options Header

   The Destination Options header is used to them via a control protocol, a hop-by-hop option, or other
means.  For example, upon receiving a packet from a particular source
with an unknown, non-zero flow label, carry optional information
   that need be examined only by a router may process its IPv6 packet's destination node(s).  The
   Destination Options header and any necessary extension headers as if the flow label were
zero.  That processing would include determining the next-hop interface,
and possibly other actions, such as updating is identified by a hop-by-hop option,
advancing the pointer and addresses Next Header value of 60
   in a Routing header, or deciding on
how to queue the packet based on its Priority field.  The router may
then choose to "remember" immediately preceding header, and has the results of those processing steps and
cache that information, using the source address plus following format:

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

   Next Header          8-bit selector.  Identifies the flow label as type of header
                        immediately following the cache key.  Subsequent packets with Destination Options
                        header.  Uses the same source address and flow
label may then be handled by referring to the cached information rather
than examining all those fields that, according to values as the requirements IPv4
                        Protocol field [RFC-1700 et seq.].

   Hdr Ext Len          8-bit unsigned integer.  Length of the previous paragraph, can be assumed unchanged from the first packet
seen
                        Destination Options header in 8-octet units,
                        not including the flow.

Cached flow-handling state first 8 octets.

   Options              Variable-length field, of length such that is set up opportunistically, as
discussed in the last paragraph, must be discarded no more than 6
seconds after it
                        complete Destination Options header is established, regardless an
                        integer multiple of whether 8 octets long.  Contains
                        one or not packets of  more TLV-encoded options, as described
                        in section 4.2.


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

   Note that there are two possible ways to arrive.  If another packet with encode optional destination
   information in an IPv6 packet: either as an option in the same
source address Destination
   Options header, or as a separate extension header.  The Fragment
   header and flow label arrives after the cached state has been
discarded, Authentication header are examples of the packet undergoes full, normal processing (as if its flow
label were zero), which may result in the re-creation of cached flow
state for that flow.

The lifetime of flow-handling state that is set up explicitly, for
example by a control protocol or a hop-by-hop option, must latter
   approach.  Which approach can be specified
as part of the specification used depends on what action is
   desired of the explicit set-up mechanism; it may
exceed 6 seconds.

A source must not re-use a flow label for a new flow within the lifetime
of any flow-handling state destination node that might have been established for does not understand the
prior use of that flow label.  Since flow-handling state with a lifetime
of 6 seconds may be established opportunistically for any flow, optional
   information:

      o  if the
minimum interval between desired action is for the last packet of one flow and destination node to discard
         the first packet of a new flow using and, only if the same flow label packet's Destination Address is 6 seconds.  Flow
labels used for explicitly set-up flows with longer flow-state lifetimes
must remain unused for those longer lifetimes before being re-used for
new flows.

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 multicast address, send an
earlier flow whose lifetime may not have expired yet.  This ICMP Unrecognized Type message to
         the packet's Source Address, then the information may be
accomplished by recording flow label usage on stable storage so that it
can be remembered across crashes,
         encoded either as a separate header or by refraining from using any flow
labels until the maximum lifetime of any possible previously established
flows has expired (at least 6 seconds; more if explicit flow set-up
mechanisms with longer lifetimes might have been used).  If as an option in the minimum



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time for rebooting the node is known (often more than 6 seconds), that
time can be deducted from


         Destination Options header whose Option Type has the necessary waiting period before starting
to allocate flow labels.

There is no requirement that all, value 11
         in its highest-order two bits.  The choice may depend on such
         factors as which takes fewer octets, or even most, packets belong to flows,
i.e., carry non-zero flow labels.  This observation which yields better
         alignment or more efficient parsing.

      o  if any other action is placed here to
remind protocol designers and implementors not to assume otherwise.  For
example, it would desired, the information must be unwise to design a router encoded
         as an option in the Destination Options header whose performance would
be adequate only if most packets belonged to flows, Option
         Type has the value 00, 01, or to design a 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 compression scheme or any
   extension header indicates that only worked on packets there is nothing following that belonged to
flows.


7.  Priority

The 4-bit Priority
   header.  If the Payload Length field in of the IPv6 header enables a source to identify indicates the desired delivery priority
   presence of its packets, relative to other packets
from octets past the same source.  The Priority values are divided into two ranges:
Values 0 through 7 are used to specify the priority end of traffic for which a header whose Next Header field
   contains 59, those octets must be ignored, and passed on unchanged if
   the source packet is providing congestion control, i.e., traffic forwarded.































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5. Packet Size Issues

   IPv6 requires that "backs
off" every link in response to congestion, such as TCP traffic.  Values 8 through
15 are used to specify the priority internet have an MTU of traffic 576
   octets or greater.  On any link that does not back off cannot convey a 576-octet packet
   in
response to congestion, e.g., "real-time" packets being sent one piece, link-specific fragmentation and reassembly must be
   provided at a
constant rate.

For congestion-controlled traffic, the following Priority values are
recommended for particular application categories:

      0 - uncharacterized traffic
      1 - "filler" traffic (e.g., netnews)
      2 - unattended data transfer (e.g., email)
      3 - (reserved)
      4 - attended bulk transfer (e.g., FTP, NFS)
      5 - (reserved)
      6 - interactive traffic (e.g., telnet, X)
      7 - internet control traffic (e.g., routing protocols, SNMP)

For non-congestion-controlled traffic, layer below IPv6.

    From each link to which a node is directly attached, the lowest Priority value (8)
should node must
   be used for those able to accept packets as large as that the sender is most willing link's MTU.  Links that
   have a configurable MTU (for example, PPP links [RFC-1661]) must be
   configured to have
discarded under conditions an MTU of congestion (e.g., high-fidelity video
traffic), and the highest value (15) should be used for those packets at least 576 octets; it is recommended
   that the sender a larger MTU be configured, to accommodate possible
   encapsulations (i.e., tunneling) without incurring fragmentation.

   It is least willing strongly recommended that IPv6 nodes implement Path MTU
   Discovery [RFC-1191], in order to have discarded discover and take advantage of
   paths with MTU greater than 576 octets.  However, a minimal IPv6
   implementation (e.g., low-fidelity
audio traffic).  There is in a boot ROM) may simply restrict itself to
   sending packets no relative ordering implied between the
congestion-controlled priorities larger than 576 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 non-congestion-controlled
priorities.






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8. Upper-Layer Protocol Issues

8.1 Upper-Layer Checksums

Any transport or other upper-layer protocol that includes Fragment header to fragment the addresses
from packet at the IP header 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 checksum computation packets to fit the measured path MTU (i.e., down to 576
   octets).

   A node must be modified for use
over IPv6, able to include accept a fragmented packet that, after
   reassembly, is as large as 1500 octets, including the 128-bit IPv6 addresses instead header.  A
   node is permitted to accept fragmented packets that reassemble to
   more than 1500 octets.  However, a node must not send fragments that
   reassemble to a size greater than 1500 octets unless it has explicit
   knowledge that the destination(s) can reassemble a packet of 32-bit IPv4
addresses. that
   size.

   In particular, the following illustration shows the TCP and
UDP "pseudo-header" for IPv6:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                         Source Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      zero     |  Next Header  |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   o  If the response to an IPv6 packet contains a Routing header, the Destination Address
      used in the pseudo-header that is sent to an IPv4 destination
   (i.e., a packet that of the final destination.  At undergoes translation from IPv6 to IPv4), the
   originating system, IPv6 node may receive an ICMP Packet Too Big message
   reporting a Next-Hop MTU less than 576.  In that address will be in the last element
      of the Routing header; at case, the recipient(s), that address will be
      in IPv6 node
   is not required to reduce the Destination Address field size of subsequent packets to less than
   576, but must include a Fragment header in those packets so that the IPv6 header.

   o  The Next Header
   IPv6-to-IPv4 translating router can obtain a suitable Identification
   value to use in resulting IPv4 fragments.  Note that this means the pseudo-header identifies the upper-
      layer protocol (e.g., 6 for TCP, or 17
   payload may have to be reduced to 528 octets (576 minus 40 for UDP).  It will differ
      from the Next Header value in the
   IPv6 header and 8 for the Fragment header), and smaller still if there are
   additional extension headers between the are used.




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      header.

   o  The Payload Length used Specification              December 1995


        Note: Path MTU Discovery must be performed even in the pseudo-header cases where a
        host "thinks" a destination is attached to the length of the
      upper-layer packet, including the upper-layer header.  It will be
      less than the Payload Length in the IPv6 header if there are
      extension headers between the IPv6 header and the upper-layer
      header.



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   o same link as
        itself.

        Note: Unlike IPv4, when UDP packets are originated by an IPv6 node, the
      UDP checksum it is not optional.  That is, whenever originating a UDP
      packet, an unnecessary in IPv6 node must compute to set a UDP checksum over "Don't
        Fragment" flag in the packet
      and header in order to perform Path MTU
        Discovery; that is an implicit attribute of every IPv6 packet.
        Also, those parts of the pseudo-header, and, if RFC-1191 procedures that computation yields involve use of
        a result table of
      zero, it must be changed MTU "plateaus" do not apply to hex FFFF for placement in the UDP
      header.  IPv6 receivers must discard UDP packets containing a zero
      checksum, and should log IPv6, because the error.

The IPv6
        version of ICMP [IPV6-ICMP] 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 "Datagram Too Big" message always identifies the change is
        exact MTU to protect ICMP from misdelivery or corruption
of those fields of the be used.








































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6.  Flow Labels

   The Next Header 24-bit Flow Label field in the pseudo-header IPv6 header may be used by a
   source to label those packets for ICMP contains the value 58, which
identifies it requests special handling
   by the IPv6 version of ICMP.


8.2 Maximum Packet Lifetime

Unlike IPv4, routers, such as non-default quality of service or
   "real-time" service.  This aspect of IPv6 nodes are not required to enforce maximum packet
lifetime.  That is the reason is, at the IPv4 "Time time of writing,
   still experimental and subject to Live" field was renamed
"Hop Limit" change as the requirements for flow
   support in IPv6.  In practice, very few, if any, IPv4
implementations conform to the requirement Internet become clearer.  Hosts or routers that they limit packet
lifetime, so this is do not a change in practice.  Any upper-layer protocol
that relies on
   support the internet layer (whether IPv4 or IPv6) to limit packet
lifetime ought functions of the Flow Label field are required to be upgraded set the
   field to provide its own mechanisms for
detecting zero when originating a packet, pass the field on unchanged
   when forwarding a packet, and discarding obsolete packets.


8.3 Maximum Upper-Layer Payload Size

When computing ignore the maximum payload size available field when receiving a
   packet.

   A flow is a sequence of packets sent from a particular source to a
   particular (unicast or multicast) destination for upper-layer data,
an upper-layer protocol must take into account which the larger size of source
   desires special handling by the
IPv6 header relative intervening routers.  The nature of
   that special handling might be conveyed to the IPv4 header.  For example, in IPv4, TCP's
MSS option is computed routers by a control
   protocol, such as the maximum packet size (a default value or a
value learned through Path MTU Discovery) minus 40 octets (20 octets for resource reservation protocol, or by information
   within the minimum-length IPv4 header and 20 octets for flow's packets themselves, e.g., in a hop-by-hop option.
   The details of such control protocols or options are beyond the minimum-length TCP
header).  When using TCP over IPv6, 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 MSS 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 computed as chosen (pseudo-)randomly and uniformly from the
maximum packet size minus 60 octets, because
   range 1 to FFFFFF hex.  The purpose of the minimum-length IPv6
header (i.e., an IPv6 header with no extension headers) random allocation is 20 octets
longer than to
   make any set of bits within the Flow Label field suitable for use as
   a minimum-length IPv4 header.









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Appendix A. Formatting Guidelines hash key by routers, for Options

This appendix gives some advice on how to lay out looking up the fields in options state associated with the
   flow.

   All packets belonging to the same flow must be used in sent with the same
   source address, destination address, priority, 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 or
   contents (excluding the Destination Next Header field of the Hop-by-Hop Options
   header).  If any of those packets includes a Routing 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 then
   they all must 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 originated with the Hop-by-Hop or Destination
      Options header take same contents in all extension
   headers up as little space as possible, subject to and including the
      requirement that the Routing header be an integer multiple of 8 octets
      long.

   o  It may be assumed that, when either of (excluding the option-bearing headers Next
   Header field in the Routing header).  The routers or destinations are present, they carry
   permitted, but not required, to verify that these conditions are
   satisfied.  If a very small number of options, usually
      only one.

These assumptions suggest the following approach violation is detected, it should be reported to laying out the
fields of
   source by an option: order the fields from smallest ICMP Parameter Problem message, Code 0, pointing to largest, with no
interior padding, then derive the alignment requirement for the entire
option based on the alignment requirement
   high-order octet of the largest Flow Label field (up (i.e., offset 1 within the
   IPv6 packet).


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   Routers are free to "opportunistically" set up flow-handling state
   for any flow, even when no explicit flow establishment information
   has been provided to them via a
maximum alignment of 8 octets).  This approach is illustrated in the
following examples:


Example 1

If control protocol, a hop-by-hop
   option, or other means.  For example, upon receiving a packet from a
   particular source with an option X required two data fields, one of length 8 octets unknown, non-zero flow label, a router may
   process its IPv6 header and one
of length 4 octets, it any necessary extension headers as if the
   flow label were zero.  That processing would be laid out include determining the
   next-hop interface, and possibly other actions, such as follows:


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


Its alignment requirement is 8n+2, updating a
   hop-by-hop option, advancing the pointer and addresses in a Routing
   header, or deciding on how to ensure that queue the 8-octet field ends



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up packet based on a multiple-of-8 offset from its Priority
   field.  The router may then choose to "remember" the start results of those
   processing steps and cache that information, using the enclosing header.  A
complete Hop-by-Hop or Destination Options header containing this one
option would look source address
   plus the flow label 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, the cache key.  Subsequent packets with the
   same source address and one of length 1 octet, it would be laid out flow label may then be handled by referring
   to the cached information rather than examining all those fields
   that, according to the requirements of the previous paragraph, can be
   assumed unchanged from the first packet seen in the flow.

   Cached flow-handling state that is set up opportunistically, as
follows:


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


Its alignment requirement
   discussed in the preceding paragraph, must be discarded no more than
   6 seconds after it is 4n+3, established, regardless of whether or not
   packets of the same flow continue to ensure that arrive.  If another packet with
   the 4-octet field ends same source address and flow label arrives after the cached state
   has been discarded, the packet undergoes full, normal processing (as
   if its flow label were zero), which may result in the re-creation of
   cached flow state for that flow.

   The lifetime of flow-handling state that is set up on explicitly, for
   example by a multiple-of-4 offset from control protocol or a hop-by-hop option, must be
   specified as part of the start specification of the enclosing header. explicit set-up
   mechanism; it may exceed 6 seconds.

   A
complete Hop-by-Hop or Destination Options header containing this source must not re-use a flow label for a new flow within the
   lifetime of any flow-handling state that might have been established
   for the prior use of that flow label.  Since flow-handling state with
   a lifetime of 6 seconds may be established opportunistically for any
   flow, the minimum interval between the last packet of one
option would look flow and
   the first packet of a new flow using the same flow label is 6
   seconds.  Flow labels used for explicitly set-up flows with longer
   flow-state lifetimes must remain unused for those longer lifetimes
   before being re-used for new flows.

   When a node stops and restarts (e.g., 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       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



draft-ietf-ipngwg-ipv6-spec-01.txt 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 (at least 6 seconds; more if explicit


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Example 3

A Hop-by-Hop


   flow set-up mechanisms with longer lifetimes might have been used).
   If the minimum time for rebooting the node is known (often more than
   6 seconds), 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.


7.  Priority

   The 4-bit Priority field in the IPv6 header enables a source to
   identify the desired delivery priority of its packets, relative to
   other packets from the same source.  The Priority values are divided
   into two ranges:  Values 0 through 7 are used to specify the priority
   of traffic for which the source is providing congestion control,
   i.e., traffic that "backs off" in response to congestion, such as TCP
   traffic.  Values 8 through 15 are used to specify the priority of
   traffic that does not back off in response to congestion, e.g.,
   "real-time" packets being sent at a constant rate.

   For congestion-controlled traffic, the following Priority values are
   recommended for particular application categories:

         0 - uncharacterized traffic
         1 - "filler" traffic (e.g., netnews)
         2 - unattended data transfer (e.g., email)
         3 - (reserved)
         4 - attended bulk transfer (e.g., FTP, NFS)
         5 - (reserved)
         6 - interactive traffic (e.g., telnet, X)
         7 - internet control traffic (e.g., routing protocols, SNMP)

   For non-congestion-controlled traffic, the lowest Priority value (8)
   should be used for those packets that the sender is most willing to
   have discarded under conditions of congestion (e.g., high-fidelity
   video traffic), and the highest value (15) should be used for those
   packets that the sender is least willing to have discarded (e.g.,
   low-fidelity audio traffic).  There is no relative ordering implied
   between the congestion-controlled priorities and the non-congestion-
   controlled priorities.





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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                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Payload Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      zero                     |  Next Header  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      o  If the 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 Payload Length used in the pseudo-header is the length of
         the upper-layer packet, including the upper-layer header.  It
         will be less than the Payload Length in the IPv6 header (or in


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         the Jumbo Payload option) if there are extension headers
         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 [RFC-1885] 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 Destination Options a value learned through Path MTU Discovery) minus 40
   octets (20 octets for the minimum-length IPv4 header containing both options X and
Y from Examples 1 and 2 would have one of 20 octets
   for the two following formats,
depending on which option appeared first:


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








draft-ietf-ipngwg-ipv6-spec-01.txt minimum-length TCP header).  When using TCP over IPv6, the
   MSS must be computed as the maximum packet size minus 60 octets,
   because the minimum-length IPv6 header (i.e., an IPv6 header with no
   extension headers) is 20 octets longer than a minimum-length IPv4
   header.




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Appendix B.  Changes from Previous Draft

Changes from draft-hinden-ipng-ipv6-spec-00.txt, October 1994:

   o  Changed "cluster address" to "region address".

   o  Added definitions of "upper layer" and "packet" to Terminology
      section.

   o  Changed all references of "transport layer" to "upper layer".

   o  Changed name of "TClass" field to "Priority", changed name of
      "Flow ID" field A. Formatting Guidelines for Options

   This appendix gives some advice on how to "Flow Label", and dropped the use of lay out the name
      "Flow Label" to refer fields when
   designing new options to be used in the combination of those two fields.

   o  Added note that Hop-by-Hop Options must be processed by source and
      destination nodes, as well header or
   the Destination Options header, as intermediate nodes along a delivery
      path. described in section 4.2.  These
   guidelines are based on the following assumptions:

      o  Specified  One desirable feature is that unknown Next Header values, as well as a Next Value
      of zero in any header other than multi-octet fields within the
         Option Data area of an IPv6 header, option be aligned on their natural
         boundaries, i.e., fields of width n octets should invoke be placed at
         an
      ICMP Parameter Problem message.

   o  Changed name integer multiple of "End-to-End Options" to "Destination Options", and
      specified 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 occur twice in a
      packet, once before be assumed that, when either of the option-bearing
         headers are present, they carry a Routing Header and once before very small number of options,
         usually only one.

   These assumptions suggest the upper-
      layer header.

   o  Changed text regarding advisability 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 violating recommended
      ordering for extension headers ("be conservative in what you send;
      be liberal 8 octets).  This approach is
   illustrated in what you receive").

   o  Specified that the following examples:


   Example 1

   If an unrecognized option triggers an ICMP Parameter
      Problem, Code 2, message, not an "ICMP Unrecognized Type" message.

   o  The third-highest-order bit 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 codes, which indicates
      whether or not an option's data can change en-route, now applies Type=X |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         8-octet field                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Its alignment requirement is 8n+2, to Destination Options as well as Hop-by-Hop Options, because
      Destination Options can now precede ensure that the 8-octet field
   starts at a Routing header and thus may
      be modified en-route.

   o  Added multiple-of-8 offset from the Jumbo Payload hop-by-hop option.

   o  Deleted prohibition of en-route insertion start of Routing headers
      (though I still think it's a bad idea).

   o  Added Strict/Loose Bit Map to the Type 0 Routing header.




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   o  Deleted IPv6-in-IPv6 Encapsulation section -- moved to a separate
      document.

   o  Added the "no next header"


   header.  A complete Hop-by-Hop or Destination Options header
   containing this one option would look as follows:


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header type.

   o  Added a recommendation that links with configurable MTU, such as
      PPP links, be configured with  | Hdr Ext Len=1 | Option Type=X |Opt Data Len=12|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         4-octet field                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         8-octet field                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   Example 2

   If an MTU larger than the minimum (576)
      so as to accommodate encapsulations (tunneling) without incurring
      fragmentation.

   o  Split the Flow Label and Priority discussion into two sections.

   o  Changed the description option Y required three data fields, one of length 4 octets,
   one of the fields that must not change within
      a flow to include all headers up to length 2 octets, and including the Routing
      header.

   o  Added discussion one of "opportunistic" flow state set-up, and added
      requirement that such state must length 1 octet, it would be discarded within 6 seconds of
      being established.  Also discussed source behavior 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 avoid
      reusing an active flow label after a reboot.

   o  Added a warning about assuming ensure that most packets will belong to
      flows.

   o  In making the distinction between the two sub-ranges of Priority
      values, changed the terminology 4-octet field
   starts at a multiple-of-4 offset from "flow-controlled" to
      "congestion-controlled".

   o  Deleted statement about flow set-up mechanisms possibly redefining the semantics start of the Priority (formerly TClass) field.

   o  Rearranged some text in the Upper-Layer (formerly Transport-Layer)
      Checksums section, added requirement that 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 1883                   IPv6 hosts discard UDP
      packets with zero checksum, Specification              December 1995


   Example 3

   A Hop-by-Hop or Destination Options header containing both options X
   and changed the ICMP pseudo-header to
      be the same as Y from Examples 1 and 2 would have one of the TCP/UDP pseudo-header.

   o  Added a small section about upper-layer maximum payload size.

   o  Updated references to newer documents.

   o  Put Deering's name back two following
   formats, depending on as an editor.










draft-ietf-ipngwg-ipv6-spec-01.txt which option appeared first:


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


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








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Security Considerations

   This document specifies that the format of an IP Authentication header, which is
part of the machinery intended to provide end-to-end authentication Header [RFC-1826]
   and
integrity assurance for IPv6 packets.  Non-repudiation may be provided
by an authentication algorithm used with the Authentication option, but
it is not provided with all authentication algorithms that might IP Encapsulating Security Payload [RFC-1827] be used with this option.  Usage of the option is specified
   IPv6, in [IPV6-AUTH]. conformance with the Security Architecture for the Internet
   Protocol [RFC-1825].

Acknowledgments

   The document editors 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.


Document Editors'

Authors' Addresses

   Stephen E. Deering                   Robert M. Hinden
   Xerox Palo Alto Research Center      Ipsilon Networks, Inc.
   3333 Coyote Hill Road                   2465 Latham Street,                2191 E. Bayshore Road, Suite 100
   Palo Alto, CA 94304                     Mt. View,                  Palo Alto, CA 94040 94303
   USA                                  USA

   phone:

   Phone: +1 415 812 4839                  phone:               Phone: +1 415 528 846 4604
   fax:
   Fax:   +1 415 812 4471                  fax:               Fax:   +1 415 528 4653
   email: 855 1414
   EMail: deering@parc.xerox.com           email:        EMail: hinden@ipsilon.com























draft-ietf-ipngwg-ipv6-spec-01.txt



























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References

[IPV6-AUTH] R.

   [RFC-1825]   Atkinson, IPv6 R., "Security Architecture for the Internet
                Protocol", RFC 1825, Naval Research Laboratory, August
                1995.

   [RFC-1826]   Atkinson, R., "IP Authentication Header, March Header", RFC 1826,
                Naval Research Laboratory, August 1995.

[IPV6-ICMP] A. Conta

   [RFC-1827]   Atkinson, R., "IP Encapsulating Security Protocol
                (ESP)", RFC 1827, Naval Research Laboratory, August
                1995.

   [RFC-1885]   Conta, A., and S. Deering, ICMP "Internet Control Message
                Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6), October 1994.

[IPV6-TRAN] R. Gilligan and E. Nordmark, Transition Mechanisms for IPv6
            Hosts and Routers, March
                (IPv6) Specification", RFC 1885, Digital Equipment
                Corporation, Xerox PARC, December 1995.

[IPV6-ADDR] R.

   [RFC-1884]   Hinden, Editor, IP R., and S. Deering, Editors, "IP Version 6
                Addressing Architecture,
            March Architecture", RFC 1884, Ipsilon Networks,
                Xerox PARC, December 1995.

   [RFC-1191]  J. Mogul   Mogul, J., and S. Deering, Path "Path MTU Discovery, RFC-1191, Discovery", RFC
                1191, DECWRL, Stanford University, November 1990.

   [RFC-791]   J.    Postel, Internet Protocol, RFC-791, J., "Internet Protocol", STD 5, RFC 791,
                USC/Information Sciences Institute, September 1981.

   [RFC-1700]  J. Reynolds   Reynolds, J., and J. Postel, Assigned Numbers, RFC-1700, "Assigned Numbers", STD 2,
                RFC 1700, USC/Information Sciences Institute, October
                1994.

[RFC-1548]  W.

   [RFC-1661]   Simpson, The W., Editor, "The Point-to-Point Protocol (PPP), RFC-1548,
            April
                (PPP)", STD 51, RFC 1661, Daydreamer, July 1994.




























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