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Versions: 01 RFC 1883

INTERNET DRAFT                                    S. Deering, Xerox PARC
March 17, 1995                                        R. Hinden, Ipsilon
Obsoletes: draft-hinden-ipng-ipv6-spec-00.txt                    Editors


                  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 an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, 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
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as ``work in progress.''

To learn the current status 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).

Distribution of this memo is unlimited.














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Contents

   Status of this Memo..............................................1

   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
       4.6 Authentication Header...................................18
       4.7 Destination Options Header..............................19
       4.8 No Next Header..........................................20

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

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

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

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

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

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

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

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

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

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









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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 address" is defined, to identify topological
      regions rather than individual 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
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 of IPv6 are specified in
separate documents, including the following:





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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.

   host        - any node that is not a router.

   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.





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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].

   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].



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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] and section 4.4.


4.  IPv6 Extension Headers

In IPv6, optional internet-layer information is encoded in separate
headers that may be placed between the IPv6 header and the upper-layer
header in a packet.  There 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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header determine whether or not to proceed to the next header.

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

If, while 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 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.




























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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)
        Routing header
        Fragment header
        Authentication header
        Destination Options header (2)
        upper-layer header

         (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) 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.













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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 (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 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 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 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 change en-route must be excluded from the
integrity assurance computation performed when the Authentication header
is present.

   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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boundaries.  The alignment requirement of an option is specified using
the notation xn+y, meaning the Option Type must appear at an integer
multiple of x octets from the start of the header, plus y octets.  For
example:

    2n    means any 2-octet offset from the start of the header.
    8n+2  means any 8-octet offset from the start of the header,
          plus 2 octets.

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


   Pad1 option  (alignment requirement: none)

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

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

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


   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.








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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].

   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 the length of a
       Hop-by-Hop Options header containing only this option, with no
       additional headers or data; however, use of this option for
       packets with payloads less than 65,535 octets is not recommended.

       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 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 is encountered
       in a packet that contains a 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.
































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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  |  Routing Type |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   .                                                               .
   .                       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].

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

   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 IPv6 node that is processing a Routing header does not recognize
the Routing Type value, it must discard the packet and, only if the
packet's Destination Address is not a multicast address, send an ICMP
Parameter Problem, Code 0, message to the packet's Source Address,
pointing to the unrecognized Routing Type.

The Type 0 Routing header has the following format:










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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |Routing Type=0 |   Num Addrs   |   Next Addr   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Reserved    |             Strict/Loose Bit Mask             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                           Address[0]                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                           Address[1]                          +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                               .                               .
   .                               .                               .
   .                               .                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Address[Num Addrs - 1]                    +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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            8-bit unsigned integer.  Number of addresses in
                        the Routing header.  Maximum legal value = 24.

   Next Addr            8-bit unsigned integer.  Index of next address
                        to be processed; initialized to 0 by the
                        originating node.




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

   Strict/Loose Bit Mask
                        24-bit bit-mask, numbered 0 to 23, left-to-right.
                        If bit n is 1, then the packet may be forwarded
                        to Address[n] by the node that places Address[n]
                        in the IPv6 Destination Field only if the
                        interface identified by Address[n] is a neighbor
                        of the forwarding node.  If bit n is 0, then
                        Address[n] need not be a neighbor of the
                        forwarding node.


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 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] = 0 or if the new
      destination address is known to be a neighbor of this node,
      increment Next Addr by one and re-submit the packet to the IPv6
      module for forwarding to the new destination, else send an ICMP
      Destination Unreachable, Not a Neighbor message to the Source
      Address and discard the packet.

   o  If Next Addr = Num Addrs, dispatch to the next header processing
      module, as identified by the Next Header field in the Routing
      header.

   o  If Next Addr > Num Addrs, send an ICMP Parameter Problem, Code 0,
      message to the Source Address, pointing to the Num Addrs field,
      and discard the packet.

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












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4.5  Fragment Header

The Fragment header is used by an IPv6 source to send payloads larger
than would fit in the path MTU 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, 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 the Fragment header.
                        Uses the same values as the IPv4 Protocol field
                        [RFC-1700].

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

   Fragment Offset      13-bit unsigned integer.  The offset, in 8-octet
                        units, of the following payload, relative to the
                        start 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 the destination
node(s)) is divided into fragments, each, except possibly the last,
being an integer multiple of 8 octets long.  Each fragment is prepended
with a Fragment header and sent in a separate IPv6 packet.  The M
("more") flag is set to 1 on all fragments of the same payload except
the last.  The original payload is assigned an Identification value that
is different than that of any other fragmented payload sent recently*
with the same IPv6 Source Address, IPv6 Destination Address, and
Fragment Next Header value.  (If a Routing header is present, the IPv6
Destination Address is that of the final destination.)  The
Identification value is carried in the Fragment header of all of the



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original payload's fragments, and is used by 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 from source to destination and time spent
     awaiting reassembly with other fragments of the same payload.
     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 payload must be
     fragmented.  It is an implementation choice whether to maintain a
     single counter for the node or multiple counters, e.g., one for
     each of the node's possible source addresses, or one for each
     active (source address, destination address, 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 the original, unfragmented payload.
































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4.6  Authentication Header

The Authentication header is used to provide authentication and
integrity assurance for IPv6 packets.  Non-repudiation may be provided
by an authentication algorithm used with the Authentication header, but
it is not provided with all authentication algorithms that might be used
with this header.  The Authentication header is identified by a Next
Header value of 51 in the immediately preceding header, and has the
following format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Payload Type | Auth Data Len |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Security Association ID                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                      Authentication Data                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Payload Type         8-bit selector.  Identifies the type of header
                        immediately following the Authentication header.
                        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.

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

   Security Assoc. ID   32 bits.  When combined with the IPv6 Destination
                        Address, identifies to the receiver(s) the
                        pre-established security association to which
                        this packet belongs.

   Authentication Data  Variable-length field, an integer multiple of
                        8 octets long.  Algorithm-specific information
                        required authenticate the source of the packet
                        and assure its integrity, as specified for the
                        pre-established security association.


Use of the Authentication header is specified in [IPV6-AUTH].  All IPv6
nodes are required to support the keyed MD5 algorithm used with the
Authentication header as described in 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).  The
Destination Options header is identified by a Next Header value of TBD
in the immediately preceding header, and has the following format:


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

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

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

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


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

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

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



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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.

   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 Header

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






























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5. Packet Size Issues

IPv6 requires that every link in the internet have an MTU 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 the one in IPv4.  In
     IPv4, the minimum link MTU is 68 octets [RFC-791, page 25]; 576
     octets is the minimum reassembly buffer size required in an IPv4
     node, which has nothing to do with link MTUs.

 From each link to which a node is directly attached, the node must be
able to accept packets as large as that link's MTU.  Links that have a
configurable MTU (for example, PPP links [RFC-1548]) must be configured
to have an MTU of at least 576 octets; it is recommended that a larger
MTU be configured, to accommodate possible encapsulations (i.e.,
tunneling) without incurring fragmentation.

IPv6 nodes 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 packets no 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
IPv6 Fragment header to fragment the packet at the source and have it
reassembled at the destination(s).  However, the use of such
fragmentation is discouraged in any application that is able to adapt
its packets to fit the measured path MTU (i.e., down to 576 octets).  A
node must not send a packet larger than the path MTU (i.e., fragments
that reassemble to a size larger than the path MTU) unless it has
explicit knowledge that the destination(s) can reassemble a packet of
that size.

In response to an IPv6 packet that is sent to an IPv4 destination (i.e.,
a packet that undergoes translation from IPv6 to IPv4), the originating
IPv6 node may receive an ICMP Packet Too Big message reporting a Next-
Hop MTU less than 576.  In that case, the IPv6 node is not required to
reduce the size 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 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 for the IPv6 header and 8 for 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 is attached to the same link as itself.



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     Note: Unlike IPv4, it is unnecessary in IPv6 to set a "Don't
     Fragment" flag in the packet header in order to perform Path MTU
     Discovery; that is an implicit attribute of every IPv6 packet.
     Also, those parts of the RFC-1191 procedures that involve use of a
     table of MTU "plateaus" do not apply to IPv6, because the IPv6
     version of the "Datagram Too Big" message always identifies the
     exact MTU to be used.












































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6.  Flow Labels

The 24-bit Flow Label field in the IPv6 header may be used by a source
to label those packets for which it requests special handling by the
IPv6 routers, such as non-default quality of service or "real-time"
service.  This aspect of IPv6 is, at the time of writing, still
experimental and subject to change as the requirements for flow support
in the Internet become clearer.  Hosts or routers that do not support
the functions of the Flow Label field are required to set the field to
zero when originating a packet, pass the field on unchanged when
forwarding a packet, and ignore the field when receiving a packet.

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

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

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

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

Routers are free to "opportunistically" set up flow-handling state for
any flow, even when no explicit flow establishment information has been



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provided 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, a router may process its IPv6
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 a hop-by-hop option,
advancing the pointer and addresses in a Routing header, or deciding on
how to queue the packet based on its Priority field.  The router may
then choose to "remember" the results of those processing steps and
cache that information, using the source address plus the flow label as
the cache key.  Subsequent packets with 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 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
discussed in the last paragraph, must be discarded no more than 6
seconds after it is established, regardless of whether or not packets of
the same flow continue to arrive.  If another packet with the 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 explicitly, for
example by a control protocol or a hop-by-hop option, must be specified
as part of the specification 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 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 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 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 flow set-up
mechanisms with longer lifetimes might have been used).  If the minimum



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


   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 system, 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 if there are
      extension headers between the IPv6 header and the upper-layer
      header.



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   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 [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 the change is to protect ICMP from misdelivery or corruption
of those fields of the IPv6 header on which it depends, which, unlike
IPv4, are not covered by an internet-layer checksum.  The Next Header
field in the pseudo-header for ICMP contains the value 58, which
identifies the IPv6 version of ICMP.


8.2 Maximum Packet Lifetime

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


8.3 Maximum Upper-Layer Payload Size

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

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

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

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

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

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


Example 1

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


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


Its alignment requirement is 8n+2, to ensure that the 8-octet field ends



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up on a multiple-of-8 offset from the start of the enclosing header.  A
complete Hop-by-Hop or Destination Options header containing this one
option would look as follows:


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



Example 2

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


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


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


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



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Example 3

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


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  | Hdr Ext Len=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                         +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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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 to "Flow Label", and dropped the use of the name
      "Flow Label" to refer to the combination of those two fields.

   o  Added note that Hop-by-Hop Options must be processed by source and
      destination nodes, as well as intermediate nodes along a delivery
      path.

   o  Specified that unknown Next Header values, as well as a Next Value
      of zero in any header other than an IPv6 header, should invoke an
      ICMP Parameter Problem message.

   o  Changed name of "End-to-End Options" to "Destination Options", and
      specified that the Destination Options header may occur twice in a
      packet, once before a Routing Header and once before the upper-
      layer header.

   o  Changed text regarding advisability of violating recommended
      ordering for extension headers ("be conservative in what you send;
      be liberal in what you receive").

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

   o  The third-highest-order bit of Option Type codes, which indicates
      whether or not an option's data can change en-route, now applies
      to Destination Options as well as Hop-by-Hop Options, because
      Destination Options can now precede a Routing header and thus may
      be modified en-route.

   o  Added the Jumbo Payload hop-by-hop option.

   o  Deleted prohibition of en-route insertion 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" Next Header type.

   o  Added a recommendation that links with configurable MTU, such as
      PPP links, be configured with 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 of the fields that must not change within
      a flow to include all headers up to and including the Routing
      header.

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

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

   o  In making the distinction between the two sub-ranges of Priority
      values, changed the terminology from "flow-controlled" to
      "congestion-controlled".

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

   o  Rearranged some text in the Upper-Layer (formerly Transport-Layer)
      Checksums section, added requirement that IPv6 hosts discard UDP
      packets with zero checksum, and changed the ICMP pseudo-header to
      be the same as 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 on as an editor.










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Security Considerations

This document specifies the format of an Authentication header, which is
part of the machinery intended to provide end-to-end authentication 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 be used
with this option.  Usage of the option is specified in [IPV6-AUTH].


Acknowledgments

The document editors 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' Addresses

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

   phone: +1 415 812 4839                  phone: +1 415 528 4604
   fax:   +1 415 812 4471                  fax:   +1 415 528 4653
   email: deering@parc.xerox.com           email: hinden@ipsilon.com























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References

[IPV6-AUTH] R. Atkinson, IPv6 Authentication Header, March 1995.

[IPV6-ICMP] A. Conta and S. Deering, ICMP for the Internet Protocol
            Version 6 (IPv6), October 1994.

[IPV6-TRAN] R. Gilligan and E. Nordmark, Transition Mechanisms for IPv6
            Hosts and Routers, March 1995.

[IPV6-ADDR] R. Hinden, Editor, IP Version 6 Addressing Architecture,
            March 1995.

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

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

[RFC-1700]  J. Reynolds and J. Postel, Assigned Numbers, RFC-1700,
            October 1994.

[RFC-1548]  W. Simpson, The Point-to-Point Protocol (PPP), RFC-1548,
            April 1994.




























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