RFC 1883                   IPv6 Specification              December 1995


4.5  Fragment Header

   The Fragment header is used by an IPv6 source to send packets 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 initial header
                        type of the Fragmentable Part of the original
                        packet (defined below).  Uses the same values
                        as the IPv4 Protocol field [RFC-1700 et seq.].

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

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

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

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

   Identification       32 bits.  See description below.

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

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

      * "recently" means within the maximum likely lifetime of a packet,
        including transit time from source to destination and time spent



 
RFC 1883                   IPv6 Specification              December 1995


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

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

   original packet:

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

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

      The Fragmentable Part consists of the rest of the packet, that is,
      any extension headers that need be processed only by the final