RFC 1883                   IPv6 Specification              December 1995


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.

    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-1661]) 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.

   It is strongly recommended that IPv6 nodes 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
   adjust its packets to fit the measured path MTU (i.e., down to 576
   octets).

   A node must be able to accept a fragmented packet that, after
   reassembly, is as large as 1500 octets, including the IPv6 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 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.





 
RFC 1883                   IPv6 Specification              December 1995


        Note: Path MTU Discovery must be performed even in cases where a
        host "thinks" a destination is attached to the same link as
        itself.

        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.