RFC2460 violation of RFC1122

["Templin, Fred L" <[e-mail]>]

Section 5 of RFC2460 states:

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

RFC2460 therefore requires the IPv4 destination to be able to
reassemble at least 1280 bytes minus 28 (since the translation
from an IPv6 header plus fragment header to an IPv4 header
incurs a 28 byte size reduction). However, section 3.3.2 of
RFC1122 states:

         "We designate the largest datagram size that can be reassembled
         by EMTU_R ("Effective MTU to receive"); this is sometimes
         called the "reassembly buffer size".  EMTU_R MUST be greater
         than or equal to 576, SHOULD be either configurable or
         indefinite, and SHOULD be greater than or equal to the MTU of
         the connected network(s)."

By assuming an EMTU_R of greater than 576 bytes, RFC2460
is therefore in violation of RFC1122, which could lead to
communication failures. How do we reconcile this?

Thanks - Fred
[e-mail]