In message <e1829b60731d1740bb7a0626b4faf0a65d8f4c8...@xch-nw-01v.nw.nos.boeing
.com>, "Templin, Fred L" writes:
> 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?
You live with it. Nobody has said that IPv6 to IPv4 translation
will work in all circumstances.
> Thanks - Fred
> [email protected]
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--
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: [email protected]
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