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<TITLE>Re: [users:00488] RFC 1981 undersized PMTU test</TITLE>
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On Fri, 25 Jan 2008, Yinghui Yao wrote:<BR>
> Self_Test_1.4.9 PMTU test #9 seems to test RFC 1981 section 4 =
statement<BR>
><BR>
> "A node MUST NOT reduce its estimate of the Path MTU below the =
IPv6<BR>
> minimum link MTU.<BR>
><BR>
> Note: A node may receive a Packet Too Big =
message reporting a<BR>
> next-hop MTU that is less than the IPv6 =
minimum link MTU. In that<BR>
> case, the node is not required to reduce =
the size of subsequent<BR>
> packets sent on the path to less than the =
IPv6 minimun link MTU,<BR>
> but rather must include a Fragment header =
in those packets [IPv6-<BR>
> SPEC]. "<BR>
><BR>
> Could anyone please shed some light on why the RFC requires this. =
Why should<BR>
> a node pay attention to an malformed ICMP packet, let alone adding =
a frag<BR>
> header to a unfrag packet? Why? Appreciate it.<BR>
<BR>
RFC 2460 answers your question:<BR>
<BR>
In response to an IPv6 packet that is sent to an IPv4 =
destination<BR>
(i.e., a packet that undergoes translation from IPv6 =
to IPv4), the<BR>
originating IPv6 node may receive an ICMP Packet Too =
Big message<BR>
reporting a Next-Hop MTU less than 1280. In =
that case, the IPv6 node<BR>
is not required to reduce the size of subsequent =
packets to less than<BR>
1280, but must include a Fragment header in those =
packets so that the<BR>
IPv6-to-IPv4 translating router can obtain a suitable =
Identification<BR>
value to use in resulting IPv4 fragments. Note =
that this means the<BR>
payload may have to be reduced to 1232 octets (1280 =
minus 40 for the<BR>
IPv6 header and 8 for the Fragment header), and =
smaller still if<BR>
additional extension headers are used.<BR>
<BR>
- ---<BR>
Mark Atkinson<BR>
m.atkinson@f5.com<BR>
(!wired)?(coffee++):(wired);<BR>
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