Subject: pmtu across translators

From: [email-masked]
Date: Wed, 28 Mar 2001 21:34:13 +0300

hello,
ok i made a mistake in my previous mail. Actually, pmtu discovery fails if:

1. there is a link in the ipv6 internet which has mtu < 1280 (which
shouldn't be possible anyway).
2. if there is a link in the ipv4 side across the translator which has mtu <
1280.

So, now my re-framed question is :

Do I need to add the frag header to every packet when the pmtu fails??

PS: ignore all the other stuff in previous mail:)

regards,
imran
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From: Erik Nordmark <[email-masked]>
Date: Thu, 29 Mar 2001 00:57:17 -0800 (PST)

> Do I need to add the frag header to every packet when the pmtu fails??

RFC 2460 section 5 last paragraph says:
   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.

Does that answer your question?

  Erik