Re: PMTUD and MTU < 1280

[Mark Andrews <[e-mail]>]

In message <[e-mail]>, Philip Homburg writes:
> In your letter dated Wed, 20 Jul 2011 17:35:31 -0400 you wrote:
> >I am not sure the specs insist that an IPv6 implementation 
> >must treat an ICMPv6 Packet-Too-Big for less than 1280 bytes 
> >as "unrecoverable". (I haven't re-read the IPv6 specs recently.)
> 
> Some services, like big DNS server cannot afford to do PMTU. It requires them
> to store the original DNS reply just in case an ICMP comes back.

This was originally noted in draft-ietf-ipngwg-bsd-frag-00 (January
1998) and became IPV6_USE_MIN_MTU (RFC 3542, May 2003).  Now if we
can just get it into POSIX ....

And, yes, nameservers do set this option.  FreeBSD's implementation
however is partially broken.  See:
http://www.freebsd.org/cgi/query-pr.cgi?pr=158307.

> Without that, the host will see that its first request fails and has to retry
> .
> Over time, this will happen again and again because big servers cannot
> maintain all PMTU state forever.

It takes multiple requests usually and yes the state does get lost
within minutes on busy nameservers.

What should have been specified is that hosts need to fragment
packets into roughly equal sizes fragments.  This reduces the
probabilty of having to deal with multiple PTB packets.

Mark
-- 
Mark Andrews, ISC
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