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Re: Use cases for PMTUD and PLPMTUD (was: RE: 6MAN: Adoption call on draft-hinden-6man-rfc1981bis-01)

Michael Richardson <mcr+ietf@sandelman.ca> Sun, 07 February 2016 16:24 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: Mark Andrews <marka@isc.org>
Subject: Re: Use cases for PMTUD and PLPMTUD (was: RE: 6MAN: Adoption call on draft-hinden-6man-rfc1981bis-01)
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Date: Sun, 07 Feb 2016 11:19:24 -0500
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Cc: Fernando Gont <fgont@si6networks.com>, 6man WG <ipv6@ietf.org>, "Fred Baker (fred)" <fred@cisco.com>, Bob Hinden <bob.hinden@gmail.com>
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Mark Andrews <marka@isc.org> wrote:
    >> Except for within well-managed administrative domains where
    >> RFC1981-only > is sufficient. They do exist; I am typing this message
    >> from within one righ t now.
    >>
    >> I'd expect 6man to produce protocols that work everywhere, rather than
    >> just on some subset scearios...

    > RFC 1981 works for TCP unless you *deliberately* break it by dropping
    > PTB as TCP retries.

Yes, and we know that many networks do this.

{It appears that the Linux IPv6 XFRM/NetKey IPsec stack also fails to send
 PTBs when operating in one-arm router mode, but I haven't fully convinced
 myself that this is where the failure is yet}

    > RFC 1981 does NOT work in general for UDP.  DNS/UDP is a perfect
    > example as it is the responding server that get the PTB and the DNS
    > client is trying multiple servers.  Often by the time the server is
    > tried again especially when a server is anycast the MTU knowledge has
    > been lost.

I didn't think PLPMTUD worked any better in this situation, or are I wrong here?

i.e: both are broken for UDP, and in the DNS/UDP case, the server never
     retransmits either... so absent using TCP, it will always just fail.

--
Michael Richardson <mcr+IETF@sandelman.ca>, Sandelman Software Works
 -= IPv6 IoT consulting =-

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