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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> Mon, 08 February 2016 18:21 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: Mon, 08 Feb 2016 13:21:08 -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:
    >> I didn't think PLPMTUD worked any better in this situation, or are I
    >> wrong he re?

    > Conceptually you do PLPMTUD by adjusting the EDNS UDP size advertised
    > in the query until you get a response.  This isn't strict PMTUD as it
    > also accounts for firewalls that are blocking fragments, firewalls that
    > still think DNS is limited to 512 byte UDP payloads.  Unfortunately you
    > only have about 3 seconds to do this all in as well as talking to other
    > servers.  The DNS doesn't have a way of signaling "fragment at this
    > size".

I'd forgotten about doing it this way.
I now recall you explaining it many times before :-)

It would seem that ICMP based PMTUD is useless for DNS too, and that we
should do away with 1981?

    > Named tracks successful answers/timeouts with different EDNS udp sizes
    > as well as the actual response sizes that have got through.

    > ; [edns success/4096 timeout/1432 timeout/1232 timeout/512 timeout]

    > Too many timeouts with a particlar size and we stop offering a EDNS UDP
    > buffer of that size and fallback to the next break point.  These values
    > are set for 4in6/6in4 + a UDP header.  timeouts with a smaller EDNS
    > buffer size also count against larger sizes.

    >> 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.

    > But the client re-queries leading to a re-transmission.

    >> --
    >> Michael Richardson <mcr+IETF@sandelman.ca>, Sandelman Software Works
    >> -= IPv6 IoT consulting =-
    >>
    >>
    >>
    >>
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    > --
    > Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia
    > PHONE: +61 2 9871 4742 INTERNET: marka@isc.org

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

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