[v6ops] Re: [tsvwg] Re: Carrying large DNS packets over UDP in IPv6 networks

"C. M. Heard" <heard@pobox.com> Mon, 17 June 2024 14:30 UTC

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From: "C. M. Heard" <heard@pobox.com>
Date: Mon, 17 Jun 2024 07:30:08 -0700
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To: Suresh Krishnan <suresh.krishnan@gmail.com>, Mohamed Boucadair <mohamed.boucadair@orange.com>
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Subject: [v6ops] Re: [tsvwg] Re: Carrying large DNS packets over UDP in IPv6 networks
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On Mon, Jun 17, 2024 at 6:15 AM Mohamed Boucadair wrote:

> [ ... ]
> > De : Suresh Krishnan <suresh.krishnan@gmail.com>
> > Envoyé : lundi 17 juin 2024 12:10
> > À : BOUCADAIR Mohamed INNOV/NET <mohamed.boucadair@orange.com>
> > Cc : v6ops@ietf.org; tsvwg@ietf.org
> > Objet : Re: [v6ops] Carrying large DNS packets over UDP in IPv6
> > networks
> >
> >
> > Hi Med,
> >
> > > On Jun 14, 2024, at 10:10 AM, mohamed.boucadair@orange.com
> > wrote:
> > >
> > > Hi Suresh, all,
> > >
> > > (ccing tsvwg)
> > >
> > >  FWIW, large DNS packets was the main driver for the FRAG UDP option:
> https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-udp-options-32#section-11.4
> .
> >
> > Thanks for this pointer. I read the draft based on your message
> > as well as Joe's mail downthread. It is very interesting and I
> > would love to learn more about the implementations on the client
> > side and whether it would be a viable alternative in the near
> > term.
> >
>
> [Med] I'm aware of
> https://datatracker.ietf.org/meeting/103/materials/slides-103-tsvwg-sessa-51-cco-and-implementation-status-for-udp-options-02
> (or long version
> https://datatracker.ietf.org/meeting/103/materials/slides-103-maprg-a-tale-of-two-checksums-tom-jones-00)
> Gorry may have more to share here I guess.
>
>

For a more complete version of that work see in
https://tma.ifip.org/2020/wp-content/uploads/sites/9/2020/06/tma2020-camera-paper70.pdf


That work found that approximately 20% of packets with UDP options sent to
DNS servers included in the test failed to elicit a response and concluded
that the cause was on-path filtering.

As I pointed out earlier in this thread, there is a short draft that
proposes a backward compatible and incrementally deployable method for
using UDP options to allow DNS over UDP to transmit large DNS responses.
One of the key features of the proposal is that an option client signals
its ability to accept UDP fragmentation by including the MRDS option (
https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-udp-options-32#section-11.6)
in its requests (or at least in those expected to elicit a large response).
As long as the network does not drop packets with UDP options, this should
work very well: options-aware servers would send large responses using UDP
fragmentation, uptions-unaware servers would truncate large  responses
(causing fallback to TCP), and small responses would be sent normally. But
if a significant number of client requests with the MRDS option get dropped
in the network, then the client would need to implement some kind of
probing method to determine if the path supports UDP options. It is then
unclear whether one might not end up better off to just rely on truncation
and fallback to TCP. See
https://datatracker.ietf.org/doc/html/draft-heard-dnsop-udp-opt-large-dns-responses-00#section-6
for
more discussion.

Mike Heard