Re: [tsvwg] UDP-Options: UDP has two ???maximums???

Paul Vixie <paul@redbarn.org> Sat, 03 April 2021 20:23 UTC

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Date: Sat, 3 Apr 2021 20:23:13 +0000
From: Paul Vixie <paul@redbarn.org>
To: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: Re: [tsvwg] UDP-Options: UDP has two ???maximums???
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> > On Fri, Apr 02, 2021 at 12:29:55PM +0100, Gorry Fairhurst wrote:
> > > ... Whereas a sender could anticipate a common fragment size - e.g.
> > > 1200B, or use dplpmtud to discover this, there is no real way of
> > > determining the largest datagram that would be the option. ...

> On 03/04/2021 00:12, Paul Vixie wrote:
> > i think some form of discovery/negotiation ought to be required here.

On Sat, Apr 03, 2021 at 10:20:31AM +0100, Gorry Fairhurst wrote:
> I think I agree, is this something like you were suggesting:
> 
> https://tools.ietf.org/html/draft-fairhurst-tsvwg-udp-options-dplpmtud-04

as long as it's not an april fools' joke, and to the extent that it does
not accidentally preclude TCP from being treated as a packetization layer
(so that TCP MSS can also scale with PMTU rather than first-hop MTU), yes.

PLPMTUD is exactly what i wanted for DNS over UDP, and we refer to it here:

https://www.ietf.org/archive/id/draft-ietf-dnsop-avoid-fragmentation-04.txt

note, a lot of others expect DNS to move to HTTP/3 or TCP, but even in
those cases i would like to use the largest discrete PDUs that will fit.

-- 
Paul Vixie