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

Paul Vixie <paul@redbarn.org> Fri, 02 April 2021 23:01 UTC

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Date: Fri, 2 Apr 2021 23:01:20 +0000
From: Paul Vixie <paul@redbarn.org>
To: Joseph Touch <touch@strayalpha.com>
Cc: Gorry Fairhurst <gorry@erg.abdn.ac.uk>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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Subject: Re: [tsvwg] UDP-Options: UDP has two ???maximums???
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On Fri, Apr 02, 2021 at 10:59:12AM -0700, Joseph Touch wrote:
> > On Apr 2, 2021, at 4:29 AM, Gorry Fairhurst <gorry@erg.abdn.ac.uk> wrote:
> > Does this help?
> 
> I had assumed that would be at useful.

> The question that remains is whether it is useful to also have an MSS
> option in the same spirit as TCP. That would argue for two different
> MSS values:
> 
> 	UDP path MSS
> 	UDP EMTU_R
> 
> Are both actually useful? Is the TCP one useful? (esp. given it really
> isn???t path info)?

is everyone here familiar with RFC 8899 and RFC 8900? while RFC 8900 talks
about IP layer fragmentation, one of its stated concerns would apply to
app-layer (QUIC) fragmentation (see section 3.5), and there's a well known
micro-burst problem (shows up in NFS) where app-layer message transmission
isochrony won't be maintained for packets issued by the fragmenter.

RFC 8899 specifies an ICMP-free method for packetization layer path MTU
discovery (PLPMTUD), and i think making QUIC dependant on PLPMTUD would
avoid either duplication or waste of effort, as well as informing the
UDP MSS discussion of this thread.

-- 
Paul Vixie