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

Joseph Touch <touch@strayalpha.com> Sat, 03 April 2021 19:07 UTC

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From: Joseph Touch <touch@strayalpha.com>
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Date: Sat, 3 Apr 2021 12:07:21 -0700
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To: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: Re: [tsvwg] =?utf-8?q?UDP-Options=3A_UDP_has_two_=E2=80=9Dmaximums?= =?utf-8?b?4oCd?=
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> On Apr 3, 2021, at 2:17 AM, Gorry Fairhurst <gorry@erg.abdn.ac.uk> wrote:
> 
>> 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)?
>> 
>> Joe
> Hi Joe,
> 
> I'll argue also that the maximum size a remote transport receiver can process, isn't so helpful, because the transport endpoint often does not know the set of encaps/tunnel/link-MTU along the path.

Yes - except for the PLPMTUD case. In that case, the receiver COULD report back the largest MTU received in the “UDP path MSS”.

> That's why I argue we should have a "default" (as in "For IPv4, EMTU_S is the smaller of 576 bytes and the first-hop MTU [RFC1122]. For IPv6, EMTU_S is 1280 bytes [RFC2460]"), that could be used - and might be expected to usually work, *or* a "probe" to find out what the path actually supports: knowing the remote receiver can process X bytes does not really help that much, and actually could be unhelpful when information if it is not correct. So I'll argue against sharing this from the receiver.

If it’s not needed for PLPMTUD and has no corollary to how it’s used in TCP, then agreed.

Anyone else have a position??

Joe