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

Joseph Touch <touch@strayalpha.com> Fri, 02 April 2021 17:59 UTC

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From: Joseph Touch <touch@strayalpha.com>
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Date: Fri, 2 Apr 2021 10:59:12 -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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Hi, Gorry (et al.),

> On Apr 2, 2021, at 4:29 AM, Gorry Fairhurst <gorry@erg.abdn.ac.uk> wrote:
> 
> In the last IETF TSVWG meeting you asked for throughts about what MSS should mean?
> 
> I have now thought - the key unknown here is how large a datagram segment the remote receiver is willing to accept - i.e., what is the largest fragmented datagram I can send that would be reassembled. This might be any number, depending on the way in which the receiver is designed or configured. 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.
> 
> I think you suggested this as: Max reassembly size, a hard upper bound, similar to MSS_R.
> 
> 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)?

Joe