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Re: [tsvwg] A review of draft-ietf-tsvwg-udp-options-12

Joseph Touch <touch@strayalpha.com> Mon, 14 June 2021 00:04 UTC

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From: Joseph Touch <touch@strayalpha.com>
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Date: Sun, 13 Jun 2021 17:04:03 -0700
Cc: "C. M. Heard" <heard@pobox.com>, Gorry Fairhurst <gorry@erg.abdn.ac.uk>, TSVWG <tsvwg@ietf.org>
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To: Paul Vixie <paul@redbarn.org>
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Subject: Re: [tsvwg] A review of draft-ietf-tsvwg-udp-options-12
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> On Jun 13, 2021, at 3:56 PM, Paul Vixie <paul@redbarn.org> wrote:
> 
> On Sun, Jun 13, 2021 at 03:00:35PM -0700, C. M. Heard wrote:
>> Responding to just this one specific point ...
> 
> same.
> 
>> On Sun, Jun 13, 2021 at 12:55 PM Joseph Touch wrote:
>> 
>>> And no, I do not think we should rely on RMSS to indicate that this is
>>> available. Note that a receiver would have to indicate RMSS and know the
>>> path MTU to know how many fragments it would actually take; we don???t
>>> require path MTU discovery for UDP options, so that???s an undue burden.
>>> Besides again injecting state into UDP.
>> 
>> For IPv6 -- where fragmentation has the greatest issues -- we are
>> guaranteed a minimum MTU of 1280 bytes. A DNS server operating over IPv6
>> would be able to serve up a fragmented response with the right number of
>> 1280 byte fragments (or else set the TC bit) if it received the MRSS option
>> in the request from the client.
> 
> computing UDP payload size by subtraction requires either that one be inside
> the network stack (sometimes this means "in the kernel") so as to know the
> interface MTU and perhaps the path MTU, as well as the IP6 header and option
> sizes.

That’s correct; that’s exactly how TCP MSS works. The UDP stack can make this information available to the user.

> when this information is unavailable, for example when running inside
> a DNS responder,

A DNS responder would not be creating UDP option packets; it creates user that it passes to UDP. The same way TCP keeps ithis info on a per-socket basis, UDP is expected to.

Joe

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