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

Joseph Touch <touch@strayalpha.com> Sun, 04 April 2021 16:07 UTC

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From: Joseph Touch <touch@strayalpha.com>
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Date: Sun, 4 Apr 2021 09:07:13 -0700
Cc: Gorry Fairhurst <gorry@erg.abdn.ac.uk>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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To: Paul Vixie <paul@redbarn.org>
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Subject: Re: [tsvwg] UDP-Options: UDP has two ???maximums???
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Hi, Paul,

> On Apr 3, 2021, at 9:57 PM, Paul Vixie <paul@redbarn.org> wrote:
> 
>> PLPMTUD intends to cache MTUs per-endpoint exactly the same way
>> that PMTUD did.
> 
> that's interesting! so, a DNS protocol speaker (framer of messages operating
> at the packetization layer) who uses DNS/UDP messages of various sizes in
> order to discover the PMTU, because the kernel routing table had no hint of
> the MTU for the endpoint under consideration, will be expected to report its
> finding to the kernel, which can store it in the routing table, where TCP,
> QUIC, NFS and so on can find it?

From RFC4821:

   ...Furthermore, the natural mechanism to
   share Path MTU information between concurrent or subsequent
   connections is a path information cache in the IP layer.  The various
   Packetization Protocols need to have the means to access and update
   the shared cache in the IP layer.  

Joe