Re: [TLS] ECH and resumption - what to put in SNI?

Ben Schwartz <bemasc@google.com> Fri, 25 June 2021 14:37 UTC

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From: Ben Schwartz <bemasc@google.com>
Date: Fri, 25 Jun 2021 10:36:54 -0400
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Subject: Re: [TLS] ECH and resumption - what to put in SNI?
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On Fri, Jun 25, 2021 at 10:21 AM Stephen Farrell <stephen.farrell@cs.tcd.ie>
wrote:

>
> Hiya,
>
> If a client established a session to foo.example.com
> using ECH with a public_name of example.com, what ought
> the client put in the SNI when resuming?
>
...

> I guess the client could do ECH again


That's my expectation.

...

> might cause interesting failures for a library
> client that can't do fresh DNS queries from within the
> library (and might never see the TTL of the HTTPS/SVCB
> RR in any case).
>

I expect the application would pass an ECHConfigList to the TLS library
with each new connection request, and new requests might be resumed from a
ticket cache.  The TLS library doesn't need to know anything about the TTL,
because the ECHConfigList is single-use, and independent from whether the
resumption cache was used.