Re: [TLS] 0-RTT in DTLS 1.3

Martin Thomson <mt@lowentropy.net> Sun, 23 May 2021 23:58 UTC

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Date: Mon, 24 May 2021 09:57:22 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/6TQth68GYFmjA202vhrBWAQ-PyI>
Subject: Re: [TLS] 0-RTT in DTLS 1.3
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On Sun, May 23, 2021, at 16:05, Hanno Becker wrote:
> 1) In DTLS 1.3, it would seem common for the server to send an HRR for 
> the sake of return routability checking. TLS 1.3 forbids the use of 
> 0-RTT after an HRR. So, 0-RTT can't be used in DTLS 1.3 if the server 
> requires return routability checking -- is this understanding correct? 
> Should this be stated more explicitly?

This is not the model that QUIC uses.  Binding return routability information into session tickets allows 0-RTT to be used, albeit at some risk.  Managing that risk might take a few forms, the most common being limiting the total amount of response data and limiting the period over which 0-RTT is accepted (more than the 7 days).
 
> 2) Not allowing 0-RTT after an HRR, or rather not allowing 0-RTT 
> *twice*, seems important for DTLS 1.3 as we'd otherwise overload epoch 
> 1. Is this worth stating?

TLS already says that HRR automatically causes 0-RTT to be rejected.  "Early data is not permitted after a HelloRetryRequest." (RFC 8446, Section 4.1.2)