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Re: [TLS] RFC8446 backward compatibility question

"Salz, Rich" <rsalz@akamai.com> Thu, 05 August 2021 21:17 UTC

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From: "Salz, Rich" <rsalz@akamai.com>
To: Toerless Eckert <tte@cs.fau.de>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] RFC8446 backward compatibility question
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Date: Thu, 05 Aug 2021 21:16:37 +0000
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Subject: Re: [TLS] RFC8446 backward compatibility question
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>    I am trying to figure out if every implementation compliant with
    RFC8446 is also necessarily interoperable with an RFC5246 peer, or if this
    is just a likely common, but still completely optional implementation choice.

It is possible to have a single stack that implements TLS.[123].  OpenSSL, among many others does this.  Some have implemented ONLY TLS 1.3; that code tends to be cleaner (in a nerd esthetic sense) than code that implements multiple protocols. Some servers even "hand off" pre-1.3 protocols to separate implementations (libraries); FB and Amazon used to do that.

The wire protocol for TLS 1.3 uses some deliberately-reserved extension fields so that a server which doesn't do 1.3 can fail cleanly, and a server that DOES will work. And also the other way, a 1.3 client can work fine with both a 1.3 server and a 1.[12] server.

It's easy to rationale 1.3-only for clients. It is harder to rationalize 1.3-only for servers if you are intending those servers to be generally accessible on the public Internet.

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