Re: [TLS] RFC8446 backward compatibility question

Eric Rescorla <> Thu, 05 August 2021 21:26 UTC

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From: Eric Rescorla <>
Date: Thu, 5 Aug 2021 14:26:13 -0700
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To: "Salz, Rich" <>
Cc: Toerless Eckert <>, "" <>
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Subject: Re: [TLS] RFC8446 backward compatibility question
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FWIW, the situation Rich outlines has been true for all prior versions of
TLS as well; it's just that TLS 1.3 is so different that there's a lot more
advantage to doing TLS 1.3 only than (say) doing TLS 1.2 but not TLS 1.1.

This isn't the question asked, but for the most part, ClientHellos
generated by stacks which support both TLS 1.3 and TLS 1.2 will be
compatible with unmodified TLS 1.2 servers, thus allowing for clients to
upgrade even when servers have not.

The exception is that if a TLS 1.3 client is attempting 0-RTT (which it can
only do if it has already successfully connected with TLS 1.3 to the
server), then this is not compatible with TLS 1.2. Thus, if you have
multiple servers associated with a given address you need to ensure that
they all speak TLS 1.3 prior to advertising 0-RTT. It is safe, however to
upgrade only some of them to TLS 1.3 as long as you do not advertise 0-RTT
because the ClientHello will still be TLS 1.2 compatible.


On Thu, Aug 5, 2021 at 2:17 PM Salz, Rich <>

> >    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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