Re: [TLS] Flags extension and announcing support

Eric Rescorla <> Fri, 22 January 2021 14:18 UTC

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From: Eric Rescorla <>
Date: Fri, 22 Jan 2021 06:17:28 -0800
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To: Nick Harper <>
Cc: Martin Thomson <>, "<>" <>
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Subject: Re: [TLS] Flags extension and announcing support
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On Fri, Jan 22, 2021 at 1:55 AM Nick Harper <> wrote:

> On Thu, Jan 21, 2021 at 9:46 PM Martin Thomson <> wrote:
>> In other words, each flag is treated just like an empty extension: you
>> can initiate an exchange with it, but you can only answer with it if it was
>> initiated with it.
>> I agree that this is the correct guiding principle for handling flags. We
> should allow unsolicited flags in the same places we allow unsolicited
> extensions. Going by section 4.2 of RFC 8446, that would be ClientHello,
> CertificateRequest, and NewSessionTicket.

FWIW, this is what I was trying to say as well, though I'm prepared to
believe I didn't say it.

There is also a separate but related question of whether you can send an
unsolicited flags *extension* regardless of the contents of that extension
in these messages. I believe you should be able to for the reason Nick and
Martin indicates above.