Re: [TLS] RFC 6066 - Max fragment length negotiation

Peter Gutmann <pgut001@cs.auckland.ac.nz> Fri, 17 March 2017 11:21 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Martin Thomson <martin.thomson@gmail.com>
CC: "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] RFC 6066 - Max fragment length negotiation
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Date: Fri, 17 Mar 2017 11:21:12 +0000
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Subject: Re: [TLS] RFC 6066 - Max fragment length negotiation
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Martin Thomson <martin.thomson@gmail.com> writes:

>Plaintext records don't have any such limits.  I explicitly excluded them.

Hmm, it's somewhat disguised in the text, technically all records are
"protected records" (if you use EMS, everything is at least integrity-
protected).  So if you mean "this only applies to application_data" then you
should probably say so (alerts and CCS are too short for it to matter, and I'm
assuming no rehandshake, so only application_data will be affected by the
length constraints).

However, this then leads to a problem where it doesn't actually solve the
constrained-client/server issue, if a client asks for 2K max record size and
the server responds with a 4K hello then it's going to break the client even
if later application_data records are only 2K.  So it would need to apply to
every record type, not just application_data.

Peter.