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

Peter Gutmann <pgut001@cs.auckland.ac.nz> Sat, 18 March 2017 07:35 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Thomas Pornin <pornin@bolet.org>
CC: Martin Thomson <martin.thomson@gmail.com>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] RFC 6066 - Max fragment length negotiation
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Date: Sat, 18 Mar 2017 07:35:42 +0000
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Subject: Re: [TLS] RFC 6066 - Max fragment length negotiation
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Thomas Pornin <pornin@bolet.org> writes:

>having some guarantees on non-fragmentations may help some implementations
>that are very constrained in ROM size and must stick to the simplest possible
>code.

It's not the code size, it's attack surface.  There have been endless attacks
on IP fragmentation and fragment reassembly, not to mention equally numerous
firewall/IDS bypasses by creative fragmentation.  So in my case not doing
fragmentation is a security thing, not a code-size thing (as was not doing
rehandshake, compression, and a pile of other things that have caused problems
in SSL/TLS in the past).

The fact that I've never encountered any embedded/SCADA device that handles or
requires fragmentation makes it ever easier.

Peter.

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