Re: [TLS] custom lower limit of record_size_limit

Nikos Mavrogiannopoulos <nmav@redhat.com> Mon, 21 January 2019 08:03 UTC

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From: Nikos Mavrogiannopoulos <nmav@redhat.com>
To: Martin Thomson <mt@lowentropy.net>, tls@ietf.org
Date: Mon, 21 Jan 2019 09:03:13 +0100
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Subject: Re: [TLS] custom lower limit of record_size_limit
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On Mon, 2019-01-21 at 16:13 +1100, Martin Thomson wrote:
> On Sat, Jan 19, 2019, at 19:02, Daiki Ueno wrote:
> > My interpretation is that, if the client sent "record_size_limit"
> > but
> > didn't receive the extension from the server, that would mean the
> > extension was not negotiated and the server may not respect the
> > limit.
> > 
> > Is this correct, or 64 is really mandatory to implement?
> 
> Unfortunately, if you want your peer to respect your limit, then you
> have to be willing to generate very small records.
> 
> BTW, 64 is entirely an arbitrary number.  It's at the point where the
> overheads get really noticeable, so performance is probably pretty
> bad well before you get to this point.  But we didn't get any
> indication that it was impossible to go that low.
> 
> If there had been feedback about it being too small, I'm fairly sure
> that a large number would have been fine.  Do you know why 64 is
> considered too hard to implement?

I do not think that 64 is not hard to implement, but I think it is very
hard to implement it in a way that it is efficient. I have not measured
specifically TLS with 64 byte packets, but my experience with ATM
(telephony) which has 53-byte packets, suggest to avoid small
packetizations if you want use your CPU for something other than
packing and unpacking frames.

regards,
Nikos