Re: [TLS] Working Group Last Call for draft-ietf-tls-tls13-18

Adam Langley <agl@imperialviolet.org> Wed, 09 November 2016 20:22 UTC

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From: Adam Langley <agl@imperialviolet.org>
Date: Wed, 9 Nov 2016 12:22:20 -0800
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Subject: Re: [TLS] Working Group Last Call for draft-ietf-tls-tls13-18
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On Wed, Nov 9, 2016 at 11:42 AM, Martin Rex <mrex@sap.com>; wrote:

> in reality, the Google servers perform pathological fragmentation
> of the AppData and use a whooping 36 TLS records for the response
> (curiously no TLS AppData record split between header and body).
>

At the beginning of a connection we aim to size TLS records so that they
don't span packets. Since none of a TLS record can be processed until the
whole record is received, spanning packets can mean spanning TCP windows or
loss events and thus add significant latency. Web browsers will
shallow-parse data as it arrives in order to start subresource fetches as
soon as possible, so delays can have a cascading effect. See also
https://hpbn.co/transport-layer-security-tls/#optimize-tls-record-size.

If you download a lot of data on a connection (i.e. some MBs) then you
should find the the record sizes adjust to optimise for throughput.


Cheers

AGL