Subject: [TLS] Padding extension and 0-RTT

From: Martin Thomson <[email-masked]>
Date: Sun, 30 Oct 2016 21:03:41 +1100

(Trivial optimization warning)

Just perusing my draft and noticed that NSS pads a 0-RTT handshake,
which is not that surprising given that it's fairly beefy (it will get
even larger in -18).  Since a 0-RTT handshake will break servers that
don't at least superficially understand TLS 1.3, maybe we could avoid
pading in this case.  Is there any reason we shouldn't include that
advice in the draft?

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From: David Benjamin <[email-masked]>
Date: Sun, 30 Oct 2016 19:52:20 +0000

Sounds reasonable.

One concern is the F5 bug's failure mode was a timeout rather than an
error, so, if you take away padding, the allowance in C.3 will not save
heterogenous deployments where some servers do 1.3 and some are old F5
servers. But given we're talking about a straight-up server bug now, it
seems reasonable for a client to say, okay, I will try to account for
heterogenous 1.2 and 1.3 deployments because that's kinda operationally
tricky, but if you've got that F5 bug, please fix it already.

David

On Sun, Oct 30, 2016 at 6:03 AM Martin Thomson <[email-masked]>
wrote:

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From: Eric Rescorla <[email-masked]>
Date: Sun, 30 Oct 2016 12:58:27 -0700

I'm ambivalent on this. OTOH, you're technically right, but OTOH it's just
one more conditional to save a few bytes (you need padding to exist
anyway), and if you're doing 0-RTT, you're about to send a lot more bytes
anyway.

-Ekr

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From: Martin Thomson <[email-masked]>
Date: Mon, 31 Oct 2016 08:13:23 +1100

On 31 October 2016 at 06:58, Eric Rescorla <[email-masked]> wrote:
> one more conditional to save a few bytes (you need padding to exist anyway),
> and if you're doing 0-RTT, you're about to send a lot more bytes anyway.

0-RTT happens when you have a small CWND.  I thought that it was maybe
worth it.  The space I'm seeing padded is enough for an extra request,
maybe two.

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From: Eric Rescorla <[email-masked]>
Date: Sun, 30 Oct 2016 14:29:43 -0700

On Sun, Oct 30, 2016 at 2:13 PM, Martin Thomson <[email-masked]>
wrote:

> anyway),
> 0-RTT happens when you have a small CWND.

Fair enough.

> I thought that it was maybe

Note: this will change somewhat with draft-18, because ClientHello is going
to expand
by ~32 octets.

-Ekr