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RE: CONEX? (was: Re: Q and L loss bits)

"Lubashev, Igor" <ilubashe@akamai.com> Thu, 21 November 2019 07:23 UTC

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From: "Lubashev, Igor" <ilubashe@akamai.com>
To: "Brian Trammell (IETF)" <ietf@trammell.ch>, Lars Eggert <lars@eggert.org>
CC: IETF QUIC WG <quic@ietf.org>, Christian Huitema <huitema@huitema.net>, "alexandre.ferrieux@orange.com" <alexandre.ferrieux@orange.com>, Dmitri Tikhonov <dtikhonov@litespeedtech.com>
Subject: RE: CONEX? (was: Re: Q and L loss bits)
Thread-Topic: CONEX? (was: Re: Q and L loss bits)
Thread-Index: AQHVoDbg9/uTCNZtvUmvO6ZyW7zzuaeVklGA//+feYA=
Date: Thu, 21 Nov 2019 07:23:06 +0000
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References: <20191119143218.GB2789@ubuntu-dmitri> <24388_1574174932_5DD400D4_24388_372_7_1574174948.30247.49.camel@orange.com> <20191119150829.GD2789@ubuntu-dmitri> <20191119192622.GE2789@ubuntu-dmitri> <a21bc373-1fda-836e-7efc-599832ae575d@huitema.net> <20191120014850.GC16017@ubuntu-dmitri> <cd2e9389a61d49eabad2b0fac2f4db96@ustx2ex-dag1mb5.msg.corp.akamai.com> <A5687771-2F40-419F-A8A2-00FFF919B3FA@eggert.org> <A6DB3084-447B-467A-B164-310C83A6526C@trammell.ch>
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A few things about the ConEx vs QL-lossbits

1. ConEx has a mechanism that is similar to L-bit (we mention ConEx RFC7713 in the draft).  But it has no equivalent of the Q-bit signal.  It is very hard to tell upstream loss from downstream loss with ConEx.

2. ConEx reports bytes, while QL-loss reports packets.  There is a trade-off here, as bytes may be more valuable, but reporting bytes can be imprecise, if the packets marked are of different size from the packets lost.  Overall, both work well enough.

3. ConEx requires more bits than what we have in QUICv1, so it would be a significant disruption to QUICv1 header format to properly implement all ConEx bits.  The Q- and L- bits are enough for the purpose.

4. CWR is an interesting signal, and I actually have a separate CWR bit (I call it "E-bit") in https://tools.ietf.org/html/draft-ferrieuxhamchaoui-tsvwg-lossbits-02#section-5 -- the -tsvwg- version of the draft.  However, it is not very closely related to loss for networks with ECN.  On an ECN-capable network for ECN-capable flows, downstream loss would become indistinguishable from downstream congestion that marks CE.  Ultimately, it seemed that given just two bits to work with, Q-bit plus a pure loss bit seemed preferable to Q-bit plus "loss-or-CE_echo" bit.  But I am happy to discuss this!

- Igor

> On Thu, Nov 21, 2019 at 2:44 PM, Brian Trammell (IETF) <ietf@trammell.ch> wrote:
> 
> hi Lars,
> 
> This is kind of where I was going with the "expose CWR-like semantics for loss"
> thing; "just do conex" is a much better shorthand for that.
> 
> Thanks, cheers,
> 
> Brian
> 
> > On 21 Nov 2019, at 14:42, Lars Eggert <lars@eggert.org> wrote:
> >
> > Hi,
> >
> > I'm wondering whether specifying a CONEX mapping for QUIC would satisfy
> the needs that quic-lossbits attempts to address? We could reuse an existing
> piece of IETF work.
> >
> > See https://tools.ietf.org/html/rfc7713 for an overview and
> https://tools.ietf.org/html/rfc7786 for the TCP mapping.
> >
> > Lars
> >

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