Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt

Ingemar Johansson S <ingemar.s.johansson@ericsson.com> Fri, 05 March 2021 13:51 UTC

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From: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
To: Bob Briscoe <ietf@bobbriscoe.net>, "tsvwg@ietf.org" <tsvwg@ietf.org>
Thread-Topic: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tsvwg/JQbRZ-XwGuwtX_TLH52qMtE3rJM>
Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt
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Hi

Inline [IJ2]

/Ingemar

> -----Original Message-----
> From: Bob Briscoe <ietf@bobbriscoe.net>
> Sent: den 5 mars 2021 11:33
> To: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>om>;
> tsvwg@ietf.org
> Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt
>
> Ingemar, see [BB] inline
>
> On 05/03/2021 09:48, Ingemar Johansson S wrote:
> > Hi
> >
> > Please see inline
> >
> >> -----Original Message-----
> >> From: Bob Briscoe <ietf@bobbriscoe.net>
> >> Sent: den 4 mars 2021 19:21
> >> To: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>om>;
> >> tsvwg@ietf.org
> >> Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-ecn-l4s-id-13.txt
> >>
> >> Ingemar,
> >>
> >> Sorry, you probably thought this thread had ended this morning, but...
> >>
> >> I've had someone suggest (off list), that the conclusion that RTCP
> >> datagrams can be ECN-capable ought to be written (informatively) into
> >> ecn-l4s-id. It would go in Appendix A.2.1, which would have to be
> >> retitled: "Setting ECT in -TCP- Control Packets and Retransmissions".
> >>
> >> Before doing that, I checked RFC6679. It says "RTCP reports MUST NOT
> >> be ECT marked, since ECT-marked traffic may be dropped if the path is
> >> not ECN compliant."
> >>
> >> This seems rather paranoid, given the path is first checked for ECN
> >> support before using it on RTP media. It justifies this by saying the
> >> path might change to one that black-holes ECN-capable traffic.
> >> Whatever, if RFC6679 (proposed
> >> standard) says this, I don't want to contradict it.
> >> Do you think it would be OK to say:
> >>
> >> "RFC6679 precludes the use of ECT on RTCP datagrams, in case the path
> >> changes after it has been checked for ECN traversal. L4S experiments
> >> will need to observe this rule. Nonetheless, as ECN usage becomes
> >> more widespread, it would be useful to gather data on whether such
> >> caution is necessary."
> > [IJ] Yes. it makes sense. I guess the text was written at a time when it 
> > was
> uncertain how likely ECT traffic is to become black holed. I understand that
> today these cases are quite rate.
> > Still, if it happens, then real time streaming applications should 
> > implement
> circuit brakers  (https://tools.ietf.org/html/rfc8083) . This should make it 
> safe
> to use ECT for RTCP traffic as well.
>
> [BB] In a 2017 measurement study, we found ECN was wiped on 0.23% of
> paths from residential fixed access vantage points. But with mobile access,
> we found 10.5 / 18 of the EU mobile networks we tested wipe the ECN field
> at the first IP hop incoming to their network (the 0.5 of an operator was
> solely wiping on port 80).
>      See https://protect2.fireeye.com/v1/url?k=6d2c03bd-32b73afe-6d2c4326-
> 861fcb972bfc-b249fdfe321805e2&q=1&e=908b3683-d82e-493c-a2bc-
> 159f5fe358e2&u=https%3A%2F%2Fbobbriscoe.net%2Fpubs.html%23measur
> e-generalized-ecn
> Some follow-up contacts with the operator found this was probably not
> deliberate wiping; most likely bugs in kit wiping the DS field.
[IJ2] I guess by "wiping" you refer to the case that the ECN bits are set to 
zero ?. Yes, that seems to happen when packets are marked ECT in UL, at least 
in one experiment I ran in a controlled lab setup. I suspect that this is a 
configuration matter but it could be bugs. In any case I believe that this 
will sort itself out for the better when L4S is on the table in 3GPP, 
hopefully mid-year.

>
> So we're probably not out of the woods yet.
>
>
> Bob
>
> >
> >> Does that seem reasonable?
> >>
> >>
> >> Bob
> >>
>
> --
> __________________________________________________________
> ______
> Bob Briscoe 
> https://protect2.fireeye.com/v1/url?k=170b85e9-
> 4890bcaa-170bc572-861fcb972bfc-6795715d08611dd8&q=1&e=908b3683-
> d82e-493c-a2bc-159f5fe358e2&u=http%3A%2F%2Fbobbriscoe.net%2F