Re: [Dart] [tsvwg] I-D Action: draft-ietf-tsvwg-rtcweb-qos-02.txt

"Black, David" <david.black@emc.com> Mon, 21 July 2014 13:02 UTC

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From: "Black, David" <david.black@emc.com>
To: Michael Welzl <michawe@ifi.uio.no>, Brian E Carpenter <brian.e.carpenter@gmail.com>
Date: Mon, 21 Jul 2014 09:02:17 -0400
Thread-Topic: [tsvwg] I-D Action: draft-ietf-tsvwg-rtcweb-qos-02.txt
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Cc: "Karen E. Egede Nielsen" <karen.nielsen@tieto.com>, rmcat WG <rmcat@ietf.org>, "dart@ietf.org" <dart@ietf.org>, "tsvwg@ietf.org" <tsvwg@ietf.org>
Subject: Re: [Dart] [tsvwg] I-D Action: draft-ietf-tsvwg-rtcweb-qos-02.txt
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> >> Hence the relative mix of those drop precedences can be expected to affect
> >> the overall drop rate/probability seen in a session where those precedences
> >> are mixed.
> >
> > Yes. And it was clearly aimed at layered video, such that there was
> > a benefit to the end user in dropping the least important layer
> > during lossy periods. With anything like a TCP-like congestion
> > control algorithm in sight, the effects may be completely bizarre.
> 
> Reordering could make this even worse. Could different drop precedences cause
> reordering?

No, packets MUST NOT be reordered among the three drop precedences within
an AF class (e.g., AF2x).  See RFC 2597 and draft-york.

Thanks,
--David

> -----Original Message-----
> From: Michael Welzl [mailto:michawe@ifi.uio.no]
> Sent: Monday, July 21, 2014 8:14 AM
> To: Brian E Carpenter
> Cc: Black, David; Karen E. Egede Nielsen; rmcat WG; dart@ietf.org;
> tsvwg@ietf.org
> Subject: Re: [tsvwg] I-D Action: draft-ietf-tsvwg-rtcweb-qos-02.txt
> 
> 
> On 21. juli 2014, at 13:08, Brian E Carpenter <brian.e.carpenter@gmail.com>
> wrote:
> 
> > On 21/07/2014 14:00, Black, David wrote:
> >> Karen,
> >>
> >>>> What's the rationale for "certainly isn't viable" regarding different
> drop
> >>>> precedences within a single SCTP session?
> >>> [Karen] Ok - "certainly not viable" is too strong formulation.
> >>> Having different drop precedences within the same SCTP CC context would,
> >>> given that SCTP CC presently is drop driven only,
> >>> results in that the whole data channel is impacted, from a CC perspective,
> >>> by the highest drop precedence. But I realize that whether this is
> >>> acceptable is a question indeed.
> >>
> >> I think you're on to something here.  If the drop precedences only came
> into
> >> play after the decision to drop a packet had been made, their effect would
> >> probably be minimal, as they would only affect what to drop in a session,
> not
> >> how much to drop.  OTOH, DiffServ AF is not intended to operate in that
> fashion
> >> - the rationale for the 3 drop precedences per AF class was based on
> envisioning
> >> the output of two-threshold rate shaper:
> >> 	AFx1 - Within base or committed traffic profile
> >> 	AFx2 - Beyond base/committed, but within excess or burst traffic profile
> >> 	AFx3 - Completely out of profile
> >>
> >> Hence the relative mix of those drop precedences can be expected to affect
> >> the overall drop rate/probability seen in a session where those precedences
> >> are mixed.
> >
> > Yes. And it was clearly aimed at layered video, such that there was
> > a benefit to the end user in dropping the least important layer
> > during lossy periods. With anything like a TCP-like congestion
> > control algorithm in sight, the effects may be completely bizarre.
> 
> Reordering could make this even worse. Could different drop precedences cause
> reordering?
> 
> Cheers,
> Michael