On 6 Mar 2012, at 07:52, Harald Alvestrand wrote:

On 03/06/2012 12:32 AM, Colin Perkins wrote:

Here's our initial attempt at a "circuit breakers" draft for RTCWeb. Comments welcome - this is very much a straw-man for discussion, rather than a final solution.

Colin



Begin forwarded message:

From: internet-drafts@ietf.org
Subject: I-D Action: draft-perkins-avtcore-rtp-circuit-breakers-00.txt
Date: 5 March 2012 20:17:59 GMT
To: i-d-announce@ietf.org
Reply-To: internet-drafts@ietf.org

A New Internet-Draft is available from the on-line Internet-Drafts directories.

	Title           : RTP Congestion Control: Circuit Breakers for Unicast Sessions
	Author(s)       : Colin Perkins
                         Varun Singh
	Filename        : draft-perkins-avtcore-rtp-circuit-breakers-00.txt
	Pages           : 14
	Date            : 2012-03-05

  The Real-time Transport Protocol (RTP) is widely used for telephony,
  video conferencing, and telepresence applications.  These
  applications are often used over best-effort UDP/IP networks.  If
  congestion control is not implemented then network congestion will
  deteriorate the user's multimedia experience.  This document does not
  propose a congestion control algorithm.  Instead, it specifies a
  minimal set of "circuit-breakers".  Circuit-breakers are conditions
  under which an RTP flow should cease to transmit media to protect the
  network from excessive congestion.  It is expected that all RTP
  applications running on best-effort networks will be able to run
  without triggering these circuit breakers in normal operation.

A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-perkins-avtcore-rtp-circuit-breakers-00.txt

Thanks for this work!

A few questions (to make sure I get them noted down):

Section 4.1:

   Accordingly, if a
   sender of RTP data packets receives two or more consecutive RTCP RR
   packets from the same receiver that correspond to its transmission
   and have a non-increasing extended highest sequence number received
   field, then that sender SHOULD cease transmission.

If I see RTCP packets with

1: highest sequence number = 2
2: highest sequence number = 2
3: highest sequence number = 2

do I cease transmission after packet 3 has arrived, or after packet 2 has arrived?
I *think* the logical time is after packet 3 has arrived, but I'm a little unsure that the words are
unambiguously saying that; it's not 100% clear to me whether packet 1 is considered included in the set of "non-increasing highest sequence number".

We should clarify. My feeling is that either way would work, depending on how aggressive we want the circuit breaker to be.

Section 4.2: Is it reasonable to replace, for the purposes of this calculation, "an order of magnitude" with "a factor of ten"? (for those who don't have a physics background, putting text somewhere that says that an order of magnitude is "somewhere around a factor of ten" might be appropriate.)

Sure, will clarify.

We might also want to add the words about doing a dramatically reduced rate if we can from section 4.1 here, factor it out as a general statement, or say that it is not appropriate here if it's not.

Yes.