[Dart] DART dscp-rtp draft: Proposed RTCP text

"Black, David" <david.black@emc.com> Thu, 28 August 2014 19:45 UTC

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From: "Black, David" <david.black@emc.com>
To: "dart@ietf.org" <dart@ietf.org>, "avt@ietf.org WG (avt@ietf.org)" <avt@ietf.org>
Date: Thu, 28 Aug 2014 15:45:10 -0400
Thread-Topic: DART dscp-rtp draft: Proposed RTCP text
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Cc: "ben@nostrum.com" <ben@nostrum.com>, "Black, David" <david.black@emc.com>, "Colin Perkins \(csp@csperkins.org\)" <csp@csperkins.org>
Subject: [Dart] DART dscp-rtp draft: Proposed RTCP text
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Here's the proposed new section 5.4, with the RTCP multi-stream reporting
optimizations text moved into that new section from its current location
at the end of 5.3, plus the rewritten Section 6 guideline bullet:

5.4.  DiffServ and RTCP

   The RTP Control Protocol (RTCP) [RFC3550] is used with RTP to monitor
   quality of service and convey information about RTP session
   participants.  A sender of RTCP packets that also sends RTP packets
   (i.e., originates an RTP stream) should use the same DSCP marking for
   both types of packets.  If an RTCP sender doesn't send any RTP
   packets, it should mark its RTCP packets with the DSCP that it would
   use if it did send RTP packets.  If the RTCP sender uses or would use
   multiple DSCPs that differ only in drop precedence for RTP, then it
   should use the DSCP with the least likelihood of drop for RTCP to
   increase the likelihood of RTCP packet delivery.

   This guidance may result in different DSCP markings for RTP streams
   and RTCP receiver reports about those RTP streams.  The resulting
   variation in network QoS treatment by traffic direction could result
   in unrepresentative round trip time (RTT) estimates that don't
   correspond to consistent network QoS treatment in both directions.
   RTCP receiver reports may be relatively infrequent (e.g., may be sent
   only once per video frame rendered) and hence the resulting RTT
   estimates are of limited utility for congestion control (although
   they have other important uses, see [RFC3550]).  For this reason, it
   is not important that RTCP receiver reports receive the same network
   QoS treatment as the RTP stream or streams being reported on.

   RTCP multi-stream reporting optimizations for an RTP session
   [I-D.ietf-avtcore-rtp-multi-stream-optimisation] assume that the RTP
   streams involved experience the same network behavior, including
   packet loss.  This mechanism is highly inappropriate when the RTP
   streams involved use different DSCPs, even if the corresponding PHBs
   differ solely in drop precedence.

6.  Guidelines

[... snip ...]

   o  Should use a single DSCP for RTCP packets, which should be a DSCP
      used for RTP packets that are or would be sent by the same sender,
      see Section 5.4.

Thanks,
--David
----------------------------------------------------
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953             FAX: +1 (508) 293-7786
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