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

"Black, David" <david.black@emc.com> Thu, 28 August 2014 20:36 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 16:36:25 -0400
Thread-Topic: DART dscp-rtp draft: Proposed RTCP text
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Cc: "ben@nostrum.com" <ben@nostrum.com>, "Colin Perkins \(csp@csperkins.org\)" <csp@csperkins.org>
Subject: Re: [Dart] DART dscp-rtp draft: Proposed RTCP text
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Minor edit to proposed text below:

OLD
    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.
NEW
   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 with media similar to the RTP traffic
   that it receives.

Thanks,
--David


> -----Original Message-----
> From: Black, David
> Sent: Thursday, August 28, 2014 3:45 PM
> To: dart@ietf.org; avt@ietf.org WG (avt@ietf.org)
> Cc: Black, David; Colin Perkins (csp@csperkins.org); ben@nostrum.com
> Subject: DART dscp-rtp draft: Proposed RTCP text
> 
> 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
> david.black@emc.com        Mobile: +1 (978) 394-7754
> ----------------------------------------------------
>