Re: [Dart] [AVTCORE] WGLC: draft-ietf-dart-dscp-rtp-02

Colin Perkins <> Mon, 18 August 2014 15:17 UTC

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Cc: Ben Campbell <>, " WG" <>,
Subject: Re: [Dart] [AVTCORE] WGLC: draft-ietf-dart-dscp-rtp-02
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[I’m not on the DART list, so please cc me in replies]

This draft looks good overall, but I did have some comments, primarily about the discussion of RTP multiplexing. 

The four bullet points in Section 2.1 that follow “The RTCWEB protocol suit encompasses a number of forms of multiplexing” seem to confuse RTP sessions,
RTCP, and transport-layer flows. I suggest they be changed to:

>    The RTP streams that comprise a WebRTC session can be multiplexed 
>    together in a number of ways:
>    1.  Individual source streams are carried in one or more individual
>        RTP streams. These RTP streams can be multiplexed onto a single
>        transport-layer flow or sent as separate transport-layer flows.
>        This memo only considers the case where the RTP streams are to be
>        multiplexed onto a single transport-layer flow, forming a single
>        RTP session;
>    2.  The RTP Control Protocol (RTCP) [RFC3550] may be multiplexed onto
>        the same transport-layer flow as the RTP stream with which it is 
>        associated, as described in [RFC5761], or it may be sent on a 
>        separate transport-layer flow;
>    3.  The RTP and RTCP traffic can be multiplexed with other protocols 
>        via UDP encapsulation over a common pair of UDP ports as described 
>        in [RFC5764] as updated by
>        [I-D.petithuguenin-avtcore-rfc5764-mux-fixes]; and
>    4.  The data may be further encapsulated via STUN [RFC5389] and TURN
>        [RFC5766] for NAT (Network Address Translator) traversal.

In the following paragraph (penultimate paragraph in Section 2.1), I suggest changing “unidirectional UDP packet flow” to “transport-layer flow” since it will not be unidirectional (since RTCP also flows in the reverse direction), and might not use UDP. 

In the first paragraph of section 2.2, I suggest changing “multiplexed over RTP sessions” to “multiplexed in a single RTP session” and “multiplexing of source streams in RTP sessions” to “multiplexing of source streams in a single RTP session”. 

In Section 6, the second bullet point refers to “an RTCP session”. There is no such thing. Rather, within a single RTP session there are RTCP packets sent that give information about the RTP streams that are being sent, and that report on the reception quality of RTP streams being received. Using a single PHB and DSCP for all RTCP packets within an RTP session might make sense, but it’s important to note that one role of RTCP is to provide an estimate of the round-trip time seen by the media, so the PHB/DSCP will have to be chosen with care to avoid biasing that estimate too much.

Editorial: the protocol is called WebRTC, but the working group is RTCWEB. Accordingly, I think all instances of “RTCWEB” in this draft need to change to “WebRTC”.


On 9 Aug 2014, at 19:48, Ben Campbell <> wrote:
> Hi,
> The DART working group has started a last call for draft-ietf-dart-dscp-rtp-02, ending on August 22. This informational draft describes considerations for the use of DiffServ code points in situations where one multiplexes multiple RTP packet streams, and potentially other protocol streams, that share the same 5-tuple. 
> Since this draft is likely of interest to several working groups, I would like to solicit additional reviews from participants of TSVWG, AVTCORE, MMUSIC, CLUE, RTCWEB, and RMCAT. Please send any feedback (including feedback to the effect of "this is ready to progress") to the DART working group mailing list.
> Thanks!
> Ben.
> Begin forwarded message:
>> Resent-From:
>> From: Ben Campbell <>
>> Subject: WGLC: draft-ietf-dart-dscp-rtp-02
>> Date: August 9, 2014 at 12:50:16 PM CDT
>> Resent-To:,,
>> To:
>> Cc:,, Richard Barnes <>
>> This is a DART Working Group Last Call of draft-ietf-dart-dscp-rtp-02. It's available on the data tracker at the following URL:
>> The WGLC will conclude on 22 August, 2014. Please send your comments to the authors and the DART mailing list. If you've reviewed it and think it's good to go, please say so.
>> Thanks!
>> Ben.
> _______________________________________________
> Audio/Video Transport Core Maintenance

Colin Perkins