Re: [Dart] I-D Action: draft-york-dart-dscp-rtp-00.txt

Harald Alvestrand <> Wed, 11 June 2014 11:09 UTC

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Subject: Re: [Dart] I-D Action: draft-york-dart-dscp-rtp-00.txt
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On 06/10/2014 09:44 PM, Black, David wrote:
> Hi Ruediger,
>> thanks, your team created a well written draft.
> Great - thanks for taking a look.
>> If a MediaStream is carried in a RTP session, the text may also explicitely
>> say that (it says MediaStreamTrack == media flow, which is carried in an
>> individual RTP packet stream)
> As I read Section 11 of draft-ietf-rtcweb-rtp-usage-15, for RTCWEB,
> that would happen only when the MediaStream contains exactly one
> MediaStreamTrack.  The use of these terms in bullet item 1 in section 2
> is intended to be specific to RTCWEB, but we could add text elsewhere to
> point out that non-RTCWEB usage of MediaStreams could use an RTP packet
> stream for each Media Stream, independent of how many MediaStreamTracks
> each MediaStream contains.
> Could you suggest a reference that could be cited for usage of an RTP
> packet stream for each MediaStream independent of how many
> MediaStreamTracks each MediaStream contains?

Careful - "RTP packet stream" is one of those concepts that can be 
meaningless without citing a specific definition.

draft-ietf-avtext-rtp-grouping-taxonomy is probably the best reference 
at the moment. It says this about "Packet stream":

2.1.10.  Packet Stream

    A stream of RTP packets containing media data, source or redundant.
    The Packet Stream is identified by an SSRC belonging to a particular
    RTP session.  The RTP session is identified as discussed in
    Section 2.2.2.

Under this definition, it's impossible to put more than one 
MediaStreamTrack into a packet stream.
In a lot of cases (FEC, redundancy, SVC, simulcast), there will be 
multiple RTP packet streams associated with one MediaStreamTrack.

The term is used only once in the draft, but unfortunately defines a new 
term to mean the same thing:

    The most common protocol used for real time media is the Real-Time
    Transport Protocol (RTP)[RFC3550].  RTP defines the mechanism by
    which real-time data is transmitted between hosts on the Internet.
    With most applications, a single media type (e.g., audio) is
    transmitted within a single RTP session.  However, it is possible to
    transmit multiple, distinct media flows over the same RTP session as
    individual RTP packet streams.  This is referred to as RTP

    For the purposes of this draft, the term "media flow" refers to a
    sequence of packets that is transmitted as a single RTP packet

The term "media flow" doesn't sound to me like a good term to use for 
this concept.
Would the authors be willing to consider switching to "packet stream"?