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

"Black, David" <david.black@emc.com> Wed, 11 June 2014 14:11 UTC

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From: "Black, David" <david.black@emc.com>
To: Harald Alvestrand <harald@alvestrand.no>, "dart@ietf.org" <dart@ietf.org>
Date: Wed, 11 Jun 2014 10:10:45 -0400
Thread-Topic: [Dart] I-D Action: draft-york-dart-dscp-rtp-00.txt
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Subject: Re: [Dart] I-D Action: draft-york-dart-dscp-rtp-00.txt
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Harald,

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

+1

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

Thank you for the pointer, we'll be happy to cite and use that definition.

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

Speaking only for myself, I don't think so, because there are two
different concepts involved - IMHO, this draft needs two terms to refer to:

	a) What the application sends.
	b) How RTP carries that traffic.

"RTP packet stream" is clearly the right RTP term for the latter, and the
draft was written that way.  I would've liked to have used "media stream"
for the former, but W3C has defined MediaStream to not be a single stream
of media (go figure ...).  "media flow" seemed to be as good a term as
any for that concept, and "media flow" is already used for that concept
in draft-ietf-tsvwg-rtcweb-qos (i.e., it was not invented for this draft).

If you have an alternative term, please suggest it ... and then the authors
of any other draft that uses "media flow" (starting w/the rtcweb-qos draft)
will have to go make corresponding changes.

Thanks,
--David

> -----Original Message-----
> From: Dart [mailto:dart-bounces@ietf.org] On Behalf Of Harald Alvestrand
> Sent: Wednesday, June 11, 2014 7:09 AM
> To: dart@ietf.org
> Subject: Re: [Dart] I-D Action: draft-york-dart-dscp-rtp-00.txt
> 
> 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
>     multiplexing.
> 
>     For the purposes of this draft, the term "media flow" refers to a
>     sequence of packets that is transmitted as a single RTP packet
>     stream.
> 
> 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"?
> 
> 
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