Subject: [AVT] [MMUSIC]Heterogeneous flows in draft-ietf-mmusic-fid-06.txt

From: Rajesh Kumar <[email-masked]>
Date: Mon, 18 Mar 2002 23:16:28 -0800

"Media flow consists of a single media instance, e.g., an audio
    stream or a video stream as well as a single whiteboard or shared
    application group. When using RTP, a media flow comprises one or
    more RTP sessions."

draft-ietf-mmusic-fid-06 seems to imply that the media streams (MID's) in a 
flow (FID) must be of the same media type.

While this is OK for cell phones, consider the case in which a flow 
transitions from media type = audio to media type = image (T.38 fax relay) 
or media type = application (MoIP).

In these cases, there is an association between the different, 
heterogeneous 'm' lines since you send one and not all the media streams. I 
suggest that draft-ietf-mmusic-fid-06 be made FoIP/MoIP-compatible by 
allowing this case. This will be useful in the case in which a 
heterogeneous flow switches between audio, FoIP or MoIP, while using 
different ports.

The case when audio, FoIP and MoIP use the same port (although at different 
times)has been covered by Flemming Andreasen in 
http://search.ietf.org/internet-drafts/draft-andreasen-mmusic-sdp-simcap-05.txt. 
The only problem I see with this I-D is that it requires (not just permit) 
that active media properties be reiterated within the capability set, if 
there is one. I believe this redundant information. It is sufficient for 
all practical purposes if the capability set describes latent capabilities 
without requiring the active properties to be repeated. Repeating them does 
no harm, it only adds "fluff" to the description. So if we look at the 
following para:

"The capability set MUST begin with a single sequence number followed by 
one or more capability descriptions listing all media formats the endpoint 
is currently able and willing to support. More specifically, if a media 
format is included in a media ("m=") line, then by definition the media 
format MUST be included in either a session capability or a media stream 
capability for that media line."

I'd suggest this be re-written as:

"The capability set MUST begin with a single sequence number followed by 
one or more capability descriptions listing all media formats the endpoint 
is currently able and willing to support, and which are not included in a 
media ("m=") line. It MAY also include, as a session capability or a media 
stream capability, any media format also included in a media ("m=") line."

Thanks,

Rajesh Kumar
Cisco Systems

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Gonzalo Camarillo <[email-masked]>
Date: Wed, 27 Mar 2002 00:50:35 +0200

Hi,

inline:

> heterogeneous flow switches between audio, FoIP or MoIP, while using

Could you be more specific and illustrate the scenario you have in mind
with an example of session description?

FID as it is already covers voice band data, but I guess you are
thinking about transmitting the actual image of a fax, rather than its
encoding in the telephone line, right?

Thanks,

Gonzalo