Subject: [MMUSIC] Codec equality in FID

From: Flemming Andreasen <[email-masked]>
Date: Thu, 25 Apr 2002 14:46:47 -0400

The fid-draft says:

" The application encodes the media using the current codec and checks
   one by one all the "m" lines that are part of the flow. If a
   particular "m" line contains the codec being used and the direction
   attribute is "sendonly" or "sendrecv" a copy of the encoded media is
   sent to the address/port specified in that particular media stream."

The question I have here is how we define "codec". To be more specific,
let's say I have one media line using RTP and another media line using
something different (e.g. SRTP or perhaps a completely different type of
transport) but it just so happens that they all list PCMU as a valid
encoding method. Now do I need to send PCMU to all the destinations, or
just the one with the transport (e.g. RTP) I have chosen to use ?
Needless to say properly, I hope the answer is the latter.

-- Flemming

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From: Gonzalo Camarillo <[email-masked]>
Date: Fri, 26 Apr 2002 08:27:21 +0300

Hello,

> Needless to say properly, I hope the answer is the latter.

I guess you like the latter solution because it is easier to generate
for the user agent, but I would say that the proper thing to do is the
former.

For instance, we are in the same intranet. You support AMR and G.711. I
want you to send me AMR using RTP to my mobile phone directly. However,
if you send G.711 at a particular point of time, I want you to send it
to a transcoder that will send me AMR. This transcoder is outside our
intranet, so I want you to use SRTP to communicate with it.

Therefore, the demultiplexing is performed based on the codec in use
(i.e., format of the media), and not on the transport in use.

Does this make sense to you?

Thanks,

Gonzalo
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From: Flemming Andreasen <[email-masked]>
Date: Fri, 26 Apr 2002 09:06:12 -0400

> (i.e., format of the media), and not on the transport in use.

Understood - my question was whether it was on the combination of transport
and codec or just codec. If it is done based on both transport and codec, then
FID provides a useful way of negotiating both codecs and transport in a single
round-trip. If not, then we'll have to do multiple round-trips (e.g. based on
SIMCAP information) for this, which I was hoping to avoid. Apart from this, I
think we are in danger of getting into fuzzy semantics when we start having
more fundamental changes in transports, e.g. ATM - when is a "codec" the same
and when is it not ? Can you elaborate more on what the motivation for doing
it based on only codec is ?

> Does this make sense to you?

Not sure. We'll call you G, me F, and the transcoder T. If I understand the
example correctly, then either
a) G sends an SDP with two m lines containing AMR (one RTP and the other
SRTP), or
b) G sends an SDP with a single m line containing AMR, and somebody in the
middle adds another m line for the transcoder.

In case a), G may end up receiving both the RTP and SRTP stream if F supports
both, which I don't see why you would want to. In case b), there are no
duplicate codecs on the m line, and hence the example doesn't apply (F sends
AMR to G or F sends PCMU to T). Did I miss something ?

-- Flemming