Subject: [MMUSIC] Multiple 'm' lines with the same port number

From: Rajesh Kumar <[email-masked]>
Date: Fri, 25 Apr 2003 11:40:36 -0700

Colin, Steve,
Is the use of multiple 'm=' lines with the same port number within an SDP 
descriptor permitted?

In SDP, as it is defined, all the media formats on an 'm=' line are of the 
same media type (e.g. audio, image, text etc.).

Now consider the case in which two RTP media formats can use a destination 
port but are registered under different MIME types. For example, text/t140 
and audio/PCMU. In this case, there is no recourse except to use multiple 
media lines with the same port number. For example:

m=audio 49232 RTP/AVP 0
m=text 49232 RTP/AVP 100
a=rtpmap:100 t140

My thought is that although this is unsual, it is not proscribed by SDP 
syntax in RFC 2327 or in its latest internet draft.

Thanks,

Rajesh

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From: Stephen Casner <[email-masked]>
Date: Fri, 25 Apr 2003 11:57:23 -0700 (PDT)

Rajesh,

> Is the use of multiple 'm=' lines with the same port number within an SDP

Presuming that the multiple m= lines are all RTP/AVP, then doing so
causes all of the payload formats listed on the m= lines to become
part of a single RTP session (unless they are distinguished by some
other aspect of addressing).  Section 5.2 of the RTP spec addresses
NOT multiplexing different media in the same RTP session.

> media lines with the same port number. For example:

The "can use a destination port" is the tricky part.  We may not agree.

> syntax in RFC 2327 or in its latest internet draft.

I believe this is proscribed by section 5.2 of the RTP spec because
these two media should not be multiplexed in one RTP session.

My bet is that the real example you are thinking about is fax or modem
over IP.  We've been there before.

                                                        -- Steve

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From: Rajesh Kumar <[email-masked]>
Date: Fri, 25 Apr 2003 13:16:03 -0700

Steve,
The real examples I am thinking of are all RTP-encapsulated payloads such 
as text/t140 and the new t38-facsimile-over-RTP work that I am involved in 
with Mario Garakani and Paul Jones and others. In all these cases, the 
payload is encaspulated in RTP and uses the RTP/AVP profile. So this is not 
a repeat of an old issue.

So if facsimile were encapsulated in RTP, you would need a new MIME 
audio/t38 to multiplex it with PCMU into the same UDP port? The current 
MIME for T.38 facsimile over IP is image/t38. Or does the standard 
proscribe the multiplexing into a single port of  PCMU and 
RTP-encapsulate-facsimile altogether? I do not think so.

Thanks,

Rajesh

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From: Flemming Andreasen <[email-masked]>
Date: Fri, 25 Apr 2003 16:39:39 -0400

> The "can use a destination port" is the tricky part.  We may not agree.

We also seem to be entering a bit of a gray area here in terms of offer/answer
(RFC 3264). It's not clear that the definition of "m=" lines and media streams
allow for something like this; for sure the parties would have to recognize that
the two "m=" lines in fact are one media stream and hence should not be sending
to "both" at the same time.  However, I'm not aware of such behavior being
specified anywhere.

-- Flemming

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From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 28 Apr 2003 19:10:53 -0400

inline.

> to "both" at the same time.  However, I'm not aware of such behavior being

Using two m lines to represent alternate encodings for a stream is not 
consistent with the offer/answer model, IMHO. RFC 3264 has mechanisms 
for transitioning from audio to fax, for example, but these require a 
re-invite to do:

8.3.3 Changing Media Types

    The media type (audio, video, etc.) for a stream MAY be changed.  It
    is RECOMMENDED that the media type be changed (as opposed to adding a
    new stream), when the same logical data is being conveyed, but just
    in a different media format.  This is particularly useful for
    changing between voiceband fax and fax in a single stream, which are
    both separate media types.  To do this, the offerer creates a new
    media description, with a new media type, in place of the description
    in the previous SDP which is to be changed.

-Jonathan R.
-- 
Jonathan D. Rosenberg, Ph.D.                600 Lanidex Plaza
Chief Scientist                             Parsippany, NJ 07054-2711
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