Subject: [SIP] What does m-line mean here?

From: "Victor A Paulsamy" <[email-masked]>
Date: Fri, 16 Feb 2001 10:56:26 -0500

Hi,

I'm having difficulty in understanding the following from RFC2327, SDP.

     "When a list of payload formats is given, this implies that all of
     these formats may be used in the session, but the first of these
     formats is the default format for the session." -- SDP Standards Track,
page 20.

What does default mean here?

When a m-line reads as "m=audio 49230 RTP/AVP 3 4 5", does this mean that
the remote UA will be requested to use GSM (PT 3) if it could support it and
if it didn't support GSM, the local UA will negotiate for G723 (PT 4) with
the remote UA and so on? (It's clear that I need to understand the above
phrase!).

Thanks for your help and valuable time.

Regards,

--victor

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---

From: "Culpepper, Bert" <[email-masked]>
Date: Fri, 16 Feb 2001 13:12:44 -0600

comments inline..

> -----Original Message-----
> From: Victor A Paulsamy [mailto:[email-masked]]
> Sent: Friday, February 16, 2001 10:56 AM
> To: SIP
> Cc: Victor A Paulsamy
> Subject: [SIP] What does m-line mean here?
> 
> 
> Hi,
> 

The SDP in the sender's message indicates its capabilities and
where any recipients (of the message) are to send the media to. In 
the above case, the sender is indicating it can handle any of 3 different 
media formats all on the same port (49230).  The recipient is free to 
send one or more of the three formats listed.  (I wonder how often a m= 
line like above will be seen in practice.  The COTS hw and sw I'm familiar 
with can't handle multiple simultaneous media encoding formats on a 
single port, or even switching encoding format without being told.)

There is no subsequent media negotiation in SIP in a single session 
establishment request.  Each party "declares" what media formats it 
can support.  Now UAs can negotiate media format in separate session 
establish requests.  One way is for the UAS to respond to an INVITE 
with 415.  The requestor can send another INVITE with a different 
media format and see if the other end accepts it.  Another way is 
for the UAS to accept the request and send a reINVITE with a 
different SDP.

This is my understanding of SDP and its use in SIP.  Others may 
respond if I'm mistaken.

regards,
Bert

---

From: Itamar Gilad <[email-masked]>
Date: Mon, 19 Feb 2001 10:39:37 +0200

My understanding of 'default' in this context is "I support all of the
following payload types, but I prefer using the first one".  Please comment
if this is a wrong interpretation.

  Itamar

> Sent: Fri, February 16, 2001 5:56 PM
> To: SIP
> Cc: Victor A Paulsamy