Subject: RE: [SIP] Multiple sessions in SDP payload

From: Christian Huitema <[email-masked]>
Date: Mon, 17 Jul 2000 22:43:10 -0700

The problem is, there is a fundamental contradiction between two
requirements. If I include 3 "m=audio" lines, do they describe 3 ways to
pass audio, such as IPv4, IPv6 and POTS, or do I describe 3 audio channels
that should be deployed in parallel? My take so far would be to use multiple
instance of the same media within a single SDP payload to describe
alternative encodings, and to use multiple SDP payloads, using MIME
multipart, to encode several parallel sessions.
 
Christian Huitema

-----Original Message-----
From: William Lee [mailto:[email-masked]]
Sent: Monday, July 17, 2000 10:15 PM
To: 'Philip Levis'
Cc: [email-masked]
Subject: RE: [SIP] Multiple sessions in SDP payload

Yes, according to the spec, multiple SDP sessions may be specified.  
At the 2nd SIP bake-off, we tested a trial implementation of our 
SDP parser/statemachine that sent and received multiple SDP sessions. 
No one supported this, and as far as I know no one else does today. 

William Lee 
[email-masked] 

	-----Original Message----- 
From:   Philip Levis [SMTP:[email-masked]] 
Sent:   Monday, July 17, 2000 4:53 PM 
To:     [email-masked] 
Subject:        [SIP] Multiple sessions in SDP payload 

	I've looked over 2543 several times, and been unable to determine
whether 
an SDP payload can have multiple sessions specified within it. RFC 2327 
(SDP) states that multiple sessions may be specified unless annnounced by 
SAP. 

	I take this to mean that multiple SDP sessions may be specified. Is
this 
correct? 

	The next question is, what would multiple SDP sessions signify? 

	Phil 

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 18 Jul 2000 10:16:34 -0400

Christian Huitema wrote:
> The problem is, there is a fundamental contradiction between two requirements. If I include 3 "m=audio" lines, do they describe
> 3 ways to pass audio, such as IPv4, IPv6 and POTS, or do I describe 3 audio channels that should be deployed in parallel? My
> take so far would be to use multiple instance of the same media within a single SDP payload to describe alternative encodings,
> and to use multiple SDP payloads, using MIME multipart, to encode several parallel sessions.

I think three lines of the same type means the same thing it does in
SAPs usage - parallel media streams, not alternatives. This, in fact, is
necessary as per the other thread on sending DTMF using rfc2833 to third
party application servers.

As for multiple sessions per SDP, I don't know what the value of this
would be. rfc2543 is silent on this, and I would suggest it be
disallowed within SIP.

-Jonathan R.

-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

---

From: Philip Levis <[email-masked]>
Date: Tue, 18 Jul 2000 09:28:04 -0600 (MDT)

> would be. rfc2543 is silent on this, and I would suggest it be

There is a simple value; it provides additional information at little
cost.

In short, it allows for an explicit distinction to be made between
multiple streams, and multiple options for a stream. There's a very
pronounced (and useful) difference between:

v=0
...
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
m=application 32416 udp wb
...

and
v=0
...
m=audio 49170 RTP/AVP 0
...
v=0
...
m=video 51372 RTP/AVP 31
...
v=0
...
m=application 32416 udp wb
...

It would allow a UA to make more intelligent decisions on what SDP to
reply with, if some codecs are more desirable than others (say, because of
hardware decoding).

Phil

---

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 18 Jul 2000 11:58:53 -0400

> pronounced (and useful) difference between:

Well, this isn't how it works.

Multiple media streams in SDP means exactly that, multiple media
streams. Thats the usage in SAP, thats been the usage for SIP. I don't
want to change this. If you want alternatives for the same stream, you
list multiple codecs on the same m line. Thats the defined usage.

-Jonathan R.

---

From: Philip Levis <[email-masked]>
Date: Tue, 18 Jul 2000 10:08:46 -0600 (MDT)

> list multiple codecs on the same m line. Thats the defined usage.

You are correct. A closer reading of 2327 even states that the desired
functionality (preference) is already present in 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." (pg. 20)

In light of this, I agree that that the presence of multiple SDP sessions
is not useful (at least, in any circumstance I can come up with).

Phil