Subject: Alice and Bob asking questions :-)

From: "Shahar Pavel" <[email-masked]>
Date: Thu, 29 Mar 2001 12:56:09 +0200

Hi

My name is Shahar and I am a part of a team which develop video conference
using SIP.

During implementation of SIP I faced some question I hope this forum would
help me with.

Lets say I am a user who would like to initiate AUDIO/VIDEO  session using
SIP.

My name is Alice and the target user name is Bob.

In order to begin the session Alice sends an Invite to Bob with SIP+SDP
message.
Lets say Alice can support 5 different Vocoders and Bob only 2.

1.	Questions :

1.1	What exactly  does the SDP's  media descriptor describes ? Is it what
Alice capable to receive ( e.g. Vocoders types )?  is it what Alice would
like to send ? both maybe ?
1.2	Is it possible for Alice to declare that she is going to send MPEG4
(with out the capability to receive MPEG4 ) and capable to receive MPEG2
(with out the capability to send MPEG2 )

Now Bob receives the message .

2.	Questions :

2.1	 What exactly  does this SDP's  media descriptor describes ?
2.2	what are the possible conversion Bob can do to the  SDP message ?
2.3	What does it means when Bob delete a Media Description Line ? Add a
media description line ? change a Media Description line ?
2.4	What Bob would do if he can support only part of the Media Description.

After Bob convert the SDP message he would send it back with the response to
Alice.

Alice receive the message with the SDP message.

3.	Questions :

3.1	What exactly  does this SDP's  media descriptor describes ?
3.2	In case Alice "dislike" the SDP message would she Initiate BYE method or
ACK and then BYE.
3.3	In case Alice reply with ACK , would she attach a SDP message to the ACK
message ? What for ?
3.4	What this SDP message means ?
3.5	What would Bob understand from Alice not sending an SDP message with the
ACK message.

Thank you very much for your help

shahar

Pavel Shahar
Emblaze Research Ltd.
[email-masked]
Phone: +972-9-8658570/320  Cell: +972-53-253576

---

From: Gonzalo Camarillo <[email-masked]>
Date: Sun, 01 Apr 2001 01:48:27 +0300

Hello,

Comments inline:

> like to send ? both maybe ?

For sendrecv streams it means the codecs you can both send AND receive.
Note that a m line without any "a" line attribute is considered to be
sendrecv.

> (with out the capability to send MPEG2 )

You cannot do this using plain-SDP. You might have to use the fid
attribute, for instance. The fid draft will be out very soon from now.

> 2.1      What exactly  does this SDP's  media descriptor describes ?

Since the message that Bob receives is the same as Alice sent, it means
the same :o)

> 2.2     what are the possible conversion Bob can do to the  SDP message ?

Bob does not do any conversion. It prepares its own SDP and sends it in
a 200 OK response.
It it does not want to accept a media stream, it lists the m line with
port number 0. Otherwise, it lists the codecs he can both send AND
receive also.

> media description line ? change a Media Description line ?

Bob cannot delete a media description line, since in order to align
media streams SIP clients match the nth m lines in the SDP. It cannot
add an m line either. It will have to re-INVITE in order to do that.

> 2.4     What Bob would do if he can support only part of the Media Description.

Bob lists the codecs he can can send AND receive. If they are a sub set
of Alice's codecs, the response is 200 OK, because establishing the
session is possible. If Bob does not support any codec supported by
Alice, the response is 488 or 606. The session cannot be established.

This method of using SDP in SIP leaves outside some scenarios in which
it would be possible to establish a session, but both SIP entities think
that it is not. However, these are corner cases (Bob can send but not
receive a codec that Alice can receive but not send and the other way
around with another codec...)

> After Bob convert the SDP message he would send it back with the response to

Bob does not convert the SDP. It just provides his own SDP. It has to
follow some rules, like respecting the number of m lines, but it is not
called SDP conversion.

> 3.1     What exactly  does this SDP's  media descriptor describes ?

Again, unless somebody is changing the messages on-the-wire, it means
the same as when Bob sent it.

> 3.2     In case Alice "dislike" the SDP message would she Initiate BYE method or

ACK it and then BYE.

> 3.3     In case Alice reply with ACK , would she attach a SDP message to the ACK

SDP can be carried either in the INVITE or in the ACK. NOT in both.

You can, for the time being, read appendix B of RFC 2543. When the new
bis draft is released, you will be able to find there this behavior
explained in detail.

Best regards,

Gonzalo

-- 
Gonzalo Camarillo                    Phone :   +1 212 939 71 71
Columbia University                  Mobile:  +358 40 702 35 35
472 Computer Science Building        Fax   :  +358  9 299 30 52
1214 Amsterdam Ave., Mail Code 0401  http://www.hut.fi/~gonzalo
New York, NY 10027                   
USA                              [email-masked]