Subject: RE: [SIP] SDP: Sub-set or not in the 200 OK?

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Tue, 3 Apr 2001 03:34:16 -0700

Victor,

I think there is some confusion here: selecting different vocoders for each
direction is entirely permissible. However, Gonzalo's point is illustrated
in the following example:
A can receive 0,1,2,3 but can only send 0. And B can send and receive only 2
& 3. In this case a bidirectional session is not possible because they is no
vocoder that can be sent from A->B.

Two approaches:
-	Use stardard SDP receive only SDP vocoder lists and then report
incompatibilities in the SIP plane (one side can always detect this
non-bidirectional situation and can terminate the call if desired - it may
be happy to just receive). This is still more flexible and simpler in my
mind.
-	Only place send and receive in the SDP list which creates big
headaches for vocoders which can only be send or received (eg video or DTMF)
(eg requires use of fid draft so that multiple media announcements can be
linked to the same flow id). 

Regards,

Robert.

-----Original Message-----
From:	Victor A Paulsamy [mailto:[email-masked]]
Sent:	Monday, April 02, 2001 9:51 PM
To:	Gonzalo Camarillo
Cc:	SIP; Victor A Paulsamy
Subject:	RE: [SIP] SDP: Sub-set or not in the 200 OK?

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of Gonzalo Camarillo
Sent: Monday, April 02, 2001 1:54 PM
To: Fairlie-Cuninghame, Robert
Cc: 'Jonathan Rosenberg'; sip; 'Robert Sparks'; dov; Michael Thomas;
Victor A Paulsamy
Subject: Re: [SIP] SDP: Sub-set or not in the 200 OK?

Gonzalo Camarillo wrote:

>My proposal is to go for solution number 2. This gives a symmetrical
>negotiation. Besides, it covers more cases than solution number 1). It
>can happen that for a particular codec, the callee can send AND receive
>it but the caller can just send it. Therefore, the caller does not
>include it in the INVITE. However, if the callee lists it in the 200 OK,
>the caller will be able to send it.

Victor A Paulsamy wrote:

> This holds good *only* for sending DTMF tones. Otherwise, this would mean
> two media streams encoded *differently* (one in each direction) in a
> session. Is my understanding correct?

Gonzalo Camarillo wrote:

>No. What you want to do can be done using the SDP fid attribute (I will
>be releasing the new version of the draft today or tomorrow). Here we
>are dealing with plain SDP.

Let me give an example. Using codec '0' the caller can only send and the
callee can send and receive.

The caller sends out codec 3, 4, 5 in the INVITE. The callee responds with
0, 3, 4, 5 in the 200 OK.
If the caller choses to send its media stream using codec '0' (since it's
included in 200 OK), it has to use *another codec*
to receive the media stream as it cannot receive using codec '0'. This would
give rise to two codecs, one for transmission and another for reception.

This fits the model of transmitting DTMF tones, where PCM codecs will be
used to transmit it rather than speech codecs.

Warm regards,

--victor

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---

From: "Victor A Paulsamy" <[email-masked]>
Date: Tue, 3 Apr 2001 10:56:05 -0400

RE: [SIP] SDP: Sub-set or not in the 200 OK?Hi Robert,

  Selecting two different vocoders for each direction is *possible* with the
standard proposed by Gonzalo. However I'm not seeing how it's possible with
the existing one.

  Here is an example. A caller sends 3, 4, 5 in the INVITE. Callee will
check to see if these formats can be supported. For a session to be set up
successfully, the callee should support at least one of these formats.
Otherwise, it would report with "client-error: 415 Unsupported Media Type."
Selecting two different codecs for each direction will not arise here.

  BTW, why would any one designate a codec as receive only and send only?
It's supposed to be used for both.

  Warm regards,

  --victor

   -----Original Message-----
  From: Fairlie-Cuninghame, Robert [mailto:[email-masked]]
  Sent: Tuesday, April 03, 2001 6:34 AM
  To: 'Victor A Paulsamy'; Gonzalo Camarillo
  Cc: SIP
  Subject: RE: [SIP] SDP: Sub-set or not in the 200 OK?

  Victor,

  I think there is some confusion here: selecting different vocoders for
each direction is entirely permissible. However, Gonzalo's point is
illustrated in the following example:

  A can receive 0,1,2,3 but can only send 0. And B can send and receive only
2 & 3. In this case a bidirectional session is not possible because they is
no vocoder that can be sent from A->B.

  Two approaches:

    a.. Use stardard SDP receive only SDP vocoder lists and then report
incompatibilities in the SIP plane (one side can always detect this
non-bidirectional situation and can terminate the call if desired - it may
be happy to just receive). This is still more flexible and simpler in my
mind.
    b.. Only place send and receive in the SDP list which creates big
headaches for vocoders which can only be send or received (eg video or DTMF)
(eg requires use of fid draft so that multiple media announcements can be
linked to the same flow id).

  Regards,

  Robert.

  -----Original Message-----
  From:   Victor A Paulsamy [mailto:[email-masked]]
  Sent:   Monday, April 02, 2001 9:51 PM
  To:     Gonzalo Camarillo
  Cc:     SIP; Victor A Paulsamy
  Subject:        RE: [SIP] SDP: Sub-set or not in the 200 OK?

  -----Original Message-----
  From: [email-masked]
  [mailto:[email-masked]]On Behalf Of Gonzalo Camarillo
  Sent: Monday, April 02, 2001 1:54 PM
  To: Fairlie-Cuninghame, Robert
  Cc: 'Jonathan Rosenberg'; sip; 'Robert Sparks'; dov; Michael Thomas;
  Victor A Paulsamy
  Subject: Re: [SIP] SDP: Sub-set or not in the 200 OK?

  to receive the media stream as it cannot receive using codec '0'. This
would

  Warm regards,

  --victor