Subject: [SIP] T.38 and multiple streams

From: Christer Holmberg <[email-masked]>
Date: Mon, 19 Feb 2001 10:08:22 +0200

Hi,

I have a couple of questions about the t.38 call flow draft. I looked at
the fax discussions in the archieve, but I could not find anything
related to this. It is, however, related to SDP in general, so if it has
been discussed I would be happy if someone could point me to the thread
(and, hopefully, the solution :)

1. F11 INVITE IFTGW UA -> PROXY

In this re-INVITE we add the fax stream to the call. However, t.38 for
SIP recommendation it says that the existing voice stream should remain,
so shouldn't the original voice parameters (m line etc) be included in
the re-INVITE SDP?

2. This questions is depending on the first.

Like I said, the recommendation says that the existing voice stream
should remain, but that it should be muted during the fax transmission.
The problem is that SDP uses the c line, which is for the WHOLE call
(not a single stream), to mute stream(s), so I wonder if it will be
possible to do this on a protocol level, or if it has to be in the
implementation?

Regards,

Christer Holmberg
Ericsson Finland

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From: Jean-Francois Mule <[email-masked]>
Date: Tue, 20 Feb 2001 23:11:14 -0800

Thanks for the comments & questions.  Reply in the text below - in short I
mostly agree with Bert's comments.

Basically, the current ID on sip t.38 was a first text to stimulate
comments.   A couple of other errors have already been raised (Cseq,
Record-route headers).  From this discussion, I note the following:
  - sip-t38 text should be updated to better represent the possible choices
(a/ fax only comm; b/ voice then fax then voice again; c/ voice, then both
voice+fax back to voice only)
  - call flow should be updated also to follow the recommendations made in
t.38 annex D for sip (note that the current call flow represents the voice
then fax then voice again case which is a possible real-life case).

Jean-Francois Mule
Clarent Corp.

> so shouldn't the original voice parameters (m line etc) be included in
In the current ID, we terminate the audio stream and negotiate just a fax
stream (case b/ above).
Yes, we should probably not shut down the voice stream to mute it, i agree
;-)
--> if there is an agreement that the best practice is c/, we'll update the
Internet-draft.  Note however that the current call flow is not illegal...

> should remain, but that it should be muted during the fax
*should*
yes, we should correct that by "adding" a fax stream.  Will propose new call
flow for review in the next couple of days.

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---

From: Christer Holmberg <[email-masked]>
Date: Thu, 22 Feb 2001 00:28:58 +0200

Hi,

>Internet-draft.  Note however that the current call flow is not illegal...

No, it's not illegal from a protocol point of view, but like you also
mentioned, it doesn't follow the recommendation.
 
>flow for review in the next couple of days.

Great :)

Regards,

Christer Holmberg
Ericsson Finland