Subject: RE: [SIP] SIP - H.323 - INVITE - No media line.

From: "Maddux, Michel" <[email-masked]>
Date: Wed, 15 Nov 2000 16:41:03 -0600

RFC 2543 Section B.4 states:
 
<!--StartFragment-->
B.4 Delayed Media Streams

   In some cases, a caller may not know the set of media formats which
   it can support at the time it would like to issue an invitation. This
   is the case when the caller is actually a gateway to another protocol
   which performs media format negotiation after call setup. When this
   occurs, a caller MAY issue an INVITE with a session description that
   contains no media lines. The callee SHOULD interpret this to mean
   that the caller wishes to participate in a multimedia session
   described by the session description, but that the media streams are
   not yet known. The callee SHOULD return a session description
   indicating the streams and media formats it is willing to support,
   however. The caller MAY update the session description either in the
   ACK request or in a re-INVITE at a later time, once the streams are
   known.

The I-D - Singh/Schulzrinne states: "... [snip - to page 14...] The default
session description MUST be either empty or conatin media descriptio (m=)
lines indicating lthe minimal capabilities of any H.323 terminal handled by
the IWF."

The sip UAC that I'm using for testing either doesn't respond correctly
(e.g. with the appropriate media),
or the I-D is in conflict with the spec on this item, or perhaps I'm sending
an inappropriate value for the "empty"
string in the SDP m= line.  From my reading, I interpret the phrase "empty"
to mean the following line:
 
c=IN IP4 0.0.0.0
m=audio 0 RTP/AVP 0
 
Yet when I send this line, instead of getting back the expected line:
 
c=IN IP4 real.ip.address.here
m=audio 8000 RTP/AVP 8
 
I receive: m=audio 0 RTP/AVP 0
 
A blank media line is either:
 
m=
or no m= line at all.  In both case the UAC issues a rejection.  
 
so I'm still unable to ascertain how to setup the port.  Any comments?
thanks. /m.

-----Original Message-----
From: Vladislav Zubarev [mailto:[email-masked]]
Sent: Tuesday, November 14, 2000 6:25 PM
To: Igor Slepchin
Cc: Maddux, Michel; [email-masked]; Lawhorn, Tom; Byrne, Sam;
Hurlbert, John
Subject: Re: [SIP] SIP - H.323 - INVITE - No media line.

Again, call scenarios with different issues resolving (like this) can 
be found in the following document: 

http://search.ietf.org/internet-drafts/draft-singh-sip-h323-01.txt
<http://search.ietf.org/internet-drafts/draft-singh-sip-h323-01.txt>  

Thanx. 

Igor Slepchin wrote: 

> 1) When 323 calls, wait until it opens first media channel. 
> Then issue the 
> INVITE, noting only a media line of that media type but with 
> a phony port. 
> When Sip replies (200 OK presumed), complete the 323 media 
> channel being 
> negotiated, giving it the Sip's media port. Then negotiate the reverse 
> channel with 323, saving its port, which is then transmitted 
> to Sip via an 
> ACK with a message body correcting the 323 media port. 

This is illegal in SIP (see section 4.2.1 in 2543bis). You should use 
re-INVITE if you want to modify the session offered in INVITE request. 

Thank you, 
Igor Slepchin 

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-- 

Vladislav Zubarev         mailto:[email-masked]
<mailto:[email-masked]> 

Software Engineer   

Vovida Networks, Inc.     http://www.vovida.com <http://www.vovida.com> 

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From: Vladislav Zubarev <[email-masked]>
Date: Wed, 15 Nov 2000 16:28:15 -0800

"Empty" means SIP message does NOT contain SDP AT ALL, which I guess
means the same as it says in RFC2543 (no media lines).

The problem is the most SUAs do not support this "empty" INVITE right now.

The UA, which supports this feature for sure is CISCO IP PHONE 7960,
with the latest load. There could be some other implementations out there
as well.

Thanx.

--
Vladislav Zubarev        mailto:[email-masked]
Software Engineer
Vovida Networks, Inc.    http://www.vovida.com