Subject: [MMUSIC] Media Attribute lines in answer for unsupported media lines

From: M Subramanya-A5007C <[email-masked]>
Date: Thu, 23 Jun 2005 17:31:46 +0530

Hi,
	we have a scenario like...

	<------ offer with 2 media lines
	
	m=audio 50001 RTP/AVP 8 
	a=rtpmap:8 SIPUA_G711U/8000
	m=audio 50001 RTP/AVP 100
	a=rtpmap:100 SIPUA_DTMF/8000 

	--------> ? Now in the answer is it mandatory to send out media
attribute lines(a lines) for unsupported media line 	like shown below 

	m=audio 0 RTP/AVP 8 		is this valid...?
	m=audio 50001 RTP/AVP 100
	a=rtpmap:100 SIPUA_DTMF/8000 

-Subbu

	

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 23 Jun 2005 11:23:24 -0400

inline.

> 	a=rtpmap:100 SIPUA_DTMF/8000 

These m-lines are strange in that they indicate the same port for two 
different media streams. We've been through many discussions on that and 
I believe the conclusion is that it is not allowed.

> 	a=rtpmap:100 SIPUA_DTMF/8000 

note section 6 of rfc3264:

An offered stream MAY be rejected in the answer, for any reason.  If
    a stream is rejected, the offerer and answerer MUST NOT generate
    media (or RTCP packets) for that stream.  To reject an offered
    stream, the port number in the corresponding stream in the answer
    MUST be set to zero.  Any media formats listed are ignored.  At least
    one MUST be present, as specified by SDP.

so what you list in the answer is fine.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
Director, Service Provider VoIP Architecture   Parsippany, NJ 07054-2711
Cisco Systems
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