Subject: [Megaco] doubt about usage of cpar in sdp parameters

From: "Ganesh Kumar" <[email-masked]>
Date: Fri, 23 Dec 2005 13:03:46 +0530

Hello,

            I confused in usage of cpar parameters in sdp.

            In specs it is given that multiple cpar is allowable.

            Which one of the following is correct?

For the following 

m=audio $ RTP/AVP 18 8 0 4

> a=sqn:0

> a=cdsc:1 image udptl t38

> a=cpar: a=T38FaxVersion:0

1.

> a=cpar: a=T38FaxVersion:0 a= T38FaxVersion:0         /* Is it
permissible */

2.

> a=cpar: a=T38FaxVersion:0 

> a=cpar: a=T38FaxVersion:0

       Please clarify which way to use.

Regards,

Ganesh

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From: "Joe Stone \(joestone\)" <[email-masked]>
Date: Fri, 23 Dec 2005 17:11:22 -0800

> a=cpar: a=T38FaxVersion:0

First, I'm glad to see someone using RFC 3407 syntax in the context of
Megaco/H.248 and T.38. I posted to this mailer back in September trying
to find out if anyone was using RFC 3407 in this way. I'd be interested
in better understanding how you intend to use RFC 3407 syntax. I'd like
to see a move towards RFC 3407 in order to create a "strict" T.38 mode
between MGCP, TGCP, NCS, Megaco/H.248 and SIP. Of course, I'm assuming
that you are in fact using RFC 3407 syntax in the context of T.38.

Second, a media format that is included in a media ("m=") line MUST be
included in either a session capability or a media stream capability for
that media line. In other words, assuming that you don't have a session
capability, your example should have been,

	m=audio $ RTP/AVP 18 8 0 4
	a=sqn:0
	a=cdsc: 1 audio RTP/AVP 18 8 0 4 <<< media formats from media
("m=") line
	a=cdsc: 5 image udptl t38
	a=cpar: a=T38FaxVersion:0

> a=cpar: a=T38FaxVersion:0 a=T38FaxVersion:0         /* Is it
permissible */

This is not permissible.

> a=cpar: a=T38FaxVersion:0

This format is correct. I'm not sure what you're trying to accomplish
with respect to T38FaxVersion. You wouldn't want to repeat the same
attribute <type> (i.e., T38FaxVersion) with the same <value> (i.e., 0).
In the case of T38FaxVersion, you only need to advertise the highest
version supported. You would not include multiple capability parameter
lines. I'm guessing that you just grabbed T38FaxVersion as an example
<type>.