Subject: RE: [Megaco] SDP parameters

From: "Wainwright, John" <[email-masked]>
Date: Mon, 14 Nov 2005 06:20:14 -0800

Thanks.  LocalControl mode property is described with respect to the
exterior of the context so is it a correct assumption to say that the
relevant SDP data is the same ?
Would an MGC ever change the mode using only the SDP data and not the
LocalControl data ?

John

-----Original Message-----
From: Christian Groves [mailto:[email-masked]] 
Sent: Sunday, November 13, 2005 5:38 PM
To: Wainwright, John
Cc: [email-masked]
Subject: Re: [Megaco] SDP parameters

Hello John,

The following was added H.248.1v3 for clarification:
7.1.7 LocalControl
...
The LocalControl Mode Property takes precedence over any mode specified 
in the Local and Remote Descriptors.
...

Regards, Christian

Wainwright, John wrote:

>Can anyone help me with the relationship of the SDP parameter with 
>values "a=sendrecv", "a=sendonly" , "a=recvonly" and the LOCALCONTROL / 
>MODE descriptor value? What action is taken if these values are not 
>consistent with each other ? Does the Megaco data override the SDP data 
>?
>
>Thanks
>John
>
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---

From: "Joe Stone \(joestone\)" <[email-masked]>
Date: Mon, 14 Nov 2005 09:44:48 -0800

> Thanks.  LocalControl mode property is described with respect
> to the exterior of the context so is it a correct assumption
> to say that the relevant SDP data is the same ?

Consider the case in which a SIP endpoint sends a re-INVITE offer with
a=sendonly corresponding to call hold (the recommended SIP call hold
technique). Based on the following "clarification",

	The LocalControl Mode Property takes precedence over any
	mode specified in the Local and Remote Descriptors.

the MGC must specify a LocalControl Mode Property that is consistent
with the a=<mode> SDP attribute, typically receive-only. This would be
reflected in the LocalDescriptor returned by the IP termination,
a=recvonly, and ultimately as part of the SIP re-INVITE answer.

I think that it's worth noting that CableLabs/PacketCable opted to parse
and ignore and not generate the a=<mode> SDP attribute with TGCP/NCS.
This makes it clear that the interworking responsibility lies with the
MGC. You avoid this conflict between LocalControl (i.e., ConnectionMode
in the case of TGCP/NCS) and the a=<mode> SDP attribute. This is part of
a broader MGC Megaco/H.248-SIP interworking function including the need
to cache the last LocalDescriptor in order to satisfy the SIP
offer/answer model in the event that the Megaco/H.248 termination does
not return a LocalDescriptor in response to a particular command.

Joe Stone