Subject: RE: [Megaco] Clarification on MG handling of Local & RemoteDescriptors

From: "Kevin Boyle" <[email-masked]>
Date: Wed, 6 Jul 2005 11:18:22 -0400

Comments inline. [KJBII]

Kevin 

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Kamitses, Jerry
Sent: Wednesday, July 06, 2005 11:02 AM
To: [email-masked]
Subject: RE: [Megaco] Clarification on MG handling of Local &
RemoteDescriptors

... additional comment on this topic (after additional research and
discussion) ...

A particular case for the MG supplying information in the returned Local
Descriptor beyond that which was present in the received Local
Descriptor occurs when the received Local Descriptor is underspecified
through use of CHOOSE in a property value (as discussed in Section 7.1.8
Local and Remote descriptors).  

So for the payload selections in the "m=audio ..." parameter, if the MGC
sends $ as the payload list, 
          m=audio 5000 rtp/avp $
then the MG can return as many payload numbers as it likes, ordered by
preference. 
In particular, if the MG wishes to include 2833 as an option, it would
return the info necessary in the SDP along with whatever other codecs it
may wish to choose.

Consequently the gist of original question is now:

(1) whether there are any other instances where the MG can exercise this
option to supply payload numbers beyond those in the Local Descriptor
from the MGC, and
[KJBII] The MG may only choose options within the constraints set by the
MGC.  If the MGC sends $, then the MG can choose whatever it wants.

(2) for the particular example of CHOOSE above, are their any guidelines
or additional implied requirement for the MG when responding to such a
choose? 
For example, should/must the MG attempt to include payload values
present in the last received Remote descriptor. 
[KJBII] There is no requirement to do so, though many MGs do try to
align the Remote and Local descriptors.

One final question pertains to the following text from the standard
(from Section 7.1.7 LocalControl descriptor)

   If the value of a Reserve property is False, the MG SHALL choose 
   one of the alternatives specified in the Local descriptor (if
present)
   and one of the alternatives specified in the Remote descriptor (if
   present).  ... Finally, the MG shall send a reply to the MGC 
   containing the alternatives for the Local and/or Remote descriptor
   that it selected.  

Is the above text claiming that the MG can send a Remote Descriptor as
well as Local Descriptor in the reply message ?
[KJBII] If the Remote descriptor is underspecified, then yes.

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf
Of Kamitses, Jerry
Sent: Tuesday, July 05, 2005 7:48 PM
To: [email-masked]
Subject: [Megaco] Clarification on MG handling of Local & Remote
Descriptors

A question has arisen during interop testing regarding the extent 
to which the content of Local and Remote Descriptors sent to an MG 
termination by the MGC are intended to limit the content of any 
Local Descriptor returned by that MG termination in a response message. 

The question concerns whether the Local Descriptor in the MG
termination's 
response must always be a subset of the last Local Descriptor sent to
that 
termination or whether there are circumstances where the MG termination
is 
free to include additional information in the response Local Descriptor.

For example, are there situations where the content of the Remote
descriptor 
must or may effect the content of the Local Descriptor the MG returns? 

In particular, if the last received Remote descriptor contains
information 
not present in the last received Local descriptor then is the MG free to

use any/all of the Remote Descriptor contents in addition to the 
last received Local Descriptor when selecting the Local Descriptor to 
return?

Specifically, what are the rules regarding SDP media parameters in the
response. 
If, for example, the Local and Remote descriptors 
sent to the MG are
     Local {
          v=0
          o=- $ 0 IN IP4 $
          c=IN IP4 $
          m=audio $ RTP/AVP 8
     },
     Remote {
          v=0
          o=- 0 0 in ip4 100.23.14.82
          c=in ip4 100.23.14.82
          m=audio 5000 rtp/avp 18 101
	    a=rtpmap:101 telephone-event
          a=fmtp:101 0-15
	} 

then can the MG choose to return the following Local Descriptor?
     Remote {
          v=0
          o=- 0 0 in ip4 98.14.23.123
          c=in ip4 98.14.23.123
          m=audio 6348 rtp/avp 18 101
	    a=rtpmap:101 telephone-event
          a=fmtp:101 0-15
	} 

Do dynamic payloads based on RFC 2833 handled require any special
treatment 
in this regard? For example, if the Remote Descriptor contains a
telephone-event
rtpmap and the Local Descriptor does not, then can the MG choose to
include 
a telephone-event rtpmap in the Local Descriptor it returns?

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---

From: "Pascal Lambers" <[email-masked]>
Date: Wed, 6 Jul 2005 20:49:03 +0200

Hi,
I thought that underspecifiying the codec list as done below by means of
"m=audio 5000 rtp/avp $ " is not a valid syntax, according to RFC 2327?

Kind regards,
Pascal

----- Original Message ----- 
From: "Kevin Boyle" <[email-masked]>
To: "Kamitses, Jerry" <[email-masked]>; <[email-masked]>
Sent: Wednesday, July 06, 2005 5:18 PM
Subject: RE: [Megaco] Clarification on MG handling of Local &
RemoteDescriptors