Subject: Media Format Negotiation in MGCP

From: dav0703 <[email-masked]>
Date: Sun, 24 Oct 1999 23:58:38 +0200

Hi,
    Sorry to bother with what may be a trivial question,

In a basic MGCP audio call setup (e.g. TGW to TGW) both originator and
terminator may return a LocalConnectionDescriptor containing several
possibile media formats:

        200 1203 OK

        v=0
        c=IN IP4 128.96.63.25
        m=audio 1296 RTP/AVP 0 96
        a=rtpmap:96 G726-32/8000

As a gateway, how do I know what format will have the media
I am going to receive ? Should I wait for the first RTP packet
and see the payload type, or is there a better/faster way ?

When should I allocate the resouces (i.e. codec, etc.) ?

Thanks and regards,
Dan

---

From: Christer Holmberg <[email-masked]>
Date: Mon, 25 Oct 1999 13:19:58 +0300

Hi,

I am not sure this mail will answer your question, but I'll give it a try
:)

There is no "good" codec negotiation technique in MGCP, and it's also an
(one of many) inter-MGC communication problem.

If  you are the MG, you can return a list of codecs that you support, and
then you have to analyse the RTP packets to find out which of the listed
codecs the remote party uses.

Or, you could, instead of a list, only return one codec, which you then
expect the remote party to use when coding packets sent to you . Of
course, you should pick the codec from the list in the given remote
connection descriptor, so you know that the remote party will support that
specific codec.

However, this is not always a good way to do it, beucase now the remote
party will think that this is the only codec that you support, and it will
affect scenarios where the remote party would like to, for some reason,
change the codec.

I guess which way you use depends much on how your MG works technically.

Regards,

Christer