asterisk users - Nov 2004 - Question involving Windows Messenger 5.0 and Asterisk (SDP related)

All,

I am running an Asterisk server (1.0-RC1) with great success using Cisco 
7960 SIP phones.

I was bored so I started playing with Windows Messenger 5.0 and it's SIP 
implementation with Asterisk.   After some basic  configuration I have 
the following working:

* Two PC's can "register" with Asterisk
* One PC trys to send a IM to the other user : 

In this INVITE message, the originating PC sends the following line in 
it's SDP info (under the Media Description sub field)

"m=x-ms-message 5060 sip null"


The PC then gets a "100 Trying" message from Asterisk

You then see the invite get sent to the PC wjhere the other user lives, 
in the SDP that gets sent to the far-end PC, you see:

"m=audio 15802 RTP/AVP"

At this point the PC either says it's an incoming voice  call (which 
it's really not - but the SDP was changed to look like one) or a 
different PC sends a "488 Not Acceptable" SIP response code.


The config in the sip.conf are as follows:

[6970]
host=dynamic
;context=local-im
context=local
type=friend
;secret=password
insecure=yes

[6971]
host=dynamic
;context=local-im
type=friend
;secret=password
insecure=yes

In the extensions.conf file:

exten => 6970,1,Dial(SIP/6970)
exten => 6971,1,Dial(SIP/6971)

Why is asterisk rewriting the SDP with some sort of "default" SDP
info?
(From x-ms-message to audio)
Is my asterisk configuration looking OK for Microsoft Messenger?
Are there any hooks I can throw into the code to make this subfield 
(x-ms-message extension to SDP) valid in asterisk?

Thanks for any info,

Dave DeChellis