CUCM stripping SDP and Asterisk


My customer is running Asterisk 12.8.2 and is connecting to CUCMs running 8.6.2. The setup is very basic.

Phone A <–> CUCM A <–> Asterisk 12.8.2 <–> CUCM B <–> Phone B

Phone A sends INVITE to CUCM A, indicating in the SDP that it supports presentation sharing. CUCM A, for reasons unknown, removes the SDP from the Invite before sending it to Asterisk. i.e. Content-Length:0 Phone B responds with a 200 OK indicating in the SDP it supports presentation sharing which CUCM B sends to Asterisk. My issue is that I need to let CUCM A know that the called phone supports presentation sharing, i.e. I want to copy that information into the 200 OK Asterisk will send to CUCM A. I modified Asterisk to store the information in the sip_pvt structure for the Asterisk <–> CUCM B dialog, but I can’t determine how to get that information into the sip_pvt structure for the Asterisk <–> CUCM A dialog. i.e. how do I pass information from handle_response_invite to sip_answer? Hoping I have overlooked something obvious.

The customer cannot modify the CUCMs, so the problem must be resolved in the Asterisk code.


CUCM uses later offer SDP if it doesn’t have the option of allocating an MTP (spelling?). That’s because, without an MTP, it can’t transcode, so needs to know what codec the other side can use before it selects one.

The only way you might make it work is if you configure CUCM to always use MTPs and allocate enough of them.

With late offer SDP, it sends an INVITE with no SDP, waits for the SDP in the OK, and then sends its offer in the ACK.

That worked like a charm. Thank you very much.