Can IOS CUBE Gateway be configured to send RTP in response to 180 or 183 early media?

[TONY SMITH]

Hi,

We have a scenario where the ITSP only sends RTP after first receiving RTP from the customer gateway.   They do this to work around any firewall or NAT issues.   This causes a problem on outbound calls when the far end tries to use early media either 180/SDP or 183/SDP, because in those cases the Cisco gateway has no audio to send and therefore does not send RTP.   The effect is that early media is not received, and the caller hears silence during this time until the call connects.

In the past I have seen that some end-user SIP devices (such as Siemens Gigaset) work in exactly the way required by this ITSP.  When a Gigaset makes an outgoing call and receives 180/SDP or 183/SDP then the Gigaset immediately starts sending RTP.  

My question is whether it's possible to configure and IOS CUBE gateway to behave in the same way?

It's a 2911 running 15.2(3)T

Thanks,  Tony S

[Jonathan Schulenberg]

Have you enabled PRACK on the CUCM SIP trunk's SIP Profile? Without this, there is no way to respond to the 1xx message and complete the SDP handshake.

Another way is to use Early Offer Forced on the ITSP-facing dial-peer. I would try PRACK first though.

[TONY SMITH]

Thanks for the suggestion.  

PRACK is troublesome because this ITSP doesn't properly support it.   Invites from the ITSP for inbound calls advertise 100rel, but then when the CUBE sends 180 ringing the ITSP doesn't send the expected PRACK.  Incoming calls therefore fail unless I configure "disable rel1xx".    However for test purposes I removed this, and made an outbound call.  I can see the CUBE sending PRACK after receiving 180/SDP but not early media is received.

Regarding early offer, that is how it is currently working at the moment.  However for completeness I tried adding early offer forced on the outbound dial peer, but as expected that made no change in either observed signalling nor in the early media operation.

[TONY SMITH]

Some more dialogue with the ITSP, they now have 100rel working properly so PRACKs are being send in the correct places both ways.  No change in the symptoms and the ITSP is still of the opinion that two-way audio is needed at the early media stage.