Hm, as far as I can see, the call comes via H.323 and is relayed by SIP, so it's up to router whether to send SDP in INVITE or one of the subsequent messages.

Regarding INVITEs w/o SDP, yes, it can be. The spec says the offer MUST be in the INVITE or the first reliable non-failure message. It does NOT say that it can ONLY be in the INVITE or the first reliable non-failure message.

And such thing amy have some special appliances, e.g. draft-ietf-sipping-3pcc refers to INVITE messages sent without SDP body so as to manage third party call control.

Maybe, but we had H.323-to-SIP scenario and the table reads n/a in this column. I hope you'll post your findings here.

we just found the same. we have an scenario CCM -H323--IPIPGW--SIP--- Softswitch and the INVITE message always lacks of the SDP field. I think this is not supported for now.

I don't have IPIPGW to experiment now, but Cisco claims that since 12.3(11)T IOS supports interworking between H.323 Fast-Start and SIP early-media signaling. I would try the following to enable it: create voice class h323 like:

voice class h323 1

call start fast

!

and enter voice-class h323 1 command on the dial-peer.

Have in mind that if you do transcoding in your IPIPGW H323 fast start falls back to slow start

http://cisco.com/en/US/products/sw/voicesw/ps5640/products_configuration_guide_chapter09186a00803fecdc.html#wp1315453

"The transcoding process on the IP-to-IP gateway will always drop fast-start calls down to slow-start between H.323 endpoints even when the H.323 terminating endpoints support fast-start calls."

BR,

Stoyan