[cisco-voip] Inconsistent 79x0 SIP behavior -- RTP response to 183 w/SDP

[Mark R. Lindsey]

I've observed some strange behavior with the 79x0's running SIP (7.1 
and 7.3). Suppose this sequence occurs --

Cisco SIP Phone --INVITE w/ SDP----------------> SIP Proxy
Cisco SIP Phone <-100 Trying-------------------- SIP Proxy
Cisco SIP Phone <-183 Session Progress w/ SDP--- SIP Proxy

In some cases, the Cisco phone starts sending RTP to the connection 
point identified in the SDP immediately at this point. In other cases, 
it doesn't.

The only pattern we can find is that if the SIP UAS that generated the 
SDP is a Lucent LCS 9000, the SIP phone starts sending RTP. But if it's 
a Cisco AS5400, the Cisco doesn't send RTP. The codecs and ordering are 
the same . . . except that the Lucent's SDP includes an a=ptime entry.

Why might this occur?

Cisco folks -- this is SR 600978978. A pcap trace demonstrating this 
behavior is attached to that ticket.

Thanks for any ideas.