Good articles on the subject…,
http://www.voip-calculator.com/protocols.html
http://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.html
RTP header not only refers to the RTP, but generally also refers to the IP/UPD part of the header also. Thus cRTP (compression of RTP – the whole header) can reduce the entire 40bytes (20-IP/8-UDP/12-RTP) into either 2 to 5 bytes.
Here is a Cisco explaination of how cRTP works:
http://www.cisco.com/warp/public/732/Tech/real/index.html
In essence, it is shorthand. The whole header(s) are sent the first time. Both ends of the link remember the header, then each subsequent frame includes a shorthand reference to that header plus a small amount of info concerning what bits should be different for each time. The changes in the header propogate to the *remembered* header thus each change builds upon the previous.
It is very CPU intensive and is link specific so both ends (routers) need to know about the other’s cRTP encoding.
Sharing other IP/TCP/UDP traffic get tricky. Be more specific as to what you are looking for and I will try to give some pointers.
Dwayne