How wireshark can guess the protocol inside the packets?


Hello everybody,

I’m trying to develop a program to calculate the delay in a VoIP
conversation, using the RTCP packet ( and java/jpcap in order to intercept it ).

The problem is, I can’t filter on the RTCP packet with jpcap, I must
take the UDP one. I don’t know how to recognize a RTCP packet. I’ve
studied the RTCP protocol with the RFC and wireshark, but I didn’t
find any way the know the protocol in the UDP packet.

How wireshark can “guess” the protocol inside the UDP packet ?
Can somebody help me ?

thanks in advance, sorry for my bad english.