Sip traffic

hello all
i am supposed to investigate the sip protocol. and i need to write a short program that will capture ALL sip messages.

i think i have done a pretty good job of capturing the general behavior of the protocol
i have identified EVERY sip message as having the following fields (data):

Call-ID (or i)
From (or f)
To (or t)
Via (or v) followed by SIP/

i have received a response that it will dot capture sip messages from a private label sip client (e.g gold seal) talking to a Asterisk server.

how can a sip message be different ?
how does the above scenario change the message format ?
meaning will it have traffic that i havent mentioned, or have i mentioned feilds that will not be in such traffic?

if anyone understands my problem , a quick answer will help a lot
thank you