[Serusers] Reg. SIP Parsing, SIP Protocol

Hi, I am writing a software to parse SIP messages. I have couple of questions that I would like to clarified. 1. How to recognise if a packet is a SIP or a non-SIP packet?. SIP uses TCP, UDP as underlying transport. 2. How to know the direction of the SIP message if it is from SIP proxy or from SIP client?. I ask this question because in most of the SIP messages, the header fields are copied from previous messages and so the from, To header fileds doesn't always signify he direction 3. Can Call-ID be used to find the number of distinct phone call taken place between different SIP UA's?. 4. What is the significance of dialog-ID? and how can it be used? Thanks, Balaji