Headers are used to transport the information to the SIP entities. The main fields are:
- Via: shows the transport protocol used and the request route, each proxy adds a line to this field
- From: shows the address of the caller.
- To: show the called user address of the request.
- Call-Id: Unique identifier for each call and contains the host address. It must be the same for all the messages within a transaction.
- Cseq: begins with a random number and it identifies in a sequential way each message.
- Contact : shows one (or more) address than can be used to contact the user
- User Agent: The client agent who deals the communication.
Via: SIP/2.0/UDP 192.168.0.100:5060;rport;branch=z9hG4bK646464100000007343c52679000020a600000e45
CSeq: 1 ACK
User-Agent: SJphone/1.60.289a (SJ Labs)