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.

Message Header
Via: SIP/2.0/UDP 192.168.0.100:5060;rport;branch=z9hG4bK646464100000007343c52679000020a600000e45
Content-Length: 0
Call-ID: 911D32E5-EEDF-4572-B0B2-61B294636E88@192.168.0.100
CSeq: 1 ACK
From: "Prueba"<sip:20000@miasterisk.com>;tag=8922404614682
Max-Forwards: 70
Route: <sip:20001@192.168.0.1>
To: <sip:20001@miasterisk.com>;tag=as0a27b928
User-Agent: SJphone/1.60.289a (SJ Labs)
Contact: <sip:20100@192.168.0.100:5060>;expires=3600