Core SIP Header Fields

In the Session Initiation Protocol (SIP), header fields are essential components that convey key information about the communication session, its participants, and the desired actions. This part focuses on the core SIP header fields, which are fundamental to initiating, managing, and terminating SIP sessions as defined in RFC 3261.

Via (V)
The Via header field indicates the transport protocol and intermediaries a request has passed through. The Via header has the ‘branch’ parameter, which format is mandated by RFC 3261 to always start with "z9hG4bK" to ensure compliance. This prefix is critical for distinguishing SIP transactions and preventing message loops, underscoring the importance of adhering to this format for protocol integrity and effective routing.
Format Example: Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKhjhs8assdfd