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

From (F) and To (T)
The From header field identifies the initiator of the request, potentially including a display name and definitely including a SIP URI or globally routable URI (GRUU) that represents the sender's address.
The To header field specifies the intended recipient of the request. It also includes a display name and SIP URI or GRUU.
Format Example: From: "Alice" <sip:alice@example.com>;tag=9fxced76sl and To: "Bob" <sip:bob@example.com>

Call-ID (I)
The Call-ID header is a unique identifier for a SIP session, ensuring that all participants in a session can refer to the same context.
Format Example: Call-ID: a84b4c76e66710

CSeq
The Command Sequence (CSeq) header field numbers the requests within a dialog, combining a sequence number and a SIP method to ensure proper ordering of requests and responses.
Format Example: CSeq: 314159 INVITE

Max-Forwards
This header limits the number of times a request can be forwarded by SIP proxies, preventing infinite loops.
Format Example: Max-Forwards: 70

Contact (M)
The Contact header specifies the URI at which the sender of the request can be reached; it is essential for establishing future requests within a dialog.
Format Example: Contact: <sip:alice@example.com>

These core SIP header fields are indispensable for the basic operations of SIP-based communication, providing mechanisms for routing, session identification, message sequencing, and content specification. Understanding and correctly implementing these headers is foundational for any SIP user agent or application to participate in SIP signaling.

For comprehensive details on each header and examples of their use in various SIP scenarios, the original RFC 3261 document and subsequent amendments and extensions should be consulted.