What is the function of the 'From' header in a SIP message?

The 'From' header in a SIP (Session Initiation Protocol) message plays a crucial role in indicating the sender of the message. This header contains essential information that identifies the originating user or entity initiating the SIP communication. It typically includes a SIP URI that represents the source, allowing the recipient to understand who is initiating the session or request.

By clearly defining the sender, the 'From' header establishes identity in SIP communications, which is particularly important for managing session states, routing responses correctly, and handling registrations. This header can also support features such as replies and redirects, as it provides context for how the signaling is to be interpreted based on the sender's identity.

The other options do not encompass the primary function of the 'From' header. For instance, the destination of the SIP message is determined by the 'To' header, while session details would typically be found elsewhere in the message, such as in the body. Authentication information, if necessary, is generally managed through other headers or mechanisms in SIP, rather than being included in the 'From' header itself.