What is involved in "session establishment" in SIP?

Session establishment in the context of the Session Initiation Protocol (SIP) refers to the initial procedures that facilitate the setup of a communication session between participants. This process is crucial as it involves signaling and negotiation elements necessary for both parties to agree on the parameters of the communication, such as codecs, media types, and other session details. By successfully completing session establishment, participants can ensure that they are on the same page regarding how they will communicate, which is fundamental for ensuring smooth interactions.

The other options do not encapsulate the essence of session establishment. For instance, changing session characteristics post-establishment deals with modifications made after a session has been established, while terminating an active communication session is associated with ending an ongoing interaction, rather than starting one. Establishing a data network for SIP could imply infrastructure setup rather than the active negotiation and initiation of an individual communication session. Thus, the primary focus of session establishment is correctly recognized in the answer choice that highlights its negotiation and preparation aspects.