Dialogic 6.2 Manuale Utente

Understanding the SIP Protocol

November 2009

245

Numerous protocols carry various forms of real-time multimedia 

session data such as voice, video, or text messages. The Session 

Initiation Protocol (SIP) works in concert with these protocols by 

enabling internet endpoints (called user agents) to discover one 

another and to agree on a characterization of a session they would 

like to share. 

For locating prospective session participants, and for other 

functions, SIP enables the creation of an infrastructure of network 

hosts (called proxy servers) to which user agents can send 

registrations, invitations to sessions, and other requests. SIP is an 

agile, general-purpose tool for creating, modifying, and terminating 

sessions that works independently of underlying transport protocols, 

regardless of the type of session being established.

SIP is an application-layer control protocol that can establish, 

modify, and terminate multimedia sessions (conferences) such as 

internet telephony calls. SIP can also invite participants to already 

existing sessions, such as multicast conferences. Media is added to 

(and removed from) an existing session. SIP transparently supports 

name mapping and redirection services, supporting personal 

mobility - users can maintain a single externally visible identifier 

regardless of their network location.

SIP supports five facets of establishing and terminating multimedia 

communications: 

„

User location: determination of the end system to be used for 

communication;

„

User availability: determination of the willingness of the called 

party to engage in communications;

„

User capabilities: determination of the media and media 

parameters to be used;

„

Session setup: “ringing”, establishment of session parameters at 

both called and calling party;

„

Session management: including transfer and termination of 

sessions, modifying session parameters, and invoking services.