HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

Session Initiation Protocol (SIP) 1
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SESSION INITIATION PROTOCOL (SIP)
INTRODUCTION The Session Initiation Protocol (SIP) is a signaling protocol for initiating,
managing and terminating voice and video sessions across packet networks.
Borrowing from Internet protocols, such as HTTP and SMTP, SIP is text-
encoded and highly extensible. SIP can be extended to accommodate features
and services such as call control services, mobility and interoperability with
existing telephony systems.
SIP is being standardized by the SIP Working Group and others, within the
Internet Engineering Task Force (IETF). The protocol is published as RFC 3261
and includes numerous extensions to the basic SIP Protocol.
This section describes the key constituents of SIP.
SIP ENTITIES A SIP network is composed of five types of logical SIP entities. Each entity has
specific functions and participates in SIP communication as a client (initiates
requests), as a server (responds to requests), or as both. One “physical device”
can have the functionality of more than one logical SIP entity. For example, a
network server working as a Proxy server can also function as a Registrar at the
same time.
The logical SIP entities are:
 User Agent
 Proxy Server
 Redirect Server
 Registrar Server
 Back-to-Back User Agent (B2BUA)