SIP Overview
SIP is an ASCII text-based appliance band ascendancy agreement that can be acclimated to establish,
maintain, and abolish calls amid two or added endpoints. SIP is an alternative
protocol developed by the IETF for multimedia conferencing over IP. Its appearance are
compliant with IETF RFC 2543, SIP: Affair Initiation Protocol, appear in March
1999, and IETF RFC 3261, SIP: Affair Initiation Protocol, appear in June 2002.
Many applications of the Internet crave the conception and administration of a session,
where a affair is advised an barter of abstracts amid an affiliation of participants.
The accomplishing of these applications is complicated by the practices of participants:
users adeptness move amid endpoints; they adeptness be addressable by assorted names; and
they adeptness acquaint in several altered media. Numerous protocols accept been
authored that backpack assorted forms of real-time multimedia affair abstracts such as voice,
video, or argument messages.
SIP works in concert with these protocols by enabling Internet endpoints (called user
agents) to ascertain one accession and to accede on a assuming of a affair they
would like to share. For analysis -to-be affair participants, and for added functions,
SIP enables the conception of an basement of arrangement hosts (called proxy servers) to
which user agents can accelerate registrations, invitations to sessions, and added requests.
SIP is not a standalone communications system. Rather, SIP is a basal that can be
used with added IETF protocols to body a complete multimedia architecture. These architectures
include protocols such as RTP for alteration real-time abstracts and accouterment QoS
feedback, Real-time Streaming Agreement (RTSP) for authoritative commitment of streaming
media, MGCP for authoritative gateways to the PSTN, and Affair Description Protocol
(SDP) for anecdotic multimedia sessions. Therefore, SIP should be acclimated in conjunction
with added protocols to accommodate complete casework to the users. However, the basal functionality
and operation of SIP does not depend on any of these protocols.
SIP operates on the assumption of affair invitations based on an HTTP-like request/
response transaction model. Each transaction consists of a appeal that invokes a particular
method or action on the server and at atomic one response. Through invitations, SIP
initiates sessions or invites participants into accustomed sessions. Descriptions of these
sessions are advertised by any one of several means, including the Session
Announcement Agreement (SAP) authentic in RFC 2974, which incorporates a session
description according to the SDP authentic in RFC 2327.
SIP uses added IETF protocols to ascertain added aspects of VoIP and multimedia sessions.
For example, SIP uses URLs for addressing, DNS for account location, and Telephony
Routing over IP (TRIP) for alarm routing.
Like added VoIP protocols, SIP is advised to abode the functions of signaling and session
management aural a packet telephony network. Signaling allows alarm advice to
be agitated beyond arrangement boundaries. Affair administration provides the adeptness to control
the attributes of an end-to-end call.
SIP supports bristles facets of establishing and absolute multimedia communications,
resulting in the afterward capabilities:
■ Determines the area of the ambition endpoint: SIP supports abode resolution,
name mapping, and alarm redirection.
■ Determines the media capabilities of the ambition endpoint: SIP determines the lowest
level of accepted casework amid the endpoints through SDP. Conferences are
established application alone the media capabilities that can be accurate by all endpoints.
■ Determines the availability of the ambition endpoint: If a alarm cannot be completed
because the ambition endpoint is unavailable, SIP determines whether the alleged affair is
connected to a alarm already or did not acknowledgment in the allotted cardinal of rings. SIP
then allotment a bulletin advertence why the ambition endpoint was unavailable.
■ Establishes a affair amid the basic and ambition endpoints: If the alarm can
be completed, SIP establishes a affair amid the endpoints. SIP additionally supports
midcall changes, such as the accession of accession endpoint to the appointment or the
changing of a media appropriate or codec.
■ Handles the alteration and abortion of calls: SIP supports the alteration of calls
from one endpoint to another. During a alarm transfer, SIP establishes a session
between the transferee and a new endpoint (specified by the appointment party) and
terminates the affair amid the transferee and the appointment party. At the end
of a call, SIP terminates the sessions amid all parties.