Information Technology Reference
In-Depth Information
SIP
SIP is a text-based, peer-to-peer, application layer control signaling protocol that is used for setting up, modifying, and tearing down
multimedia sessions between participants. It is defined in RFC 3261 (which obsoletes RFC 2543) and takes advantage of elements of
HTTP, SMTP, and the Session Description Protocol (SDP).
SIP uses SDP to negotiate media type and format, such as audio and video codecs, and transport protocol parameters, such as RTP
and UDP parameters and ports. SDP operates in an offer/answer model such that the session initiating endpoint indicates desired
session parameters (such as supported codecs), and the receiving endpoint replies with matching session parameters (excluding codecs
that are not supported).
In the process of setting up and tearing down sessions, SIP supports the capability to determine a user's location and the user's
willingness to communicate, and ascertain appropriate media parameters. By default, SIP uses port 5060 for TCP/UDP and port 5061
for TLS over TCP.
Various elements contribute to SIP's capability to establish, manage, and terminate sessions, and these include the following:
User agent (UA): This endpoint can act as both a user agent client (UAC) and a user agent server (UAS). A UA could, for
example, be a SIP IP phone.
UAC: This logical entity initiates and sends requests, such as those specifying the INVITE method. The UAC is a logical
role, so it lasts only for the duration of a SIP transaction.
A transaction comprises all messages beginning with the initial request message and ending with the final response message.
UAS: This entity responds to a SIP request by accepting, rejecting, or redirecting the request. The UAS role lasts only for the
duration of the transaction.
Redirect server: This is a (user agent) server that provides address translation and redirects clients to alternative destination
addresses. It does this by sending 3xx responses to requests.
Proxy server: A SIP proxy server's primary role is to provide routing, but it can also enforce policies, provide features, and
authenticate and authorize users.
 
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