NOTES, RESOURCES, ADDITIONAL INFORMATION | 69
The SIP message protocol is similar to the Web’s HTTP (Hypertext Transfer Protocol), and
shares some of its design principles: It is human readable and request-response structured. SIP
even shares many HTTP status codes, including the familiar “404 not found.”
Here is a typical SIP message:
INVITE sip:skip@there.com SIP/2.0
Via: SIP/2.0/UDP 4.3.2.1:5060
To: Skip Pizzi <sip:skip@there.com>
From: Steve Church <sip:stevec@here.com>
Call-ID: 4678995554545@4.3.2.1
CSeq: 1 INVITE
Contact: <sip:stevec@4.3.2.1>
Content-Length: 126
This is how Steve’s SIP client would signal to Skip’s that he wants to connect and speak with him.
SIP works together with several other protocols and is only involved in the signaling portion
of a communication session. SIP is a carrier for the SDP (Session Description Protocol), which
describes the media content of the session, e.g. the codec being used, the bitrate, etc.
SIP provides the following capabilities:
♦ Determines the location of the endpoint: SIP supports address resolution, name map-
ping, and call redirection.
♦ Determines the media capabilities of the endpoint, i.e., which codecs are available and
supported: During a negotiation, SIP determines the best codec that can be used by the
parties on the call.
♦ Determines the availability of the called endpoint: If a call cannot be completed because
the target endpoint is unavailable, SIP returns a message indicating this and why.
♦ Establishes a session between the originating and called endpoints (if the call can be
completed).
♦ Handles the transfer and termination of calls: SIP supports the transfer of calls from
one endpoint to another. During a call transfer, SIP establishes a session between the
transferee and a new endpoint (specified by the transferring party), and terminates the
session between the transferee and the transferring party.
The Parts of a SIP System
Like most things based on IP, SIP was designed to be modular. Implementers can pick-and-
choose among the following elements to build the system they need.
SIP Clients: Sometimes called User Agents or endpoints. These can be hardware phones or
“softphones” (phone applications running on PCs).
Gateways: When needed, gateways translate between the IP network side and the switched-
circuit Telco side, providing physical, electrical, signaling and audio interface.
Proxy server: Receives SIP requests from a client and forwards them on the client’s
behalf. Basically, proxy servers receive SIP messages and forward them to the next SIP
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