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Introduction to SIP Alan Percy, Director of Business Development Alan.firstname.lastname@example.org 716-662-2769. Agenda. SIP Overview SIP Messages SIP Call Flows SIP versus H323 Further Reading. What is SIP?.
Alan Percy, Director of Business Development
SIP is an application layer control protocol that can establish, modify an terminate multimedia sessions or calls.Multimedia sessions include: - multimedia conference. - Distance learning. - Internet telephony.
SIP can invite parties to both unicast and multicast sessions.SIP supports either UDP or TCP transports.SIP supports name mapping and redirection services (“Personal Mobility”). The initiator of SIP request is called a SIP client and the responding entity is called a SIP server
Media Session (RTP)
Convention:user @ domain (SIP URL).
User can be : user name or TEL number.
Domain can be : domain name or numeric network address.
Sometimes the SIP URL can be guessed from the email address.
A SIP URL address can designate :
an individual ,
first available person from a group of individuals,
a whole group.
A SIP network is composed of four 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.
A User Agent (UA) is the endpoint entity.
It initiate and terminate sessions by exchanging requests and responses. RFC2543 defines the User Agent as an application, which contains both a User Agent client and User Agent server, as follows:
A client application that initiates SIP requests.
A server application that contacts the user when a SIP request is received and that returns a response on behalf of the user.
Devices that function as UA in a SIP network:
A Proxy server acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced either internally or by passing them on, possibly after translation, to other servers. A Proxy interprets, and, if necessary, rewrites a request message before forwarding it.
It functions as a kind of a Router.
A Redirect Server is a server that accepts a SIP request, maps the SIP address of the called party to a new addresses and returns them to the client. Unlike Proxy servers, Redirect Servers do not pass the request on to other servers.
A Registrar is a server that accepts REGISTER requests for the purpose of updating a location database with the contact information of the user specified in the request.
General basic fields:
Client uses the register method to register its address with the SIP server.
The client needs to register at startup and then periodically thereafter.
Recommended to register via multicast.
Multicast registration has a well known “all SIP servers” multicast address: “sip.mcast.net” (18.104.22.168)
Indicates that the user or service is being invited to participate in a session.
The ACK request confirms that the client has received a final response to an INVITE request
The client uses “BYE” to indicate to the server that it wishes to release the call.
A party to a call should send a BYE request before hanging up.
The CANCEL request cancels a pending request with the same CALL-ID, TO, FROM and Cseq.
It doesn’t affect a completed request nor an existing call.
The server (callee) is being queried as to its capabilities.
Let’s take a look at an actual call flow
IP and lower layers
SIP Demystifiedby Gonzalo Camarillo
Understanding the Session Initiation Protocol by Alan B. Johnston
Carrier Grade Voice over IPby Daniel Collins