SIP protocol overview audio and video tutorial

SIP is an application control signaling protocol proposed by IETF. As the name implies, it is used to initiate a session. It can be used to create, modify, and end multimedia session processes attended by multiple participants. Participants can communicate with each other through multicast, unicast, or network connection.

There are clients and servers in the SIP. A client is an application that establishes a connection with the server to send requests to the server. The User Agent (User Agent) and Proxy) contain clients. A server is an application used to provide services and send responses to requests sent from a client. There are four types of basic servers:

· User proxy server: When a SIP request is received, it contacts the user and returns a response on behalf of the user.

· Proxy server: initiates requests on behalf of other clients, acting as both a server and a media program of the client. Before forwarding a request, it can rewrite the content in the original request message.

· Redirect server: it receives the SIP request and maps the original address in the request to zero or multiple new addresses and returns them to the client.

· Registration server: it receives client registration requests to complete user address registration. User Terminal programs usually need to include user proxy clients and user proxy servers. Proxy Server, redirection server, and registration server are public network servers. The concept of locating a server is also often mentioned in SIP, but locating a server does not belong to the SIP service.

SIP fully considers the scalability of other protocols. It supports many address descriptions and addressing methods, including username @ host address: callednumber @ PSTN gateway address: Tel: 010-62281234 common phone descriptions. In this way, the SIP caller can identify the location of the called phone on the traditional telephone network according to the called address, and then initiate and establish a call through a gateway connected to the traditional telephone network. The most powerful feature of SIP is the user positioning function. SIP itself contains the registration function to the registration server. It can also enhance its positioning function by using the positioning services provided by other locating server DNS and LDAP.

SIP provides six types of signaling: INVITE, ACK, CANCEL, OPTIONS, BYE, and REGISTER. INVITE and ACK are used to establish a call, complete three-way handshake, or change session properties after the call is established. BYE is used to end the session. OPTIONS is used to query server capabilities; CANCEL is used to CANCEL a request that has been sent but has not ended. REGISTER is used to send a message such as a user location registered with the registration server.

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