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Lecture 3

EE 284 Lecture 3: sip notes

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San Jose State University
Electrical Engineering
EE 284

find more resources at SIP SIP is used in conjunction with several other IETF protocols such as the Session Description Protocol (SDP), the Real-Time Streaming Protocol (RTSP), and the Session Announcement Protocol (SAP). SIP defines two basic classes of network entities: clients and servers. Strictly speaking, a client (also known as a user agent client) is an application program that sends SIP requests. A server is an entity that responds to those requests. Thus, SIP is a client-server protocol. Four different types of servers exist: proxy server, redirect server, user- agent server, and registrar. Proxy server The proxy server here is similar to that used in the LAN networks, when a user sends a request, the proxy server either handles that request or forwards it to some other servers perhaps after performing some translation, In this way the user identity is hidden from the outside servers. Redirect server  This server accepts SIP requests, maps the destination address to zero or more new addresses, and returns the new address to the originator of the request. Thereafter, the originator of the request can send requests directly to the address(es) returned by the redirect server.  A redirect server does not initiate any SIP requests of its own.  This can be another means of providing the call-forwarding/follow-me service that can be pro- vided by a proxy server.  The difference is that, in the case of a redirect server, the originating client does the actual forwarding of the call.  The redi- rect server simply provides the information necessary to enable the origi- nating client to correctly route the call, after which the redirect server is no longer involved. find more resources at find more resources at Registrar is a server that accepts SIP REGISTER requests. Sip uses the user registration for maintaining the access location of the user, e.g if the user log in to the office sip machine then a request is generated about his location, similarly if he changes the location the subsequent location changes are updated. find more resources at find more resources at Call Establishment in SIP Advantages of SIP SIP offers a great deal of flexibility. For example, SIP does not care what type of media is to be exchanged during a session or the type of trans- port to be used for the media. In fact, SIP itself can be carried over several different transport protocols. In other words, SIP provides more flexibility than is found in typical telecommunications protocols. That flexibility can be exploited to enable custom services and features. SIP messages can include a number of optional fields, which can contain user-specified information. This approach enables users to share nonstandard user-specific information, which enables users and devices to make intelligent call-handling decisions.  For example, a SIP INVITE can include a subject field. A person who receives an INVITE might decide to accept or reject the call depending on who is calling and what the subject happens to be. One can imagine a situation where an INVITE contains a text string such as “I know you are there. Please answer the phone.” If the call is rejected, the response might contain a text string such as “Stop calling me!”  Imagine another scenario where a call is directed to a user who is cur- rently not available. Obviously, the SIP response will indicate that the user is unavailable (no big surprise). The response could, however, include an indication that the user expects to be available a
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