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Lecture 7

EE 284 Lecture 7: sip notes3

6 Pages

Electrical Engineering
Course Code
EE 284

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Registration The very first request issued by a client is likely to be REGISTER, because this is the request that provides the server with an address at which the user can be reached for SIP sessions. The REGISTER request is somewhat similar to the Registration Request between a terminal and a gatekeeper in H.323. • In this scenario, Collins has logged in to host This causes a REGISTER request to be sent to the local registrar. • Note the format of the Via: header and in particular the fact that it specifies the transport being used. The default is the User Datagram Protocol (UDP). • Note also that the Via: header contains a branch parameter which will discusses in proxy servers. • The From: header field indicates the address of the individual who has initiated the registration. • The To: header field indicates the “address of record” of the user being registered and is the address that the registrar will store for that user. In other words, this is the address at which the user wants to be contactable. • The From: and To: fields will be identical if a user is registering himself. The two fields need not be identical, however, which means that one individual can perform a registration on behalf of another. • Note that the From: field in our example contains a tag parameter, which is a pseudoran- dom value generated at the sender of the request and is used for identifica- tion purposes. • The Call-ID: header field is set by the originating client. In order to avoid the possibility that different clients might choose the same Call-ID, the recommended syntax for the Call-ID is [email protected], thereby making the Call-ID host specific. • In Figure 5-8, Collins has inserted a Contact: header field that indicates that future SIP messages should be routed to sip:[email protected] In other words, messages that are addressed to sip:Collins should be forwarded to sip:[email protected] • The REGISTER request does not contain a message body, since the mes- sage is not used to describe a session of any kind. Therefore, the Content- Length: field is set to 0 Response • The response to the REGISTER request is positive, as indicted by the 200 (OK) in the response line. • Note that the content of the Via: header field is copied from request to response. The content of the CSeq: (or command sequence) header field is also copied from request to response. The CSeq: header field indicates the method used in the request and an integer num- ber. • the use of the CSeq is very important in other requests where dif- ferent requests can be issued for the same Call-ID, in which case the CSeq is used to avoid ambiguity. A response to any request must use the same value of CSeq as used in the request itself. • Through the use of the Expires: header, Collins has requested that the registration be effective for 2 hours. The registrar can change this time. • If it is specified as a number of seconds, the maximum value is such that a registration may be active for up to approximately 136 years, • If a REGISTER request contains an Expires: header with a value that is too short (the registration is to be active for too short a time), then the REGISTER request will be rejected with the status code 423 (Interval too brief). The response will include the Min-Expires: header field, which specifies the minimum registration interval that the registrar will accept. To cancel a registration request it can be done by sending another REGISTER request for the same address of record and Contact and specifying a registration interval of 0. In this case, the REG- ISTER request would be identical to the first REGISTER request, with the exceptions that the CSeq integer is incremented and the Expires header field contains the value 0. If Collins wanted to cancel all existing registra- tions, then he would send a REGISTER message with the Expires header field set to 0 and the Contact header field populated with the wildcard char- acter *. The Expires header value of 0 indicates the cancellation of a Regis- tration and the Contact: header value of * indicates that the request is applicable to all contact information for Collins. • TheTo:headerfieldisnotusedtocontaintheaddress of the registrar. 
 • TheRegistrar’saddressisindicatedinthefirstlineof the request. 
 • TheREGISTERrequestdoesnotcontainamessage body, since the message is not used to describe a session of any kind. Therefore,theContent-Length:fieldissetto 0. Invitation The INVITE request is the most fundamental and important SIP request, as it is the request used to initiate a session (i.e., establish a call). SIP Call Sequence: Invitation • TheINVITErequestisthemostfundamentaland important SIP request, as it is the request used to initiate a session (i.e., establish a call). • Figureshowsatwo-partycall • – The INVITE request is issued to 
[email protected] 
 • – this signaling flow does not traverse any proxies. Otherwise the message would be addressed to sip:manag
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