The request-URI does not necessarily have to be
the end destination. This is dependent on routing in the IMS. Usually the request-URI
designates the next hop in the network, as identified in the ROUTE header. Details
about routing are explained in Chapter 5 as well.
As discussed previously when we talked about the SIP message format, the headers
contain parameters that then describe the request in more detail. These headers are
identified in the following sections.
Session Initiation Protocol (SIP) 65
SIP Responses
No request can be completed without a response. There are numerous types of responses.
Each of the responses falls into one of six classifications, identified by a preceding
number. The first digit of the number identifies the class of response, while the
following two digits identify the specific response being given. The response types are:
?– 1xx Provisional
?– 2xx Successful
?– 3xx Redirect
?– 4xx Client Failure
?– 5xx Server Failure
?– 6xx Global Failure
1xx Provisional Responses When a request is made to establish a dialog, a timer is set
within the device. When the timer expires, the request is sent again. This is to ensure
that a request does not get lost in the network and the device simply waits forever.
However, this can also create a lot of traffic within the SIP network if the timer expires
quickly, since the device will simply repeat the request continually until a response is
received.
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