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56 • Web Server Integration
[ S N O M 4 S N A T F I L T E R ]
• The parameter “explanation” contains the explaining text that is
added behind the code in the SIP response. Typical values are “Ok”
or “Not Found”.
• The parameter “contact” contains the contact that should be re
-
turned by the registration response. This parameter contains the
expiry time that this contact will have. Typical values look like “<sip
:1.2.3.4:5060;ua=345a20f784c9284>;expires=3600”. This param-
eter may be present more than once if the application server has
several bindings. If there is no binding, the parameter can be omit-
ted.
If the UA does not reregister, the SBC will send a register request
with an expiry time of zero seconds. This emulates the deregistration re
-
quest of the user agent.
5.4 Call Initiation
If the “Http URL for call” setting is set in the system settings, the
SBC will consult the application server for every call that is being initi-
ated.
A new call is identified by an unknown Call-ID and starts with
an INVITE request. The SBC will perform the authentication steps before
the call is initiated (see above). For a new call, the request from the SBC
contains the following parameters:
• The parameter “action” is set to “start”. By looking at this param
-
eter, the application server can easily find out that it should do a call
start.
• The parameter “from_uri” contains the URI part of the From-header.
Typically, this has the form “sip:albert@einstein”. Note that the port
number and parameters may also be present in this parameter.
• The parameter “to_uri” contains the URI part of the To-header like
the “from_uri” parameter.
• The parameter “uri” contains the request-URI.
• The parameter “callid” contains the Call-ID of the call.
• The parameter “from_ua” is set to “true” if the SBC believes that the
call comes from a client endpoint.
5.