When the I-CSCF receives the INVITE, it must first query the HSS to determine
which S-CSCF has been assigned to the called party. The I-CSCF will also store the
value in the P-CHARGING-VECTOR header, and if there is no IMS CHARGING ID
(ICID) present, create a new one.
132 Chapter 5
If the I-CSCF queries the HSS and the SLF, but cannot locate the subscriber, the ICSCF
will return the response 404 Not Found to the request originator. If the subscriber
is not a user of the home network, the I-CSCF will send the response 604 Does Not
Exist Anymore. If the subscriber is a valid subscriber in the network but is not currently
registered, the I-CSCF will respond with 480 Temporarily Unavailable.
The INVITE is then forwarded to the assigned S-CSCF. The S-CSCF then determines
the location of the called party. Prior to forwarding the message to the called party, the
S-CSCF will add the P-CHARGING-FUNCTION-ADDRESSES header to the request
(as long as the request is being forwarded within the S-CSCF network). Charging headers
and parameters are not typically sent to outside networks by any entity unless it is
a trusted network. The request is then forwarded to the device.
Not all requests require strict routing and the use of the RECORD-ROUTE headers.
Some services, for example, may bypass the S-CSCF entirely, such as Presence. The
general rule of thumb cited by 3GPP specifications is that any services requiring media
control, CDR generation, and possibly network-initiated session termination require
strict routing to be used (and subsequently the use of RECORD-ROUTE headers).
Pages:
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276