If the message is legitimate, the message is forwarded on. The P-CSCF can then
add its own address in the form of domain name and IP address to the VIA header,
and its SIP URI to the RECORD-ROUTE header. The P-CSCF will also delete the PPREFERRED-
IDENTITY header and replace it with the P-ASSERTED-IDENTITY
header populated with the public user identity of the requestor. The P-CHARGINGVECTOR
populated with the ICID is also added to the message before forwarding the
message on to the I-CSCF for routing to the proper S-CSCF.
By using the domain of the called party, the P-CSCF can locate the address of the ICSCF
for the network domain being called; it then forwards the INVITE to the I-CSCF.
The P-CSCF does not know any of the addresses within other networks, only the addresses
of the I-CSCF for other networks.
There are several ways to address the I-CSCF. Some operators may choose to assign a
common SIP URI to all of their I-CSCF entities (to facilitate load sharing, for example),
while others may choose to implement unique addressing. Each I-CSCF will also be assigned
its own unique IP address, which is what the SIP URI gets resolved to.
When topology hiding is implemented, outside networks will only know the address
of the I-CSCF (or multiple I-CSCFs, as will be the case in most networks). The I-CSCF
must then determine how to route requests and responses within its own network.
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