Loose routing
allows the various proxies to determine the best route for requests and responses to
take, based on current traffic demands and various other conditions.
In the IMS, loose routing represents security issues, which is why strict routing is
recommended for routing between the CSCFs in the network. One of the first steps in
IMS is to enforce the use of ROUTE and RECORD-ROUTE headers. In SIP domains,
this is an elective, but in IMS this is required. Using the RECORD-ROUTE header to
record the path taken during registration, and then storing this information as part of
the registration, ensures that messages are not inserted from other parts of the network
to hijack a session, or change a registration.
The RECORD-ROUTE and ROUTE headers ensure that all requests/responses are
forced through the same proxies (call session controllers in the IMS) acting as gateways.
How these are handled between networks should also be taken into consideration.
Security Procedures in the IMS 159
We talked about this process with the P-CSCF where this is enforced. Since the
P-CSCF is the first point of contact within the IMS, it only makes sense that the
P-CSCF be mentioned here as a means of protecting the network domain.
The information that gets exchanged between networks will depend on whether or
not the other network is a trusted network and has formal agreements in place.
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