Interrogating CSCF

Interrogating CSCF

The Interrogating-CSCF (I-CSCF) has the role of a stateless proxy that, by using the indicated public identities of the caller or the callee, queries the Home Subscriber Server (HSS) and based on responses routes the message to the correct S-CSCF. It implements the Cx interface [1] of an I-CSCF to the HSS. Therefore it supports the required Diameter commands to locate the user-assigned S-CSCF or to select, based on capabilities, a new S-CSCF and check identities, roaming authorizations as specified in 3GPP TS 29.228.

After receiving a successful answer for the Diameter query the I-CSCF forwards the SIP messages in a transactional mode. It is optimized for speed and minimalist state information is kept in it. To protect the home network, it has a firewalling capacity that only allows signaling messages coming from trusted networks through Network Domain Security (NDS).

Features of the Open Source IMS I-CSCF:

• full Cx interface support (LIR, UAR)
• S-CSCF selection based on user capabilities
• Serial forking for forwarding to S-CSCF
• Visited-Network-ID header support and roaming permission verification
• Topology Hiding (THIG)
• Network Domain Security (NDS)