CSCF - Call Session Control Function
Several roles of SIP proxies are collectively called as CSCF which are used to process SIP signalling packets in IMS.
Proxy-CSCF (P-CSCF) is a SIP pro that is the first point of contact for the IMS terminal. it can be located either in the visited network (in full IMS networks) or in the home network (when the visited network is not IMS Compliant yet) . Some networks may use SDB for this function. The P-CSCF is at the core a specialised SBC for the User-Netwrok interface which not only protected the network but also the IMS terminal. The use of an additional SBC between the IMS terminal and the P-CSCF is un-necessary and infeasible due to the signalling being encrypted on this leg. The terminal discovers its P-CSCF with either DHCP, or it may be configured (e.g. during initial provisioning or via 3PP IMS management Object (MO)) or in the ISIM or assigned in the PDP context (in GPRS)
- It is assigned to an IMS terminal before registration, and does not change for the duration of registration.
- It sits in the path for all the signalling and can inspect every signal; the IMS terminal must ignore must ignore any other unencrypted signalling.
- It provides subscriber authentication and may establish IPSec or TLS security association with the IMS terminal. This prevents spoofing attacks and replay attacks and protects the privacy of the subscriber.
- It inspects the signalling and and ensures that the IMS terminals do not misbehave (e.g. change the signalling routes, do not obey home networks routing policy)
- It can also compress and de-cpmpress SIP messages using SigComp, which reduces the round trip over the slow radio links.
- It may include a policy decision function (PDF) which authorises the media plane resources for e.g. QoS over the media plane. It is used for policy control, bandwidth management, etc. The PDF can also be a separate function.
- It also generates charging records.
References:
http://en.wikipedia.org/wiki/IP_Multimedia_Subsystem
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