This is to prevent hijacking and other scenarios where
a hacker may capture a SIP message and use it as a duplicate to obtain services from
another portion of the network. This would result in the message being received by the
P-CSCF with a different set of addresses in the routing headers.
The P-CSCF is able to provide this function because when an entity registers in
the network, the P-CSCF saves all of the addresses provided in the routing headers.
While the addresses do not have to be in order, they do have to be present as addresses
that were identified during registration. This function prevents some types of attacks
Architecture of the IMS 13
and is another form of security provided by the IMS core. Other information saved by
the P-CSCF includes the device address (IP address) and the public and private user
identities.
If a device loses its connection in the IP network, the P-CSCF is notified and releases
all sessions within the IMS by sending a CANCEL to any entities that are part of the
session. Since the P-CSCF is the first point of contact for all devices in the IMS, the
P-CSCF has knowledge of all sessions created through it. The P-CSCF is stateful, so it
also has knowledge about the state of each session.
There are times when the P-CSCF can function as a SIP User Agent (UA) when
subscribing to event notification, or other non-session-related subscriptions.
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