The P-CSCF provides some protection to the S-CSCF and the HSS. This means that
the P-CSCF will do some screening to ensure that the subscriber trying to communicate
with the network is registered and authorized to access the network. It does this
through security associations.
Once a device has been authenticated and registration has been completed, the
P-CSCF assigns secure ports for the device to use for all subsequent communications.
Port 5060 and port 5061, typically used for SIP communications, are only used for registration.
The P-CSCF discards any INVITES or other SIP message types received on
these ports.
This ensures that communications into the network are from trusted devices, and not
from unauthorized devices. This is one of several different methods used for security
within the IMS, but it??™s not by any means the only measure. This is one of the primary
methods used by the P-CSCF to protect the resources of the HSS and the S-CSCF.
If you are located in a non-SIP network, other devices are used to provide the gateway
function between the IMS and legacy networks. We have already talked about
some of these devices in earlier chapters. The media gateway control function (MGCF)
provides the gateway function between the SS7 network and the SIP IMS network,
Establishing a Session in the IMS 109
but it is providing more than simply protocol conversion.
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