P-CSCF: An introduction.

 April 30, 2008 — theimsjungle

The P-CSCF is
the entry point to the IMS domain and serves as the outbound proxy server for
the UE. The UE attaches to the P-CSCF prior to performing IMS registrations and
initiating SIP sessions. The P-CSCF may be in the home domain of the IMS
operator, or it may be in the visiting domain, where the UE is currently
roaming. For attachment to a given P-CSCF, the UE performs the P-CSCF discovery
procedures. Attachment to the P-CSCF is necessary for the UE for initiating IMS
registrations and sessions.

 

In these procedures, the UE first establishes the
IP connectivity access network (IP-CAN) bearer. Then, the UE sends a query to
the DHCP server for retreiving the IP addresses and FQDN (Fully Qualified Domain
Name) of the P-CSCF. After the DHCP query, the UE performs a DNS query on the
FQDN received from the DHCP server. In response to the DNS query, the IP address
of the P-CSCF is returned. This is known as the DHCP-DNS procedure for P-CSCF
discovery. However, in some cases, it may be possible that the FQDN of the
P-CSCF is pre-configured in the UE. In these scenarios, the UE can directly
query the DNS server and get the IP address of the P-CSCF.

Subsequent to P-CSCF discovery, the UE can send a
SIP REGISTER request to register itself in the IMS core network. The P-CSCF sets
up IPSec security associations with the UE, which are facilitated with the
two-way registration handshake (i.e. REGISTER-401-REGISTER-200 OK). The IPSec
security assocaiations (SAs) setup four protected ports between the UE and the
P-CSCF and ensure that all subsequent signaling traffic passes through the
protected ports. The four protected ports negotiated during IMS registration
are: The protected server port at the P-CSCF, the protected server port at the
UE, the protected client port at the P-CSCF and the protected client port at the
UE. The Protected client ports are used by the UE and P-CSCF to send requests
while the server ports are used to receive incoming requests. IMS registration
will be discussed seperately in another post.

The P-CSCF also translates the SDP body contained
in the SIP INVITE message from the UE into DIAMETER over the Rx interface and
sends it to the PCRF. The PCRF is responsible for policy control in the IMS core
network. If the SDP offer contains a codec or any other media characteristics
that are not allowed as per the policies of the IMS operator, the operator can
choose to disallow that session setup.

Subsequent posts will dicuss the P-CSCF and PCRF
interaction in more detail.

 

From: http://theimsjungle.wordpress.com/2008/04/30/p-cscf-an-introduction/