P-CSCF discovery

P-CSCF discovery can be achieved through two mechanisms:

Method 1: IP-CAN IMS signaling bearer ConstructionCubeType;
UE sends a PDP activation request to sgsn, which indicates requesting a P-CSCF address in PDP activation;
Sgsn selects the corresponding ggsn Based on the APN and forwards the request to the ggsn;
Ggsn obtains the IP addresses of one or more P-CSCF in this network;
Ggsn forwards the P-CSCF address to sgsn;
Sgsn includes the IP address of the P-CSCF In the PDP activation response and sends it to UE;
UE selects a P-CSCF from the response returned (if multiple P-CSCF addresses are returned), subsequent registration requests are routed first to the P-CSCF, and if the P-CSCF is unavailable, if there are multiple optional P-CSCF, UE can select another P-CSCF for registration.

Method 2: DHCP + DNS query.
UE establishes a bearing channel with the IP-CAN;
UE requests the P-CSCF domain name and dns ip address from the DHCP server, if the DHCP server response carries the IP address of the P-CSCF, there is no process below;
If DHCP does not directly return the IP address of the P-CSCF, but returns the fqdn of the P-CSCF (a fully qualified domain name), ue must use the returned DNS server to resolve the FQDN to an IP address, this may involve multiple DNS query processes.
After receiving the domain name and IP address of the P-CSCF, ims ue can initiate access to IMS.

 

 

 

1. in IMS, the discovery of P-CSCF can be achieved through the dhcp dns mechanism (UE can obtain a list of P-CSCF of an access network through DHCP request ), the IP address of a P-CSCF is then found by querying the DNS. (I am not very clear about the detailed search mechanism)
2. ue begins to construct register request, request-Uri is the domain name of its domain, Route Header writes the address of the P-CSCF
3. After receiving the P-CSCF, remove yourself from the route header. Then through the request-Uri Domain Name Belonging to the domain to the DNS query, get the address of the I-CSCF, here
Note that the P-CSCF does not add the I-CSCF to the Route Header because the P-CSCF does not know if the I-CSCF can be used as a loose route.
4. After the I-CSCF receives the register request, it will find S-cscf serving the registered user through SLF and HSS. And add the S-CSCF address in the route header.
5. The S-CSCF receives the Register
The request will be authenticated on the UE, the process and the above, in this process, the S-CSCF will download the UE information from the HSS. Note that in the second register
Call-ID, from tag, CSeq number, branch is different from the first one.
6. When the UE registration is successful, the S-CSCF returns a 200 OK response,
The S-CSCF adds a service-Route Header in the response, with the aim that all messages except register of UE do not need to be re-queried through the I-CSCF s-
The address of cscf. Finally, the S-CSCF needs to change the user registration status in the HSS,

One more thing to note here is the path header (which is the address of the P-CSCF), The S-CSCF will add this header in the response after the registration is successful, because
For security considerations in IMS networks, the UE cannot directly send messages to the S-CSCF and must pass through P-cscf. In addition, the P-CSCF also executes a media-related strategy.
This means that even in the IMS domain, it is also necessary to pass through the P-CSCF to s-cscf.

 

 

 

The methods for P-CSCF discovery are:
I. when using IPv4, employ the Dynamic Host Configuration Protocol (DHCP) RFC 2132 [20f], the dhcpv4 options for SIP servers RFC 3361 [35a], and RFC 3263 [27A] as described in subclause 9.2.1. when using IPV6, employ Dynamic Host Configuration Protocol for IPv6 (dhcpv6) RFC 3315 [40], the dhcpv6 options for SIP servers RFC 3319 [41] And dhcpv6 options for domain name servers (DNS) RFC 3646 [56C] as described in subclause 9.2.1.

 

Ii. Transfer P-CSCF address (es) within the PDP context activation procedure.
The UE shall indicate the request for a P-CSCF address to the ggsn within the Protocol configuration options information element of the activate PDP context request message or activate secondary PDP context request message.

 

If the ggsn provides the UE with a list of P-CSCF IPv4 or IPv6 addresses in the activate PDP context accept message or activate secondary PDP context accept message, the UE shall assume that the list is prioritised with the first address within the Protocol configuration options information element as the P-CSCF address with the highest priority.