This list is associated with the public user identity within the P-CSCF. The PCSCF
also stores all of the public user identities identified in the P-ASSOCIATED-URI
header in the response.
This is an important security function of the P-CSCF. Should a registration become
hijacked and a perpetrator sends an INVITE or other message using the identity already
registered, the message will be routed using this routing information regardless
114 Chapter 5
of the SIP headers. The P-CSCF will change the routing of the request to follow the
route as registered. This prevents someone from hijacking their service from another
point in the network.
The SERVICE-ROUTE header is used in the response to a REGISTER (200 OK). The
values are derived from the PATH header contained in the REGISTER message and
identify the addresses of all the network elements used in the path of the message. This
is the only time this header is used.
Upon receipt of the response 200 OK from the S-CSCF, the device will extract the following
parameters from the response and store them for session establishment:
?– Expiration time for each public identity
?– List of URIs found in the P-ASSOCIATED-URI header
?– The first URI in the P-ASSOCIATED-URI as its default public user identity
?– RECORD-ROUTE headers for a route list
?– Security association lifetime to either the previous value or 30 seconds longer than
the EXPIRES value
The information sent over the Cx interface shown in Figure 5.
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