The S-CSCF routes the response using the same path as the request. The response
200 OK will be populated as follows:
?– PATH headers indicating the addresses used to reach the S-CSCF
?– P-ASSOCIATED-URI containing all the public user identities the user is authorized
to use for this subscription
?– The default public user identity listed as the first entry in the P-ASSOCIATEDURI
(as identified by the HSS)
?– SERVICE-ROUTE header populated with the address of the S-CSCF assigned to
the subscriber as well as the I-CSCF address
?– The address of the I-CSCF providing topology hiding populated as the first address
in the SERVICE-ROUTE header if applicable
?– The P-CHARGING-FUNCTION-ADDRESS header provided by the HSS if the PCSCF
identified in the P-VISITED-NETWORK-ID header is in the same network
as the S-CSCF
If the S-CSCF does not respond to the REGISTER, or if it returns the response 3xx
or 480 Temporary Unavailable, the I-CSCF will select another S-CSCF (depending on
operator policy). If there are no available S-CSCFs to receive the request, then the ICSCF
returns the response 600 Busy Everywhere.
When the P-CSCF receives the response from the S-CSCF (through the I-CSCF)
the P-CSCF stores the SERVICE-ROUTE header in the order received and uses the
addresses in the headers to establish a routing list to be used for all subsequent requests.
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