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Ti+  for  TS 24.228§7.2.2§7.3.2§7.4.2
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IMS S1 signalling flow - Session Initiation: Mobile origination and termination roaming, with different network operators - IMS S1-1c. SIP INVITE request (UE1 to UE2): from S-CSCF1 to I-CSCF2
  • Request-URI: In the case where the Request-URI of the incoming INVITE request to S-CSCF contains a TEL-URL, it has to be translated to a globally routable SIP-URL before applying it as Request-URI of the outgoing INVITE request. For this address translation the S-CSCF uses the services of an ENUM-DNS protocol according to RFC 2916, or any other suitable translation database.
  • As S-CSCF1 does not know whether the I-CSCF at is a loose router or not, it does not introduce a Route header.
  • P-Asserted-Identity: S-CSCF1 inserts the corresponding TEL URL to the P-Asserted-Identity header in order that the TEL URL is known to the destination network in case the INVITE is forwarded to a MGCF.
  • P-Charging-Vector: S-CSCF1 adds the identifier of its own network to the originating Inter Operator Identifier (IOI) parameter of this header.
  • I-CSCF2 sends a query to the HSS (the public user identity is provided in the Request-URI) to find out the S-CSCF of the called user. The HSS responds with the address of the current S-CSCF for the terminating subscriber.