Note the spi is only required when initiating and

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NOTE: The SPI is only required when initiating and deleting SAs in the P-CSCF. The SPI is not exchanged between IPsec and the SIP layer for incoming or outgoing SIP messages. 2. The SIP application at the P-CSCF shall check upon receipt of a protected REGISTER message that the source IP address in the packet headers coincide with the UE’s IP address inserted in the Via header of the protected REGISTER message. If the Via header does not explicitly contain the UE's IP address, but rather a symbolic name then the P-CSCF shall first resolve the symbolic name by suitable means to obtain an IP address. 3. The SIP application at the P-CSCF shall check upon receipt of an initial REGISTER message or a re- REGISTER message that the pair (UE_IP_address, UE_protected_client_port), where the UE_IP_address is the source IP address in the packet header and the protected client port is sent as part of the security mode set-up procedure (cf. clause 7.2), has not yet been associated with entries in the "SA_table". In addition, if the P-CSCF has detected that the UE is located behind a NAT (cf. Section 7.2), the P-CSCF shall check upon receipt of an initial (unprotected) REGISTER message, or a REGISTER message protected with UDP encapsulated tunnel mode, that the pair (UE_IP_address, UE_protected_server_port) has not yet been associated with entries in the "SA_table". Here the UE_IP_address is the source IP address in the packet header, and the protected client and server ports are sent as part of the security mode set-up procedure (cf. clause 7.2). NOTE: In case of multiple UEs behind the same NAT, the same public IP address may be assigned by the NAT to two different UEs. Therefore, the P-CSCF shall not accept registration attempts from UEs with the same address and protected server port in order to ensure unambiguous addressing of SIP messages sent towards the UE, using the protected server port. Furthermore, the P-CSCF shall check that, for any one IMPI, no more than six SAs per direction are stored at any one time. If these checks are unsuccessful the registration is aborted and a suitable error message is sent to the UE. NOTE: According to clause M.7.4 on SA handling, at most six SAs per direction may exist at a P-CSCF for one user at any one time. 4. For each incoming protected message the SIP application at the P-CSCF shall verify that the correct inbound SA according to clause M.7.4 on SA handling has been used. The SA is identified by the triple (UE_IP_address, 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 65 Release 12
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UE_protected_port, P-CSCF_protected_port) in the "SA_table". The SIP application at the P-CSCF shall further ensure that the user associated with the SA, which was used to protect the incoming message from the UE, is identical to the user who is associated at SIP level with the message sent by the P-CSCF towards the network.
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