Note 12the spi is only required when initiating and

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NOTE 12: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". 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 13:According to clause 7.4 on SA handling, at most six SAs per direction per registered contact may exist at a P-CSCF for one IMPI 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 7.4 on SA handling has been used. The SA is identified by the triple (UE_IP_address, 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. NOTE 14:Not all SIP messages necessarily contain public or private identities, e.g. subsequent messages in a dialogue. Other information, e.g. a dialogue identifier, may be used to associate the message with a user at SIP level. 5. For each unidirectional SA which has been established and has not expired, the SIP application at the UE stores at least the following data: (UE_protected_port, P-CSCF_protected_port, SPI, lifetime) in an "SA_table". The pair (UE_protected_port, P-CSCF_protected_port) equals either ( port_uc, port_ps ) or ( port_us, port_pc ). 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 36 Release 12
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NOTE 15:The SPI is only required to initiate and delete SAs in the UE. The SPI is not exchanged between IPsec and the SIP layer for incoming or outgoing SIP messages. 6. When establishing a new pair of SAs (cf. clause 6.3) the SIP application at the UE shall ensure that the selected numbers for the protected ports do not correspond to an entry in the "SA_table".
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