What is the purpose of the SPI field in an IPsec packet?
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A. B. C. D.A.
The SPI (Security Parameter Index) field in an IPsec packet serves to identify the Security Association (SA) that is used to process the packet. The SA contains the security parameters, such as the encryption and authentication algorithms and keys, that are negotiated during the IPsec setup phase.
The SPI is a 32-bit field that is included in the IPsec header of each packet. When a packet is received, the SPI is used to look up the corresponding SA in the Security Association Database (SAD). The SAD contains a list of active SAs that have been established between the two communicating hosts.
Once the correct SA is identified, the security parameters in the SA are used to process the packet. This can include decrypting the packet, verifying its integrity, and optionally applying additional security services such as authentication or anti-replay protection.
Therefore, option A is not correct as the SPI does not identify a transmission channel. Option B is partially correct, as the SPI is used in conjunction with other security measures, such as sequence numbers, to provide anti-replay protection. However, the primary purpose of the SPI is to identify the SA. Option C is also partially correct, as the SPI is used in combination with the security parameters in the SA to ensure data integrity. Option D is not correct, as the SPI does not contain a shared session key. The shared session key is negotiated during the establishment of the SA using a key exchange protocol such as IKE (Internet Key Exchange).