> # IPsec and IKE Terminology

> The terms used in this chapter are explained in this section.

> - Security association (SA)— An agreement between two participating peers on the entries required to encrypt and decrypt IP packets. Two SAs are required for each peer in each direction (inbound and outbound) to establish bidirectional communication between the peers. Sets of bidirectional SA records are stored in the SA database (SAD). IPsec uses IKE to negotiate and bring up SAs. Each SA record includes the following information:
> - Security parameter index (SPI)—A number which, together with a destination IP address and security protocol, uniquely identifies a particular SA. When using IKE to establish the SAs, the SPI for each SA is a pseudo-randomly derived number.
> - Peer—A switch or other device that participates in IPsec. For example, a Cisco MDS switch or other Cisco routers that support IPsec.
> - Transform—A list of operations done to provide data authentication and data confidentiality. For example, one transform is the ESP protocol with the HMAC-MD5 authentication algorithm.
> - Session key—The key used by the transform to provide security services.
> - Lifetime—A lifetime counter (in seconds and bytes) is maintained from the time the SA is created. When the time limit expires the SA is no longer operational and, if required, is automatically renegotiated (rekeyed).
> - Mode of operation—Two modes of operation are generally available for IPsec: tunnel mode and transport mode. The Cisco NX-OS implementation of IPsec only supports the tunnel mode. The IPsec tunnel mode encrypts and authenticates the IP packet, including its header. The gateways encrypt traffic on behalf of the hosts and subnets.
>   The Cisco NX-OS implementation of IPsec does not support transport mode.

> Note The term tunnel mode is different from the term tunnel, which is used to indicate a secure communication path between two peers, such as two switches connected by an FCIP link.

> - Anti-replay—A security service where the receiver can reject old or duplicate packets to protect itself against replay attacks. IPsec provides this optional service by use of a sequence number combined with the use of data authentication.
> - Data authentication—Data authentication can refer either to integrity alone or to both integrity and authentication (data origin authentication is dependent on data integrity).
> - Data integrity—Verifies that data has not been altered.
> - Data origin authentication—Verifies that the data was actually sent by the claimed sender.
> - Data confidentiality—A security service where the protected data cannot be observed.
> - Data flow—A grouping of traffic, identified by a combination of source address and mask or prefix, destination address mask or prefix length, IP next protocol field, and source and destination ports, where the protocol and port fields can have any of these values. Traffic matching a specific combination of these values is logically grouped together into a data flow. A data flow can represent a single TCP connection between two hosts, or it can represent traffic between two subnets. IPsec protection is applied to data flows.
> - Perfect forward secrecy (PFS)—A cryptographic characteristic associated with a derived shared secret value. With PFS, if one key is compromised, previous and subsequent keys are not compromised, because subsequent keys are not derived from previous keys.
> - Security Policy Database (SPD)—An ordered list of policies applied to traffic. A policy decides if a packet requires IPsec processing, if it should be allowed in clear text, or if it should be dropped.
> - The IPsec SPDs are derived from user configuration of crypto maps.
> - The IKE SPD is configured by the user.

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