HP 6125XLG R2306-HP 6125XLG Blade Switch Security Configuration Guide - Page 209

Figure 72 Security protocol encapsulations in different modes Mode Protocol Transport Tunnel AH IP AH Data IP AH IP Data ESP IP ESP Data ESP-T IP ESP IP Data ESP-T AH-ESP IP AH ESP Data ESP-T IP AH ESP IP Data ESP-T Security association A security association (SA) is an agreement negotiated between two communicating parties called "IPsec peers." An SA comprises a set of parameters for data protection, including security protocols (AH, ESP, or both), encapsulation mode (transport mode or tunnel mode), authentication algorithm (HMAC-MD5 or HMAC-SHA1), encryption algorithm (DES, 3DES, or AES), and shared keys and their lifetimes. An SA is unidirectional. At least two SAs are needed to protect data flows in a bidirectional communication. If two peers want to use both AH and ESP to protect data flows between them, they construct an independent SA for each protocol in each direction. An SA is uniquely identified by a triplet, which consists of the security parameter index (SPI), destination IP address, and security protocol identifier. An SPI is a 32-bit number that identifies an SA. It is transmitted in the AH/ESP header. An SA can be set up manually or through IKE. • Manual mode-Configure all parameters for the SA through commands. This configuration mode is complex and does not support some advanced features (such as periodic key update), but it can implement IPsec without IKE. This mode is mainly used in small and static networks or when the number of IPsec peers in the network is small. • IKE negotiation mode-The peers negotiate and maintain the SA through IKE. This configuration mode is simple and has good expansibility. In medium- and large-scale dynamic networks, HP recommends setting up SAs through IKE negotiations. A manually configured SA never ages out. An IKE-created SA has a lifetime, which comes in two types: • Time-based lifetime-Defines how long the SA can be valid after it is created. • Traffic-based lifetime-Defines the maximum traffic that the SA can process. If both lifetime timers are configured for an SA, the SA becomes invalid when either of the lifetime timers expires. Before the SA expires, IKE negotiates a new SA, which takes over immediately after its creation. Authentication and encryption Authentication algorithms IPsec uses hash algorithms to perform authentication. A hash algorithm produces a fixed-length digest for an arbitrary-length message. IPsec peers respectively calculate message digests for each packet. The receiver compares the local digest with that received from the sender. If the digests are identical, the receiver considers the packet intact and the sender's identity valid. IPsec uses the Hash-based Message 200