AH and ESP protocols
IPSec uses two distinct protocols, Authentication Header (AH) and Encapsulating Security Payload (ESP), which are defined by the IETF.
The AH protocol provides a mechanism for authentication only. AH provides data integrity, data origin authentication, and an optional replay protection service. Data integrity is ensured by using a message digest that is generated by an algorithm such as HMAC-MD5 or HMAC-SHA. Data origin authentication is ensured by using a shared secret key to create the message digest. Replay protection is provided by using a sequence number field with the AH header. AH authenticates IP headers and their payloads, with the exception of certain header fields that can be legitimately changed in transit, such as the Time To Live (TTL) field.
The ESP protocol provides data confidentiality (encryption) and authentication (data integrity, data origin authentication, and replay protection). ESP can be used with confidentiality only, authentication only, or both confidentiality and authentication. When ESP provides authentication functions, it uses the same algorithms as AH, but the coverage is different. AH-style authentication authenticates the entire IP packet, including the outer IP header, while the ESP authentication mechanism authenticates only the IP datagram portion of the IP packet.
Either protocol can be used alone to protect an IP packet, or both protocols can be applied together to the same IP packet. The choice of IPSec protocol is determined by the security needs of your installation, and is configured by the administrator. It does not have to be applied system-wide, and can be configured differently for each set of connection endpoints. For a dynamic tunnel, the choice of IPSec protocol is configured using the IpDataOffer statement in an IP security policy configuration file. For a manual tunnel, the choice of IPSec protocol is configured using the IpManVpnAction statement in an IP security policy configuration file. For more details about the IpDataOffer statement and the IpManVpnAction statement, see z/OS Communications Server: IP Configuration Reference.
Encryption Protocol | Encryption Algorithm | Authentication Protocol | Authentication Algorithm |
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None | None. | ESP or AH | Any of the following algorithms:
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ESP | Any of the following algorithms:
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ESP or AH | Any of the following algorithms:
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ESP | Any of the following algorithms:
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ESP | NULL (AES_GCM provides built-in authentication) |