Authentication Header (AH)
Encapsulating Security Payload (ESP)
AH provides data integrity by using an authentication algorithm. It does not encrypt the packet.
ESP typically protects the packet with an encryption algorithm and provides data integrity with an authentication algorithm. Some encryption algorithms provide both encryption and authentication, such as AES GCM.
The AH protocol cannot be used with network address translation (NAT).
The authentication header provides data authentication, strong integrity, and replay protection to IP packets. AH protects the greater part of the IP packet. As the following illustration shows, AH is inserted between the IP header and the transport header.
The transport header can be TCP, UDP, SCTP, or ICMP. If a tunnel is being used, the transport header can be another IP header.
The encapsulating security payload (ESP) protocol provides confidentiality over what the ESP encapsulates. ESP also provides the services that AH provides. However, ESP does not protect the outer IP header. ESP provides authentication services to ensure the integrity of the protected packet. Because ESP uses encryption-enabling technology, a system that provides ESP can be subject to import and export control laws.
In a TCP packet, the ESP header is authenticated and it encapsulates the TCP header and its data. If the packet is an IP-in-IP packet, ESP protects the inner IP packet. Per-socket policy allows self-encapsulation, so ESP can encapsulate IP options when necessary.
Self-encapsulation can be used by writing a program that uses the setsockopt() system call. If self-encapsulation is set, a copy of the IP header is made to construct an IP-in-IP packet. For example, when self-encapsulation is not set on a TCP socket, the packet is sent in the following format:
[ IP(a -> b) options + TCP + data ]
When self-encapsulation is set on that TCP socket, the packet is sent in the following format:
[ IP(a -> b) + ESP [ IP(a -> b) options + TCP + data ] ]
For further discussion, see Transport and Tunnel Modes in IPsec.
The following table compares the protections that are provided by AH and ESP.
IPsec security uses two types of algorithms, authentication and encryption. The AH protocol uses authentication algorithms. The ESP protocol can use encryption as well as authentication algorithms. You can obtain a list of the algorithms on your system and their properties by using the ipsecalgs command. For more information, see the ipsecalgs(8) man page. You can also use the functions that are described in the getipsecalgbyname(3C) man page to retrieve the properties of algorithms. IPsec uses the Cryptographic Framework to perform encryption and authentication.