IPsec/IPsec-ESP/IPsec-AH

Internet Protocol Security (IPSec)	Internet Protocol security.
Encapsulating Security Payload (IPsec-ESP)	The encapsulating security payload (ESP) encapsulates its data, enabling it to protect data that follows in the datagram.
Internet Protocol security Authentication Header (IPsec-AH)	AH protects the parts of the IP datagram that can be predicted by the sender as it will be received by the receiver.

IPsec is a set of protocols developed by the IETF to support secure exchange of packets at the IP layer. IPsec has been deployed widely to implement VPNs.

IPsec supports two encryption modes: Transport and Tunnel. Transport mode encrypts only the data portion (payload) of each packet, but leaves the header untouched. The more secure Tunnel mode encrypts both the header and the payload. On the receiving side, an IPSec-compliant device decrypts each packet.

For IPsec to work, the sending and receiving devices must share a public key. This is accomplished through a protocol known as Internet Security Association and Key Management Protocol/Oakley (ISAKMP/Oakley), which allows the receiver to obtain a public key and authenticate the sender using digital certificates.