IPsec - Protocol
Short for IP Security, a set of protocols being developed by the IETF to support secure exchange of packets at the IP layer. Once it's completed, IPsec is expected to be deployed widely to implement Virtual Private Networks (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.