IPsec

Internet Protocol Security (IPsec) is a way of making Internet communications more secure and private.
IPsec is a collection of protocols for securing Internet Protocol (IP) communications by authenticating (and optionally encrypting) each of a data stream. IPsec also includes protocols for establishing mutual authentication between agents at the beginning of the session and negotiation of cryptographic keys to be used during the session. IPsec can be used to protect data flows between a pair of hosts (e.g. computer users or servers), between a pair of security gateways (e.g. routers or firewalls), or between a security gateway and a host. 
IPsec is an end-to-end security solution and operates at the Internet Layer of the Internet Protocol Suite, comparable to Layer 3 in the OSI model. Other Internet security protocols in widespread use, such as SSL, TLS and SSH, operate in the upper layers of these models. This makes IPsec more flexible, as it can be used for protecting all the higher level protocols, because applications do not need to be designed to use IPsec, whereas the use of TLS/SSL or other higher-layer protocols must be built-in the application.
The term "IPsec" is officially defined by the Internet Engineering Task Force (IETF). This definition includes the form of capitalization used for the term; it is often incorrectly spelled "IPSec".