PPP (Point-to-Point Protocol) refers to a family of computer networking protocols that provide a standard way to transport multiprotocol data over point-to-point links. PPP has three main components: a way to encapsulate multiprotocol datagrams; a Link Control Protocol to establish, configure and test the data-link connection; and a group of network control protocols that establish and configure different types of network-layer protocols.
PPP also tunnels Internet Protocol (IP) or other network Layer 3 data between two directly connected nodes over a physical connection, or over a direct link. Since IP and Transmission Control Protocol (TCP) do not support point-to-point connections, the use of PPP can enable them over Ethernet and other physical media.
In terms of the OSI model, PPP provides Layer 2, or data-link, service. PPP is a full-duplex protocol that can be used on a variety of physical media, including twisted pair copper wire, fiber optic lines or satellite links. PPP can provide services over everything, from a dial-up modem connection to a Secure Sockets Layer (SSL) encrypted virtual private network (VPN) connection. PPP uses a variation of High-level Data Link Control (HDLC) for packet encapsulation.
For example, a high-security application on a company network connects to the network via the VPN and establishes an SSL link. The client for the application can then establish a PPP tunnel on top of that, which will carry IP packets to the application's server.
Point-to-Point Protocols are sometimes considered a member of the TCP/IP suite of protocols. Variations of PPP exist for running over Ethernet using the PPPoE specification and for asynchronous transfer mode (ATM) using the PPPoA specification.
PPP is sometimes hidden from view -- for example, it has been used to connect Digital Subscriber Line (DSL) and cable modems to their back-end services. Its visible use has been declining steadily over time, along with dial-up modem services.
PPP is also known as RFC 1661 by the Internet Engineering Task Force (IETF), which initially created the protocol.
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