3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.3 Point-to-Point Protocol (PPP)
A limited number of point-to-point WAN links were possible in the 1980s due to a lack of a standard Internet encapsulation protocol. The Point-to-Point Protocol (PPP) was designed to solve this problem. In addition to solving the problem of standardized Internet encapsulation of IP over point-to-point links, PPP was also designed to address the issues of assignment and management of IP addresses, asynchronous (start/stop) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for such capabilities as network-layer address negotiation and data compression negotiation. PPP addresses these issues by providing an extensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) to negotiate optional configuration parameters and facilities. Today, PPP supports other protocols besides IP, including IPX and DECnet.

PPP provides a method for transmitting datagrams over serial point-to-point links. It has three main components:

  1. A method for encapsulating datagrams over serial links-PPP uses the High-Level Data Link Control (HDLC) protocol as a basis for encapsulating datagrams over point-to-point links.
  2. An extensible LCP to establish, configure, and test the data-link connection.
  3. A family of NCPs for establishing and configuring different network-layer protocols. PPP is designed to allow the simultaneous use of multiple network-layer protocols.

In order to establish communications over a point-to-point link, the originating PPP station first sends LCP frames to configure and (optionally) test the data link. After the link has been established and optional facilities have been negotiated as needed by the LCP, the originating PPP sends NCP frames to choose and configure one or more network-layer protocols. Once each of the chosen network-layer protocols has been configured, packets from each network-layer protocol can be sent over the link. The link remains configured for communications until explicit LCP or NCP frames close the link or until some external event occurs (for example, an inactivity timer expires or a user intervenes).

PPP can operate across any DTE/DCE interface (for example, EIA/TIA-232, EIA/TIA-422, EIA/TIA-423, and ITU-T V.35). The only absolute requirement imposed by PPP is the provision of a duplex circuit, either dedicated or switched, that can operate in either an asynchronous or synchronous bit-serial mode, transparent to PPP link-layer frames. PPP does not impose any restrictions regarding transmission rate, other than those imposed by the particular DTE/DCE interface in use.