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 The Point To Point Protocol

Written by: Les Harper - Sep 7, 2008

PPP – The Point-to-Point Protocol

The Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes (computers, servers, routers etc.). It can connect computers using serial cable, phone line, trunk line, cellular telephone, specialized radio links, or fiber optic links. Most Internet Service Providers (ISP) use PPP for customers' dial-up access to the Internet.

PPP was designed to enable the transmission of different protocols over one point-to-point link by utilizing encapsulation. Encapsulation is the process of storing packets from the different protocol inside PPP frames.

PPP is more stable than the older SLIP (Serial Line Internet Protocol, is a mostly obsolete encapsulation of the Internet Protocol designed to work over serial ports and modem connections) and provides error checking features. Besides providing error checking, PPP also supports multiple protocols, allows IP addresses to be negotiated at connection time, permits authentication, and many others.

The main three features provided by PPP are: 1. A framing method that clearly defines end of one frame and the start of the next one. 2. A Link Control Protocol (LCP) for bringing lines up, testing them, negotiating options, and bringing them down. The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. 3. A way to negotiate network layer options in a way that is independent of network layer protocols. PPP works in Data Link Layer, the 2nd layer of OSI model. A PPP Network Control Protocol must be defined for each type of network packet which is to be encapsulated and transmitted across the PPP link. Several NCP implementations from different vendors like Novell, Apple are available.

To establish communications over a point-to-point link, the originating PPP 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. When each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link. The link will remain 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).

A key advantage of PPP is that it is an extensible protocol suite. Over the years new protocols have been added to the suite, to provide additional features or capabilities. For example, PPP is designed not to use just a single authentication protocol, but to allow a choice of which protocol is used for this purpose. Windows Server 2003 and Windows XP support the following PPP authentication protocols: 1. PAP: Password Authentication Protocol 2. CHAP: Challenge-Handshake Authentication Protocol

PPP is capable of operating across any type of interfaces. PPP does not impose any restriction regarding transmission rate other than those imposed by the particular interface in use.

In summery, Point-to-Point Protocol is a multi-protocol framing mechanism suitable for use over modems, serial lines, and other physical layers. It supports error detection, option negotiation, compression, and reliable transmission.