The Point-to-Point Protocol reference article from the English Wikipedia on 24-Jul-2004
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Point-to-Point Protocol

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The Point-to-Point Protocol, or PPP, is commonly used to establish a connection between two computers using a phone line. Many ISPs use PPP when providing customers with dial-up access, where it has largely superseded an older protocol known as SLIP.

PPP is commonly used to act as a layer 2 (Layer 2 is the Data Link layer of the OSI model) protocol for connection over synchronous and asynchronous circuits. PPP was designed to work with several network layer protocols, such as, IP, IPX and AppleTalk protocol and as a replacement for the non-standard layer 2 protocol SLIP.

PPP was designed much later than the original HDLC specifications. As a result, the creators of PPP included many additional features that had not been seen in WAN data-link protocols up to that time.

Enhanced Error Detection

PPP uses FCS fields to determine if an individual frame has an error, however PPP monitors the frequency with which frames are received in error, and it can be configured to take down an interface if too many errors occur.

Looped Link Detection

LCP (Link Control Protocol, an integral part of PPP and defined in the same RFC) notices looped links using a feature called magic numbers. When using PPP, router sends PPP LCP messages, these messages include a magic number, which is different on each router. If a line is looped, the router receives an LCP message with its own magic number instead of getting a message with the other router's magic number.

PPP also provides hooks for automatically configuring the network interfaces at each end (setting IP address, default gateway, etc.) and for authentication.

PPP is described by IETF RFC 1661. Numerous documents on PPP have been published through the RFC process since July 1990, including various authentication, encryption and compression methods and the use of PPP in conjunction with other network protocols

RFC 1994 describes CHAP, the Challenge Handshake Authentication Protocol which is commonly used when establishing dialup connections with ISPs.

RFC 2516 describes PPPoE, a method for transmitting PPP over Ethernet which is sometimes used with DSL.

RFC 2364 describes PPPoA, a method for transmitting PPP over ATM Adaptation Layer 5 (AAL5) known as or PPPoATM for PPP over ATM.

See also HDLC, SLIP, EAP.