network protocols handbook.pdf

182Protocols GuideWAN - PPP ProtocolsProtocol NameLCP: PPP Link Control ProtocolProtocol DescriptionThe Link Control Protocol (LCP) is used to automatically agreeupon the encapsulation format options, handle varying limits onsizes of packets, detect a looped-back link and other commonmisconfiguration errors, and terminate the link. Other optionalfacilities provided are authentication of the identity of its peer onthe link, and determination when a link is functioning properlyand when it is failing. The Link Control Protocol LCP in PPP isversatile and portable to a wide variety of environment.There are three classes of LCP packets:1. Link Configuration packets used to establish and configurea link (Configure-Request, Configure-Ack, Configure-Nakand Configure-Reject).2. Link Termination packets used to terminate a link (Terminate-Requestand Terminate-Ack).3. Link Maintenance packets used to manage and debuga link (Code-Reject, Protocol-Reject, Echo-Request,Echo-Reply, and Discard-Request).In the interest of simplicity, there is no version field in the LCPpacket. A correctly functioning LCP implementation will alwaysrespond to unknown Protocols and Codes with an easily recognizableLCP packet, thus providing a deterministic fallbackmechanism for implementations of other versions.Regardless of which Configuration Options are enabled, all LCPLink Configuration, Link Termination, and Code-Reject packets(codes 1 through 7) are always sent as if no Configuration Optionswere negotiated. In particular, each Configuration Optionspecifies a default value. This ensures that such LCP packetsare always recognizable, even when one end of the link mistakenlybelieves the link to be open.Exactly one LCP packet is encapsulated in the PPP Informationfield, where the PPP Protocol field indicates type hex c021 (LinkControl Protocol).Protocol Structure5 Terminate-Request.6 Terminate-Ack.7 Code-Reject.8 Protocol-Reject.9 Echo-Request.10 Echo-Reply.11 Discard-Request.12 Link-Quality Report.• Identifier - Decimal value which aids in matching requestsand replies.• Length - Length of the LCP packet, including theCode, Identifier, Length and Data fields.• Data - Variable length field which may contain one ormore configuration options.Related protocolsPPP, PPPoE, PPPoA, SLIP, CHAP, HDLC, NCPSponsor SourceLCP is defined by IETF (http://www.ietf.org) .Referencehttp://www.javvin.com/protocol/rfc1570.pdfPPP LCP Extensions.http://www.javvin.com/protocol/rfc1661.pdfThe Point-to-Point Protocol (PPP)8 16 32bit VariableCode Identifier Length Data• Code - Decimal value which indicates the type ofLCP packet:1 Configure-Request.2 Configure-Ack.3 Configure-Nak.4 Configure-Reject.