In computer networking, the Link Control Protocol (LCP) forms part of the Point-to-Point Protocol (PPP), within the family of Internet protocols. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine the standards of the ensuing data transmission.
The protocol: checks the identity of the linked device and either accepts or rejects the device determines the acceptable packet size for transmission searches for errors in configuration can terminate the link if requirements exceed the parameters LCP packets are sent during the Link Establishment Phase of PPP, after the physical layer has been made ready from the Link Dead state. No transmission of network-layer protocols such as IP may be transmitted across PPP until after the Link Establishment or Authentication phases are complete. When a connection is requested by a peer to be terminated (or if Authentication fails), the Link Termination Phase is performed, and PPP returns to the Link Dead state.
Structure OffsetsOctet0123OctetBit012345678910111213141516171819202122232425262728293031Point-to-Point Protocol00Flag (0x7E)Address (0xFF)Control (0x03)Protocol (0xC0)432Protocol, cont. (0x21)Link Control Protocol00CodeIdentifierLengthLink ConfigurationCode1NameConfigure-RequestDescriptionPeer requesting connection with the specified options over link defaults.2Configure-AckPeer requesting connection with the specified options over link defaults.3Configure-NakPeer recognizes given options, but some are not acceptable.4Configure-RejectPeer does not recognize or is prohibited from accepting some given options.432List of option data, size variableLink TerminationCode5NameTerminate-RequestDescriptionPeer requesting to close the connection.6Terminate-AckPeer accepts connection closure, or requires re-negotiation.432Uninterpreted data by the sender, size variableLink MaintenanceCode7NameCode-RejectDescriptionPeer does not recognize a provided LCP code.432Faulting LCP Packet, size variableLCP-Specific CodesCode8NameProtocol-RejectDescriptionPeer does not recognize a protocol used within the LCP Opened state.432Rejected PPP ProtocolFaulting Packet, size variableCode9NameEcho-RequestDescriptionPeer is requesting an Echo-Reply.10Echo-ReplyPeer is responding to an Echo-Request (only acceptable when in LCP Opened.)11Discard-RequestPeer is sending data to be discarded ("sinked") by the remote peer.432Magic Number (0, if Configure-Request / Configure-Ack have not established one yet)864Uninterpreted data by the sender, size variable
Link Configuration Option OffsetsOctet0123OctetBit01234567891011121314151617181920212223242526272829303100TypeLengthType0NameVendor SpecificDescriptionVendor specific data not defined by the PPP specification.216Vendor's Organizationally Unique IdentifierKind (OUI Subtype)648Vendor defined data, size variableType1NameMaximum-Receive-UnitDescriptionDefines a peer's MRU, overriding the default of 1,500 octets.216The maximum-receive-unit for this peer's implementationType3NameAuthentication-ProtocolDescriptionDefines an authentication protocol for a remote peer to authenticate with before transitioning phases.216The requested authentication protocol, assigned by IANAAdditional data for this protocol, size variableType4NameQuality-ProtocolDescriptionDefines a quality protocol for link quality monitoring.216The requested quality protocolAdditional data for this protocol, size variableType5NameMagic-NumberDescriptionDefines a unique identifier for a peer.216Magic numberType7NameProtocol-Field-CompressionDescriptionNo data. Refer to RFC 1661 Section 6.5 for details.Type8NameAddress-and-Control-Field-CompressionDescriptionNo data. Refer to RFC 1661 Section 6.6 for details.
References
External links : PPP LCP Extensions : The Point-to-Point Protocol (PPP) : PPP Reliable Transmission