276 V7122 GatewayUser Guidenegotiable at this phase, including MRU (maximum receive unit), Authentication Protocol,and others.Once the link is established (each side sends a ‘configure ack’ message to the other side),the authentication phase may begin. The authentication phase is not mandatory. However, itis negotiated in the link configuration phase. A host may ask other hosts for authenticationusing Password Authentication Protocol (PAP) or Challenge Handshake AuthenticationProtocol (CHAP).The PAP sends the username and password to the remote host unencrypted.The CHAP is a more sophisticated method of authentication. The two hosts share a ‘secret’.The authenticator sends a ‘challenge’ to the host requesting authentication. The hostperforms a calculation (one-way hash) using the challenge received from the authenticatorand the shared ‘secret’, and sends the result to the authenticator. The authenticator verifiesthe host if the result of the calculation is correct; otherwise it is rejected.The last configuration phase, immediately after the authentication phase (or after the LinkConfiguration) is the Network Control Protocol. There is a family of control protocols forestablishing and configuring different network-layer protocols, for example, IPCP (PPPInternet Protocol Control Protocol), IPv6CP (PPP IP v6 Control Protocol), and BCP (PPPBridging Control Protocol). Each of them handles and manages the specific needs requiredby their respective network-layer protocol.When working in an IP network, IPCP is used as the Network Configuration Protocol. TheIPCP is used to configure the network layer of the hosts, requesting/declaring on IPAddresses.Further information on PPP Protocol is available on the IETF website(http://www.ietf.org/rfc/rfc1661.txt). Further information on Password Authentication Protocolis available on the IETF website (http://www.ietf.org/rfc/rfc1334.txt). Further information onChallenge Handshake Authentication Protocol is available on the IETF website(http://www.ietf.org/rfc/rfc1994.txt). Further information on PPP Internet Protocol ControlProtocol (IPCP) is available on the IETF website (http://www.ietf.org/rfc/rfc1332.txt).PPPoE OverviewPPPoE is a method of sending the Point-to-Point Protocol over Ethernet network. PPPoEprovides the ability to connect a network of hosts over a simple bridging access device to aremote Access Concentrator. Access control, billing and type of service can be done on aper-user, rather than a per-site, basis.A common use of the PPPoE is in the ADSL market: The home PC is connected to a modemusing Ethernet, and the PC uses the PPPoE to ‘simulate’ as if it was directly connected tothe remote host on a point-to-point connection.Since PPPoE frames are sent over Ethernet, each PPP session must learn the Ethernetaddress of the remote peer, as well as establish a unique session identifier. The PPPoEstandard describes a discovery protocol that provides this. A PPPoE session begins with a
