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the network layer can be exchanged The reason is that PPP supports multiple protocols at the network layer If a node is running multiple protocols simultaneously at the network layer, the receiving node needs to know which protocol will receive the data Open In the open phase, data transfer takes place When a connection reaches this phase, the exchange of data packets can be started The connection remains in this phase until one of the endpoints wants to terminate the connection Terminate In the termination phase the connection is terminated Several packets are exchanged between the two ends for house cleaning and closing the link
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Although PPP is a data link layer protocol, PPP uses another set of other protocols to establish the link, authenticate the parties involved, and carry the network layer data Three sets of protocols are defined to make PPP powetful: the Link Control Protocol (LCP), two Authentication Protocols (APs), and several Network Control Protocols (NCPs) At any moment, a PPP packet can carry data from one of these protocols in its data field, as shown in Figure 1134 Note that there is one LCP, two APs, and several NCPs Data may also come from several different network layers
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LCP: OxC021 AP: OxC023 and OxC223 NCP: Ox8021 and Data: Ox0021 and
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LCP: Link Control Protocol AP: Authentication Protocol NCP: Network Control Protocol
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Link Control Protocol The Link Control Protocol (LCP) is responsible for establishing, maintaining, configuring, and terminating links It also provides negotiation mechanisms to set options between the two endpoints Both endpoints of the link must reach an agreement about the options before the link can be established See Figure 1135 All LCP packets are carried in the payload field of the PPP frame with the protocol field set to C02 1 in hexadecimal The code field defines the type of LCP packet There are 11 types of packets as shown in Table 112
SECTION 117 POINT-TO-POINT PROTOCOL
LCP packet encapsulated in aframe
Variable Infonnation Payload (and padding)
Table 112 LCP packets
Code
OxOl Ox02 Ox03 Ox04 Ox05 Ox06 Ox07 Ox08 Ox09 OxOA OxOB
Packet Type
Coofigure-request Configure-ack Configure-nak Configure-reject Terminate-request Terminate-ack Code-reject Protocol-reject Echo-request Echo-reply Discard-request
Description
Contains the list of proposed options and their values Accepts all options proposed Announces that some options are oot acceptable Announces that some options are not recognized Request to shut down the line Accept the shutdown request Announces an unknown code Announces an unknown protocol
A type of hello message to check if the other end is alive
The response to the echo-request message A request to discard the packet
There are three categories of packets The first category, comprising the first four packet types, is used for link configuration during the establish phase The second category, comprising packet types 5 and 6, is used for link tennination during the termination phase The last five packets are used for link monitoring and debugging The ID field holds a value that matches a request with a reply One endpoint inserts a value in this field, which will be copied into the reply packet The length field defines the length of the entire LCP packet The information field contains information, such as options, needed for some LCP packets There are many options that can be negotiated between the two endpoints Options are inserted in the information field of the configuration packets In this case, the information field is divided into three fields: option type, option length, and option data We list some of the most common options in Table 113
Table 113
Common options Option Default
1500 None
Maximum receive unit (payload field size) Authentication protocol Protocol field compression Address and control field compression
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Authentication Protocols
Authentication plays a very important role in PPP because PPP is designed for use over dial-up links where verification of user identity is necessary Authentication means validating the identity of a user who needs to access a set of resources PPP has created two protocols for authentication: Password Authentication Protocol and Challenge Handshake Authentication Protocol Note that these protocols are used during the authentication phase
PAP The Password Authentication Protocol (PAP) is a simple authentication procedure with a two-step process:
1 The user who wants to access a system sends an authentication identification (usually the user name) and a password 2 The system checks the validity of the identification and password and either accepts or denies connection Figure 1136 shows the three types of packets used by PAP and how they are actually exchanged When a PPP frame is carrying any PAP packets, the value of the protocol field is OxC023 The three PAP packets are authenticate-request, authenticate-ack, and authenticate-nak The first packet is used by the user to send the user name and password The second is used by the system to allow access The third is used by the system to deny access
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