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University of education township campus lahore, Pakistan

Lecture 05

PPP (POINT TO POINT PROTOCOL)

Internet Architecture and Protocols, University of education township campus, lahore

Internet Architecture and Protocols

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Lecture Objectives• WAN Basics

– Three types of WAN Services

• Introduction to Point-to-Point protocol (PPP)

• Basic Components of PPP– Encapsulation, LCP, NCP

• PPP Frame Format• PPP Authentication

– PAP, CHAP

• General Operation of PPP• PPP Services

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WAN Point-to-Point Link

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WAN ServicesTerm Description

Leased lines A dedicated, always on circuit between two end points. The service provider just passes a constant rate bit stream. Generally is more expensive than packet switching today

Circuit switched/dial

Provide dedicated bandwidth between two points, but only for the duration of the call. Typically used as a cheaper alternative to leased lines, particularly when connectivity is not needed all the time. Also useful for backup when a leased line or packet-switched service fails

Packet-switched

Provides virtual circuits between pairs of sites, with contracted traffic rates for each VC. Each site’s physical connectivity consists of a leased line from the site to a device in the provider’s network. Generally cheaper than leased lines.

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WAN Data-Link Protocols for Point-to-Point Links

Leased line

Circuit-switched

Packet-switched

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• A simplified look inside the WAN cloud• The router also uses a WAN central office

An Overview of Wide-Area Services

Central Office(CO)Central Office(CO)

CO

Basic Telephone Service

Basic Telephone Service

Time-Division Multiplexed Circuits(56/64 kbps or T1/E1)

BC

A E

X.25/Frame RelayNetworks

Call SetupCall Setup(SS7 or other)(SS7 or other)

X.25 / Frame RelayX.25 / Frame RelayNetworksNetworks

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PPP over the Internet• Today, million of Internet users who need to

connect their home PCs to the server of an ISP use PPP

• The Internet needs a PPP for a variety of purposes, including router-to-router traffic (leased line) and home-user to ISP (dial-up) traffic.

• Majority users have a traditional modem, DSL modem or a cable modem which connects them to the Internet through a Telephone line or a TV cable connection.

• These lines provide a physical link, but to control and manage the transfer of data, there is a need of PPP

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PPP over the Internet• For both connections, router-to-router (leased line

connections) and home-user to ISP (dial-up connections) some data link protocol is required on the line for:– Framing or encapsulation– Error control and other data link layer functions

• If two devices are directly connected to each other, then they are said to be in point-to-point configuration

• Point-to-point protocol is a data link layer protocol which is used to control the communication over such links

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Definition - PPP• PPP is defined in 1661.• It originally emerged as an encapsulation

protocol for transporting IP traffic over point-to-point links.

• PPP also established a standard for– assigning and managing IP addresses– asynchronous and synchronous encapsulation– network protocol multiplexing– link configuration & link quality testing– error detection, option negotiation

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PPP Components• Main components of PPP include:

– Encapsulation/Framing– Link Control Protocol (LCP)– Network Control Protocol (NCP)

• These components help PPP to define the frame formats to be exchanged, how the 2 devices will negotiate with each other to establish a link and for exchanging the data, how the link is configured and terminated.

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1. Encapsulation• It is a method for encapsulating datagrams

of network layer over serial links

• A framing mechanism defines the boundaries of the start and end of the PPP frame

• It also helps in error detection

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Encapsulation…

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2. Link Control Protocol• LCP is used to establish, configure and test the

data link connections• Its main purpose is:

– to bring the lines up, test them– negotiate options– and bringing them gracefully down again when they are

no longer needed• It supports synchronous and asynchronous circuits

• LCP configuration options include:– Maximum frame size or MTU (maximum transmission

unit)– Authentication protocol specifications (if any)– Options to skip control and address fields of PPP frame

• PPP negotiates configuration parameters at the start of the connection using LCP.

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LCP packet encapsulated in a frame

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LCP Phase: Common options

Option Default

Maximum receive unit 1500

Authentication protocol None

Protocol field compression Off

Address and control field compression

Off

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Link Control Protocol (LCP)

• 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.

• When PPP is carrying a LCP packet, it is either in establishing or terminating state

• No user data is exchanged during this state• All LCP packets are carried in the data field

of the PPP frame

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Link Control Protocol (LCP)

• LCP frames are categorized into 3 classes– Link establishment frames– Link terminating frames– Link maintenance frames

• Options– There are many options that can be negotiated

between the two endpoints.– Options are inserted in the information field of

the configuration packets

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LCP packets and their codes

Code Packet Type Description

0116 Configure-requestContains the list of proposed options and their values

0216 Configure-ack Accepts all options proposed

0316 Configure-nak Announces that some options are not acceptable

0416 Configure-reject Announces that some options are not recognized

0516 Terminate-request Requests to shut down the line

0616 Terminate-ack Accepts the shut down request

0716 Code-reject Announces an unknown code

0816 Protocol-reject Announces an unknown protocol

0916 Echo-requestA type of hello message to check if the other end is alive

0A16 Echo-reply The response to the echo-request message

0B16 Discard-request A request to discard the packet

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PPP Authentication…

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Authentication Protocols• The term authentication refers to a set

of security functions that help one device ensure that it is communicating with the correct other device

• PPP use two authentication protocols– Password Authentication Protocol (PAP)– Challenge Handshake Authentication Protocol

(CHAP)

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Password Authentication Protocol

• Password Authentication Protocol (PAP)– PAP sends the username and password in clear

text.– The user who wants to access a system sends

username and password– The system checks the validity of the

username and password and either accepts or denies connection.

– For those systems that require greater security, PAP is not enough; a third party with access to the link can easily pick up the password and access the system resources

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Password Authentication Protocol

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Selecting a PPP Authentication Protocol

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Challenge Handshake

Authentication Protocol • Challenge Handshake Authentication

Protocol (CHAP)– It is a three handshaking authentication protocol

that provides greater security that PAP.– In this method the password is kept secret, it is

never sent online.– The system sends to the user a challenge packet

containing a challenge value, usually a few byte.– The user applies a predefined function (Message

Digest-5, one way hash function) that takes the challenge value and the user’s own password and creates a result. The user sends the result in the response packet to the system

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CHAP…• Challenge Handshake Authentication

Protocol (CHAP)…– The system does the same. It applies the same

function to the password of the user (known to the system) and the challenge value to create a result. If the result created the same as the result in the response packet, access is granted; otherwise it is denied

– CHAP is more secure than PAP, especially if the system continuously changes the challenge value. Even if the intruder learns the challenge value and the result, the password is still secret.

– CHAP authentication is difficult to break

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CHAP…

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Selecting a PPP Authentication Protocol