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Lec # 18, 19Computer Communication

& Networks

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History of Internet• In 1960s the telephone network was the worlds

most dominant communication network• Uses Circuit switching which is appropriate for

voice traffic by supporting constant data rates

• Three research groups laid the foundations of packet switching notion for computers communications:– MIT (Leonard Kleinrock)– Rand Institute (Paul Baran)– National Physical Laboratory (NPL)

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History of Internet• 1957 by Paul Baran and others give Idea of

Packet Switching, Principles of Packet Switching.

• 1961--- First Paper by him on Packet Switching.

• Paul Baran used first time Digital Computer Technology for Communication between Switching Networks and divided the data into “Message Blocks” and reassembled at destination with some error detection technique.

• 1968--- First Packet Switching Network was designed and Implemented

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The Internet’s Infancy: 1960s

• 1957 DARPA (Defense Advanced Research Project Agency) was established as an outcome of the launch in by NASA (National Aeronautics and Space Administration), formally known as ARPA.

• 1966--- First Wide Area Computer Network was developed.

• 1967--- First Packet Switching Router in the form of IMP (Interface Message Processor) was proposed; about a size of refrigerator.

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The Internet’s Infancy: 1960s

• 1969--- Network Working Group (NWG) was formed to ensure the stability of communication protocols. Steve Crocker wrote first minutes of meetings.

• Device Drivers were proposed to enable communication between different operating systems and hardware

• The IMPs (Interface Message Processors) connected both host computers and other IMPs and functioned to:

– Receive data– Check for errors– Retransmit, if error exists– Route the packets– Verify that packet are sent to intended receivers

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The Internet Early Years: 1970s

• 1970--- NCP (Network Control Protocol) was designed; used Stop and Wait flow control. It was the first host-to-host communication protocol that is used between the ARPANET end systems

• 1973--- TCP (Transmission Control Flow Control) was designed for data transmission and Checksum was used for error detection

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The Internet Early Years: 1970s

• Protocol Stack

APPLICATION

NCP

DEVICE DRIVER

IMP

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The Internet Growth Begins: 1970 - 1980s

• 1973--- Ethernet was proposed as a LAN Technology.

• 1974 --- First Ethernet protocol was developed.

• 1978 --- IP was proposed for Addressing purposes.

• 1980--- TCP/IP Protocol Suite was designed.

• 1st January 1983--- It was decided to replace NCP to TCP/IP for all Networks that gives birth to INTERNET- FTP, SMTP, DNS,UDP were introduced.

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The Internet Growth Begins: 1980s

• UDP comes into play for Real time Applications like Voice and Video.

• 1986--- All Super Computers were connected to form a Backbone Network for 56Kbps. and in 1988 was converted to T1 Line I.e., 1.544Mbps

• 1988--- First Internet Worm was invented effecting around 60,000 Hosts

• 1992--- WWW was created by Berners-Lee who also created First Web Server and Browser (Also designed HTTP later)

• 1993--- Clinton received first email at president@Whitehouse.gov

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Internet Privatization: 1990s

• 1994--- E-business started at Internet, NAPs (Network Access Points) and giving permission to ISPs to connect to NAPs

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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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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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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 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

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Internet Service Providers• What is an ISP?

– An ISP is an organization that connects business or residential customers to Internet (backbone).

– An Internet Service Provider (ISP) is a company that provides access to the Internet.

– Their customers can be businesses, individuals or organizations.

– The arrival of ISPs has made connecting to the Internet an affordable and convenient option for general people

• Internet structure is roughly hierarchical• In the public Internet, access networks situated at

the edge of the Internet are connected to the rest of the Internet through a tiered hierarchy of Internet Service Providers (ISPs)

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ISP Categories• ISP Categories

– Tier-1 ISPs (Internet Backbone)– Tier-2 ISPs– Tier-3 ISPs

• Backbone Providers / Tier-1 ISPs– These ISPs are nationwide or multinational organizations

that control Internet routing.– They often own significant pieces of backbone itself

• National Providers / Tier-2 ISPs– These ISPs buy capacity (bandwidth) and routing

services from backbone providers and run Points Of Presence (POP: location of access points to the Internet) across the country.

• Local Providers / Tier-3 ISPs– These ISPs operate in the same way as the national ISPs,

but on a smaller geographical area

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Points of Presence (POPs)• POPs are private peering points of ISPs• Within an ISPs network, the physical location /

points at which the ISP connect to other ISPs are known as Points of Presence (POPs)

• A POP is simply a group of one or more routers in the ISP’s network at which routers in other ISPs can connect.

• The POP is in the ISP’s switch site or in a collocation space, the contents will always contain “access” equipment and an IP router.

• At the core of the POP is a router that acts as the central hub for routing within the POP and is also used to terminate high capacity connections.

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Network Access Points (NAPs)

• NAPs are public peering points of ISPs• When two ISPs are directly connected to each

other, they are said to peer with each other.• The NAP can be owned and operated by either

some third-party telecommunications company or by an Internet backbone provider.

• NAPs exchange huge quantities of traffic among many ISPs

• Often a NAPs uses high speed ATM switching technology, with IP running on the top of ATM

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Backbone Providers / Tier-1 ISPs• Tier-1 ISPs

– Also known as Internet Backbone– Exists at the center of the Internet Architecture – Directly connected to each of the other tier-1

ISPs– Connected to a large number of tier-2 ISPs and

other customer networks– International in coverage– Two tier-1 ISPs can also peer with each other by

connecting together a pair of POPs, one from each of the two ISPs.

– The trend is for the tier-1 ISPs to interconnect with each other directly at private peering points.

– Examples (e.g., UUNet, BBN/Genuity, Sprint, AT&T)

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Internet structure: Tier-1 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)

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National Providers / Tier-2 ISPs

• Tier-2 ISPs– Provides smaller coverage as compared to tier-1– National Coverage– Connect to one or more tier-1 ISPs– Connect to other tier-2 ISPs as well.– Tier-2 ISPs typically have regional or national

coverage and connects only to a few of tier-1 ISPs

– A tier-2 ISP is said to be a customer of the tier-1 ISP to which it is connected, and the tier-1 ISP is said to be a provider to its customer.

– The trend for tier-2 ISPs is to interconnect with other tier-2 ISPs and with tier-1 ISPs at NAPs

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Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

Internet structure: Tier-2 ISPs

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Local Providers / Tier-3 ISPs

• Tier-3 ISPs– last hop (“access”) network (closest to end

systems)– Local Coverage– Below tier-2 ISPs are the lower-tier ISPs,

which connect to the larger Internet via one or more tier-2 ISPs

– Users and content providers are the customers of lower-tier ISPs and lower-tier ISPs are the customers of higher-tier ISPs

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Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Internet structure: Tier-3 ISPs

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Internet structure: network of networks

• a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISP

localISP

localISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

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Delay Packet Switched Networks

• Considering what can happen to a packet as it travels from its source to its destination.– As a packet travels from one node to other

node (host or end system), it suffers from several types of delays at each node along the path

• Most important types of delays are:– Processing Delay– Queuing Delay– Transmission Delay– Propagation Delay

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Types of Delay• Processing Delay

– The time required to process (examine the packet’s header and determine where to direct the packet) is part of the processing delay

– Processing delay in high-speed routers is typically on the order of microseconds or less.

– After this nodal processing, the router directs the packet to the queue that precedes the link to the next router.

– Processing Delay depends on the processing speed of a router.

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Types of Delay• Queuing Delay

– At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link.

– The queuing delay of a packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link

– If queue is empty, and no other packet is being transmitted, the queuing delay will be zero

– If traffic is heavy and many other packets are waiting to be transmitted, the queuing delay will be long

– Thus, queuing delay depends on the intensity and nature of traffic arriving at the queue.

– Queuing delays can be in the order of microseconds to milliseconds in practice

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Types of Delay• Transmission Delay

– It is the amount of time required to push an entire packet into the link

– The time taken by a transmitter to send out all the bits of a packet onto the medium

– Also called Store and Forward Delay– Node receives complete packet before

forwarding– Transmission delays are typically in the order

of microseconds to milliseconds in practice

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Types of Delay• Propagation Delay

– Time it takes a bit to propagate from one node to the next.

– The time required by a bit to propagate from the beginning of the link to the next router is called propagation delay

– The bit propagates at the propagation speed of the link which depends on the physical medium being used.

– In wide area networks, propagation delays are on the order of milliseconds

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Packet Loss• In reality a queue has a finite capacity• As the traffic intensity approaches 1, a packet can

arrive to find a full queue.• With no place to store such a packet, a router will

drop that packet; that is the packet will be lost• The fraction of lost packets increases as the

traffic intensity increases• Thus, a node performance also includes the

probability of packet loss• A lost packet may be retransmitted on an end-to-

end basis, either the application or transport layer protocol.

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End-to-End Delay• The total delay from source to destination

is referred to as end-to-end delay– Example:

• Suppose that the queuing delay is negligible as the network is uncongested, then the end-to-end delay between the source and destination having N-1 routers in between will be:

dend-end = N (dproc + dtrans + dprop )

R R R

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