Chapter 1:Introduction Computer

Networking: A Top Down Approach ,

4th edition. Jim Kurose, Keith

Ross

Computer Networks

Chapter 1: IntroductionOur goal: get “feel” and

terminology more depth, detail

later in course approach:

use Internet as example

Overview: what’s the Internet? what’s a protocol? network edge; hosts, access

net, physical media network core: packet/circuit

switching, Internet structure performance: loss, delay,

throughput Protocol layers, service

models History of Internet

What’s the Internet: “nuts and bolts” view

Interconnects millions of computing devices: hosts = end systems running network

apps Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wirelesslaptop

cellular handheld

wiredlinks

access points

communication links fiber, copper, radio,

satellite transmission rate =

bandwidth (bits/sec) routers: forward

packets (chunks of data)

What’s the Internet: “nuts and bolts” view protocols control sending,

receiving of msgs e.g., TCP, IP, HTTP, Skype

Internet: “network of networks” public Internet versus

private intranet

Internet standards IETF: Internet Engineering

Task Force RFC: Request For Comments

IETF standard DocumentsMore than 5000 RFCs

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

What’s the Internet: A Service View

Communication infrastructure that provides services to applications: Involve multiple end system

that exchange data with each other.

Web, VoIP, email, games, e-commerce, file sharing

Applications do not run on the routers

Communication services provided to applications: reliable data delivery from

source to destination (connection oriented)

“best effort” (unreliable) data delivery (connection less)

What’s a protocol?Human Protocols: “what’s the time?” “I have a question” Interview

… specific msgs sent… specific actions

taken when msgs received, or other events

Network Protocols: machines rather than

humans all communication

activity in Internet governed by protocols

protocols define format, order of msgs sent and

received among network entities, and actions taken on msg transmission, receipt

What’s a protocol?a human protocol and a computer network protocol:

Hi

Hi

Got thetime?

2:00

TCP connection request

TCP connectionresponseGet http://www.awl.com/kurose-ross

<file>time

The Network Edge: end systems (hosts):

run application programs e.g. Web, email at “edge of network”

client/server

peer-peer

client/server model client host requests,

receives service from always-on server

e.g. Web browser/server; email client/server Peer-Peer model:

minimal (or no) use of dedicated servers

e.g. Skype, BitTorrent, Kazaa

More in Chapter 2

Network AccessAccess Networks:

The physical link that connects an end system to its “edge router”.

Q: How to connect end systems to

edge router?Access Networks can be loosely

classified into three categories Residential access networks

Dial up, DSL etc Institutional access networks (school,

company) Ethernet

Mobile access networks

Wireless LAN (WiFi)

Wider-area wireless access (WiMAX)

Reading Assignment

The Network Core

Mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone networks

packet-switching: data sent through network in discrete “chunks” called packets

Hotel reservation analogy

Network Core: Circuit SwitchingEnd-end resources reserved for

theduration of the call Three phases

Establish, Transfer, Disconnect

dedicated resources: no sharing

Guaranteed performance Very Reliable Developed for Telephone

networks Inefficient

Channel capacity dedicated for duration of connection

If no data, capacity wasted Set up (connection) takes time

Network Core: Packet SwitchingToday’s internet is a packet switched network Each end-end data stream divided into packets

Each packet contains a portion of user data plus some control info.

Each packet has to find its own route to the destination

No predetermined path Decision as to which node to hop to in the next step is

taken only when a node is reached. Resources used as needed Congestion : packets queue, wait for link use Reliability Less reliable Store and forward switching Routers receives complete packet before forwarding Question: What is Cut through Switching?

Protocol Layers

Networks are complex!

• many “pieces”:– hosts– routers– links of various

media– applications– protocols– hardware,

software

Question: Is there any way of organizing network

architecture?

Answer:

Yes possible with a layered architecture

Organization of air travel

• A series of steps(actions)

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Layering of Airline Functionality

• Airline functionality can be divided into layers, providing a frame work in which we can discuss air travel.

• At the ticketing layer and below– Airline-counter-to-airline-counter transfer of a person.

• At the gate layer– Departure-gate –to-arrival-gate transfer of a person is

accomplishedLayers: each layer implements a service

– via its own internal-layer actions– Combined with the services directly below it

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Why layering?Dealing with complex systems:Discuss a well defined, specific part of a

large and complex systemModularization eases maintenance,

updating of systemChange of implementation of layer’s

service transparent to rest of systeme.g. change in gate procedure doesn’t

affect rest of system

Internet Protocol Stack To provide structure to design

of network protocols, network designers organize protocols in layers

Service – says what a layer does

Protocol – says how the service is implemented

Advantages Drawbacks When taken together the

protocols of various layers are called the Protocol Stack.

Internet Protocol Stack consists of Five layers

Physical, Link, Network, Transport and Application layers .

Organization of Book

Internet Protocol Stack

• Application Layer:

Network applications and their application layer protocols reside.

Provides user interfaces and support for services such as e-mail, file transfer etc.

Hyper Text Transfer Protocol (HTTP)File Transfer Protocol (FTP)Session Initiation Protocol (SIP)

An application layer protocol is distributed over multiple end systems

The packets of information at the application layer is called as a message.

Internet Protocol Stack• Transport Layer:

Transports application-layer messages between application end points.

Transport layer packet is called as a segment Breaks long messages into shorter segments There are two Transport Layer Protocols Transmission Control Protocol (TCP)

Connection Oriented serviceGuaranteed delivery of application layer messagesFlow controlCongestion Control

User Datagram Protocol (UDP)Connectionless serviceNo reliability, flow control and congestion control