7/29/2019 Week 2 Lec 2-Bit

1/30

Chapter 1:IntroductionComputer Networking:A Top Down Approach ,

4th edition.Jim Kurose, Keith Ross

Computer Networks

7/29/2019 Week 2 Lec 2-Bit

2/30

Chapter 1: Introduction

Our goal: get feel and

terminology more depth, detail

laterin course approach:

use Internet asexample

Overview: whats the Internet?

whats a protocol?

network edge; hosts, accessnet, physical media

network core: packet/circuitswitching, Internet structure

performance: loss, delay,throughput

Protocol layers, service models

History of Internet

7/29/2019 Week 2 Lec 2-Bit

3/30

Throughput

The rate (bits/sec) at which bits are transferredbetween sender/receiver

Difference between Bandwidth and Throughput?

ISPs sell bandwidth In computer networks, the throughput is less

than the bandwidth for several reasons The channel may be shared by other users Packet loss due to congestion

Packet loss due to bit errors Noise in the channel Transmission rates of the link over which the

data flows.

7/29/2019 Week 2 Lec 2-Bit

4/30

Throughput

Rs< Rc What is average end-end throughput?

Rsbits/sec Rcbits/sec

Rs> Rc What is average end-end throughput?

Rsbits/sec Rcbits/sec

Throughput is min {Rs,Rc}Transmission Rate of thebottleneck link

7/29/2019 Week 2 Lec 2-Bit

5/30

How do loss and delay occur?

packets queuein router buffers

packets queue, wait for turn queue (aka buffer) has finite capacity

packet arriving to full queue dropped (aka lost)

lost packet may be retransmitted

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets

dropped (loss) if no free buffers

7/29/2019 Week 2 Lec 2-Bit

6/30

Four sources of packet delay

1. Processing Delay: Time required to

examine packet headerand determine outputlink

check bit errors High Speed Routers

Microseconds or less

A

B

propagation

transmission

processing queueing

2. Queuing Delay:

Time waiting at outputlink for transmission

depends on congestionlevel of router

If queue empty no delay Microseconds to

milliseconds

7/29/2019 Week 2 Lec 2-Bit

7/30

Delay in packet-switched networks

3. Transmission Delay:

R=link bandwidth (bps)

L=packet length (bits)

time to push all of

packets bits into thelink = L/R

Microseconds tomilliseconds

4. Propagation Delay:

Time to propagate fromthe beginning of the linkto the other router(node)

propagation delay = d/s d = length of physical link

s = propagation speed inmedium (~3x108 m/sec)

A

B

propagation

transmission

nodal

processing queueing

7/29/2019 Week 2 Lec 2-Bit

8/30

Queuing Delay

R=Transmission Rate (bps) L=packet length(bits)

a=average packet arrivalrate(packets/sec)

Traffic Intensity = (Average rate at which bits arrive at the queue) =LaTransmission Rate R

La/R ~ 0: average queuing delay small

La/R -> 1: delays become large (queue begins to get larger)

La/R > 1: average rate at which bits arrive at the queue exceeds therate at which the bits can be transmitted from the queue.

more work arriving than can be serviced

Packet Loss will occur

When is Queuing Delay large andwhen it is insignificant?

Rate at which traffic arrives at thequeue

Transmission rate of the link

Nature of the arriving traffic

7/29/2019 Week 2 Lec 2-Bit

9/30

Queuing Delay

Consider the case La/R1Nature of arriving traffic impacts queuing delay in such acase

(Find about Queuing Theory?)

In reality the arrival process to a queue is random andarrivals do not follow any pattern.

7/29/2019 Week 2 Lec 2-Bit

10/30

Protocol Layers

Networks are complex! many pieces:

hosts

routers

links of variousmedia

applications

protocols

hardware,software

Question:Is there any way of

organizingnetworkarchitecture?

Answer:

Yes possible with alayered architecture

7/29/2019 Week 2 Lec 2-Bit

11/30

Why layering?

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

large and complex system

Modularization eases maintenance,updating of system

Change of implementation of layersservice transparent to rest of system

e.g. change in gate procedure doesntaffect rest of system

7/29/2019 Week 2 Lec 2-Bit

12/30

InternetProtocolStack To provide structure to design of

network protocols, networkdesigners organize protocols inlayers

Service says what a layer doesProtocol says how the service is

implementedAdvantagesDrawbacksWhen taken together the

protocols of various layers are

called the Protocol Stack.Internet Protocol Stack consistsof Five layers

Physical, Link, Network,Transport and Application layers .

Organization of Book

7/29/2019 Week 2 Lec 2-Bit

13/30

Internet Protocol Stack

Application Layer:

Network applications and their application layer protocolsreside.

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 endsystems

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

7/29/2019 Week 2 Lec 2-Bit

14/30

Internet Protocol Stack Transport Layer:

Transports application-layer messages between applicationend 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 service

Guaranteed delivery of application layer messages

Flow control

Congestion Control

User Datagram Protocol (UDP)Connectionless service

No reliability, flow control and congestion control

7/29/2019 Week 2 Lec 2-Bit

15/30

Internet Protocol Stack Network Layer:

Responsible for moving network layer packetsknown as datagrams from one host to another.

Transport layer passes a transport layer

segment and a destination address to thenetwork layer.

Network layer includes IP Protocol

Defines the fields in the datagram as well as

how end systems and routers act on thesefields

Different routing protocols.

Determine the route that datagrams take

between sources and destinations

7/29/2019 Week 2 Lec 2-Bit

16/30

Internet Protocol Stack Link Layer:

Moves a packet from one node (host or router)to the next node in the route.

Divide the stream of bits received from the

network layer into manageable data units calledframes.

Error Detection and Correction

Transforms a raw transmission facility to a

reliable link.Mechanism to detect and retransmit damaged

or lost frames

Example of link layer protocols include WiFi,

Ethernet etc.

7/29/2019 Week 2 Lec 2-Bit

17/30

Internet Protocol Stack Physical Layer:

The job of this layer is to move the individual bits with in framesfrom one node to next.

Representation of bits

Physical Layer data consists of a stream of bits (0 or 1)

To be transmitted bits must be encoded into signals. Thephysical layer defines the type of encoding.

The protocol in this layer depend on the actual transmissionmedium of the link.

7/29/2019 Week 2 Lec 2-Bit

18/30

Internet Protocol Stack Application:Provides user interfaces and

support for services such as e-mail, filetransfer etc. FTP, HTTP

Transport:Transports application-layermessages between application end points.

Segmentation and reassembly TCP, UDP

Network:Routing of Datagrams fromsource to destination IP, routing protocols

Link:Move a packet from one node (host orrouter) to the next node in the route.

Ethernet, WiFi

Physical:Move the individual bits with in

frames from one node to next

Application

Transport

Network

Link

Physical

7/29/2019 Week 2 Lec 2-Bit

19/30

OSI Reference Model

In 1970 International Organization forStandardization proposed a seven layeredmodel called Open SystemsInterconnection (OSI) model.

Presentation Layer: Provide services such

as data encryption, compression. Session Layer: Synchronization points

(checkpointing) and recovery of dataexchange.

Internet stack missing these layers!

these services, if needed, must beimplemented in the application by theapplication developer.

7/29/2019 Week 2 Lec 2-Bit

20/30

source

applicationtransport

networklinkphysical

segment

datagram

destination

applicationtransportnetwork

linkphysical

router

switch

Encapsulationmessage

Ht

Hn

M

Ht

HtHnHl M

HtHn M

Ht MM

networklink

physical

linkphysical

Ht

Hn

Hl

M

HtHn M HtHn M

HtHnHl M

M

Ht M

Hn

frame

7/29/2019 Week 2 Lec 2-Bit

21/30

Chapter 2Application LayerComputer Networking: A

Top Down Approach,

4th edition.

Jim Kurose, Keith RossAddison-Wesley, July

2007.

7/29/2019 Week 2 Lec 2-Bit

22/30

Chapter 2: Application layer

2.1 Principles ofnetwork applications

2.2 Web and HTTP

2.3 FTP 2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P applications

2.7 Socket programming

7/29/2019 Week 2 Lec 2-Bit

23/30

Chapter 2: Application Layer

Our goals: conceptual,

implementationaspects of network

application protocols client-server

paradigm

peer-to-peer

paradigm

learn about protocolsby examining popularapplication-levelprotocols

HTTP FTP

SMTP / POP3 / IMAP

DNS

programming networkapplications

socketprogramming

7/29/2019 Week 2 Lec 2-Bit

24/30

Some Network Applications

E-mail

Web

Instant messaging

Remote login P2P file sharing

Multi-user network games

Streaming stored video clips

Voice over IP Real-time video conferencing

Many more interesting applications

7/29/2019 Week 2 Lec 2-Bit

25/30

Creating a Network Application

write programs that run on (different) end systems

communicate over network

e.g., web server softwarecommunicates with browser

softwareNo need to write software fornetwork-core devices

Network-core devices do not runuser applications

Function at lower layers

Basic design is to confineapplication software to the endsystems

Facilitate rapid networkapplication development and

deployment

application

transport

network

data link

physical

application

transport

network

data linkphysical

application

transport

network

data link

physical

7/29/2019 Week 2 Lec 2-Bit

26/30

Application Architectures

Application Architecture isDesigned by the application developer

Dictates how the application isstructured over various end systems

Two architectures used in modern dayNetwork Applications:Client-Server Architecture

Peer-to-Peer (P2P) Architecture

7/29/2019 Week 2 Lec 2-Bit

27/30

Client-Server ArchitectureServer:

Always-on host

permanent well defined IPaddress

A single server is incapable ofkeeping up with all the requestsof the clients.

A cluster of hosts referred asserver farm is often used.

Clients:

Communicate with server

May have dynamic IP addresses

Do not communicate directly witheach other

Client/Server architecture isinfrastructure intensive

Require service providers toinstall and maintain servers.

Client/Server

7/29/2019 Week 2 Lec 2-Bit

28/30

P2P Architecture

Direct communication

between pairs ofintermittently connectedhosts called peers

Peers are not owned by anyservice provider

P2P Peers communicate without

passing through anydedicated server

e.g. Bit Torrent, eMule, Skype

Security issues Highly Distributive

Cost Effective

Detailed study later on in the

chapter

peer-peer

Wh t T t S i d

7/29/2019 Week 2 Lec 2-Bit

29/30

What Transport Service does anApplication need?

Data Loss

Loss Tolerant Applications Some apps (e.g., audio, VoIP)

can tolerate some loss 2% tolerable for VoIP

Other apps (e.g., file transfer,

email) require 100% reliabledata transfer

Timing Application may require

timing guarantee Tight timing constraints

multiplayer games, VoIP,teleconferencing.

In Non-real time lower delaysare preferred but no tightconstraint on end-to-enddelays.

Throughput

Bandwidth sensitiveapplications (e.g., multimedia)require minimum amount ofthroughput

Other apps (elastic apps)

make use of whateverthroughput they get e.g .Email, file transferSecurity Encryption and decryption

7/29/2019 Week 2 Lec 2-Bit

30/30

Transport Service Requirements of CommonApplications

Application

file transfer

e-mail

Web documentsreal-time audio/video

stored audio/video

interactive games

Data loss

no loss

no loss

no lossloss-tolerant

loss-tolerant

loss-tolerant

Throughput

elastic

elastic

elasticaudio: 5kbps-1Mbps

video:10kbps-5Mbps

same as above

few kbps -10kbps

Time Sensitive

no

no

no

yes, 100s msec

yes, few secs

yes, 100s msec