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Chapter 1:Introduction Computer Networking: A Top Down Approach , 4 th edition. Jim Kurose, Keith Ross Computer Networks

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Page 1: Week 2 Lec 1-Bit

Chapter 1:Introduction Computer

Networking: A Top Down Approach ,

4th edition. Jim Kurose, Keith Ross

Computer Networks

Page 2: Week 2 Lec 1-Bit

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)

Page 3: Week 2 Lec 1-Bit

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

Page 4: Week 2 Lec 1-Bit

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

Page 5: Week 2 Lec 1-Bit

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)

Page 6: Week 2 Lec 1-Bit

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

Page 7: Week 2 Lec 1-Bit

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

Page 8: Week 2 Lec 1-Bit

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?

Page 9: Week 2 Lec 1-Bit

Throughput The rate (bits/sec) at which bits are transferred

between 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.What is Goodput?

Page 10: Week 2 Lec 1-Bit

Throughput

Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

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

Page 11: Week 2 Lec 1-Bit

Throughput

10 connections share bottleneck link R

Rs

Rs

Rs

Rc

Rc

Rc

R

10 clients/servers pairs, Common link R traversed

by all 10. Rate of the link R is very

large then the throughput is min {Rs , Rc}

Rs=2Mbps, Rc=1Mbps,

R=5Mbps Common link divides

transmission rate equally among the 10 downloads

500kbps to each download

Shared Link R is now the bottleneck.

Page 12: Week 2 Lec 1-Bit

How do loss and delay occur?

packets queue in 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

Page 13: Week 2 Lec 1-Bit

Four sources of packet delay 1. Processing Delay:

Time required to examine packet header and determine output link

check bit errors High Speed Routers

Microseconds or less

A

B

propagation

transmission

processing queueing

2. Queuing Delay: Time waiting at output

link for transmission depends on

congestion level of router

If queue empty no delay

Microseconds to milliseconds

Page 14: Week 2 Lec 1-Bit

Delay in packet-switched networks3. Transmission Delay: R=link bandwidth

(bps) L=packet length

(bits) time to push all of

packets bits into the link = L/R

Microseconds to milliseconds

4. Propagation Delay: Time to propagate from

the beginning of the link to the other router (node)

propagation delay = d/s

d = length of physical link

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

A

B

propagation

transmission

nodalprocessing queueing

Page 15: Week 2 Lec 1-Bit

Nodal delay

dproc = processing delay typically a few microsecs or less

dqueue = queuing delay depends on congestion

dtrans = transmission delay = L/R

dprop = propagation delay microsecs or msecs

End to End Delay?

proptransqueueprocnodal ddddd

Page 16: Week 2 Lec 1-Bit

Queuing Delay

R=Transmission Rate (bps) L=packet length(bits) a=average packet arrival

rate(packets/sec)Traffic Intensity = (Average rate at which bits arrive at the queue) =La

Transmission 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 the rate 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 and when it is insignificant?

Rate at which traffic arrives at the queue

Transmission rate of the link Nature of the arriving traffic

Page 17: Week 2 Lec 1-Bit

Queuing DelayConsider the case La/R≤1Nature of arriving traffic impacts queuing delay in

such a case

(Find about Queuing Theory?)

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

Page 18: Week 2 Lec 1-Bit

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

Page 19: Week 2 Lec 1-Bit

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

Page 20: Week 2 Lec 1-Bit

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

Page 21: Week 2 Lec 1-Bit

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

Page 22: Week 2 Lec 1-Bit

Internet Protocol Stack

To provide structure to design of network protocols, network designers organize protocols in layers

Service – says what a layer doesProtocol – says how the service is

implementedAdvantages DrawbacksWhen 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

Page 23: Week 2 Lec 1-Bit

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.

Page 24: Week 2 Lec 1-Bit

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

Page 25: Week 2 Lec 1-Bit

Internet Protocol Stack• Network Layer:

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

Transport layer passes a transport layer segment and a destination address to the network layer.

Network layer includes IP ProtocolDefines the fields in the datagram as well

as how end systems and routers act on these fields

Different routing protocols.Determine the route that datagrams take

between sources and destinations