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Introduction 1-1

Chapter 1Introduction

Computer Networking: A Top Down Approach ,3rd/5th edition. Jim Kurose, Keith RossPearson/Addison-Wesley, April 2009.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2009J.F Kurose and K.W. Ross, All Rights Reserved

Introduction 1-2

Goals

Examine the Types of Network Models

Understand nature of an Internet Protocol

Postal System

3

• Write address• Put stamp• Close envelop

Drop letter

Take letter to local post office

• Verify postage• “Route” letter

Take letter to destination’s postal office

Check destination address

Take letter to destination• Check address

• Open envelope

Other Information Networks

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Fire signalsSending messages by birds

Other Networks

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6

Communication networks can be classified based on the way in which the nodes exchange information:

Taxonomy of Communication Networks

Skeleton View of Internet

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Communication Network

BroadcastCommunication

Network

8

• Information transmitted by any node is received by every other node in the network– Usually only in LANs (Local Area Networks)

E.g., WiFi, Ethernet (classical, but not current) E.g., lecture!

• What problems does this raise?

• Problem #1: limited range (the killer)

• Problem #2: privacy of communication

• Problem #3: coordinating access to the shared communication medium (Multiple Access Problem)

Broadcast Communication Networks

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SwitchedCommunication

Network


Network

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Information transmitted along a path of intermediary nodes“switches” or “routers”

Each switch acts as a small switchboardInformation comes in on one linkSwitch directs it out some other link

Basic issue: how do the switches figure out the next hop along the path?

Switched Communication Networks

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Network


Network

Circuit-SwitchedCommunication

Network

Introduction 1-12

Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required

Introduction 1-13

Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls

resource piece idle if not used by owning call (no sharing)

dividing link bandwidth into “pieces” frequency division time division

14

Circuit Switching (e.g., Phone Network)• Establish: source creates circuit to

destination– Nodes along the path store connection info– Nodes generally reserve resources for the

connection– If circuit not available: “Busy signal”

• Transfer: source sends data over the circuit– No destination address, since nodes know path

• Teardown: source tears down circuit when done

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Circuit Switching With Human Operator

Telephone Network

Almon Brown Strowger (1839 - 1902)1889: Invents the mechanical switching system for telephone network

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Timing in Circuit Switching

Host 1 Host 2Switch 1 Switch 2

time

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Circuit Establishment


propagation delay between Host 1 and Switch1

time

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Transmission delay

time

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Transmission delay

time

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propagation delay between Host 1 and Host 2

Transmission delay

time

22


Information


Transfer




Transmission delay

time

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Information


Transfer

Circuit Teardown




Transmission delay

time

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Circuit Switching• Node (switch) in a circuit switching network

incoming links outgoing linksNode

How do the black and orange circuits share the outgoing link?

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Circuit Switching: Multiplexing a Link

• Time-division– Each circuit

allocated certain time slots

• Frequency-division– Each circuit

allocated certain frequencies

timefr

eque

ncy

time

26

Time-Division Multiplexing Illustration

• Time divided into frames; frames into slots

• Relative slot position inside a frame determines to which conversation data belongs

– E.g., slot 0 belongs to orange conversation

• Requires synchronization between sender and receiver—surprisingly non-trivial!

• In case of non-permanent conversations– Need to dynamically bind a slot to a conversation

• If a conversation does not use its circuit the capacity is lost!

Frames

0 1 2 3 4 5 0 1 2 3 4 5Slots =

27





Network


Network


Network

Packet-SwitchedCommunication

Network

Introduction 1-28

Packet Switching

each end-end data stream divided into packets

user A, B packets share network resources

each packet uses full link bandwidth

resources used as needed

resource contention: aggregate resource

demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time Node receives complete

packet before forwarding

Bandwidth division into “pieces”

Dedicated allocationResource reservation

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Packet Switching• Data sent as chunks of formatted bit-sequences

(Packets)

• Packets have following structure:

Header and Trailer carry control information (e.g., destination address, checksum)

• Each packet traverses the network from node to node along some path (Routing) based on header info.

• Usually, once a node receives the entire packet, it stores it (hopefully briefly) and then forwards it to the next node (Store-and-Forward Networks)

Header Data Trailer (sometimes)

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Packet Switching• Node in a packet switching network

incoming links outgoing linksNode

Memory

Introduction 1-31

Packet-switching: store-and-forward

takes L/R seconds to transmit (push out) packet of L bits on to link at R bps

store and forward: entire packet must arrive at router before it can be transmitted on next link

delay = 3L/R (assuming zero propagation delay)

Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay =

15 sec

R R RL

more on delay shortly …

Introduction 1-32

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.

TDM: each host gets same slot in revolving TDM frame.

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

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Packet Switching: Multiplexing/Demultiplexing

• Data from any conversation can be transmitted at any given time

– Single conversation can use the entire link capacity if it is alone

• How to tell them apart?– Use meta-data (header) to describe data

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Network


Network


Network


Network

Datagram Network

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Datagram Packet SwitchingEach packet is independently switchedEach packet header contains full destination address

No resources are pre-allocated (reserved) in advance

Leverages “statistical multiplexing” (or stat-muxing)Gambling that packets from different conversations won’t all arrive at the same time, so we don’t need enough capacity for all of them at their peak transmission rateAssuming independence of traffic sources, can compute probability that there is enough capacity

Example: IP networks; postal system

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Timing of Datagram Packet Switching

Packet 1

Host 1 Host 2Node 1 Node 2

propagationdelay betweenHost 1 and Node 1

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Packet 1



transmission time of Packet 1at Host 1

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Packet 1

Packet 1


Packet 1 processing

delay of Packet 1 at Node 2




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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3


Packet 1

Packet 2

Packet 3

processing

delay of Packet 1 at Node 2




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Datagram Packet Switching

Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

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Network


Network


Network


Network

Datagram Network

Virtual Circuit Network

A hybrid of circuits and packets; headers include a

“circuit identifier” established during a setup phase

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Advantages of Circuit Switching• Guaranteed bandwidth

– Predictable communication performance– Not “best-effort” delivery with no real

guarantees

• Simple abstraction– Reliable communication channel between hosts– No worries about lost or out-of-order packets

• Simple forwarding – Forwarding based on time slot or frequency– No need to inspect a packet header

• Low per-packet overhead– Forwarding based on time slot or frequency– No IP (and TCP/UDP) header on each packet

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Disadvantages of Circuit Switching• Wasted bandwidth

– Bursty traffic leads to idle connection during silent period– Unable to achieve gains from “statistical multiplexing”

• Blocked connections– Connection refused when resources are not sufficient– Unable to offer “okay” service to everybody

• Connection set-up delay – No communication until the connection is set up– Unable to avoid extra latency for small data transfers

• Network state– Network nodes must store per-connection information– Unable to avoid per-connection storage and state– This makes failures more disruptive!

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Packet-Switching vs. Circuit-Switching

• Performance advantage of packet-switching over circuit switching: Exploitation of statistical multiplexing

• Reliability advantage: since routers don’t know about individual conversations, when a router or link fails, it’s:Easy to fail over to a different path

• Deployability advantage: easier for different parties to link their networks together because they’re not promising to reserve resources for one another

• Disadvantage: packet-switching must handle congestion

– More complex routers (more buffering, sophisticated dropping)

– Harder to provide good network services (e.g., delay and bandwidth guarantees)

Introduction 1-47

Packet switching versus circuit switching

1 Mb/s link each user:

100 kb/s when “active”

active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active at same time is less than .0004 (leave as exercise)

Packet switching allows more users to use network!

N users

1 Mbps link

Introduction 1-48

Packet switching versus circuit switching

great for bursty data resource sharing simpler, no call setup

excessive congestion: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed for audio/video apps

still an unsolved problem (chapter 7)

Is packet switching an outright winner?

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

CombinationsCan a packet switched network run over a circuit-switched one?

Can a circuit-switched network run over a packet-switched one?

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Introduction 1-50

What’s a protocol?human protocols: “what’s the time?” “I have a question” introductions

… 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

Introduction 1-51

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

protocol:

Q: Other human protocols?

Hi

Hi

Got thetime?

2:00

TCP connection request

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

<file>time

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What Is A Protocol?

• A protocol is an agreement on how to communicate

• Includes syntax and semantics– How a communication is specified & structured

Format, order messages are sent and received

– What a communication means Actions taken when transmitting, receiving, or when a

timer expires

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Examples of Protocols in Human Interactions

• Telephone1. (Pick up / open up the phone.)2. Listen for a dial tone / see that you have

service.3. Dial.4. Should hear ringing …5. Callee: “Hello?”6. Caller: “Hi, it’s Alice ….”

Or: “Hi, it’s me” ( what’s that about?)7. Caller: “Hey, do you think … blah blah blah …”

pause8. Callee: “Yeah, blah blah blah …” pause

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Examples of Protocols in Human Interactions

• Asking a question1. Raise your hand.2. Wait to be called on.

1. Or: wait for speaker to pause and vocalize

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Example: HyperText Transfer Protocol

GET /courses/archive/spring06/cos461/ HTTP/1.1Host: www.cs.princeton.eduUser-Agent: Mozilla/4.03CRLF

HTTP/1.1 200 OKDate: Mon, 6 Feb 2006 13:09:03 GMTServer: Netscape-Enterprise/3.5.1Last-Modified: Mon, 6 Feb 2006 11:12:23 GMTContent-Length: 21CRLFSite under construction

Request

Response

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Example: The Internet Protocol (IP)Problem:Many different network technologiese.g., Ethernet, Token Ring, ATM, Frame Relay, etc.How can you hook them together?

• n x n translations?

IP was invented to glue them togethern translationsMinimal requirements (datagram)

The Internet is founded on IP“IP over everything”

What does an internet protocol do?What would the protocol need to communicate?

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Example: IP Packet

4-bitVersion

4-bitHeaderLength

8-bit Type ofService (TOS)

16-bit Total Length (Bytes)

16-bit Identification3-bitFlags 13-bit Fragment Offset

8-bit Time to Live (TTL)

8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

20-byte20-byteheaderheader

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Example: IP: “Best-Effort” Packet Delivery ProtocolDatagram packet switchingSend data in packetsHeader with source & destination address

Service it provides:Packets may be lostPackets may be corruptedPackets may be delivered out of order

source destination

IP network

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Example: Transmission Control Protocol

• Communication service– Ordered, reliable byte stream– Simultaneous transmission in both directions

• Key mechanisms at end hosts– Retransmit lost and corrupted packets– Discard duplicate packets and put packets in order– Flow control to avoid overloading the receiver buffer– Congestion control to adapt sending rate to network load

source network destination

TCP connection

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Protocol Standardization• Ensure communicating hosts speak the same

protocol– Standardization to enable multiple implementations– Or, the same folks have to write all the software

• Standardization: Internet Engineering Task Force– Based on working groups that focus on specific issues– Produces “Request For Comments” (RFCs)

Promoted to standards via rough consensus and running code

– IETF Web site is http://www.ietf.org– RFCs archived at http://www.rfc-editor.org

• De facto standards: same folks writing the code– P2P file sharing, Skype, <your protocol here>…

Summary

Examined the types of Communication Networks

Got introduced to the concept of protocol

Acknowledgment & CopyrightThe instructor duly acknowledges the authors

of the text book “Computer Networking: A Top-down approach”, James Kurose & Keith Ross and the instructors of EE122 “Computer Networks” course at UC Berkeley, Ian Stoica, Scott Shenker, Jennifer Rexford, Vern Paxson and other instructors at Princeton University for the course material.

Please note: certain modifications may have been done to adapt the slides to the current audience -Bruhadeshwar @cs335 Spring 2011(C)

week2.1

Technology

circuit switchinghost

link switch

node switch

switch acts

circuit switchingendend

nodes exchange information

destination nodes

lans local area networks