Networks 1 Intro (1)

8/6/2019 Networks 1 Intro (1)

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An Introduction to ComputerNetworks

Some slides are from lectures by Nick Mckeown, Ion Stoica, FransKaashoek, Hari Balakrishnan, and Sam Madden

Prof. Dina Katabi

Chapter 7


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Chapter Outline

Introduction (slides and 7.A)Layered Architecture (slides and 7.B &7.D)Routing (slides and 7.D)Reliable Transmission & Flow Control(slides and read 7.E)

Congestion Control (slides and read 7.F)


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This Lecture

What is a network?Sharing the infrastructure

Circuit switching

Packet switchingBest Effort Service

Analogy: the mail systemInternets Best Effort Service


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NetworksWhy they are interesting?Overcome geographic limits

Access remote dataSeparate clients and server

Goal: Universal Communication (any to any)Design the cloud

Network


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Connectivity

LinkDSL, T1, T 3 , ...Characterized by

Capacity or bit-rate (1. 5 Mb/s, 100Mb/s, )

Propagation delay (10us, 10ms, 100ms, ..)Transfer time on a link = #bit/bit-rate + propagation delay


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Connectivity

A mesh requires N 2 links too costly


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We Have to Share the Infrastructure

Intermediate nodes called switches or routersallow the hosts to share the infrastructure


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This Lecture

What is a network?Sharing the infrastructure

Circuit switching




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Two ways to share

Circuit switching (isochronous)Packet switching (asynchronous)


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Circuit Switching

DATA

Caller Callee

BostonSwitch

LASwitch

propagationdelaybetweencaller andand Bostonswitch

processing delay at switch

Its the methodused by thetelephone network

A call has threephases:1. Establish circuit from

end-to-end (dialing),2 . Communicate,3 .

Close circuit (teardown).If circuit notavailable: busysignal

(1)

(2)

(3)


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Switch

Circuit Switching:Multiplexing/Demultiplexing

Time divided into frames and frames divided into slotsRelative slot position inside a frame determines which

conversation the data belongs toE.g., slot 0 belongs to the red conversationNeed synchronization between sender and receiver

F rames

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

One way for sharing a circuit is TDM:

L ecture notes use the word frame for slot


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Circuit SwitchingAssume link capacity is C bits/secEach communication requires R bits/sec#slots = C/RMaximum number of concurrent communications is C/RWhat happens if we have more than C/R communications?What happens if the a communication sends less/morethan R bits/sec?

Design is unsuitable for computer networks wheretransfers have variable rate (bursty)


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Internet Traffic Is Bursty

Daily traffic at an MIT-CSAIL router

M ax In:12.2 Mb/ s Avg. In: 2.5 Mb/ s

M ax Out: 12.8 Mb/ s Avg. Out: 3.4 Mb/ s


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Packet SwitchingUsed in the InternetData is sent in Packets(header contains controlinfo, e.g., source anddestination addresses)

Per-packet routingAt each node the entirepacket is received,stored, and thenforwarded ( store-and-forward networks )No capacity is allocated

H eader Data

Packet 1

Packet 2

Packet 3

Packet 1Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

processingdelay of Packet 1at Node 2

propagationdelay

et eenH ost 1 &Node 2transmissiontime of

Packet 1

at H ost 1

H ost 1 H ost 2

Node 1 Node 2


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R outer

Packet Switching:Multiplexing/Demultiplexing

Multiplex using a queueRouters need memory/buffer

Demultiplex using information in packet headerHeader has destinationRouter has a routing table that contains information aboutwhich link to use to reach a destination

Queue


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Q ueues introduce

Variable DelayDelay = Q ueuing delay + propagation delay + transmissiondelay + processing delay

LossesWhen packets arrive to a full queue/buffer they aredropped


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Packet switching also show reordering

H ost A

H ost BH ost E

H ost D

H ost C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Packets in a flow may not follow the same path (dependson routing as we will see later) packets may bereordered


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This Lecture

What is a network?Sharing the Infrastructure

Circuit switching




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The mail system

Dina Nick

M IT Stanford

Admin Admin


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Characteristics of the mail systemEach envelope is individually routedNo time guarantee for deliveryNo guarantee of delivery in sequenceNo guarantee of delivery at all!

Things get lostHow can we acknowledge delivery?Retransmission

How to determine when to retransmit? Timeout?

If message is re-sent too soon duplicates


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The mail system

Dina Nick

M IT Stanford

Admin Admin


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The Internet

Dina Nick

N ms.csail.mit.edu L eland.Stanford.edu

O.S. O.S.HeaderData HeaderDataP acket


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Characteristics of the InternetEach packet is individually routedNo time guarantee for deliveryNo guarantee of delivery in sequenceNo guarantee of delivery at all!

Things get lostAcknowledgementsRetransmission

How to determine when to retransmit? Timeout?

If packet is re-transmitted too soonduplicate


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Best Effort

No Guarantees:Variable Delay (jitter)Variable ratePacket lossDuplicatesReordering

(notes also state maximum packet length)


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Differences Between Circuit & Packet SwitchingCircuit-switching Packet-Switching

Guaranteed capacity No guarantees (best effort)

Capacity is wasted if data isbursty

More efficient

Before sending dataestablishes a path

Send data immediately

All data in a single flowfollow one path Different packets mightfollow different pathsNo reordering; constantdelay; no pkt drops

Packets may be reordered,delayed, or dropped


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This Lecture

We learned how to share the networkinfrastructure between manyconnections/flows

We also learned about the implicationsof the sharing scheme (circuit or packetswitching) on the service that the

traffic receives

Networks 1 Intro (1)

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