If the Internet is the answer, then what was the question?

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If the Internet is the answer,

then what was the question?

EE122 Fall 2011 Scott Shenker

http://inst.eecs.berkeley.edu/~ee122/

Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley

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Administrivia

• No sections next week (Monday is a holiday) • Enrollment: admitted 20 additional seniors

– We are now at our legal limit; there is nothing I can do – Tuesday 10-11 section: moved to Cory 521

• Need feedback on the index cards

– Can do it anonymously on Piazza – Or send email to me

• Plug computers arrived!

– Now we just have to get them to work…..(Go Yahel!)

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Outline for today’s class

• The telephone network

– boring

• Why does the Internet use packet switching?

– Less boring, but hardly earthshattering

• Important life lessons

– This will change your life….

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Telephone network

• Alexander Graham Bell

– 1876: Demonstrates the telephone at US Centenary Exhibition in Philadelphia

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Telephone network uses circuit switching

• Establish: source creates circuit to destination

– Nodes along the path store connection info – And 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 – Continual stream of data

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The switch in “circuit switching”

incoming links Node outgoing links

How does the node connect the incoming link to the outgoing link?

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“Modern” switches

• Almon Brown Strowger (1839 - 1902)

– 1889: Invents the “girl-less, cuss-less” telephone system -- the mechanical switching system

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Timing in Circuit Switching

Host 1 Host 2

Switch 1 Switch 2

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Timing in Circuit Switching

Circuit Establishment Host 1 Host 2 Switch 1 Switch 2 propagation delay between Host 1 and Switch1 time

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Circuit Establishment Host 1 Host 2 Switch 1 Switch 2 propagation delay between Host 1 and Switch1 Transmission delay time

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Circuit Establishment Host 1 Host 2 Switch 1 Switch 2 propagation delay between Host 1 and Switch1 Transmission delay time

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Circuit Establishment Host 1 Host 2 Switch 1 Switch 2 propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission delay time

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Information Circuit Establishment Transfer Host 1 Host 2 Switch 1 Switch 2 propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission delay time

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Information Circuit Establishment Transfer Circuit Host 1 Host 2 Switch 1 Switch 2 propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission delay time

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Sharing a link

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 time frequen c y time

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Time-Division Multiplexing/Demultiplexing

• 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 • Need to dynamically bind a slot to a conversation

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

Frames

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

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Strengths of phone system

• Predictable performance

– Known delays – No drops

• Easy to reason about

• Supports a crucial service

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Weakness #1: Not resilient to failure

• Any failure along the path prevents transmission • Entire transmission has to be restarted

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Weakness #2: Wastes bandwidth

• Consider a network application with:

– Peak bandwidth P

– Average bandwidth A

• How much does the network have to reserve for the application to work?

– The peak bandwidth

• What is the resulting level of utilization?

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Smooth vs Bursty Applications

• Some applications have relatively small P/A ratios

– Voice might have a ratio of 3:1 or so

• Data applications tend to be rather bursty

– Ratios of 100 or greater are common

• Circuit switching too inefficient for bursty apps • Generally:

– Don’t care about factors of two in performance

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Weakness #3: Designed Tied to App

• Design revolves around the requirements of voice • Not general feature of circuit switching

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Weakness #4: Setup Time

• Every connection requires round-trip time to set up

– Slows down short transfers

• In actuality, may not be a big issue

– TCP requires round-trip time for handshake – No one seems to mind….

• This was a big issue in the ATM vs IP battle

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What if we wanted a resilient network?

• How would we design it?

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Paul Baran

• Baran investigated survivable networks for USAF

– Network should withstand almost any degree of

destruction to individual components without loss of end-to-end communications.

• “On Distributed Communications” (1964)

– Distributed control

– Message blocks (packets) – Store-and-forward delivery

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What about a less wasteful network?

• How would we design it?

• This is the question Len Kleinrock asked…..

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

Taxonomy of Communication Networks

Communication Communication Communication Communication Network Network Network Network

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

Taxonomy of Communication Networks

Communication Communication Communication Communication Network Network Network Network Broadcast Broadcast Broadcast Broadcast Communication Communication Communication Communication Network NetworkNetwork Network

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• 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

• Problem #2: coordinating access to the shared communication medium

– Multiple Access Problem

• Problem #3: privacy of communication

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

Taxonomy of Communication Networks

Communication Communication Communication Communication Network Network Network Network Switched Switched Switched Switched Communication Communication Communication Communication Network NetworkNetwork Network Broadcast Broadcast Broadcast Broadcast Communication Communication Communication Communication Network NetworkNetwork Network

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Communication Communication Communication Communication Network Network Network Network Switched Switched Switched Switched Communication Communication Communication Communication Network NetworkNetwork Network Broadcast Broadcast Broadcast Broadcast Communication Communication Communication Communication Network NetworkNetwork Network Circuit CircuitCircuit

Circuit----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication Network NetworkNetwork Network

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Taxonomy of Communication Networks

Circuit----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication Network NetworkNetwork Network Packet PacketPacket

Packet----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication Network NetworkNetwork Network

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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

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Packet Switching

• Node in a packet switching network

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Packet Switching: Multiplexing/Demultiplexing

• How to tell packets apart?

– Use meta-data (header) to describe data

• No reserved resources; dynamic sharing

– Single flow can use the entire link capacity if it is alone – This leads to increased efficiency

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Simple Example: M/M/1 Queue

• Consider n flows sharing a single queue • Flow: random (Poisson) arrivals at rate λ

• Random (Exponential) service with average 1/µ

• Utilization factor: ρ = nλ/µ

– If ρ >1, system is unstable

• Case 1: Flows share bandwidth

– Delay = 1/(µ - nλ)

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Taxonomy of Communication Networks

Packet----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication Network NetworkNetwork Network Datagram Datagram Datagram Datagram Network Network Network Network

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Datagram Packet Switching

• Each packet is independently switched

– Each packet header contains full destination address

• No resources are pre-allocated (reserved) in advance

• Leverages _“statistical multiplexing_”

– 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 rate

– Assuming independence of traffic sources, can compute

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Timing of Datagram Packet Switching

Packet 1 Host 1 Host 2 Node 1 Node 2 propagation delay between Host 1 and Node 1

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Timing of Datagram Packet Switching

Packet 1 Host 1 Host 2 Node 1 Node 2 propagation delay between Host 1 and Node 1 transmission time of Packet 1 at Host 1

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Packet 1 Packet 1

Packet 1 _processing delay of Packet 1 at Node 2 Host 1 Host 2 Node 1 Node 2 propagation delay between Host 1 and Node 1 transmission time of Packet 1 at Host 1

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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 Host 1 Host 2 Node 1 Node 2 propagation delay between Host 1 and Node 1 transmission time of Packet 1 at Host 1

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Datagram Packet Switching

Host A Host B Host E Host D Host C Node 1 _{Node 2} Node 3 Node 4 Node 5 Node 6 Node 7

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Packet----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication Network NetworkNetwork Network

A hybrid of circuits and packets; headers include a

“circuit identifier”

established during a setup phase

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5 Minute Break

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If you were building a network….

• Which would you choose?

– Circuit switched? – Packet-switched?

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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 headers 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

• Network state

– Network nodes must store per-connection information – Unable to avoid per-connection storage and state

– This makes failures more disruptive!

• Connection set-up delay

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Packet-Switching vs. Circuit-Switching

• Reliability advantage: since routers don_’t know about

individual conversations, when a router or link fails, it is

easy to fail over to a different path

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

• 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

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Choosing a Design for the Internet

• If you cared about:

– Resilience

– Bursty applications

– Ease of interconnection – …

• Which would you choose? • Not so fast…..

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The paradox of the Internet’s design

• As we will discuss next lecture, one of the main design goals is to support a wide range of apps • These applications have different requirements • Shouldn’t the Internet support them all?

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Diversity of application requirements

• Size of transfers

• Bidirectionality (or not)

• Latency sensitive (or not) • Tolerance of jitter (or not)

• Tolerance of packet drop (or not) • Need for reliability (or not)

• Multipoint (or not) • …..

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Diversity of application requirements

• Size of transfers

• Bidirectionality (or not)

• Latency sensitive (or not) • Tolerance of jitter (or not)

• Tolerance of packet drop (or not) • Need for reliability (or not)

• Multipoint (or not) • …..

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What service should Internet support?

• Strict delay bounds?

– Some applications require them

• Guaranteed delivery?

– Some applications are sensitive to packet drops

• No applications mind getting good service

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Important life lessons

• People (applications) don’t always need what they think they need

• People (applications) don’t always need what we think they need

• Flexibility often more important than performance

– But typically only in hindsight!

– Example: cell phones vs landlines

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Applying lessons to Internet

• Requiring performance guarantees would limit variety of networks that could attach to Internet • Many applications don’t need these guarantees • And those that do?

– Well, they don’t either (usually)

– Tremendous ability to mask drops, delays

• And ISPs can work hard to deliver good service without changing the architecture

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Timeline

1961 Baran and Kleinrock advocate packet switching

1962 Licklider’s vision of Galactic Network

1965 Roberts connects two computers via phone 1967 Roberts publishes vision of ARPANET

1969 BBN installs first IMP at UCLA

IMP: Interface Message Processor

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The beginning of the Internet revolution

• Kleinrock’s group at UCLA tried to log on to

Stanford computer: His recollection of the event… • We typed the L…

– “Do you see the L?” – “Yes, we see the L.”

• We typed the O…

– “Do you see the O?” – “Yes, we see the O.”

• Then we typed the G…

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Timeline continued…

1972 Email invented

1972 Telnet introduced

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The Problem

• Many different packet-switching networks • Only nodes on the same network could

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Kahn’s Rules for Interconnection

• Each network is independent and must not be required to change (why?)

• Best-effort communication (why?) • Boxes (routers) connect networks

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Solution

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Kahn’s vision

• Kahn imagined there would be only a few networks (~20) and thus only a few routers

• He was wrong

– Why?

• Imagined gateways would “translate” between networks

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Timeline continued….

1973 FTP introduced

1974 Cerf and Kahn paper on TCP/IP

1980 TCP/IP adopted as defense standard 1983 Global NCP to TCP/IP flag day

198x XNS, DECbit, and other protocols 1984 Janet (British research network) 1985 NSFnet (picks TCP/IP)

198x Internet meltdowns due to congestion

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Unsung hero of Internet: David D. Clark

• Chief Architect 1981-1988 • Great consistency of vision

• Kept the Internet true to its basic design principles • Authored what became known as the End-to-end

principle (next lecture)

• Conceives and articulates architectural concepts

– Read his “Active Networking and End-To-End Arguments”

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Timeline continued…

1988 Deering and Cheriton propose multicast 1989 Birth of the web….Tim Berners-Lee

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Why did it take physicist to invent web?

• Physicists are the smartest people in the world? • Computer scientists were trying to invent nirvana

– Well, actually Xanadu (Ted Nelson)

– More generally, CS researchers focused on hyptertext

• Again, users didn’t need what we wanted to invent

– Think about it: a paper on the web design would have been rejected by every CS conference and journal

• In general, the CS research community is great at solving well-defined problems, but terrible at

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Timeline continued…..

1993 Search engines invented (Excite)

199x ATM rises and falls (as internetworking layer) 199x QoS rises and falls

1994 Internet goes commercial 1998 IPv6 specification

1998 Google reinvents search

200x The Internet boom and bust

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Next Lecture(s)

• Monday is a holiday

• Wednesday: Internet Priorities and Principles

– HW #1 assigned on Wednesday

• If you haven’t already:

– Take the survey