1 If the Internet is the answer, then what was the question? EE122 Fall 2011 Scott Shenker Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley

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 section: moved to Cory 521 Need feedback on the index cards –Can do it anonymously on Piazza –Or send to me Plug computers arrived! –Now we just have to get them to work…..(Go Yahel!) Get instructional account forms in section 2

3 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…. Internet history –From my somewhat heretical viewpoint

Telephone network Alexander Graham Bell –1876: Demonstrates the telephone at US Centenary Exhibition in Philadelphia

5 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 Teardown: source tears down circuit when done

6 The switch in “circuit switching” incoming linksoutgoing links Node How does the node connect the incoming link to the outgoing link?

7 Circuit Switching With Human Operator

“Modern” switches Almon Brown Strowger ( ) –1889: Invents the “girl-less, cuss-less” telephone system -- the mechanical switching system

9 Timing in Circuit Switching Host 1Host 2 Switch 1Switch 2 time

10 Timing in Circuit Switching Circuit Establishment Host 1Host 2 Switch 1Switch 2 propagation delay between Host 1 and Switch1 time

11 Timing in Circuit Switching Circuit Establishment Host 1Host 2 Switch 1Switch 2 propagation delay between Host 1 and Switch1 Transmission delay time

12 Timing in Circuit Switching Circuit Establishment Host 1Host 2 Switch 1Switch 2 propagation delay between Host 1 and Switch1 Transmission delay time

13 Timing in Circuit Switching Circuit Establishment Host 1Host 2 Switch 1Switch 2 propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission delay time

14 Timing in Circuit Switching Information Circuit Establishment Transfer Host 1Host 2 Switch 1Switch 2 propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission delay time

15 Timing in Circuit Switching Information Circuit Establishment Transfer Circuit Teardown Host 1Host 2 Switch 1Switch 2 propagation delay between Host 1 and Switch1 propagation delay between Host 1 and Host 2 Transmission delay time

16 Sharing a link incoming linksoutgoing links Node How do the black and orange circuits share the outgoing link?

17 Circuit Switching: Multiplexing a Link Time-division –Each circuit allocated certain time slots Frequency-division –Each circuit allocated certain frequencies time frequency time

18 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 Slots =

Strengths of phone system Predictable performance –Known delays –No drops Easy to reason about Supports a crucial service What about weaknesses? 19

Weakness #1: Not resilient to failure Any failure along the path prevents transmission Entire transmission has to be restarted “All or nothing” delivery model 20

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? –Ratio of A/P 21

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 –But when it gets to several orders of magnitude…. 22

Weakness #3: Designed Tied to App Design revolves around the requirements of voice Not general feature of circuit switching –But definitely part of the telephone network design 23

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 –But I think it is overemphasized as a key factor 24

What if we wanted a resilient network? How would we design it? This is the question Paul Baran asked…. 25

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 26

What about a less wasteful network? How would we design it? This is the question Len Kleinrock asked….. –Analyzed packet switching and statistical multiplexing 27

28 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network

29 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Broadcast Communication Network

30 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 Broadcast Communication Networks

31 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network

32 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network

33 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network

34 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)

35 Packet Switching Node in a packet switching network incoming linksoutgoing links Node Memory

36 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

Simple Example: M/M/1 Queue 37 Consider n flows sharing a single queue Flow: random (Poisson) arrivals at rate Random (Exponential) service with average 1/  Utilization factor:  = n /  –If  >1, system is unstable Case 1: Flows share bandwidth –Delay = 1/(  - n ) Case 2: Flows each have 1/n th share of bandwidth –No sharing –Delay = n/(  - n ) Not sharing increases delay by n

38 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network

39 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 probability that there is enough capacity

40 Timing of Datagram Packet Switching Packet 1 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1

41 Timing of Datagram Packet Switching Packet 1 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1 transmission time of Packet 1 at Host 1

42 Packet 1 Timing of Datagram Packet Switching Packet 1 processing delay of Packet 1 at Node 2 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1 transmission time of Packet 1 at Host 1

43 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Timing of Datagram Packet Switching Packet 1 Packet 2 Packet 3 processing delay of Packet 1 at Node 2 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1 transmission time of Packet 1 at Host 1

44 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

45 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

46 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

47 Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network A hybrid of circuits and packets; headers include a “circuit identifier” established during a setup phase

48 5 Minute Break Questions Before We Proceed?

If you were building a network…. Which would you choose? –Circuit switched? –Packet-switched? Let’s review the strengths and weaknesses 49

50 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

51 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 –No communication until the connection is set up –Unable to avoid extra latency for small data transfers

52 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 bandwidth guarantees)

Choosing a Design for the Internet If you cared about: –Resilience –Bursty applications –Ease of interconnection –…–… Which would you choose? Not so fast….. –Hindsight is great –But there were important reasons to choose differently 53

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? 54

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) ….. 55

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) ….. 56

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 –Why not require Internet support these guarantees? 57

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 Architect for flexibility, engineer for performance 58

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 If the Internet had focused on voice applications early, it might have made different choices 59

Internet History

Timeline 1961Baran and Kleinrock advocate packet switching 1962Licklider’s vision of Galactic Network 1965Roberts connects two computers via phone 1967Roberts publishes vision of ARPANET 1969 BBN installs first IMP at UCLA IMP: Interface Message Processor 1971 Network Control Program (protocol) 1972Public demonstration of ARPANET 61

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… –…and the system crashed! 62

Timeline continued… invented 1972 Telnet introduced 1972Kahn advocates Open Architecture networking 63

64 The Problem Many different packet-switching networks Only nodes on the same network could communicate

65 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 No global control at operations level (why?)

66 Solution Gateways

67 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 –We think of it as all routers supporting IP

Timeline continued… FTP introduced 1974Cerf and Kahn paper on TCP/IP 1980TCP/IP adopted as defense standard 1983Global NCP to TCP/IP flag day 198xXNS, DECbit, and other protocols 1984Janet (British research network) 1985NSFnet (picks TCP/IP) 198xInternet meltdowns due to congestion 1986Van Jacobson saves the Internet (BSD TCP) 68

Unsung hero of Internet: David D. Clark Chief Architect 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” Perhaps the only “irreplaceable” Internet pioneer 69

Timeline continued… 1988 Deering and Cheriton propose multicast 1989 Birth of the web….Tim Berners-Lee 70

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 guessing what users will actually use 71

Timeline continued… Search engines invented (Excite) 199xATM rises and falls (as internetworking layer) 199xQoS rises and falls 1994 Internet goes commercial 1998 IPv6 specification 1998 Google reinvents search 200xThe Internet boom and bust 2011 EE122 enrollment suggests boom is back! 72

73 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 Have a good holiday!