2Learning OutcomesAt the end of this session, the students should be able to:Explain what the Internet is all aboutExplain what is a protocolDescribe what comprises the network edgeDescribe what comprises the network coreExplain connection-oriented serviceExplain connectionless serviceCompare circuit-switched network against packet-switched networkAnswer the short exercises given in the session
3Introduction What’s the Internet? UNIX-based workstations laptop Digital camerasserverWhen we think of the internet, so many things come to our mind. Nowadays, the internet connects almost anything imaginable. To take a peek of the few items connected to the internet, we have… portable device assistant, and so on and so forth. You must be wondering why a toaster appears in here.. Well, out of curiosity, I looked for that myself.. :http://news.bbc.co.uk/1/low/sci/tech/ stmAutomobileWeb-page serverWebTVPDAs with wireless Internet connectionstoasterHousehold appliancesHOSTS or END SYSTEMS
4Nuts and bolts of the Internet Networking Infrastructure What’s the Internet?Nuts and bolts of the InternetHardware componentsSoftwareNetworking Infrastructureprovides services to distributed applicationsinfrastructure where new applications are being constantly invented and deployedWe shall define the Internet in two ways in our discussions. First, we shall look at it based on its working parts or elements – and so we’re using the term “nuts and bolts”, then we shall also look at it in an abstract perspective, by looking at it as the underlying foundation or basic framework where new applications are constantly invented and deployed.
5What’s the Internet? “Nuts and Bolts” View global network of networks Interconnects hundreds of millions of computing devicesprovides:Global communicationStorageComputation infrastructureThe internet is viewed as the global network of networks. It now interconnects millions of computing devices, providing global communication, storage and computation infrastructure. Before delving into the nitty-gritty part, let’s have a look at the bigger picture first in a much simpler perspective… that is, in terms of an END-to-END SYSTEM.End-to-End System:CoreEnd SystemEnd System“edge”“edge”
6SETI@HOME—MASSIVELY DISTRIBUTED COMPUTING FOR SETI What is is a scientific experiment that uses Internet-connected computers in the Search for Extraterrestrial Intelligence (SETI). You can participate by running a free program that downloads and analyzes radio telescope data.Search For Extra-Terrestial Intelligence
7The Network Structure network edge: applications and hosts network core:routersnetwork of networksaccess networks, physical media: communication links
8What’s the Internet? “Nuts and Bolts” View End-to-End SystemEnd System=HOSTCoreEnd SystemEnd SystemAccess networksPhysical mediaWhere much internet architecture complexity is placedConnect endsystems to thenetwork coreCommunication linksSwitchesTransport dataMuch of the Internet architecture can be found in the END SYSTEM and likewise, the bulk of work. Rapid application development has been seen on this component, triggering an expansion of Network technology because of the high demands of evolving applications. To mention a few, I think most of us are familiar with Peer 2 Peer networking, where you can get everything for free (that application however is barred from our network – because it disobeys copyright laws – it is illegal). Moreover, there is another interesting application called Skype that provides internet telephony – with that, you can call anyone in the world for free (if the other person is using Skype) in stereo quality. On the other hand, what can be found in the core are the communication links and switches that transport data, as well as Access networks and Physical media that are responsible for connecting the END SYSTEMS to the network core.The communication links are made up of different types of physical media like copper wire…. And are characterized in terms of bandwidth or link transmission rate, measured in terms of bits/second.Communication linksCharacterized in terms of bandwidthMade up of different types of physical media:Link transmission rateCoaxial cableMeasured in bits/secondCopper wireFiber opticsRadio Spectrum
9Where is the Network Core? Let us just identify where in the figure is the network core…. All components colored RED in the figure corresponds to the network core.The structure that connects the end systems to the internet is the network core.
10What’s in the Links? Router Router X NETWORK CORESenderReceiverRouterEnd SystemEnd SystemXEnd SystemEnd Systempath or routeWhat’s in the Links?End SystemRouterInternet uses packet switching to allow for multiple communicating end systems to share a path, or parts of a path, at the same timeTakes a chunk of information arriving on one of its incoming communication links and forwards that chunk of information on one of its outgoing communication linksThe most common component that can be found in the network core is the router. As you can see in the figure, the function of a router is to provide a path from a node on one network to a node on another network. The figure is a simplified illustration of how routers allow for the connection, but in real networks, the two networks may be actually separated by several intervening networks and, possibly, by many miles. The router provides the path by first determining a route and then providing the initial connection for the path.packet
11Now let’s see how are links formed in terms of clusters Now let’s see how are links formed in terms of clusters.. Let’s see the bigger picture
12Get http://www.massey.ac.nz/ What’s a protocol?a human protocol and a computer network protocol:TCP connection requestHiHiTCP connection replyGot the time?Get2:00<file>time
13What’s a protocol? Human Protocols: Network Protocols: Something we execute all the timeOffer a greetingWait for a responseAnalyze the responseAct accordinglyNetwork Protocols:Similar to human protocol, except that entities are machines rather than humansall communication activities in the Internet are governed by protocolsIn order for protocols to work, both entities must observe the same protocol.There is a set of conventional actions taken when messages are sent and received.Networking – understanding the what, why and how of networking protocols
14What’s a protocol? A protocol defines: format and order of messages sent and received among network entitiesand actions taken on the transmission and/or receipt of a message, or other event* All activities in the Internet that involves 2 or more communicating entities are governed by a protocol.There are protocols in:RoutersProtocols determine a packet’s path from source to destinationNIChardware-implemented protocols control the flow of the bits on the “wire”End Systemscongestion-control protocols control the rate at which packets are transmitted between sender and receiverCommunicating Entities:Hardware, Software componentsDifferent protocols are used to accomplish different communication tasks:
15What’s the Internet? “nuts & bolts” view Protocols- control the sending and receiving of information within the Internet run by End Systems, routers, etc.;TCPTwo of the most important protocols in the Internet (principal protocols)IPTCP – Transmission Control ProtocolIP – Internet Protocol – specifies the format of the packets that are sent and received among routers and end systemsThe internet is governed by a set of predefined rules in order to allow processes to communicate and exchange data.. In internet jargon, that is referred to as protocols.INTERNET STANDARDSMade possible through standards developed by (IETF) Internet Engineering Task ForceRFCs (Request for Comments)define protocols such as TCP, IP, HTTP, SMTP
16What’s the Internet? A Service View Provides a communication infrastructure that allows distributed applications running on its end systems to exchange data with each other.Remote loginWeb surfingInstant messagingInternet telephony“the Web” – distributed application that use the communication services provided by the InternetCommunication services provided to distributed applications:Connection-Oriented Reliable ServiceGuarantees that data is delivered orderly and completely (sender to receiver)Connectionless Unreliable ServiceDelivery is not guaranteed
17Question?Why would we opt for a connectionless unreliable service when there is a connection-oriented reliable service that is available?Hold on to that thought for a while…
18A closer look at the Network Edge What happens in the network edge?The sending End System doesn’t know how messages are actually sent.It only needs to know what services are provided, and so the “nuts and bolts” of the Internet serves as a “black box” that transfers messages between distributed communicating components.There is some level of abstraction that hides the nitty-gritty part of the communication process between two end systemsClient/Server Model - Most prevalent structure for Internet applications; although not all applications are purely client, or purely of server type (e.g. P2P file sharing)
19A closer look at the Network Edge What happens in the network edge?End Systems (Hosts):run application programe.g., WWW,at “edge of network”Client/Server Modelclient host requests, receivesservice from servere.g. WWW client (browser)/server; client/serverClient/Server Model - Most prevalent structure for Internet applications; although not all applications are purely client, or purely of server type (e.g. P2P file sharing)
20The Network Edge“Connection” between two End Systems: (e.g. Web application or Internet phone application)Nothing more than allocated buffers and state variables in the End-Systems“Connection”Internet provides two type of services to End-System Applications:Connection-oriented service – (TCP)App’s using TCP:HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP ( )The term connection corresponds to nothing more but allocation of buffers and state variables. Only the communicating End-Systems are the ones who knows that they’re connected.. not even the routers of the intervening packet switches knows about any connection-state information between the communicating End-Systems.Connectionless service – (UDP)App’s using UDP:streaming media, teleconferencing, Internet telephony
21Network edge: connection-oriented service performs handshakingGoal: data transfer between end system.handshaking: setup (prepare for) data transfer ahead of timeHello, hello back human protocolset up “state” in two communicating hostsQ*TCP service [RFC 793]Provides:Reliable data transfer:loss: handled using acknowledgements and retransmissionsFlow Control:Ensures that the sender won’t overwhelm receiverCongestion Control:Instructs senders to “slow down sending rate” when network is congestedPrevents gridlockAny protocol that performs handshaking is a connection-oriented service. We use the term “connection-oriented” because the end systems are connected in a loose manner, only the end systems are aware of the connection. The routers or intervening packet switches do not maintain any connection-state information about the connection.The services provided by TCP, such as reliable data transfer, flow control and congestion control are by no means requirements of a connection-oriented service. One or two of them may be missing, and yet still the network could offer connection-oriented service. Let’s just have a look at the definition here, then we shall see some animations later demonstrating them.DISCUSS THIS JUST QUICKLY – use the next animation slides to explainTransmission Control Protocol (TCP)Internet’s connection-oriented service
22Network Edge: TCP Service 3-way HandshakeControl packetCONNECTION ESTABLISHEDDATAacknowledgementrequestCLIENTSERVERCLICK TO ANIMATEReliable data transfer is achieved through acknowledgementsand retransmissions*Data is delivered without error and in proper order
23Network Edge: TCP Service Handshaking Procedure:Case: Retransmission of RequestControl packetClient assumes packet was lost, decides to retransmitClient is waiting for AcknowledgementDATAacknowledgementCLIENTSERVERCLICK TO ANIMATEReliable data transfer is achieved through acknowledgementsand retransmissions*Data is delivered without error and in proper order
24Network Edge: TCP Service Problem occurs when one communicating End-System transmits faster than the other End-SystemCLIENTThis End-System does not receive an acknowledgement yet, and so it issues another packetCLIENTControl packetCLIENTCLIENTSERVERCLICK TO ANIMATEFlow controlforces the sending End System not to send too many packets too fastfor the receiverTCP/IP provides the Flow control service
25Network Edge: TCP Service Problem: Gridlock sets-in when there is packet loss due to router congestionThe sending system’s message is lost due to congestion, and is alerted when it stops receiving acknowledgements of packets sentCLIENTSERVERDue to router congestion, the packets sent by the sending End system is lost. When that happens, the sending end system is alerted of congestion if it does not receive an acknowledgement for the packets it sent. TCP provides a congestion control service that forces the End systems to decrease the rate at which packets are sent during periods of congestion.You can view the applet from Kurose’s site demonstrating network congestion.Congestion controlforces the End Systems to decrease the rate at which packets are sent duringperiods of congestion
26Network edge: connectionless service No handshaking procedure; End-Systems just simply send the packetGoal: data transfer between end systemssame as before!UDP - User Datagram Protocol[RFC 768]: Internet’s connectionless serviceunreliable data transferno flow controlno congestion controlThere is no handshaking; therefore, data can be delivered sooner, but there is no reliable data transfer guaranteedThe sending program simply sends the packetsThe source never knows for sure which packets have arrived at the destinationInternet telephony uses the connectionless service because of the application’ demand for speed and the application doesn’t necessarily require acknowledgement of data all throughout the communication session
27Something to ponder on?Transmission rate of the link (Bandwidth) – how many bits per second a network can transportPropagation delay (Latency) – how many seconds it takes for the first bit to get from the client to the serverBesides bandwidth and latency, what other parameter is needed to give a good characterization of the quality of service offered by a network used for digitized voice traffic?
28AnswerA uniform delivery time is needed for voice, so the amount of jitter in the network is important. This could be expressed as the standard deviation of the delivery time. Having short delay but large variability is actually worse than a somewhat longer delay and low variability.Jitter – irregular random transmission time in the network
30The Network Corethe fundamental question: how is data transferred through net?circuit switching: dedicated circuit per call: telephone netpacket-switching: data sent through net in discrete “chunks”Approaches to building a Network Core:So how is data transferred in the NETWORK CORE? There are primarily two approaches, either by circuit switching or by packet switching. Circuit-switching is used by the ubiquitous telephone network, while packet switching is used by the Internet, and is said to be the future of telephone networks.
31Circuit Switching vs. Packet Switching The Network CORECircuit Switching vs. Packet SwitchingA Restaurant AnalogyWhat resources must be reserved?Circuit-switched NetworksResources are reserved for the duration of thecommunication sessionRestaurant which requires reservationWith a reservation, you can order right away when you get thereguaranteed seatsPacket-switched NetworksLet’s have a look at a simple restaurant analogy to be able to easily understand the difference between the two switching schemes. You can think of Circuit switched networks as being analogous to restaurants requiring reservations, giving you guaranteed seats. On the other hand, packet switched networks are analogous to restaurants that do not require any reservations whatsoever, therefore seats are not guaranteed; The network equivalent simply dispatch messages, which take up on resources on demand; therefore, messages may have to wait on a queue to be transported.Messages use the resources on demand; thus, may haveto wait (queue) for access to a communication linkRestaurant which does not require any reservationyou may have to wait on a queue to be servedno sure seats
32Network Core: Circuit Switching End-end resources reserved for “call”Reserved link bandwidth, switch capacitySwitches on the path between sender and receiver maintain connection state for the duration of the sessionResources are dedicated; thus, no sharingAdvantage: circuit-like (guaranteed) performancecall set-up required(unless infinite resources are available)“Circuit”The resources reserved for the duration of the call are the link bandwidth and the switch capacity.Such resources will be made available solely to a specific user in the network. Therefore, there is absolutely no sharing of resources between users or circuits. The advantage of this scheme is that circuit-like performance is guaranteed.
33Network Core: Circuit Switching How is it implemented?By dividing the link bandwidth into “pieces”frequency divisiontime divisionInefficiency: Resource piece is idle if not used by owning call(no sharing)
34Circuit Switching: FDM and TDM 4 users (or 4 circuits)Example:FDMfrequencytime4KHzNetwork dedicates a frequency band to each connection for the sessionTDMfrequencytimeFrameSlotIn FDM, the frequency spectrum is shared among the users of the link. The link dedicates a frequency band to each user for the duration of the connection. (FM radio station – share microwave frequency spectrum, Telephone networks – freq. band = 4kHz or 4,000 cycles/sec).On the other hand, in TDM, the network dedicates one time slot in every frame of the connection.Two simple multiple access control techniques.Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.As we will see, used in AMPS, GSM, IS-54/136Used solely by oneEnd SystemNetwork dedicates one time slot in every frame of the connection
35Further assume that propagation delay is negligible. Question?How long does it take to send a file of 640,000 bits from Host A to Host B over a circuit-switched network? Assume that all links in the network use TDM with 24 slots and have a bit rate of Mbps. Also suppose that it takes 500 msec. to establish an end-to-end circuit before Host A can begin to transmit the file.Further assume that propagation delay is negligible.
36Question Answer GIVEN: Size of file to send: 640,000 bits SOLUTION: How long does it take to send a file of 640,000 bits from Host A to Host B over a circuit-switched network? Assume that all links in the network use TDM with 24 slots and have a bit rate of Mbps. Also suppose that it takes 500 msec. to establish an end-to-end circuit before Host A can begin to transmit the file.GIVEN: Size of file to send: 640,000 bitsSOLUTION:Each circuit has a transmission rate of (1.536 Mbps)/24 slots= 64kbps (or 64,000 bps).So, it takes (640,000 bits)/(64,000 bps)= 10 sec. to transmit the file.Considering the circuit establishment time, we add 0.5 sec; therefore,It takes 10.5 sec. to transmit the file.The transmission time would be 10 sec. if the end-to-end circuit passed through 1 link or 100 links. (but the actual end-to-end delay also includes a propagation delay)Establishment time + transmission time
37Network Core: Packet Switching each end-end data stream divided into packetsuser A, B packets share network resourceseach packet uses full link bandwidthresources used as neededResource Contention:aggregate resource demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop at a timetransmit over linkwait turn at next linkQ*In contrast to circuit switching, packet switching allows for resource sharing. Data is transmitted on demand, without reservation, in terms of packets, using the full link bandwidth. This switching however could be fazed with a problem of catering to a multitude of packets exceeding the switch capacity, congestion, and store-and-forward delaysBandwidth division into “pieces”Dedicated allocationResource reservation
39Network Core: Packet Switching Statistical multiplexing - on-demand sharing of resources10 MbsEthernetCAstatistical multiplexingQ*1.5 MbsBqueue of packetswaiting for transmission at the output link45 MbsSender:Nodes A and BDEReceiver: Node EThe figure demonstrates a simple packet-switched network. Assuming that Hosts A and B transmit a sequence of packets towards Host E. In the first packet switch, such packets will be received in random order. The link is not reserved for any sender, but is used on demand. In the jargon of networking, this on-demand sharing of resources is referred to as statistical multiplexing. The router employs statistical multiplexing to schedule the packets for transmission (this sharply contrasts circuit switching).sequence of A & B packets has no fixed timing patternbandwidth shared on demand: statistical multiplexing.Compare this to TDM: each host gets same slot in revolving TDM frame.
40Network Core: Packet Switching Consider a message that is 7.5 x 106 bits long. Suppose that betweensource and destination, there are 2 packet switches and 3 links, and that each link has a transmission rate of 1.5 Mbps. Assuming that there is no congestion in the network and negligible propagation delay, how much time is required to move the message from source to destination with packet switching?(7.5 Mbps/1.5 Mbps) * 3 = 15 sec.Transmission delay
41Packet Switching: Store and Forward Behaviour Example:store and forward behaviour:break message into smaller chunks: “packets”Store-and-forward: switch waits until chunk has completely arrived, then forwards/routesLet us analyze how packet switching transports a message of size 5,000 packets. Take note that the unit of time used here is msec. From source to destination, taking the first packet, we see that packet one reaches the first packet switch at time = 1 msec. By the time packet 2 is transmitted, packet 1 is also being transported to the second switch. At this point, simultaneous transmission occurs, therefore, at time = 2 msec., packet 2 reaches the first switch, and packet one reaches the 2nd switch. Using this pattern, the last packet, 5,000 th packet reaches the first switch after 5,000 msec or 5 sec. adding 2 more switching, we get sec = the time to reach its destination.Pattern that can be deduced from the packet flow depicted in the Figure:Time of arrival = packet_num + 2
42Packet Switching vs. Circuit Switching Suppose that users share a 1 Mbps link, where each user alternates between generating data at a constant rate of 100 kbps, and periods of inactivity. Also assume that each user is active only 10% of the time.Compare the performance of Circuit Switching against Packet Switching.
43Packet Switching vs. Circuit Switching Packet switching allows more users to use network!Example: 1 Mbit link shared by all userseach user:Generates 100Kbps when “active”(at constant rate)active 10% of timecircuit-switching:10 users can only be supported1,000,000 bits/sec divided by 100,000 bits/sec.packet switching:with 35 users, probability > 10 are active is less than .0004probability <= 10 users are active is1 Mbps linkN usersImplies that 10 users can be using the circuit without competing, just like circuit-switching (bandwidth is equally distributed)Packet switching can cater to up to 35 users using the given 1 Mbps link. Using statistics, the probability that less than or equal to 10 users are active at a time is (almost equal to 1). When that happens, 10 users can be serviced in the same speed as circuit-switching.Packet switching allows for more than 3 times the number of users as compared to circuit-switching
44Question (Transmission delay) ? A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 10 µsec, is this likely to be a major factor in the response of a client-server system where the client is in Adelaide, Australia and the server is in Auckland, New Zealand? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum.Speed of light = 3 x 108 meters/sec.
45Question (Transmission delay) AnswerA factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 10 µsec, is this likely to be a major factor in the response of a client-server system where the client is in Adelaide and the server is in Auckland? Assume the propagation speed in copper and fiber to be 2/3 the speed of light in vacuum.No. The speed of propagation is 200,000 km/sec or 200 meters/µsec. In 10 µsec the signal travels 2 km. Thus, each switch adds the equivalent of 2 km of extra cable. If the client and server are separated by 5000 km, traversing even 50 switches adds only 100 km to the total path, which is only 2%. Thus, switching delay is not a major factor under these circumstances.
46DemoTotal delay across a link = Transmission delay + Propagation delay
47Network Core: Packet Switching Advantages: Great for bursty dataresource sharingno call set-upDrawbacks:Excessive congestion, packet delay and lossprotocols needed for reliable data transfer, congestion controlIssue: How to provide circuit-like behaviour?bandwidth guarantees needed for audio/video appsthis is still an unsolved problem!
48Access networks and physical media Q: How to connect End- Systems to edge router?residential access netsinstitutional access networks (school, company)mobile access networksKeep in mind:bandwidth (bits per second) of access network?shared or dedicated?
49Dial-up Modem uses existing telephony infrastructure telephonenetworkInternethome dial-upmodemISP modem(e.g., AOL)homePCcentral officeuses existing telephony infrastructurehome directly-connected to central officeup to 56Kbps direct access to router (often less)can’t surf, phone at same time: not “always on”Introduction 1-49
50Central Office Example: A central office in Dakota, U.S.A.
51Digital Subscriber Line (DSL) telephonenetworkDSLmodemhomePCphoneInternetDSLAMExisting phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream datasplittercentralofficeuses existing telephone infrastructureup to 1 Mbps upstream (today typically < 256 kbps)up to 8 Mbps downstream (today typically < 1 Mbps)dedicated physical line to telephone central officeWorks only within 5 to 10 miles from the CO.Introduction 1-51For more info:
52Residential access: cable modems uses cable TV infrastructure, rather than telephone infrastructureHFC: hybrid fiber coaxasymmetric:up to 30Mbps downstream,2 Mbps upstreamnetwork of cable, fiber attaches homes to ISP routerhomes share access to routerunlike DSL, which has dedicated accessIntroduction 1-52
53Residential access: cable internet access Shared broadcast mediumDiagram:Introduction 1-53
54Cable Network Architecture: Overview Typically 500 to 5,000 homescable headendhomecable distributionnetwork (simplified)Homes can be up to 100 miles from the cable headendIntroduction 1-54
58Fiber to the Home optical links from central office to the home ONTOLTcentral officeopticalsplitterShared optical fiberoptical fibersInternetOptical network terminatorOptical line terminatoroptical links from central office to the hometwo competing optical technologies:Passive Optical network (PON)Active Optical Network (AON) – switched ethernetmuch higher Internet rates (download [10,20Mbps],upload [2,10Mbps]); fiber also carries television and phone servicesIntroduction 1-58
59Ethernet Internet access 100 Mbpsinstitutionalrouterto institution’s ISPEthernetswitch100 Mbps1 Gbps100 Mbpsservertypically used in companies, universities, etc(Users:10 Mbps, 100Mbps), (Servers:1Gbps, 10Gbps Ethernet)today, end systems typically connect into Ethernet switchIntroduction 1-59
60Wireless access networks shared wireless access network connects end system to routervia base station aka “access point”wireless LANs:802.11b/g (WiFi): 11 or 54 MbpsTens of meters from access pointwider-area wireless accessprovided by telco operator~1Mbps over cellular system (3G – packet-switched wide-area wireless internet access)Tens of kilometers from access pointnext up (?): WiMAX – IEEE (10’s Mbps) over wide arearouterbasestationWiMax (Worldwide Interoperability for Microwave Access)mobilehostsIntroduction 1-60
61Home networks Typical home network components: DSL or cable modem router/firewall/NATEthernetwireless access pointwirelesslaptopsto/fromcableheadendcablemodemrouter/firewallwirelessaccesspointEthernetIntroduction 1-61
62Physical Media Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps EthernetCategory 5: 100Mbps Ethernetbit: propagates between transmitter/rcvr pairsphysical link: what lies between transmitter & receiverguided media:signals propagate in solid media: copper, fiber, coaxunguided media:signals propagate freely, e.g., radioIntroduction 1-62
63Physical Media: coax, fiber Fiber optic cable:glass fiber carrying light pulses, each pulse a bithigh-speed operation:high-speed point-to-point transmission (e.g., 10’s-100’s Gpbs)low error rate: repeaters spaced far apart ; immune to electromagnetic noiseCoaxial cable:two concentric copper conductorsbidirectionalbaseband:single channel on cablelegacy Ethernetbroadband:multiple channels on cableHFCIntroduction 1-63
64Physical media: radio Radio link types: terrestrial microwavee.g. up to 45 Mbps channelsLAN (e.g., WiFi)11Mbps, 54 Mbpswide-area (e.g., cellular)3G cellular: ~ 1 MbpssatelliteKbps to 45Mbps channel (or multiple smaller channels)280 msec end-end delaygeosynchronous versus low altitude (Low-Earth-Orbiting satellites) – future Internet accesssignal carried in electromagnetic spectrumno physical “wire”bidirectionalpropagation environment effects:reflectionobstruction by objectsinterferenceIntroduction 1-64
65Question?An image of 1024x768 pixels with 3 bytes/pixel. Assume the image is uncompressed.How long does it take to transmit over a 56-kbps modem channel?Over a 1-Mbps cable modem?Over a 10-Mbps Ethernet?Over 100-Mbps Ethernet?Clue:Mega = 1 x 106Kilo = 1 x 1031024768
66Question: transmission delay AnswerHow long does it take to transmit over a 56-kbps modem channel?Over a 1-Mbps cable modem?Over a 10-Mbps Ethernet?Over 100-Mbps Ethernet?1024768SOLUTION:The image is 1024 x 768 x 3 bytes or 2,359,296 bytes.This is 18,874,368 bits.At 56,000 bits/sec., it takes about sec.At 1,000,000 bits/sec, it takes about sec.At 10,000,000 bits/sec., it takes about sec.At 100,000,000 bits/sec., it takes about sec.
67How do loss and delay occur? packets queue in router bufferspacket arrival rate to link exceeds output link capacitypackets queue, wait for turnpacket being transmitted (delay)Afree (available) buffers: arriving packetsdropped (loss) if no free bufferspackets queueing (delay)BIntroduction 1-67
68Loss in Packet-Switched Networks - Length of Queue is finiteQueuePackets are lost when queue is fullIncoming packet is droppedQueuepacket in queue is droppedLost packet- Retransmitted by application or transport layer protocol
69Four sources of packet delay Bpropagationtransmissionnodalprocessingqueueingdnodal = dproc + dqueue + dtrans + dpropdproc: nodal processingcheck bit errorsdetermine output linktypically < msecdqueue: queueing delaytime waiting at output link for transmissiondepends on congestion level of routerIntroduction 1-69
70Four sources of packet delay Bpropagationtransmissionnodalprocessingqueueingdnodal = dproc + dqueue + dtrans + dpropdtrans: transmission delay:L: packet length (bits)R: link bandwidth (bps)dtrans = L/Rdprop: propagation delay:d: length of physical links: propagation speed of medium (~2x108 m/sec)dprop = d/sdtrans and dpropvery differentIntroduction 1-7070
71Queueing delay (revisited) R=link bandwidth (bits/sec)L=packet length (bits)a=average packet arrival rate (packets/sec)traffic intensity = La/RLa/R ~ 0: average queueing delay smallLa/R -> 1: delays become largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!There is a formula that helps us gauge the traffic intensity, in order for us to determine whether the average queuing delay is tolerable or not. The graph depicts an exponential curve telling us that as the value of traffic intensity approaches 1, queuing delay becomes large. If we will examine the formula, as the average packet arrival rate picks up, and multiply that by the Packet length, the link’s bandwidth R may not be able to cope up in transmitting the packets.This estimates the extent of queuing delay.Design your system so that traffic intensity is not greater than 1. Let’s look at a demo!
72Throughputthroughput: rate (bits/time unit) at which bits transferred between sender/receiverinstantaneous: rate at given point in timeaverage: rate over longer period of timeAverage rate of successful message delivery over a communications channellink capacityRs bits/seclink capacityRc bits/secserver sends bits(fluid) into pipeserver, withfile of F bitsto send to clientpipe that can carryfluid at rate(Rs bits/sec)pipe that can carryfluid at rate(Rc bits/sec)Introduction 1-72
73Throughput (more) Rs < Rc What is average end-end throughput? Rc bits/secRs bits/secRs > Rc What is average end-end throughput?Rs bits/secRc bits/seclink on end-end path that constrains end-end throughputbottleneck linkIntroduction 1-73
74Throughput: Internet scenario per-connection end-end throughput: min(Rc,Rs,R/10)in practice: Rc or Rs is often bottleneckRsRsRsRRcRcRce.g.10 clients downloading with 10 serversRc=1 Mbps, Rs=2Mbps, R=5Mbps10 connections (fairly) share backbone bottleneck link R bits/secIntroduction 1-74
75Delay and Routes in the Internet TraceRoute(diagnostic program) -defined in RFC 1393SOURCE HOSTDESTINATION HOSTProgramProgramSOURCE:records time elapsed (time received- time packet sent)determines the round-trip delays to all intervening routersRound-trip delays include:Router processing delayQueuing delay (varies with time)Transmission delayPropagation delayrecords name & address of router (or destination HOST) that returns the messagereconstructs the route taken by the packets (source-to-destination)If there are (N-1) routers, then SOURCE sends N special packetsEach packet is addressed to the ultimate destinationmarked 1 to NFor tracking down the route taken by packets, we can use a program like TraceRoute provided byTraceroute sends a sequence of Internet Control Message Protocol (ICMP) packets addressed to a destination host. Tracing the intermediate routers traversed involves control of the time-to-live (TTL) Internet Protocol parameter. Routers decrement this parameter and discard a packet when the TTL value has reached zero, returning an ICMP error message (ICMP Time Exceeded) to the sender.Traceroute works by increasing the TTL value of each successive batch of packets sent. The first three packets sent have a time-to-live (TTL) value of one, expecting that they are not forwarded by the first router. The next three packets have a TTL value of 2, so that the second router will send the error reply. This continues until the destination host receives the packets and returns an ICMP Echo Reply message.The traceroute utility uses the returning ICMP messages to produce a list of hosts that the packets have traversed in transit to the destination. The three timestamp values returned for each host along the path are the delay (aka latency) values, typically measured in milliseconds for each packet in the batch.When DESTINATION host receives the Nth packet:DESTINATION destroys the packet, thenreturns the message back to the sourceWhen the ith router receives the ith packet marked i:router destroys the packetSends a message containing name and address of router back to the source
76www.TraceRoute.org Trace:3x * - indicates packet loss Route trace: From MIT to Massey UniversityThree delay measurementsTrace Route from MITIMPORTANT: This tool works by sending a series of UDP packets with different port numbers and TTL (Time To Live). If you are running firewall software, your software may interpret the incoming packets as a hostile "port scan" originating from this server (jis.mit.edu). Rest assured, your system is not being attacked.1 W92-RTR-1-W92SRV21.MIT.EDU ( ) ms ms ms2 EXTERNAL-RTR-1-BACKBONE.MIT.EDU ( ) ms ms ms3 leg CHE.sprinthome.com ( ) ms ms ms( ) ms ms ms5 sl-bb21-chi-6-2.sprintlink.net ( ) ms ms ms6 sl-bb24-chi-9-0.sprintlink.net ( ) ms ms ms7 sl-bb21-sj-8-0.sprintlink.net ( ) ms ms ms8 sl-bb22-sj-15-0.sprintlink.net ( ) ms ms ms( ) ms ms ms10 sl-newzeal-1-0.sprintlink.net ( ) ms ms ms11 p5-2.sjbr1.global-gateway.net.nz ( ) ms ms ms( ) ms ms ms( ) ms ms ms14 massey-uni-ak-int.tkbr4.global-gateway.net.nz ( ) ms ms ms15 * * *16 * * *17 * * *18 * * *19 * * *20 * * *21 * * *23 * * *22 * * *24 * * *25 * * *26 * * *28 * * *27 * * *29 * * *30 * * *Trans-oceanic link* means no response (probe lost, router not replying)6 columns: n, name of router, address of router, trip delay1,trip delay2,trip delay3* - indicates packet loss
77The round-trip delay decreased between the two routers! Route trace: From MIT to Massey UniversityTrace Route from MITIMPORTANT: This tool works by sending a series of UDP packets with different port numbers and TTL (Time To Live). If you are running firewall software, your software may interpret the incoming packets as a hostile "port scan" originating from this server (jis.mit.edu). Rest assured, your system is not being attacked.1 W92-RTR-1-W92SRV21.MIT.EDU ( ) ms ms ms2 EXTERNAL-RTR-1-BACKBONE.MIT.EDU ( ) ms ms ms3 leg CHE.sprinthome.com ( ) ms ms ms( ) ms ms ms5 sl-bb21-chi-6-2.sprintlink.net ( ) ms ms ms6 sl-bb24-chi-9-0.sprintlink.net ( ) ms ms ms7 sl-bb21-sj-8-0.sprintlink.net ( ) ms ms ms8 sl-bb22-sj-15-0.sprintlink.net ( ) ms ms ms( ) ms ms ms10 sl-newzeal-1-0.sprintlink.net ( ) ms ms ms11 p5-2.sjbr1.global-gateway.net.nz ( ) ms ms ms( ) ms ms ms( ) ms ms ms14 massey-uni-ak-int.tkbr4.global-gateway.net.nz ( ) ms ms ms15 * * *16 * * *17 * * *18 * * *19 * * *20 * * *21 * * *23 * * *22 * * *24 * * *25 * * *26 * * *28 * * *27 * * *29 * * *30 * * *The round-trip delay decreased between the two routers!Can you explain why the delays sometimes decrease from one router to the next?6 columns: n, name of router, address of router, trip delay1,trip delay2,trip delay3* - indicates packet loss
78Tracert (from xtra to mit) C:\>tracert web.mit.eduTracing route to web.mit.edu [ ]over a maximum of 30 hops:ms ms msms ms msms ms ms dialup.xtra.co.nz [ ]4 * ms ms5 * ms *6 * * * Request timed out.ms * * so labr3.global-gateway.net.nz [ ]8 * * * Request timed out.9 * ms ms g core01.lax05.atlas.cogentco.com [ ]ms ms * t3-4.mpd01.lax01.atlas.cogentco.com [ ]ms ms * g9-0-0.core01.lax01.atlas.cogentco.com [ ]* ms ms p2-0.core01.dfw01.atlas.cogentco.com [ ]* * ms p15-0.core02.dfw01.atlas.cogentco.com [ ]ms * * p15-0.core01.mci01.atlas.cogentco.com [ ]* * * Request timed out.* ms * p15-0.core01.ord01.atlas.cogentco.com [ ]* ms ms p14-0.core01.alb02.atlas.cogentco.com [ ]* ms ms p6-0.core01.bos01.atlas.cogentco.com [ ]* ms ms g8.ba21.b bos01.atlas.cogentco.com [ ]* * ms MIT.demarc.cogentco.com [ ]ms * * W92-RTR-1-BACKBONE.MIT.EDU [ ]* ms * WEB.MIT.EDU [ ]* ms ms WEB.MIT.EDU [ ]Trace complete.C:\>Tracert (also known as traceroute) is a Windows based tool that allows you to help test your network infrastructure.
79Tracert (from Massey to MIT) D:\Massey Papers\159334\Codes\Game Protocol v3.6>tracert web.mit.eduTracing route to web.mit.edu [ ]over a maximum of 30 hops:1 <1 ms <1 ms <1 ms it vlan205.massey.ac.nz [ ]2 <1 ms <1 ms <1 ms it vlan801.massey.ac.nz [ ]ms <1 ms <1 msms <1 ms <1 msms ms msms ms ms abilene-1-lo-jmb-706.sttlwa.pacificwave.net [ ]ms ms ms dnvrng-sttlng.abilene.ucaid.edu [ ]ms ms ms kscyng-dnvrng.abilene.ucaid.edu [ ]ms ms ms iplsng-kscyng.abilene.ucaid.edu [ ]ms ms ms chinng-iplsng.abilene.ucaid.edu [ ]ms ms ms ge rtr.chic.net.internet2.edu [ ]ms ms ms so rtr.wash.net.internet2.edu [ ]ms ms ms ge rtr.chic.net.internet2.edu [ ]ms ms ms nox300gw1-Vl-110-NoX-ABILENE.nox.org [ ]ms ms ms nox230gw1-Vl-802-NoX.nox.org [ ]ms ms ms nox230gw1-PEER-NoX-MIT nox.org [ ]ms ms ms W92-RTR-1-BACKBONE.MIT.EDU [ ]ms ms ms WEB.MIT.EDU [ ]Trace complete.D:\Massey Papers\159334\Codes\Game Protocol v3.6>
80Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core end systems, access networks, links1.3 Network corecircuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service modelsIntroduction 1-80
81Protocol “Layers” Question: Networks are complex, with many “pieces”: hostsrouterslinks of various mediaapplicationsprotocolshardware, softwareQuestion:Is there any hope of organizing structure of network?Or at least our discussion of networks?Introduction 1-81
82An analogy: Organization of air travel ticket (purchase)baggage (check)gates (load)runway takeoffairplane routingticket (complain)baggage (claim)gates (unload)runway landingTicketed passengersBaggage-checked, Ticketed passengersBaggage-checked, Ticketed, passed through the gate passengersPassenger in-flighta series of stepsIntroduction 1-82
83Layering of airline functionality ticket (purchase)baggage (check)gates (load)runway (takeoff)airplane routingdepartureairportarrivalintermediate air-trafficcontrol centersticket (complain)baggage (claimgates (unload)runway (land)ticketbaggagegatetakeoff/landingLayers: each layer implements a servicevia its own internal-layer actionsrelying on services provided by layer belowIntroduction 1-83
84Why layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieceslayered reference model for discussionmodularization eases maintenance, updating of systemchange of implementation of layer’s service transparent to rest of systeme.g., In the air travel analogy, a change in gate procedure doesn’t affect rest of systemlayering considered harmful?Introduction 1-84
85Tasks of LayersEach layer may perform one or more of the following tasks:Error ControlFlow controlSegmentation and ReassemblyMultiplexingConnection Set-upPotential Drawbacks of Layering:Duplication of servicesPossible violation of layer dependency (conflicting information dependency among layers)
86Communication in a Layered Architecture Concept of Protocol LayeringLet’s consider 2 Network Entities (e.g. End Systems, Packet Switches)Sending sideReceiving sideLayer 4MMLayer 3H3M1H3M2H3M1H3M2Layer 2H2H3M1H2H3M2H2H3M1H2H3M2Layer 1H1H2H3M1H1H2H3M2H1H2H3M1H1H2H3M2To get a good grasp of how network entities communicate in a layered architecture,let’s consider 2 network entities, which may represent either end systems or packet switches.Let’s also assume that there are 4 layers in each network entity, and that the communication between each layer is made possible by passing layer-n messages called n-PDUs.When the SOURCE HOST creates a message M (as defined by its application) at the highest layer (Layer 4), aimed to be delivered to the DESTINATION HOST, the message M is first divided into two parts by the next lower layer (Layer 3); that is, M becomes M1 and M2. Next, Layer 3 in the SOURCE HOST appends the H3 header to M1 and M2 to indicate additional information needed by the sending and receiving sides of Layer 3 to provide the services for Layer 4. A similar process is taken by the succeeding lower layers (appending necessary headers), until the messages are sent out of the SOURCE and onto a physical link. At the receiving end system, the messages are directed up the protocol stack. During each climb on the layers, the corresponding header for that layer is removed. Eventually, M is reassembled from M1 and M2 and then passed on to the application.SOURCEDESTINATIONWhat happens when the SOURCE wants to send a message to the DESTINATION?Comprised of 4 Layers; where each layer n is governed by a protocol.Layers communicate by exchanging layer-n messages called (n-PDUs) Protocol Data Units.The contents, format, and procedure for exchanging PDUs are defined by Layer-n Protocol
88Encapsulation destination source application transport network link messageMapplicationtransportnetworklinkphysicalsegmentHtMHtdatagramHtHnMHnframeHtHnHlMlinkphysicalswitchdestinationnetworklinkphysicalHtHnMHtHnHlMMapplicationtransportnetworklinkphysicalHtHnMHtMHtHnMrouterHtHnHlMIntroduction 1-88
89Internet protocol stack application: supporting network applicationsFTP, SMTP, HTTPtransport: process-process data transferTCP, UDPnetwork: routing of datagrams from source to destinationIP, routing protocolslink: data transfer between neighboring network elementsEthernet, (WiFi), PPPphysical: bits “on the wire”applicationTransportnetworklinkphysicalIntroduction 1-89
90Internet protocol stack application: supporting network applicationsftp, smtp, httptransport: host-host data transfertcp, udpnetwork: routing of datagrams from source to destinationip, routing protocols (Hardware+Software)link: data transfer between neighbouring network elementsppp, ethernetphysical: bits “on the wire”applicationtransportnetworklinkphysicalMostly software implementedGuaranteed delivery of application layer messagesDefines fields in IP datagrams (destination address),how end systems and routers act on themMoves packets from one node (host or packet switch) to the next nodeEthernet & ATM cards implement both link and Physical LayersMove individual bits within frame from one node to the next
94Internet structure: network of networks roughly hierarchicalat center: small # of well-connected large networks“tier-1” commercial ISPs (e.g., Verizon, Sprint, AT&T, Qwest, Level3), national & international coveragelarge content distributors (Google, Akamai, Microsoft)treat each other as equals (no charges)IXPTier 1 ISPTier-1 ISPs &Content Distributors, interconnect (peer) privatelyLarge ContentDistributor(e.g., Google)Large ContentDistributor(e.g., Akamai)Tier 1 ISPTier 1 ISP… or at Internet Exchange Points IXPsIntroduction 1-94
96Chapter 1 – True or False Questions ExercisesChapter 1 – True or False Questions
97ExercisesWe are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km.Initially suppose there is only one link between source and destination. Also suppose that message switching is used, with the message consisting of the entire MP3 file. The transmission delay3 seconds3.05 seconds50 millisecondsnone of the above.3 SEC
98ExercisesWe are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km.2 . Referring to the above question, the end-to-end delay (transmission delay plus propagation delay) is3.05 seconds3 seconds6 secondsnone of the aboveA) 3.05
99ExercisesWe are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km.Referring to the above question, how many bits will the source have transmitted when the first bit arrives at the destination.1 bit30,000,000 bits500,000 bitsnone of the aboveC) 500,000 BITS
100ExercisesWe are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km.Now suppose there are two links between source and destination, with one router connecting the two links. Each link is 5,000 km long. Again suppose the MP3 file is sent as one message. Suppose there is no congestion, so that the message is transmitted onto the second link as soon as the router receives the entire message. The end-to-end delay is3.05 seconds6.1 seconds6.05 secondsnone of the aboveC) 6.05 seconds