1 Chapter 3: Networking and Internetworking From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 3, © Addison-Wesley 2001 Presentation based on slides by Coulouris et al; modified by Jens B Jørgensen and Jonas Thomsen, University of Aarhus

2 Networks – basics zA network consists of: yTransmission media (wire, cable, …). yHardware devices (routers, switches, …). ySoftware components (protocol stacks, drivers, …). yAll together defines a communication subsystem. zTerminology: yHost: Computers and other devices that use a network. yNode: Any computer or switching device attached to a network. ySubnet: Set of interconnected nodes xUnit of routing xCollection of nodes that can be reached on the same physical network

3 Networks – Design issues (1/2) zPerformance y(Network) Latency – time for one bit to traverse the network. xSoftware overheads, routing delays, load, physical distance yData Transfer Rate – how many bits pr. sec. can traverse the network. xPhysical characteristics of the network. yOverload lead to degrading performance. zScalability yAddress range, routing tables. zReliability yThe physical media is generally reliable (except for wireless) yLow level vs. application level reliability (mostly application)

4 Networks – Design issues (2/2) zSecurity yA need to produce a secure network environment xFirewalls (protect resources inside, control the use of external resources, runs on a gateway) zMobility yMovement of mobile devices require addressing and routing schemes to adapt. zQuality of service yMeet deadlines, guaranteed bandwidth, bounded latencies yDynamically: Specify minimum acceptable qos. and desired optimum. zMulticasting yTypical communication in between pairs of processes yNeed for one-to-many communication (N x one-to-one is not enough)

5 Networks – types zLAN: Local Area Network yNo routing, shared bandwidth, low latency, speeds like disc access xEthernet, token ring zWAN: Wide Area Network yDifferent organization, long distance, routing, high latency xTelephone networks, dedicated links, internet backbone zMAN: Metropolitan Area Network yLimited distance xxDSL, cable modem, fibre xADSL, StofaNet, Bolignet Aarhus, Djurslandsnet zWireless Network yWLAN (IEEE x) yWPAN (infrared, Bluetooth) yMobile networks (GSM, GPRS, UMTS) zInternetworks yLinking of different networks to provide common data communication facilities, independent of involved components, routers / gateways

6 Networks – Comparison of types RangeBandwidth (Mbps)Latency (ms) LAN1-2 kms10 – – 10 WANworldwide0.010 – [1] 100 – 500 MAN2-50 kms1 – Wireless LAN km2 – 54 [2] 5 – 20 Wireless WANworldwide0.010 – 2100 – 500 Internetworldwide0.010 – 2100 – 500 [1]: OC-192 over ATM: (OC-x (Optical Carier level x). OC-1 = 51,84 Mbps). [2]: IEEE a:

7 Networks – packet transmission zMessage: Sequence of data items of arbitrary length. zMessages subdivided into packets of restricted length yAllocate sufficient buffer space yAvoid occupying communication channel for long time during large transfers. zSwitching schemes: yBroadcast xEverything is transmitted to every node (non-switched Ethernet) yCircuit switching xPlain Old Telephone System yPacket switching xStore and forward yATM / Frame relay xAvoid switching delays, small packets (frames), switched after reception of a few bytes (5 bytes in ATM)

8 Protocols – basics zProtocol: Set of rules and formats to be used for communication between processes in order to perform a given task. zShould include specification of: ySequence of messages that must be exchanged. yFormat of the data in the messages. zImplemented by a pair of software modules in the sending and receiving computers.

9 Protocols – layers Layer n Layer 2 Layer 1 Message sent Message received Communication medium SenderRecipient zProvides a service to the layer above zExtends the service of the layer below

10 Protocols – encapsulation and headers

11 Protocols – the ISO Open Systems Interconnection (OSI) model zA framework for definition of protocols y – not a definition for a specific protocol!

12 Protocols – internetwork layers

13 Protocols – network layer routing (WAN) zPacket delivery: yDatagram xOne shot, different routes yVirtual Circuit x‘Call’ setup, same route during ‘call’ yDon’t confuse with Connection oriented / less!!! zRouting algorithms yRouters know next hop yAdaptive routing (change route if links break) zCongestion yQueues fill up, longer delays, dropped packets yControl: Informing nodes of congestion, choke packet, transmission control (TCP)

14 Internet protocols – Internetworking zInternetwork: Network which integrates a number of different subnets. zNeeds: yUnified internetwork addressing scheme (Internet: IP addresses) yProtocol defining format of internetwork packets and specifying rules for handling (Internet: IP protocol). yInterconnecting components that route packets to their destinations (Internet: Internet routers).

15 Internet protocols – the TCP/IP protocol suite Messages (UDP) or Streams (TCP) Application Transport Internet UDP or TCP packets IP datagrams Network-specific frames Message Layers Underlying network Network interface

16 Internet protocols – encapsulation and headers Application message TCP header IP header Ethernet header Ethernet frame port TCP IP

17 Internet protocols – IP zInternet Protocol. zTransmits datagrams from one host to another, if necessary via intermediate routers. zUnreliable, best-effort delivery semantics. zAddress resolution: Conversion of Internet addresses to network addresses (for a given network, e.g. ARP for Ethernet). zRouting: Each router in the Internet implements IP-layer software to provide a routing algorithm.

18 Internet protocols – IP packet layout and addressing

19 Internet protocols – TCP and UDP zBoth: Process to process communication zUDP features: yTransport-level replica of IP. yNo guarantee of delivery. yNo setup cost, no acknowledgement messages. yMessage size up to 64 Kbytes (8 Kbytes in praxis). zTCP features: yReliable delivery. yArbitrarily long sequences of bytes (stream). yConnection-oriented. yMechanisms: Sequencing, flow control, retransmission, buffering, checksum.

20 Ethernet (IEEE 802.3) – Basics zCarriers Sense Multiple Access with Collision Detection (CSMA/CD) zXerox – Ethernet zRandom Access y Stations access medium randomly z Contention yStations contend for time on medium

21 Ethernet – ALOHA zSender yGo ahead and send! yRetransmit if no ACK zProblems yCollisions yLow utilization (18%) zSlotted ALOHA is an improvement (max utilization 37%)

22 Ethernet – CSMA zCarrier Sense Multiple Access zObservations yPropagation time is much less than transmission time yAll stations know that a transmission has started almost immediately zSender yFirst listen for clear medium (carrier sense) yIf medium idle, transmit yIf two stations start at the same instant, collision yWait reasonable time yRetransmit if no ACK zMax utilization depends on propagation time (medium length) and frame length

23 Ethernet – CSMA/CD zCarrier Sense Multiple Access – Collision Detection zObservation: With CSMA, collision occupies medium for duration of transmission zSender yIf medium idle, transmit yIf busy, listen for idle, then transmit yStation listens whilst transmitting yIf collision detected, transmit jam signal, then cease transmission zAfter jam, wait random time then start again yBinary exponential back off

24 Ethernet – CSMA/CD Operation

25 Ethernet – Collision Detection zBus yCollision produces much higher signal voltage than signal yCollision detected if cable signal greater than single station signal zStar yActivity on more than one input is collision ySpecial collision presence signal

26 Summary zNetworks. zProtocols. zInternet protocols (TCP/IP). zEthernet