3rd Edition, Chapter 5 – The Fengs - Thefengs.ppt

Chapter 5Link Layer and LANsA note on the use of these ppt slides:Were making these slides freely available to all(faculty,students,readers).Theyre in PowerPoint form so you can add,modify,and delete slides (including this one)and slide content to suit your needs.They obviously represent a lot of work on our part.In return for use,we only ask the following:q If you use these slides(e.g.,in a class)in substantially unaltered form,that you mention their source(after all,wed like people to use our book!)q If you post any slides in substantially unaltered form on a www site,that you note that they are adapted from(or perhaps identical to)our slides,and note our copyright of this material.Thanks and enjoy!JFK/KWRAll material copyright 1996-2007J.F Kurose and K.W.Ross,All Rights ReservedComputer Networking:A Top Down Approach 4th edition.Jim Kurose,Keith RossAddison-Wesley,July 2007.1Chapter 5:The Data Link LayerOur goals:runderstand principles behind data link layer services:merror detection,correctionmsharing a broadcast channel:multiple accessmlink layer addressingmreliable data transfer,flow control:done!rinstantiation and implementation of various link layer technologies2Link Layerr5.1 Introduction and servicesr5.2 Error detection and correction r5.3Multiple access protocolsr5.4 Link-Layer Addressingr5.5 Ethernet and other data link layersr5.6 Hubs and switchesr5.7 PPPr5.8 Link Virtualization:ATM and MPLS3Link Layer:IntroductionSome terminology:rhosts and routers are nodesrcommunication channels that connect adjacent nodes along communication path are linksmwired linksmwireless linksmLANsrlayer-2 packet is a frame,encapsulates datagramdata-link layer has responsibility of transferring datagram from one node to adjacent node over a link4Adaptors Communicatingrlink and physical layers implemented in adaptor/NIC(Network Interface Card)mRAM,DSP chips,host bus interface,and link interfacemEthernet card,PCMCIA card,802.11 cardrsending side:mencapsulates datagram in a framemadds error checking bits,rdt,flow control,etc.rreceiving sidemlooks for errors,rdt,flow control,etcmextracts datagram,passes to upper layer at receiving siderdatagram transferred by different link protocols over different links:me.g.,Ethernet on first link,frame relay on intermediate links,802.11 on last linksendingnodeframercvingnodedatagramframeadapteradapterlink layer protocol5Protocol stack pictureapplicationtransportnetworklinkphysicalnetworklinkphysicalMMMMHtHtHnHtHnHlMHtHnHlframephys.linkdata linkprotocoladapter card6Host adaptorcontrollerphysicaltransmissioncpumemoryhost bus(e.g.,PCI)network adaptercardhost schematicapplicationtransportnetworklinklinkphysical7Link Layer FunctionsrFlow Controlmpacing between adjacent sending and receiving nodesrReliable delivery between adjacent nodesmwe learned how to do this already(chapter 3)!mseldom used on low bit error link(i.e.fiber,twisted pair)mwireless links:high error ratesEschew end-to-end principle for performancerSecurityrDemux to upper protocolrFramingmencapsulate datagram into frame,adding header,trailer8Link Layer Functions(more)rError Detectionrerrors caused by signal attenuation,noise.rreceiver detects presence of errors:rsignals sender for retransmission or drops frame rError Correctionrreceiver identifies and corrects bit error(s)without resorting to retransmissionrMedium access and quality of servicerchannel access if shared mediumrAddressingr“MAC addresses used in frame headers to identify source,dest (different from IP address)9Flow controlrPacing between sender and receiverrSender prevented from overrunning receivermReady-To-Send,Clear-To-Send signalling10Reliable deliveryrReliability at the link layerrHandled in a similar manner to transport protocolsmARQ,Stop-and-wait,Go-back-N,Selective RepeatrWhen and why should this be used?mRarely done over twisted-pair or fiber optic linksmUsually done over lossy links for performance improvement(versus correctness)11SecurityrMainly for broadcast data-link layersmEncrypt payload of higher layersmHide IP source/destination from eavesdroppersmImportant for wireless LANs especiallyParking lot attacks with 802.11bWEP,WPArIf time permits,security will be covered at the end of the course.12Demux to upper protocolrProtocol type specification interfaces to network layerrData-link layer can support any number of network layersmType field in data-link header specifies network layer of packetmEach data-link layer defines its own protocol type numbering for network layermIP is one of many network layers 13Demux to upper protocolr :/cavebear /CaveBear/Ethernet/type.htmlrSome Ethernet protocol typesr0800 DOD Internet Protocol(IP)r0806 Address Resolution Protocol(ARP)r8037 IPX(Novell Netware)r80D5 IBM SNA Servicesr809B EtherTalk(AppleTalk over Ethernet)14FramingrData encapsulation for transmission over physical linkrData embedded within a link-layer frame before transmissionrData-link header and/or trailer addedrPhysical addresses used in frame headers to。