10 - Network LayerNetwork layer transport segment from sending to receiving host d d applicationtransportnt k on sending side encapsulates segments into datagramsnetworknetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalgm on rcving side, delivers segments to transport ltknetworkdata linkphysicalnetworknetworkdata linkphysicallayer network layer protocols in everyhost, routernetworkdata linkphysicaldata linkphysicalnetworkdata linkphysicalapplicationtransportnetworkin everyhost, router Router examines header fields in all IP datagramsi th h itnetworkdata linkphysicalpassing through itKey Network-Layer FunctionsKey NetworkLayer Funct onsforwarding:move analogy:forwarding:move packets from router’s input to appropriate analogy: routing: process of li ti f pppprouter outputrouting:determine planning trip from source to destrouting:determine route taken by packets from source forwarding: process of getting through il i hpto dest. Routing algorithmssingle interchangeRouting algorithmsInterplay between routing and forwardingrouting algorithmlocal forwarding tableheader valueoutput link010001013201010111100122110111value in arrivingpacket’s header23Connection setupConnect on setup3rdimportant function in somenetwork 3important function in somenetwork architectures: ATM, frame relay, X.25y Before datagrams flow, two hosts and intervening routers establish virtual connection Routers get involvedk d l Network and transport layer cnctn service: Network: between two hostsTtbt t Transport:between two processesNetwork service modelNetwork serv ce modelQ: What service modelfor “channel” transporting dt f d t i ?datagrams from sender to receiver?Example services for Example services for a flow of datagrams:individual datagrams: guaranteed deliveryGuaranteed delivery flow of datagrams: In-order datagram deliveryGuaranteed delivery with less than 40 msec delay i.e. bounded y Guaranteed minimum bandwidth to flowdelayNetwork layer service models:ymNetworkAhit tServiceMdlBdidthNoOdTi iCongestionfdbkGuarantees ?ArchitectureInternetModelbest effortBandwidthnoneNoLossnoOrdernoTimingnofeedbackno (inferredvia loss)ATMATMCBRABRconstantrateguaranteedyesyesyesyesnovia loss)nocongestionyesATMABRguaranteed minimumnoyesnoyesNetwork layer connection and til ss s iconnection-less serviceDatagram networkprovides network-layer Datagram networkprovides networklayer connectionless service VC network provides network-layer pyconnection service Analogous to the transport-layer services, gpy,but: Service: host-to-host No choice: network provides one or the other Implementation: in the coreNetwork Layer4-8Virtual circuitsVirtual circuits“source-to-dest path behaves much like telephone circuit” performance-wisenetwork actions along source-to-dest path call setup, teardown for each call beforedata can flownetwork actions along sourcetodest path each packet carries VC identifier (not destination host address)everyrouter on source-dest path maintains “state” for each passing connection link, router resources (bandwidth, buffers) may be allocated to VCNetwork Layer4-9VC implementationVC mplementat onA VC consists of:A VC consists of:1. Path from source to destination2. VC numbers, one number for each link along gpath3. Entries in forwarding tables in routers along pathpath Packet belonging to VC carries a VC numbernumber. VC number must be changed on each link.New VC number comes from forwarding tableNetwork Layer4-10New VC number comes from forwarding tableForwarding tableVC number1222321323interfacenumberForwarding table innth st t:Incoming interface Incoming VC # Outgoing interface Outgoing VC #1 12 2 22northwest router:1 12 2 222 63 1 18 3 7 2 171 97 3 87 … … … …R ii i ifi!Network Layer4-11Routers maintain connection state information!Forwarding tableForward ng tableA connection request arrives at the router A connection request arrives at the router from port 3 with an incoming VC # of 35. The router picks output port 2 to forward pppthe packet. What should be the value of the outgoing VC #?Transport Layer3-12Virtual circuits: signaling protocolsggpused to setup maintain teardown VCused to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internetylapplicationtransportnetworkdata linkapplicationtransportnetwork1. Initiate call2 incoming call3. Accept call4. Call connected5. Data flow begins6. Receive datadata linkphysicaldata linkphysical. n t at ca2. incoming callNetwork Layer4-13Datagram networks no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host addresspackets between same source-dest pair may take packets between same source-dest pair may take different pathsapplicationtransportnetworkapplicationtransportnetworkdata linkphysicalpnetworkdata linkphysical1. Send data2. Receive dataNetwork Layer4-14Forwarding table4 billion possible entriesForward ng tableDtitiAddRLi kIt fpossible entriesDestinationAddressRangeLinkInterface11001000 00010111 00010000 00000000through 011001000 00010111 00010111 1111111111001000 00010111 00011000 00000000through1g11001000 00010111 00011000 1111111111001000 00010111 00011001 00000000through2through211001000 00010111 00011111 11111111otherwise 3Network Layer4-15Longest prefix matchingLongest pref x match ngPrefix Match Link Interface11001000 00010111 00010 011001000 00010111 00011 111001000 00010111 00011000 2otherwise 3ExamplesExamplesDA: 11001000 00010111 00010110 10100001 Which interface?DA: 11001000 00010111 00011000 10101010 Which interface?Network Layer4-16Datagram or VC network: why?gyInternetATM data exchange among computers “elastic” service, no strict evolved from telephony human conversation: strict timing reliability timing req. “smart” end systems (computers)strict timing, reliability requirements need for guaranteed service can adapt, perform control, error recovery simple inside network, service “dumb” end systems telephonesli iid p,complexity at
View Full Document