09-Network Layer, IP, NATNetwork Layer4-1Network layer transport segment from sending to receiving host d d applicationtransportnt k on sending side encapsulates segments into datagramsnetworknetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalgm on receiving side, delivers segments to tt ltknetworkdata linkphysicalnetworknetworkdata linkphysicaltransport layer network layer protocols in everyhost, routernetworkdata linkphysicaldata linkphysicalnetworkdata linkphysicalapplicationtransportnetworkin everyhost, router Router examines header fields in all IP datagrams i th h itnetworkdata linkphysicalNetwork Layer4-2passing through itKey Network-Layer FunctionsKey NetworkLayer Funct onsforwarding:move analogy:forwarding:move packets from router’s input to appropriate analogy: routing: process of li ti f pppprouter outputrouting:determine planning trip from source to destrouting:determine route taken by packets from source  forwarding: process of getting through il i hpto dest. Routing algorithmssingle interchangeNetwork Layer4-3Routing algorithmsInterplay between routing and forwardingrouting algorithmlocal forwarding tableheader valueoutput link010001013201010111100122110111value in arrivingpacket’s header23Network Layer4-4Network layer connection and til ss s iconnection-less serviceDatagram networkprovides network-layer Datagram networkprovides networklayer connectionless serviceVirtual Circuit network provides network-player connection 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 Layer4-5Virtual circuitsVirtual circuits“source-to-dest path behaves much like telephone circuit” performance-wisenetwork actions along source-to-dest path call setup, teardown for each call beforedata can flownetwork 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 Layer4-6VC 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 Layer4-7New 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!3 35 2 ?Network Layer4-8Routers maintain connection state information!Forwarding tableForward ng tableA 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 #?Network Layer4-9Datagram 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 addresspackets between same source-dest pair may take packets between same source-dest pair may take different pathsapplicationtransportnetworkapplicationtransportnetworkdata linkphysicalpnetworkdata linkphysical1. Send data2. Receive dataNetwork Layer4-10Forwarding table4 billion possible entriesForward ng tableDestinationAddressRangeLinkInterfacepossible entriesDestinationAddressRangeLinkInterface11001000 00010111 00010000 00000000through 0110010000001011100010111111111111100100000010111000101111111111111001000 00010111 00011001 00000000through 1110010000001011100011111111111111100100000010111000111111111111111001000 00010111 00011000 00000000through 21100100000010111000110001111111111001000000101110001100011111111otherwise 3Network Layer4-11Longest prefix matchingLongest pref x match ngPrefix Match Link Interface11001000 00010111 00010 011001000 00010111 00011 111001000 00010111 00011 000 2otherwise 3ExamplesExamplesDA: 11001000 00010111 00010110 10100001 Which interface?DA: 11001000 00010111 00011000 10101010 Which interface?Network Layer4-12The Internet Network layeryHost, router network layer functions:Routing protocolsIP protocolTransport layer: TCP, UDPf dinRouting protocols•path selection•RIP, OSPF, BGPp•addressing conventions•datagram format•packet handling conventionsNetworklayerforwardingtableICMP protocol•error reporting•router “signaling”layerLink layerphysical layerNetwork Layer4-13IP datagram formatIP t l iverlength32 bitsIP protocol versionnumberheader length(bytes)fortotal datagramlength (bytes)head.lentype ofservice16-bit identifierheaderchecksumtime tolivemax numberremaining hopsforfragmentation/reassembly“type” of data flgsfragmentoffsetupperlayerchecksum32 bit source IP addressgp(decremented at each router)upper layer protocoly32 bit destination IP addressdata (ibl lnthupper layer protocolto deliver payload toOptions (if any)E.g. timestamp,record routetaken, specifylist of routers (variable length,typically a TCP or UDP segment)list of routers to visit, don’tfragment bit.Network Layer4-14IP Fragmentation & ReassemblyFgm & my network links have MTU (max.transfer size) - largest ibl li kll fpossible link-level frame. different link types, different MTUs large IP datagram divided fragmentation: in: one large datagramout:3 smaller datagramslarge IP datagram divided (“fragmented”) within net one datagram becomes several datagramsout:3 smaller datagramsseveral datagrams “reassembled” only at final destination IP header bits used to reassemblyidentify, order related fragmentsNetwork Layer4-15IP Fragmentation and ReassemblyFgm myEx mplExample 4000 byte datagramMT 1500 bMTU = 1500 bytes•length–number of bytes in packetgfy p•ID– 16 bit identifier•fragflag • 0 => no more fragments• 1 => more fragments to