09-Network Layer, IP, NATNetwork layerKey Network-Layer FunctionsSlide 4Network layer connection and connection-less serviceVirtual circuitsVC implementationForwarding tableSlide 9Datagram networksSlide 11Longest prefix matchingThe Internet Network layerIP datagram formatIP Fragmentation & ReassemblyIP Fragmentation and ReassemblyIP Addressing: introductionSubnetsSlide 19Slide 20“Classful” addressingIP addressing: CIDRIP addresses: how to get one?Slide 24Hierarchical addressing: route aggregationHierarchical addressing: more specific routesIP addressing: the last word...NAT: Network Address TranslationSlide 29Slide 30Slide 31Private network addresses (RFC 3330)Circumventing the NAT firewall (if you must)Slide 34Network Layer 4-109-Network Layer, IP, NATNetwork Layer 4-2Network layertransport segment from sending to receiving host on sending side encapsulates segments into datagramson receiving side, delivers segments to transport layernetwork layer protocols in every host, routerRouter examines header fields in all IP datagrams passing through itnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalNetwork Layer 4-3Key Network-Layer Functionsforwarding: move packets from router’s input to appropriate router outputrouting: determine route taken by packets from source to dest. Routing algorithmsanalogy:routing: process of planning trip from source to destforwarding: process of getting through single interchangeNetwork Layer 4-41230111value in arrivingpacket’s headerrouting algorithmlocal forwarding tableheader valueoutput link01000101011110013221Interplay between routing and forwardingNetwork Layer 4-5Network layer connection and connection-less serviceDatagram network provides network-layer connectionless serviceVirtual Circuit network provides network-layer connection serviceAnalogous to the transport-layer services, but:Service: host-to-hostNo choice: network provides one or the otherImplementation: in the coreNetwork Layer 4-6Virtual circuitscall setup, teardown for each call before data can floweach packet carries VC identifier (not destination host address)every router on source-dest path maintains “state” for each passing connectionlink, router resources (bandwidth, buffers) may be allocated to VC“source-to-dest path behaves much like telephone circuit”performance-wisenetwork actions along source-to-dest pathNetwork Layer 4-7VC implementationA VC consists of:1. Path from source to destination2. VC numbers, one number for each link along path3. Entries in forwarding tables in routers along pathPacket belonging to VC carries a VC number.VC number must be changed on each link.New VC number comes from forwarding tableNetwork Layer 4-8Forwarding table122232132VC numberinterfacenumberIncoming interface Incoming VC # Outgoing interface Outgoing VC #1 12 2 222 63 1 18 3 7 2 171 97 3 87… … … …Forwarding table innorthwest router:Routers maintain connection state information! 3 35 2 ?Forwarding tableA connection request arrives at the router from port 3 with an incoming VC # of 35. The router picks output port 2 to forward the packet. What should be the value of the outgoing VC #?Network Layer 4-9Network Layer 4-10Datagram 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 different pathsapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysical1. Send data2. Receive dataNetwork Layer 4-11Forwarding table Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111 11001000 00010111 00011001 00000000 through 1 11001000 00010111 00011111 11111111 11001000 00010111 00011000 00000000 through 2 11001000 00010111 00011000 11111111 otherwise 34 billion possible entriesNetwork Layer 4-12Longest prefix matching Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011 1 11001000 00010111 00011 000 2 otherwise 3DA: 11001000 00010111 00011000 10101010 ExamplesDA: 11001000 00010111 00010110 10100001 Which interface?Which interface?Network Layer 4-13The Internet Network layerforwardingtableHost, router network layer functions:Routing protocols•path selection•RIP, OSPF, BGPIP protocol•addressing conventions•datagram format•packet handling conventionsICMP protocol•error reporting•router “signaling”Transport layer: TCP, UDPLink layerphysical layerNetworklayerNetwork Layer 4-14IP datagram formatverlength32 bitsdata (variable length,typically a TCP or UDP segment)16-bit identifierheader checksumtime tolive32 bit source IP addressIP protocol versionnumberheader length (bytes)max numberremaining hops(decremented at each router)forfragmentation/reassemblytotal datagramlength (bytes)upper layer protocolto deliver payload tohead.lentype ofservice“type” of data flgsfragment offsetupper layer32 bit destination IP addressOptions (if any)E.g. timestamp,record routetaken, specifylist of routers to visit, don’t fragment bit.Network Layer 4-15IP Fragmentation & Reassemblynetwork links have MTU (max.transfer size) - largest possible link-level frame.different link types, different MTUs large IP datagram divided (“fragmented”) within netone datagram becomes several datagrams“reassembled” only at final destinationIP header bits used to identify, order related fragmentsfragmentation: in: one large datagramout: 3 smaller datagramsreassemblyNetwork Layer 4-16IP Fragmentation and ReassemblyExample4000 byte datagramMTU = 1500 bytes• length – number of bytes in packet• ID – 16 bit identifier• fragflag • 0 => no more fragments• 1 => more