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Rutgers University ECE 544 - Communication Networks II

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ECE544: Communication Networks-II, Spring 2010Today’s LectureIP BasicsIP InternetService ModelFragmentation and ReassemblyExampleGlobal AddressesDatagram ForwardingAddress TranslationARP DetailsARP Packet FormatATM ARPDynamic Host Control Protocol (DHCP)Slide 15Internet Control Message Protocol (ICMP)Routing BasicsRouting ProblemTwo main approachesDistance Vector ProtocolsDistance VectorDistributed Bellman-FordExample - initial distancesE receives D’s routesE updates cost to CA receives B’s routesA updates cost to CA receives E’s routesA updates cost to C and DFinal distancesFinal distances after link failureView from a nodeDistance Vector ExampleDV Example (cont.)Slide 35DV Example – after link R2-R3 breaksSlide 37Slide 38The bouncing effectC sends routes to BB updates distance to AB sends routes to CSlide 43How are these loops caused?Avoiding the Bouncing EffectComputing Implicit PathsDistance Vector in PracticeLink State RoutingLink State Routing: Building blocksLink state packets (LSPs)Reliable floodingSPT algorithm (Dijkstra)Link State AlgorithmDijkstra/OSPF Method 1Slide 55Slide 56Dijkstra SPT Method 2Slide 58Slide 59Slide 60Slide 61Slide 62Link State in PracticeOSPF PacketsLink State CharacteristicsOSPF Sequencing and AgingProblem: Router FailureOne solution: LSP AgingToday’s HomeworkECE544: Communication Networks-II, Spring 2010D. RaychaudhuriLecture 4Includes teaching materials from L. PetersonToday’s Lecture•IP basics•Routing principles–distance vector (RIP)–link state (OSPF)IP Basics Best Effort Service ModelGlobal Addressing SchemeARP & DHCPIP Internet •Concatenation of Networks•Protocol StackR2R1H4H5H3H2H1Network 2 (Ethernet)Network 1 (Ethernet)H6Network 3 (FDDI)Network 4(point-to-point)H7 R3 H8R1ETHFDDIIPIPETHTCPR2FDDIPPPIPR3PPPETHIPH1IPETHTCPH8Service Model•Connectionless (datagram-based)•Best-effort delivery (unreliable service)–packets are lost–packets are delivered out of order–duplicate copies of a packet are delivered–packets can be delayed for a long time•Datagram formatVersion HLenTOS LengthIdent Flags OffsetTTL Protocol ChecksumSourceAddrDestinationAddrOptions (variable)Pad(variable)0 4 8 16 19 31DataFragmentation and Reassembly•Each network has some MTU•Strategy–fragment when necessary (MTU < Datagram)–try to avoid fragmentation at source host–re-fragmentation is possible –fragments are self-contained datagrams–use CS-PDU (not cells) for ATM–delay reassembly until destination host–do not recover from lost fragmentsExample H1 R1 R2 R3 H8ETH IP (1400) FDDI IP (1400) PPP IP (512)PPP IP (376)PPP IP (512)ETH IP (512)ETH IP (376)ETH IP (512)Ident = x Offset = 0Start of header0Rest of header1400 data bytesIdent = x Offset = 0Start of header1Rest of header512 data bytesIdent = x Offset = 512Start of header1Rest of header512 data bytesIdent = x Offset = 1024Start of header0Rest of header376 data bytesGlobal Addresses•Properties–globally unique–hierarchical: network + host•Dot Notation–10.3.2.4–128.96.33.81–192.12.69.77Network Host7 240A:Network Host14 161 0B:Network Host21 81 1 0C:Datagram Forwarding •Strategy–every datagram contains destination’s address–if directly connected to destination network, then forward to host–if not directly connected to destination network, then forward to some router–forwarding table maps network number into next hop–each host has a default router–each router maintains a forwarding table•Example (R2) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0Address Translation •Map IP addresses into physical addresses–destination host–next hop router•Techniques–encode physical address in host part of IP address–table-based•ARP–table of IP to physical address bindings–broadcast request if IP address not in table–target machine responds with its physical address–table entries are discarded if not refreshedARP Details •Request Format–HardwareType: type of physical network (e.g., Ethernet)–ProtocolType: type of higher layer protocol (e.g., IP)–HLEN & PLEN: length of physical and protocol addresses–Operation: request or response –Source/Target-Physical/Protocol addresses•Notes–table entries timeout in about 10 minutes–update table with source when you are the target –update table if already have an entry–do not refresh table entries upon referenceARP Packet FormatTargetHardwareAddr (bytes 2 – 5)TargetProtocolAddr (bytes 0 – 3)SourceProtocolAddr (bytes 2 – 3)Hardware type = 1 ProtocolT ype = 0x0800SourceHardwareAddr (bytes 4 – 5)TargetHardwareAddr (bytes 0 – 1)SourceProtocolAddr (bytes 0 – 1)HLen = 48 PLen = 32 OperationSourceHardwareAddr (bytes 0 – 3)0 8 16 31ATM ARPH2RH1LIS 10LIS 12ATM network10.0.0.210.0.0.112.0.0.312.0.0.5•ATM ARP for mapping IP<->ATM addr–medium is not a broadcast type unlike Ethernet–requires servers which maintain ARP tables–concept of multiple “logical IP subnets” (LIS)Dynamic Host Control Protocol (DHCP)•DHCP server per network for IP address assignment•Static list of IP<->physical addr or dynamic binding from common pool•Host boot-up via well-known address 255.255.255.255•DHCP “relay agent” can be used to avoid one server per networkDynamic Host Control Protocol (DHCP)•DHCP packet format (runs over UDP) Operation HType HLen HopsXidSecsFlagciaddryiaddrsiaddrgiaddrchaddr (16B)....Internet Control Message Protocol (ICMP)•Echo (ping)•Redirect (from router to source host)•Destination unreachable (protocol, port, or host)•TTL exceeded (so datagrams don’t cycle forever)•Checksum failed •Reassembly failed•Cannot fragmentRouting BasicsRouting Problem•Network as a GraphProblem: Find lowest cost path between two nodes•Factors–static: topology–dynamic: load43621911DAFEBCTwo main approaches•DV: Distance-vector protocols•LS: Link state protocols•Variations of above methods applied to:–Intra-domain routing (small/med networks)•RIP, OSPF–Inter-domain routing (large/global networks)•BGP-4Distance Vector Protocols•Employed in the early Arpanet•Distributed next hop computation–adaptive•Unit of information exchange –vector of distances to destinations•Distributed Bellman-Ford AlgorithmDistance Vector•Each node maintains a set of triples –(Destination, Cost, NextHop)•Exchange updates directly connected neighbors–periodically (on the order of several seconds)–whenever table


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