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CSE 30264 Computer Networks

CSE 30264 Computer Networks. Prof. Aaron Striegel Department of Computer Science & Engineering University of Notre Dame Lecture 11 – February 16, 2010. Today’s Lecture. Internetworking Local Network NAT ICMP Routing Routing in the WAN. Application. Transport. Network. Data.

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CSE 30264 Computer Networks

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  1. CSE 30264Computer Networks Prof. Aaron Striegel Department of Computer Science & Engineering University of Notre Dame Lecture 11 – February 16, 2010

  2. Today’s Lecture • Internetworking • Local Network • NAT • ICMP • Routing • Routing in the WAN Application Transport Network Data Physical CSE 30264

  3. Internetworking Outline Local Network Routing CSE30264

  4. Routing Table – netscale01 Local or next hop? Mask result Interface to pass to CSE 30264

  5. Datagram Forwarding • Strategy • Always have dest address • In IP header • Two choices • Local network (subnet) • Pass it off directly • Not on my local network • Pass to some router • Routing table • Maps network to next hop • Routing entries • Default router How do I find that next hop? Have IP, need Ethernet CSE30264

  6. Address Translation • Layer 3 -> Layer 2 Translation • IP -> Ethernet • Destination host • Next hop router • Techniques • Do nothing • Make Layer 2 part of Layer 3 address • Table • Convert Layer 3 to Layer 2 • ARP – Address Resolution Protocol • Broadcast, observe, refresh CSE30264

  7. Example – ARP Table CSE 30264

  8. ARP Key Concepts • Broadcast request • Send to FF:FF:FF:FF:FF:FF • Who is 129.74.50.20? • Says AA:BB:CC:DD:EE:FF of 129.74.153.157 • Response • Respond directly or broadcast • I am 01:34:57:AB:CD:EF, aka 129.74.50.20 • Interesting concepts • Broadcast locally only – not beyond the router • Soft state • Old entries periodically expire • Nosy neighbor • Note ARP requests of others CSE 30264

  9. ARP Details • Request Format • HardwareType: type of physical network (e.g., Ethernet) • ProtocolType: type of higher layer protocol (e.g., IP) • HLEN: Length of physical (hardware) address • PLEN: Length of protocol addresses • Operation: request or response • Source/Target-Physical/Protocol addresses • Notes • Table entries timeout in about 15 minutes • Update table with source when you are the target • Update table if already have an entry • Do not refresh table entries upon reference CSE30264

  10. 0 8 16 31 Hardware type = 1 ProtocolType = 0x0800 HLen = 48 PLen = 32 Operation SourceHardwareAddr (bytes 0 - 3) SourceHardwareAddr (bytes 4 – 5) SourceProtocolAddr (bytes 0 – 1) SourceProtocolAddr (bytes 2 - 3) TargetHardwareAddr (bytes 0 – 1) TargetHardwareAddr (bytes 2 – 5) TargetProtocolAddr (bytes 0 – 3) ARP Packet Format CSE30264

  11. Observe the Packets Wireshark www.wireshark.org CSE 30264

  12. Addressing • How do I get my initial IP address? • Static IP • System Administrator • Privatenetwork space • Know IP, mask, subnet, gateway, DNS • Dynamic IP  BOOTP / DHCP • I don’t know my IP, please tell me • Two flavors • Anybody can come in • Authenticated / MAC filtered CSE 30264

  13. DHCP • Dynamic Host Configuration Protocol CSE30264

  14. DHCP CSE30264

  15. More on DHCP • Uses UDP • Why not TCP? • Nearly all sites use DHCP • Static  server only • Other notes • Lease time for IP address • Variety of option fields • Thin clients CSE 30264

  16. NAT – Network Address Translation • Limited public IP space • Share addresses Xfinity ??? The larger Internet I’ll give you one IP address via DHCP CSE 30264

  17. Enter the NAT • Translate private address to public • Private network space • 10.* • 192.168.* • Translate via table • Use connection tuple • Src IP, Dst IP, Src Port, Dst Port DHCP Server 192.168.1.100 67.56.43.2 192.168.1.1 192.168.1.101 CSE 30264

  18. NAT Table • Must originate from inside • Map internal tuple to external tuple S: 192.168.1.100 D: 25.78.200.3 TCP SP: 9876 DP: 80 S: 67.56.43.2 D: 25.78.200.3 TCP SP: 42778 DP: 80 Mapping Table Ext: 67.56.43.2:42778 25.78.200.3:80 Int: 192.168.1.100:9876 25.78.200.3.80 67.56.43.2 192.168.1.1 CSE 30264

  19. Mapping Table • Maintain a list of all active connections • Map across the boundary • New connection • Pick an unused external port • Issues • Reach internal host from external (SSH) • DMZ – One host becomes a catch all • Scalability • UDP • IPsec / VPN CSE 30264

  20. 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 fragment CSE30264

  21. Routing Outline Distance Vector Link State CSE30264

  22. Overview • Forwarding vs Routing • forwarding: to select an output port based on destination address and routing table • routing: process by which routing table is built • Network as a Graph • Problem: Find lowest cost path between two nodes • Factors • static • dynamic CSE30264

  23. Distance Vector • Each node maintains a set of triples • (Destination, Cost, NextHop) • Directly connected neighbors exchange updates • periodically (on the order of several seconds) • whenever table changes (called triggered update) • Each update is a list of pairs: • (Destination, Cost) • Update local table if receive a “better” route • smaller cost • came from next-hop • Refresh existing routes; delete if they time out CSE30264

  24. Example Destination Cost NextHop A 1 A C 1 C D 2 C E 2 A F 2 A G 3 A CSE30264

  25. Routing Loops • Example 1 • F detects that link to G has failed • F sets distance to G to infinity and sends update t o A • A sets distance to G to infinity since it uses F to reach G • A receives periodic update from C with 2-hop path to G • A sets distance to G to 3 and sends update to F • F decides it can reach G in 4 hops via A • Example 2 • link from A to E fails • A advertises distance of infinity to E • B and C advertise a distance of 2 to E • B decides it can reach E in 3 hops; advertises this to A • A decides it can read E in 4 hops; advertises this to C • C decides that it can reach E in 5 hops… CSE30264

  26. Loop-Breaking Heuristics • Set infinity to 16 • Split horizon • Split horizon with poison reverse CSE30264

  27. Routing Information Protocol (RIP) • Distributed along with BSD Unix • Straightforward implementation of DV • Updates sent every 30 seconds • Link costs constant at 1 (16 = infinity) CSE30264

  28. Link State • Strategy • send to all nodes (not just neighbors) information about directly connected links (not entire routing table) • Link State Packet (LSP) • id of the node that created the LSP • cost of link to each directly connected neighbor • sequence number (SEQNO) • time-to-live (TTL) for this packet CSE30264

  29. Link State (cont) • Reliable flooding • store most recent LSP from each node • forward LSP to all nodes but one that sent it • generate new LSP periodically • increment SEQNO • start SEQNO at 0 when reboot • decrement TTL of each LSP • discard when TTL=0 CSE30264

  30. Link State (cont) CSE30264

  31. Route Calculation • Dijkstra’s shortest-path algorithm • Let • N denotes set of nodes in the graph • l (i, j) denotes non-negative cost (weight) for edge (i, j) • s denotes this node • M denotes the set of nodes incorporated so far • C(n) denotes cost of the path from s to node n M = {s} for each n in N - {s} C(n) = l(s, n) while (N != M) M = M union {w} such that C(w) is the minimum for all w in (N - M) for each n in (N - M) C(n) = MIN(C(n), C (w) + l(w, n )) CSE30264

  32. OSPF • Open Shortest Path First Protocol • Authentication • Additional hierarchy • Load balancing CSE30264

  33. Metrics • Original ARPANET metric • measures number of packets queued on each link • took neither latency or bandwidth into consideration • New ARPANET metric • stamp each incoming packet with its arrival time (AT) • record departure time (DT) • when link-level ACK arrives, compute Delay = (DT - AT) + Transmit + Latency • if timeout, reset DT to departure time for retransmission • link cost = average delay over some time period • Revised ARPANET metric • compressed dynamic range • replaced Delay with link utilization • Practice • static metrics (e.g., 1/bandwidth) CSE30264

  34. Routing for Mobile Hosts • Mobile IP: • home agent, home address, foreign agent • triangle routing problem CSE30264

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