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Part 3.1

Part 3.1. Wide Area Networks (WANs), Routing, and Shortest Paths. Robert L. Probert, SITE, University of Ottawa. Motivation. Connect multiple computers Span large geographic distance Cross public right-of-way Streets Buildings Railroads. Building Blocks.

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Part 3.1

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  1. Part 3.1 Wide Area Networks (WANs), Routing, and Shortest Paths Robert L. Probert, SITE, University of Ottawa CSI 4118

  2. Motivation • Connect multiple computers • Span large geographic distance • Cross public right-of-way • Streets • Buildings • Railroads CSI 4118

  3. Building Blocks • Point-to-point long-distance connections • Packet switches CSI 4118

  4. Packet Switch • Hardware device • Connects to • Other packet switches • Computers • Forwards packets • Uses addresses CSI 4118

  5. Illustration of a Packet Switch • Special-purpose computer system • CPU • Memory • I/O interfaces • Firmware CSI 4118

  6. Building a WAN • Place one or more packet switches at each site • Interconnect switches • LAN technology for local connections • Leased digital circuits for long-distance connections CSI 4118

  7. Illustration of a WAN • Interconnections depend on • Estimated traffic • Reliability needed CSI 4118

  8. Store and Forward • Basic paradigm used in packet switched network • Packet • Sent from source computer • Travels switch-to-switch • Delivered to destination • Switch • “Stores” packet in memory • Examines packet’s destination address • “Forwards” packet toward destination CSI 4118

  9. Addressing in a WAN • Need • Unique address for each computer • Efficient forwarding • Two-part address • Packet switch number • Computer on that switch CSI 4118

  10. Illustration of WAN Addressing • Two part address encoded as integer • Higher-order bits for switch number • Low-order bits for computer number CSI 4118

  11. Next-Hop Forwarding • Performed by packet switch • Uses table of routes • Table gives next hop CSI 4118

  12. Forwarding Table Abbreviations • Many entries point to same next hop • Can be condensed (default) • Improves lookup efficiency CSI 4118

  13. Source of Routing Table Information • Manual • Table created by hand • Useful in small networks • Useful if routes never change • Automatic routing • Software creates/updates table • Needed in large networks • Changes routes when failures occur CSI 4118

  14. Relationship of Routing To Graph Theory • Graph • Node models switch • Edge models connection CSI 4118

  15. Shortest Path Computation • Algorithms from graph theory • No central authority (distributed computation) • A switch • Must learn route to each destination • Only communicates with directly attached neighbors CSI 4118

  16. Illustration of Minimum Weight Path • Label on edge represents “distance” • Possible distance metric • Geographic distance • Economic cost • Inverse of capacity • Darkened path is minimum 4 to 5 CSI 4118

  17. Algorithms for Computing Shortest Paths • Distance Vector (DV) • Switches exchange information in their routing tables • Link-state • Switches exchange link status information • Both used in practice CSI 4118

  18. Distance Vector • Periodic, two-way exchange between neighbors • During exchange, switch sends • List of pairs • Each pair gives (destination, distance) • Receiver • Compares each item in list to local routes • Changes routes if better path exists CSI 4118

  19. Distance Vector Algorithm CSI 4118

  20. Distance Vector Intuition • Let • N be neighbor that sent the routing message • V be destination in a pair • D be distance in a pair • C be D plus the cost to reach the sender • If no local route to V or local routed has cost greater than C, install a route with next hop N and cost C • Else ignore pair CSI 4118

  21. Example of Distance Vector Routing • Consider transmission of one DV message • Node 2 send to 3, 5, and 6 • Node 6 installs cost 8 route to 2 • Later 3 sends update to 6 • 6 changes route to make 3 the next hop for destination 2 CSI 4118

  22. Proposal Each node discovers its neighbors and tells one specific node (the leader) what they find. The leader runs an algorithm to solve the all-pairs shortest path problem, computing routing tables for each node. The leader communicates the routing table to each node, which stores it in non-volatile memory in each of the nodes. Question: What are the problems with this method? CSI 4118

  23. Distance Vector Routing Global view A’s view:vector CSI 4118

  24. Distance Vector Routing Every node starts by building it’s own local view of what nodes are 1 hop away. Next, every node sends its vector to its directly connected neighbors. F tells A that it can reach G at cost 1. A knows it can reach F at cost 1, so it updated its own vector to indicate that it can reach G at cost 2. If A were to discover another route to G at a cost higher than 2, it would ignore it and leave its vector as it is. After a few iterations of these exchanges, the routing table converges to a consistent state. Question: How would this method deal with link failures? CSI 4118

  25. Distance Vector Routing Periodic updates: Every t seconds, send your local info to your neighbors. This allows other nodes to know that you are running. Triggered updates: Every time you learn new information from a neighbor that leads you to update your local vector, you send the recomputed vector to all your neighbors. Question: How can you detect that a node has failed? CSI 4118

  26. The Count-to-infinity Problem Link (A,E) goes down. A periodic update kicks in and A advertises that its distance to E is infinity. At about the same time, B and C tell each other that they can reach E in 2 hops. B knows now that it can’t communicate with E via A, but concludes that it can send traffic to C which says it can reach A in 2 hops. Now, B is thinking that its distance to E is 3 hops and it lets A know about that. Argh, now A is going to think it can reach E in 4 hops and it will tell C of this “fact”… Where does this end? The routing table doesn’t converge or stabilize. CSI 4118

  27. Link-State Routing • Overcomes instabilities in DV • Pair of switches periodically • Test link between them • Broadcast link status message • Switch • Receives status message • Computes new routes • Uses Dijkstra’s algorithm CSI 4118

  28. Link State Routing Assumptions: Each node can discover the state of the link to its neighbors and the cost of each link. Basic principle: If every node knows how to reach its directly connected neighbors, one can spread the local knowledge of each node to all other nodes in the network so that every node can construct a the network weighted graph. With this knowledge, a node can always determine the shortest path to any other node in the network. CSI 4118

  29. Link State Routing Mechanisms • Reliable dissemination of link-state information aflooding. • Calculation of routes from the sum of all the accumulated link-state knowledge. CSI 4118

  30. Reliable Flooding Question: If you are a node and you want to tell your neighbors the state of the links incident to you, what should be contained in the information packets that you pass to them? Underlying Problems: • A packet should reflect the state of your links at a given point in time. • You need to ensure that old link-state information eventually stops circulating around the network. CSI 4118

  31. Reliable Flooding Question: How can one guarantee that the link-state packets sent from a node will definitely arrive at its neighbors? Question: How does one deal with the possibility of having multiple, contradictory copies of a node’s link-state packets circulate the network at the same time? CSI 4118

  32. Link-State Packet (LSP) • CreatorID: identity of the packet creator. • NeighborsList: a list of tuples like <NeighborId, link cost>. • SequenceNumber: a counter value that places this packet in the context of all LSPs sent out by the creator of this packet. • TTL: time to live in number of hops. Question: When should a node send out an LSP? CSI 4118

  33. Dealing with LSPs An LSP is sent out when: • A periodic time expires. • A topology change is detected. Important: Routing messages, that is LSPs, are overhead in the network and they should be kept to a minimum. CSI 4118

  34. Route Calculation(based on Dijkstra’s shortest-path algorithm) Assumptions: • A node constructs a graph to represent the network to the best of its knowledge (received LSPs). • N: |V|, number of nodes. • l(i,j): Non-negative weight of edge (i,j). • M: nodes set of nodes incorporated so far for some node s. • C(n): cost of the path from node s to node n. Algorithm for a node s: M = {s} for each n in N-{s} do: C(n)=l(s,n) while (N not equal M) M=M U{w} such that C(w)=min{for all w in (N-M)} for each n in (N-M) do: C(n)=min{C(n), C(w)+l(w,n)} CSI 4118

  35. Example of Link-State Information • Assume nodes 2 and 3 • Test link between them • Broadcast information • Each node • Receives information • Recomputes routes as needed CSI 4118

  36. Dijkstra’s Shortest Path Algorithm • Input • Graph with weighted edges • Node, n • Output • Set of shortest paths from n to each node • Cost of each path • Called Shortest Path First (SPF) algorithm CSI 4118

  37. Dijkstra’s Algorithm CSI 4118

  38. Algorithm Intuition • Start with self as source node • Move outward • At each step • Find node u such that it • Has not been considered • Is “closest” to source • Compute • Distance from u to each neighbor v • If distance shorter, make path from u go through v CSI 4118

  39. Result of Dijkstra’s Algorithm • Example routes from node 6 • To 3, next hop = 3, cost = 2 • To 2, next hop = 3, cost = 5 • To 5, next hop = 3, cost = 11 • To 4, next hop = 7, cost = 8 CSI 4118

  40. Properties of Link-State Routing • Stabilizes quickly. • Keeps routing control traffic low. • Responds rapidly to topology changes. • Doesn’t scale well: the amoung of information stored in each node is large. CSI 4118

  41. Early WAN Technologies • ARPANET • Historically important in packet switching • Fast when invented, slow by current standards • X.25 • Early commercial service • Still Used • More popular in Europe CSI 4118

  42. Recent WAN Technologies • SMDS • Offered by phone companies • Not as popular as Frame Relay • Frame Relay • Widely used commercial service • Offered by phone companies • ATM CSI 4118

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