CMPT 371

1. CMPT 371 Data Communications and Networking Routing 1

2. Routing with Circuit Switching • Efficiency and resiliency are the key factors used to determine a route through the network. • The network will be optimized to use minimal equipment (switches + trunks) at busy times • The load may temporarily exceed the busy time load • Efficiency may dictate different paths through the network at times of lower load, or when particular components of the network are not available • Routes are usually predetermined based on previous statistics for the network • Alternate routing may be used to provide additional paths in busy times or around failures. Order alternate routes are chosen is based on historical data.

3. Alternate Routes X to Y Stallings 2003: Figure 12.1

4. Routing: Packet switching • Characteristics used to classify routing strategies are: • Correctness: The algorithm must work • Simplicity: The algorithm should use minimum processing capacity at each node, and minimum burden on links • Fairness: Should not be biased against particular types of packets. Equitable to all users • Optimality: Should provide the ‘best’ route, minimizing packet delay and maximizing throughput • Robustness: ability to deliver packets even in the presence of localized failure or overload (no losses, broken circuits) • Efficiency: minimize overhead • Stability: avoid slow reaction to congestion or oscillation

5. Routing • A local network is a group of hosts connected to a larger network through one or more routers. • Thedefault router or first hop router for a particular host is one of the routers that connects it’s local network to the larger network. • The destination router is the router that connects the destination host’s local network to the larger network • A routing algorithm is used to determine a “good” path from the source router through a series of other routers to the destination router

6. Elements of Routing techniques • Performance Criteria • Decision Time • Decision Place • Network information source • Network information update timing

7. Performance criteria • Shortest path: Minimum number of hops • Easily measured but possibly misleading • Minimum Delay: path which will get the packet there first • Maximum Throughput: maximize the amount of data transmitted • Minimum Cost: each link is assigned a cost based on delay, throughput, availability, …. More flexible than minimum hops • One of these or any combination of them (and other measures of performance) may be used to determine cost

8. A graph representing a network J 1 3 1 6 3 2 4 1 2 K 1

9. A graph representing a network J 1 3 2 3 3 6 1 6 4 3 3 2 4 1 1 1 2 K 2 1 Directed graph 1

10. Least cost path • Any router J in the network will have multiple ways to send a packet through the network to reach any another router K • A path is simply a series of links between routers that is traversed by the packet as it travels from J to K • The cost of a path from router J to router K will be the sum of the costs for all links the packet traverses • The least cost path is the path between J and K for which the sum of the costs for all links traversed is the smallest • One of the tasks of the routing algorithm is to find the least cost path between all possible pairs of routers in the network

11. Decision Time and Place • Decision Time: The path through the network can be determined • At the start of the information transfer (virtual circuit) • For each individual packet • Decision Place: The path through the network can be determined • By the sending station • By the intermediate node for each hop (distributed) • By a central coordinating node for the network (centralized)

12. Network update: source • A node may use no network information • A node may make use of local information • The costs, data rates, … to it’s immediate neighbors • A node may collect information from its immediate neighbors • Flow statistics, congestion information • Updates of routing information, for example failed links that must be routed around • A node may collect information from all nodes along any paths of interest • Allows determination of entire path by source node • Allows determination of entire path by central node

13. Network update: time • A node may not update its information • There may be no information to update (flooding) • The routing may be predetermined (static routing) • A node may update its network information continuously • Local information can be integrated as it becomes available • A node may update its network information periodically • Period and extent of updated data determined by the particular adaptive routing algorithm used

14. Global / Decentralized • Global routing algorithms • Based on a complete global knowledge of all costs for all links in the network • Requires dynamic knowledge of entire network • Used for centralized system, or system in which all routers have global knowledge • Link state algorithms • Decentralized routing algorithms • Base on knowledge about directly attached links, not of the entire network and from neighbors • Distance vector algorithms

15. Routing Strategies • Fixed Routing (Static routing) • A fixed path is defined between each source and receiver ( path is determined using some criteria) • Flooding • Packets sent to all nearest neighbors at each step • No routing table needed • Random Routing (Dynamic routing) • Packet takes a random walk through the network • Adaptive routing (Dynamic routing) • Routes vary with time, can react to changing conditions in the network

16. Fixed Routing • Useful in a reliable network with a fixed load • A single permanent route is configured for each source-destination pair of nodes in the network • At each node only need to store the next hop to each destination, not the entire routing table • All packets going from node A to node B follow the same path • No difference between datagram packets or virtual circuit • Link failures may cause failures of some paths through the network. Need to supply alternate paths to use in case of failure. (alternate routing)

17. Adaptive Routing • Used in most packet switching networks • Routing decisions are primarily influenced by: • Failure, Congestion • Cost information is usually exchanged between nodes • Routing decisions based on load/cost information may require significant computation , loading the node. • Decisions based on data gathered elsewhere in the network and/or local information must tradeoff quality and overhead. • An adaptive technique that reacts too slowly is not useful, one that reacts too fast may sacrifice stability and create oscillations.

18. Distance Vector Routing • Once every T seconds each station i, (router) sends each of its neighbors (station one hop distant) a list, of estimated delays from itself to every station j. • Similarly each station will receive routing information from each of its neighbors every T sec. • The routing data received are used to update the routing table of the station • Note: the existing routing table for station i at time t is not used directly to compute the routing table for station i at time t+T, the ‘distance’ to each neighbor station k, lki, and the delay vectors from each neighbor station j, dijare used

19. Link State Routing • Follow these steps: • Find neighbors (addresses) and measure delay to neighbors • Send a packet containing addresses and delays to neighbors information to all other routers • Receive similar packets from other routers and use the information to construct shortest path to every other router using a routing algorithm like Dijkstra’s algorithm

20. Dijkstra’s Algorithm • Initialization • Get Next Node • Update Least Cost Paths • where

21. Dijkstra’s Algorithm: • Find the minimum cost paths from station A to every other station in the network below 6 4 5 A B C D 6 2 4 4 1 4 E 2 8 F H G 4 3 5 4 2 2 4 3 2 K I J L I

23. First Iteration • Label source station A. Find the costs to all stations one hop from the source station A. Choose the minimum cost path, label the node associated with it, and select the path. (A,5) source 6 5 A B C 4 D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 (A,1) F H G 4 3 5 4 2 2 4 3 2 K I J L I

25. Second Iteration • Find the costs to all unlabeled stations one hop from station E.Choose the minimum cost path, label the node associated with it, and select the path. (A,5) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,2) (A,1) E 2 8 F H G 4 3 5 4 2 2 4 3 2 K I (E,3) J L I

27. Third Iteration • Find the costs to all unlabeled stations one hop from station F.Choose the minimum cost path, label the node associated with it, and select the path (F,6) (A,5) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H G (F,4) 4 3 5 4 2 2 (F,5) (E,3) 4 3 2 K I (E,3) J L I (F,6) (F,7)

29. Fourth Iteration • Find the costs to all remaining stations one hop from station I. Remember we do not need to consider I-F. F is already labeled. Choose the minimum cost path, label the node associated with it, and select the path (A,5) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H G (F,4) (F,4) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (I,7) (F,6) (F,7)

31. Fifth Iteration • Find the costs to all unlabeled stations one hop from station G.Choose the minimum cost path, label the node associated with it, and select the path (A,5) (A,5) (G,10) (G,8) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H (G,12) G (F,4) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (F,6) (F,7)

33. Sixth Iteration • Find the costs to all unlabeled stations one hop from station B.Choose the minimum cost path, label the node associated with it, and select the path (B,11) (A,5) (G,10) (G,8) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H (G,12) G (F,4) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (F,6) (F,6) (F,7)

35. Seventh Iteration • Find the costs to all unlabeled stations one hop from station J.Choose the minimum cost path, label the node associated with it, and select the path (A,5) (G,10) (G,8) source 5 A B 6 C 4 D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H (G,12) G (F,4) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (F,7) (F,7) (F,6) (J,9)

37. Eighth Iteration • Find the costs to all unlabeled stations one hop from station K.Choose the minimum cost path, label the node associated with it, and select the path (A,5) (G,8) (G,10) (G,8) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H (G,12) G (F,4) (K,11) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (K,9) (F,6) (F,7)

39. Ninth Iteration • Find the costs to all unlabeled stations one hop from station D.Choose the minimum cost path, label the node associated with it, and select the path (D,12) (A,5) (G,10) (G,8) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H (D,12) G (F,4) (K,11) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (K,9) (K,9) (F,6) (F,7)

41. Tenth Iteration • Find the costs to all unlabeled stations one hop from station L.Choose the minimum cost path, label the node associated with it, and select the path (A,5) (G,10) (G,10) (G,8) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 F H (L,11) G (F,4) (K,11) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (K,9) (F,6) (F,7)

43. Eleventh Iteration • There are no unlabeled stations one hop from station C.Choose the minimum cost path, label the node associated with it, and select the path (A,5) (G,10) (G,8) source 4 5 A B 6 C D 6 2 4 4 1 4 (A,2) (A,1) E 2 8 (K,11) F H G (F,4) (K,11) 4 3 5 4 2 2 4 3 2 K I (E,3) J L I (K,9) (F,6) (F,7)