Data and Computer Communications

1. Data and Computer Communications Unit 12 – Routing in Switched Data Networks Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011

2. Routing in Packet Switching Networks • key design issue for (packet) switched networks • select route across network between end nodes • characteristics required: • correctness • simplicity • robustness (delivery in face of failures and overloads) • stability (dynamic state of links) • fairness (nearby versus faraway nodes) • optimality (maximize average throughput) • efficiency (processing overhead)

3. Performance Criteria • used for selection of route • simplest is to choose “minimum hop” • can be generalized as “least cost” routing • because actual “least cost” is more flexible (with similar computational effort) it is more common than “minimum hop”

4. Elements of Routing Techniques for Packet-Switching Networks

5. Example of Packet Switched Network Costs are assigned to links to support one or more design objectives. For example, cost could be inversely related to data rate (higher data rate on a link, lower assigned cost of link) or current queuing delay on link. The shortest path (fewest hops) from node 1 to node 6 is 1-3-6 (cost is 5 + 5 = 10), but least-cost path is 1-4-5-6 (cost is 1 + 1 + 2 = 4).

6. Decision Time and Place

7. Network Information Source and Update Timing • routing decisions usually based on (static or dynamic) knowledge of network, traffic load, and link cost • distributed routing • using local knowledge, information from adjacent nodes, information from all nodes on a potential route • central routing • collect information from all nodes

8. Routing Strategies - Fixed Routing • use a single permanent route for each source to destination pair of nodes • determined using a least cost algorithm • route is fixed • until a change in network topology • based on expected traffic or capacity • cannot be based on dynamic variable (traffic) • advantage is simplicity • disadvantage is lack of flexibility • does not react to network failure or congestion

9. Fixed RoutingTables Central routing matrix (perhaps stored at network control center) – for each source-destination pair, identity of next node on the route – complete routes not stored, only first node on route From matrix, routing tables developed and stored at each node No difference between datagram and virtual circuit No flexibility unless refined via alternate next nodes provided

10. Routing Strategies - Flooding • no network information required • packet sent by node to every neighbor • eventually multiple copies arrive at destination • each packet is uniquely numbered so duplicates can be discarded • need to limit incessant retransmission of packets • nodes can remember identity of packets retransmitted • can include a hop count, such as the diameter (length of the longest minimum-hop path through the network) of the network, in packets • decrement at each node; discard packet when count reaches zero

11. Properties of Flooding

12. Flooding Example Packet to be sent from node 1 to node 6 Note: if hop count field used in each packet, such set as diameter (length of longest minimum-hop path through the network) of network - i.e. for each pair of end systems attached to network, there is a minimum hop path - length of longest such path is diameter of network 3 packets 9 packets 22 packets

13. Routing Strategies - Random Routing • simplicity of flooding with much less traffic load • node selects one outgoing path for retransmission of incoming packet • selection can be random or round robin • a refinement is to select outgoing path based on probability calculation – based on data rate or fixed link costs ( e.g. Ri/ Rj) • no network information (overhead) needed • random route is typically neither least cost nor minimum hop – network must carry a higher than optimum traffic load (not as high as flooding)

14. Routing Strategies - Adaptive Routing • used by almost all packet switching networks • routing decisions change as conditions on the network change due to failure or congestion • requires (dynamic) information about network • Disadvantages: • decisions more complex (node processing burden) • tradeoff between quality of network information and overhead (status info collected in one place, used in another) • reacting too quickly can cause oscillation • reacting too slowly means information may be irrelevant

15. Adaptive Routing Advantages

16. Classification of Adaptive Routing Strategies • on the basis of information source

17. Isolated Adaptive Routing packet arrives at node 4 from node 1 with destination node 6 – min value is on link through node 3 since Q + B6 is 4 (1 + 3) note: (outgoing) queue lengths of receiving node

18. Least Cost Algorithms • for example: Dijkstra or Bellman-Ford algorithms (Floyd-Warshall all nodes)

19. Least Cost Algorithms • Basis for routing decisions • Can minimize hop with each link cost 1 • Can have link value inversely proportional to capacity • Given network of nodes connected by bi-directional links • Each link has a cost in each direction • Define cost of path between two nodes as sum of costs of links traversed • For each pair of nodes, find a path with the least cost • Link costs in different directions may be different • e.g. length of packet queue

20. Dijkstra’sAlgorithm • Find shortest paths from given source node to all other nodes, by developing paths in order of increasing path length • N= set of nodes in the network • s = source node • T= set of nodes so far incorporated by the algorithm • w(i, j)= link cost from nodei to node j • w(i, i) = 0 • w(i, j) =  if the two nodes are not directly connected • w(i, j)  0 if the two nodes are directly connected • L(n)=cost of least-cost path from node s to node n currently known • At termination, L(n) is cost of least-cost path from s to n

21. Dijkstra’s Algorithm Method • Step 1 [Initialization] • T = {s} Set of nodes so far incorporated consists of only source node • L(n) = w(s, n) for n ≠ s • Initial path costs to neighboring nodes are simply link costs • Step 2[Get Next Node] • Find neighboring node not in T with least-cost path from s • Incorporate node into T • Also incorporate the edge that is incident on that node and a node in T that contributes to the path • Step 3[Update Least-Cost Paths] • L(n) = min[L(n), L(x) + w(x, n)]for all nÏ T • If latter term is minimum, path from s to n is path from s to x concatenated with edge from x to n • Algorithm terminates when all nodes have been added to T

22. Dijkstra’s Algorithm Notes • At termination, value L(x) associated with each node x is cost (length) of least-cost path from s to x. • In addition, T defines least-cost path from s to each other node • One iteration of steps 2 and 3 adds one new node to T • Defines least cost path from s to that node

23. Example of Dijkstra’s Algorithm 1 4 2 3 6 5

25. Dijkstra’s Algorithm

26. Dijkstra’s Algorithm Set value[0] to 0, the rest of the values to  Place node 0 (the source) into class 1 , the rest into class 2 while the sink node is not in class 1 do The last node put in class 1 is the pivotal node for all nodes j in class 2: if this node j is not connected to the pivotal node then do nothing else if the value of the pivotal node + edge value is less than value of node j then value of node j is assigned the value of the pivotal node + edge value else do nothing end for Add the node in class 2 with minimum value to class 1 end while • L(n) = min[L(n), L(x) + w(x, n)]for all nT

27. Dijkstra’s Algorithm PREDECESSORPIVOTAL NODE Class 1 = value[0] = Class 2 = value[1] = ∞ value[2] = ∞ value[3] = ∞ value[4] = ∞ value[5] = ∞ value[6] = ∞

28. 5 a b 6 2 4 1 3 7 c d e Another Example 3 1 0 4 2

29. ARPANET Routing Strategies • First Generation - 1969 • ARPANET - foundation of present-day Internet • Distributed adaptive routing • Estimated delay as performance criterion (distance-vector routing – Delay and Successor vectors for each node) • Node exchanges delay vector with neighbors (every 128 milliseconds) • Update routing table based on incoming info • Bellman-Ford algorithm (least-cost routing algorithm) • Doesn't consider link speed; just uses queue length as estimator of node delay • Queue length not a good measurement of delay • Responds slowly to congestion

30. ARPANET Routing Strategies • Second Generation - 1979 • Addressed shortcomings of first generation (i.e. in which line speed not considered – only queue length … slow response to congestion) • Used delay as performance criterion • Delay measured directly (not just as the artificial measure of queue length - using packet time-stamping - departure time minus arrival time plus transmission and propagation times – thus includes link data rate) – every 10 seconds • If significant delay changes, then flood this info out to all other nodes • (re)computation using Dijkstra’sleast-cost algorithm • works good under light and medium loads • but under heavy loads, little correlation between reported delays and those experienced

31. ARPANET Routing Strategies • Third Generation - 1987 • as network load grew, new strategy needed • Link cost calculations changed • Measure average delay over last 10 seconds – routing so that average path good instead of all routes • Normalize to value achieved on idle line (just transmission and propagation times) and based on current value versus previous results • Revised cost function keyed to utilization rather than delay • Function acts similar to delay-based metric under light loads and to capacity-based metric under heavier loads

32. Summary • routing in packet-switched networks • routing techniques • routing strategies • fixed, flooding, random, adaptive • least-cost algorithms • Dijkstra, Bellman-Ford • evolution of ARPANET routing strategies