Load Sensitive Routing Protocol for Providing QoS in Best Effort Network

1. Load Sensitive Routing Protocol for Providing QoSin Best Effort Network IIT Bombay

2. Motivation • Real time applications like audio and video conferencing, VoIP requires QoS from the Internet to have satisfactory performance. • Internet largely support best effort traffic and Open Shortest Path First (OSPF) is one of the most widely used routing protocols. • In OSPF, when a packet experiences congestion, the routing subsystem cannot send it through alternate path. Thus, it fails in providing Quality of Service. So there is a need to provide QoS routing in networks. IIT Bombay

3. Advantages of LSR algorithm • The Load Sensitive Routing algorithm implements QoS routing in a better way. • It localizes the QoS routing changes to the region where QoS has deteriorated • no flooding • Less overhead • scalability. • It chooses loop free alternate paths for routing packet • No separate loop detection • Interoperate with OSPF routers IIT Bombay

4. Control Messages • Congestion notification • Sent to all the neighbors when a link congestion is detected • When neighbors receive this congestion notification they reroute packets through alternate next hop (three different ways of finding the alternate next hops are explained later) • Congestion over • Sent to all the neighbors when a link congestion is over • Neighbors revert back to routing packets through OSPF next hops IIT Bombay

5. LSR (Contd) • LSR eligible neighbor • Different nexthop used for alternate path • Chosen based on OSPF property (which leads to loop free routing) • hop_count(ospf_nexthop, D) < hop_count(curr_node, D) • ospf_cost(ospf_nexthop, D) < ospf_cost(curr_node, D) • If Node(Q) is neighbor of Node(P) for destination Node(D) and • a’ * hop_count(Q, D) + b’ * ospf_cost(Q, D) < a’ * hop_count(P, D) + b’ * ospf_cost(P, D) (from the above ospf property) • => hop_count(Q, D) + b * ospf_cost(Q, D) < hop_count(P, D) + b * ospf_cost(P, D) Then Node (Q) will be LSR eligible neighbor for Node(P). b is called LSR Coefficient. • The task is then to determine LSR coefficient b IIT Bombay

6. LSR Contd… • Calculation of LSR coefficients • b is global (same for all nodes) for a particular destination • b is local • b is global • LSR: b is chosen such that the total number of alternate paths (for a particular destination) is maximized. • Check for each possible values of b and set it to the one that gives maximum number of alternate paths • E-LSR : Maximize total number of alternate paths subject to the constraint that maximum number of nodes have at least one alternate path IIT Bombay

7. The Efficient Load Sensitive RoutingAlgorithm (E-LSR) • Objective of LSR • Maximize total number of alternate paths in network. • Objective of E-LSR • Maximize total number of alternate paths subject to the constraint that maximum number of nodes have at least one alternate path. IIT Bombay

8. Proposed Algorithm for Coefficient Calculation • Less than and Greater than Constraints on b value. • Node(P) forwards packet to Node(Q) if • HC(Q, D) + b * OC(Q, D) < HC(P, D) + b * OC(P, D) • If (HC (Q, D) < HC (P, D) and OC(Q, D) ≤ OC(P, D)) • b ≥ 0 • If (HC(Q, D) < HC(P, D) and OC(Q, D) > OC(P, D)) • b < ((HC(P, D) – HC(Q, D) / (OC(Q, D) – OC(P, D)) • If (HC(Q, D) ≥ HC(P, D) and OC(Q, D) < OC(P, D)) • b > (HC(Q, D) - HC(P, D)) / (OC(P, D) - OC(Q, D)) IIT Bombay

9. Coefficient Calculation (Contd…) • Necessary parameters • gi: ithGreater than Constraint for destination d. • li: ithLess than Constraint of Node(i) for destination d. OC(A, C) = 6, OC(E, C) = 5, OC(F, C) = 8, OC(G, C) = 9 HC(A, C) = 2, HC(E, C) = 3, HC(F, C) = 1, HC(G, C) = 1 E – A: 3 + 5 * b < 2 + 6 * b => b > 1 (greater than constraint) F – A: 1 + 8 * b < 2 + 6 * b => b < 1/2 (less than constraint) G – A: 1 + 9 * b < 2 + 6 * b => b < 1/3 (less than constraint) IIT Bombay

10. Coefficient Calculation (Contd…) • Sort the greater than constraints such that • g1 < g2 < g3 < … < gm • Sort the less than constraints such that • l1 < l2 < l3 < … < ln IIT Bombay

11. Coefficient Calculation (Contd…) • Different Cases for Coefficient Calculation Algorithm IIT Bombay

12. Coefficient Calculation (Contd…) IIT Bombay

13. Coefficient Calculation (Contd…) • Objective Function • Calculates two parameters • n: Number of nodes having at least one alternate path. • m: Total number of alternate paths other than n. • Returns N * N * n + m IIT Bombay

14. Local Coefficient Based LSR (L-LSR) • b is local • For a particular destination, each node can choose its own local L-LSR coeffiecient denoted as b(vi,D) • but L-LSR coefficient is assigned such a way that the loop-free property is still maintained • Calculation of b is more complex • We use a graph theoretic approach IIT Bombay

15. Building QoS graph • Edges along the ospf path have a weight of infinity • For all other edges (called cross-edge) • weight is assigned as per the “out-degree” of the node • But while calculating out-degree of a node do not include any ospf edges • weights are assigned to the cross edge according to the out-degree • cross-edges are added to the sink-tree only for nodes along the QoS paths IIT Bombay

16. Example Topology IIT Bombay

17. Example sink tree IIT Bombay

18. Example QoS graph • QoS path:A-B-C-D IIT Bombay

19. Building Acyclic QoS graph • Addition of cross-edges could introduce loops • We use minimum feedback arc set (FAS) algorithm to break the cycles in the graph • we actually remove edges with maximum weight (in the cycle) while breaking cycle • we want to target a node which has more alternate path (more weight) • this acyclic graph represents the alternate paths through which nodes can send packets during congestion IIT Bombay

20. Calculating L-LSR coefficient • Calculated from the acyclic QoS graph • If Node(vi) can choose Node(vj) as its L-LSR next hop then HC(vj, D) + b(vj, D) * OC(vj,D) < HC(vi, D) + b(vi, D) * OC(vi,D) • b’(vj, D) – b’(vi, D) < weight(vj, vi) (1) where weight(vj, vi) = HC(vi, D) - HC(vj, D) (2) b’(vi, D) = b(vi, D) * OC(vi,D) IIT Bombay

21. Calculating L-LSR coefficient • (1) can be represented as a constraint graph • there is a directed edge from Node(vj) to Node(vi) with weight weigth(vj, vi) • constrained graph can be obtained by reversing the direction of edges of acyclic QoS graph and assigning weights according to (2) • Finally, the L-LSR coefficients are calculated using (1) IIT Bombay

22. Constraint Graph of example topology IIT Bombay

23. Calculating L-LSR coefficient • for assigning b traversal starts from D similar to BFS. • But a node is visited only when all its incoming edges are visited • From D we first visit Y and Z (cannot visit X from D) and compute b for Y and Z (such that (1) is satisfied) • In the next round we visit Y and then we can visit X and determine its b IIT Bombay

24. Simulation Setup • Simulation Parameters • Congestion Threshold 90% • Congestion detection Interval 1 sec • Cost of link is assigned as • Cost = 1000000 / bandwidth in bps • Traffic Scenarios • Scenario A • Voice Traffic : • CBR with bandwidth 64kbps( packet size :160bytes, Interval: 0.02 sec) • Scenario B • Data Traffic : • Exponential ON / OFF ( packet size : 576 bytes, mean ON period : 50 msec and mean OFF period :50msec, average rate : 128 kbps) • Cross Traffic • Randomly selected Source and Destination exchange Traffic which follows Poisson traffic with average rate of 32kbps (sent in both the scenario A and B) IIT Bombay

25. Simulation Topology • Two QoS paths: 0-1-2-3-4-5 10-9-8-7-6-5 IIT Bombay

26. Delay Scenario-A Path(10,5) IIT Bombay

27. Delay Scenario-B Path(10,5) IIT Bombay

28. PPD Scenario-A Path(10,5) IIT Bombay

29. PPD Scenario-B Path(10,5) IIT Bombay

30. Delay Scenario-A Path(0,5) IIT Bombay

31. Delay Scenario-B Path(0,5) IIT Bombay

32. PPD Scenario-A Path(0,5) IIT Bombay

33. PPD Scenario-B Path(0,5) IIT Bombay

34. Conclusion • We presented an OSPF-based Load Sensitive Routing protocol • Three different methods of selecting alternate paths • based on loop free property of OSPF, hence • does not need separate loop detection • can interoperate with OSPF routers • provides QoS in terms of delay and packet drop • L-LSR performs the best among the LSR family of protocols • much better performance than OSPF IIT Bombay

35. References • A. Sahoo, “An OSPF Based Load-Sensitive QoS Routing Algorithm using Alternate Paths,” in IEEE International Conference on Computer Communication Networks, October 2002. • G. Apostolopoulos, R. Guerin, S. Kamat, A. Orda, A. Przygienda, and D.Williams. “QoS routing mechanisms and OSPF extensions”. Internet Request for Comments (RFC2676), April 1999. • A. Segall, P. Bhagwat, and A. Krishna. “QoS Routing Using Alternate Paths”. Journal of High Speed Networks, 7(2): 141–158, 1998. • Z. Wang and J. Crowcroft. “Shortest path first with emergency exits”. ACM SIGCOMM 90, pages 166–176, Sept 1990. • Andrew S. Tanenbaum. Computer Networks. Prentice-Hall India, Fourth edition, 2003. • Camil Demetrescu and Irene Finocchi. Combinatorial algorithms for feedback problems in directed graphs.Inf. Process Lett. 86(3) :129-136 ,2003 IIT Bombay