Traffic Engineering (TE)

1. Traffic Engineering (TE)

2. Network Congestion • Causes of congestion • Lack of network resources • Uneven distribution of traffic caused by current dynamic routing protocols • Consequences of congestion • High loss rate • Low throughput • Long end-to-end delay • Intserv and Diffserv provide differentiated degradation of performance for different traffic when the network is congested

3. Traffic Engineering • Traffic Engineering (TE)is the process of distributing traffic flows through the network to achieve load balancing • TE leads to: • Reduced congestion • Improved bandwidth utilization

4. TE Approaches • Preplanned: • OSPF + smart link weight setting • MPLS + optimal general routing • On demand • MPLS + Constraint-Based Routing

5. OSPF Routing • Each link has a static link weight configured by the network operator. • Examples: unit weight, weight proportional to physical distance of link, weight inversely proportional to link capacity • Packets routed over the shortest path to destinations • When multiple shortest paths exist to a destination, traffic is split evenly among the paths • Drawback: may cause uneven distribution of traffic

6. OSPF Routing • Routing depends on the choice of link weights  Can control the distribution of traffic in the network by tuning the link weights.

7. Weight Tuning in OSPF • All links have same capacity, nodes q, r, s, w eachhas one unit of traffic to send to node t. • Objective: minimize the maximum link load.

8. Optimization of OSPF Link Weights • Given a network topology and a traffic matrix, find an optimal setting of the link weights so that a certain objective is achieved • Example objectives • Minimize the maximum link utilization (link utilization = link load/link capacity) • Minimize total cost of all links where the cost of a link is a function of link utilization

9. Optimization of OSPF Link Weights • Local search heuristic [Fortz and Thorup 2000] • Finding: For real networks, a good setting of the link weights can make OSPF perform almost as well as optimal general routing • General routing: traffic flow between nodes s and d can be split arbitrarily over the paths between s and d • Achievable with MPLS

10. Traffic Trunk • A traffic trunk is an aggregation of traffic flows belonging to the same class that are placed inside a LSP • Attributes of a traffic trunk • QoS requirements • Policy: include/exclude certain links

11. Constraint-Based Routing (CBR) • Given a traffic trunk, compute a path for it subject to multiple constraints • QoS constraints • Resource availability constraints • Policy constraints • Goals of CBR: • Meet QoS requirements of the traffic trunk • Increase the utilization of the network • MPLS can setup LSPs along paths determined by CBR

12. Routing Metrics • Let d(i,j) be a metric for link (i,j). For any path P = (i, j, k, …, l, m), metric d is: additive if d(P) = d(i,j) + d(j,k) + … + d(l,m) • delay, jitter, hop-count multiplicative if d(P) = d(i,j) * d(j,k) * … * d(l,m) • reliability (i.e., 1-loss rate) concave if d(P) = min{d(i,j), d(j,k), …, d(l,m)} • bandwidth

13. Complexity of CBR • Computing a route subject to constraints of two or more additive and/or multiplicative metrics is NP-complete. • The computationally feasible combinations of metrics are bandwidth and one of the other metrics.

14. Path Computation • Bandwidth and hop-count constraints are commonly used in path computation • Many real-time applications will require a certain amount of bandwidth. • The amount of resources consumed by a flow is proportional to the number of hops it traverses • Path Computation algorithm: Step 1. Prune links if: • insufficient bandwidth • violate policy constraints Step 2. Compute shortest path

15. Information Requirement of CBR • Information needed by CBR: • Network topology • Available bandwidth on links • Routers need to distribute new link state information, i.e., link available bandwidth • Extend the link state advertisements of routing protocols (OSPF, IS-IS)

16. Information Distribution • Flooding link state advertisements whenever a link’s available bandwidth changes is too expensive • A tradeoff must be made between the accuracy of link available bandwidth information and the frequency of flooding of link state advertisements.

17. Information Distribution • Periodic scheme • Periodically, a node checks if the current link status is the same as the one lastly broadcasted • If different, floods updated links status • Threshold scheme: flood LSA on significant changes of available bandwidth (e.g., more than 50% or more than 10 Mbps) • On topology changes: link addition/removal, link down/up

18. Information Distribution • LSP setup may fail due to inaccurate link information • When a node refuses to setup an LSP due to insufficient link bandwidth, it broadcasts an update of its available bandwidth

19. Tradeoff Between Resource Conservation and Load Balancing • Widest-shortest path routing: choose a path with min hop-count; if more than one such path, choose the one with the largest available bandwidth • Emphasize preserving network resources • Shortest-widest path routing: choose a path with largest available bandwidth; if more than one such path, choose the one with the min hop-count • Emphasizes load balancing