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Traffic Engineering (TE)

Traffic Engineering (TE). 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

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Traffic Engineering (TE)

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  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

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