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Published byLoren Dixon Modified about 1 year ago

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1 Traffic Engineering (TE)

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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

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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

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

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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 u neven distribution of traffic

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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.

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

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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

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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

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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

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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

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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

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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.

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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

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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)

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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.

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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

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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

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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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