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Traffic Engineering (TE)

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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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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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TE Approaches

• Preplanned:– OSPF + smart link weight setting– MPLS + optimal general routing

• On demand– MPLS + Constraint-Based Routing

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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

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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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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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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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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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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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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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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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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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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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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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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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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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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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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