Traffic Engineering Traffic Engineering

1.2.2 Traffic Engineering Traffic Engineering

The basic problem addressed in traffic engineering is as follows: Given a network and traffic demands, how can traffic flows in the network be organized so that an optimization objective is achieved? The objective may be to maximize the utiliza- tion of resources in the network or to minimize congestion in the network. Typically the optimal operating point is reached when traffic is evenly distributed across the network. With balanced traffic distribution, both queuing delay and loss rates are at their lowest points.

Obviously these objectives cannot be achieved through destination-based IP routing; there simply is not sufficient information available in IP routing to make possible such optimization. In traffic engineering, advanced route selection tech- niques, often referred to as constraint-based routing in order to distinguish them from destination routing, are used to calculate traffic trunks based on the optimi- zation objectives. To perform such optimization, the traffic-engineering system often needs network-wide information on topology and traffic demands. Thus traffic engineering is typically confined to a single administrative domain.

The routes produced by constraint-based routing are most likely different from those in destination-based IP routing. For this reason these constraint-based routes cannot be implemented by destination-based forwarding. In the past, many service providers used ATM in the backbone networks to support constraint-based routing. ATM virtual circuits can be set up to match the traffic patterns; the IP-based network is then overlaid on top of these virtual circuits. MPLS offers a better alternative since it offers similar functions yet can be tightly integrated with IP- based networks.

The existing Internet backbones have used the so-called overlay model for traffic engineering. With the overlay model, service providers build a virtual network comprising a full mesh of logical connections between all edge nodes. Using the traffic demands between the edge nodes as input, constraint-based routing selects a set of routes for the logical connections to maximize the overall resource utilization in the network. Once the routes are computed, MPLS can be used to set up the logical connections as LSPs, exactly as calculated by constraint- based routing.

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10 CHAPTER 1CHAPTER 1 Network QoS: The Big Picture

The downside of the overlay model is that it may not be able to scale to large networks with a substantial number of edge nodes. To set up a full-mesh logical network with N edge nodes, each edge node has to connect to the other ( N− 1) edge nodes, resulting in N× ₍ N− _{1) logical connections. This can add significant} messaging overheads in a large network. Another problem is that the full-mesh logical topology increases the number of peers, neighbors that routers talk to, that a routing protocol has to handle; most current implementations of routing proto- cols cannot support a very large number of peers. In addition to the increased peering requirements, the logical topology also increases the processing load on routers during link failures. Because multiple logical connections go over the same physical link, the failure of a single physical link can cause the breakdown of multiple logical links from the perspective of IP routing.

Traffic engineering without full-mesh overlaying is still a challenge. One heu- ristic approach that some service providers have used is to adjust traffic distribu- tion by changing the link weights in IP routing protocols. For example, when one link is congested, the link weight can be increased in order to move traffic away from this link. Theoretically one can achieve the same traffic distribution as in the overlay model by manipulating the link weights in the Open Shortest Path First (OSPF) routing protocol. This approach has the advantage that it can be readily implemented in existing networks without major changes to the network architecture.

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1.3 SUMMARY SUMMARY

The need for QoS capabilities in the Internet stems from the fact that best-effort service and datagram routing do not meet the needs of many new applications, which require some degree of resource assurance in order to operate effectively. Diverse customer requirements also create a need for service providers to offer different levels of services in the Internet.

The Internet community has developed a number of new technologies to address these issues. Integrated Services and Differentiated Services provide new architectures for resource allocation in the Internet. Integrated Services use res- ervation to provide guaranteed resources for individual flows. The Differentiated Services architecture takes a different approach. It combines edge policing, provisioning, and traffic prioritization to provide different levels of services to customers.

MPLS and traffic engineering address the issues of bandwidth provisioning and performance optimization in Internet backbones. The explicit route mechanism in MPLS adds an important capability to the IP-based network. Combined with constraint-based routing in traffic engineering, MPLS and traffic engineering can help network providers make the best use of available resources and reduce costs.

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1.4 RESOURCES RESOURCES

The basic principles of datagram networks and a detailed design were first described by Paul Baran in his 1964 RAND report “On Distributed Communica- tions.” Although the report was discovered after the ARPANET had already started, the current Internet is remarkably close to what Paul Baran srcinally had in mind. This 12-volume historical report is now available on-line at www.rand.org/ publications/RM/baran.list.html.

For a general introduction about data networking and the Internet, we recom- mend Peterson, L., and B. Davie, Computer Networks: A Systems Approach (Morgan Kaufmann, 1999).

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Traffi c Engineering and