Traffic engineering involves adapting the routing of traffic to the network conditions, with the joint goals of good user performance and efficient use of network resources. In this paper, we describe an approach to intradomain traffic engineering that works within the existing deployed base of Interior Gateway Protocols (IGPs), such as Open Shortest Path First (OSPF) and Intermediate System-Intermediate System (IS-IS). We explain how to adapt the configuration of link weights, based on a network-wide view of the traffic and topology within a domain. In addition, we summarize the results of several studies of techniques for optimizing OSPF/IS-IS weights to the prevailing traffic. The paper argues that traditional shortest-path routing protocols are surprisingly effective for engineering the flow of traffic in large IP networks. 1

Intra-domain traffic engineering can significantly enhance the performance of large IP backbone networks. Two important components of traffic engineering are understanding the traffic demands and configuring the routing protocols. These two components are inter-linked, as it is widely believed that an accurate view of traffic is important for optimizing the configuration of routing protocols and through that, the utilization of the network. This basic

Traffic engineering is aimed at distributing traffic so as to "optimize" a given performance criterion. The ability to carry out such an optimal distribution depends on both the routing protocol and the forwarding mechanisms in use in the network. In IP networks running the OSPF or IS-IS protocols, routing is over shortest paths, and forwarding mechanisms are constrained to distributing traffic uniformly over equal cost shortest paths. These constraints often make achieving an optimal distribution of traffic impossible. In this paper, we propose and evaluate an approach, based on manipulating the set of next hops for routing prefixes, that is capable of realizing near optimal traffic distribution without any change to existing routing protocols and forwarding mechanisms. In addition, we explore the tradeoff that exists between performance and the overhead associated with the additional configuration steps that our solution requires. The paper's contributions are in formulating and evaluating an approach to traffic engineering for existing IP networks that achieves performance levels comparable to that offered when deploying other forwarding technologies such as MPLS.

Intra-domain routing in IP backbone networks relies on link-state protocols such as IS-IS or OSPF. These protocols associate a weight (or cost) with each network link, and compute traffic routes based on these weights. However, proposed methods for selecting link weights largely ignore the issue of failures which arise as part of everyday network operations (maintenance, accidental, etc.). Changing link weights during a short-lived failure is impractical. However such failures are frequent enough to impact network performance.

Abstract — Large IP networks usually combine protection and restoration mechanisms at various layers of the protocol stack to minimize service disruption in the event of failures. Sprint has chosen an IP-based restoration approach for building a highly available tier-1 IP backbone. This paper describes the design principles of Sprint’s network that makes IP-based restoration an effective and cost-efficient approach. The effectiveness of IPbased restoration is evaluated by analyzing network failure characteristics, and by measuring disruptions in service availability during controlled failure experiments in the backbone. Current trends for improving the performance of IP-based restoration are also discussed. I.

Hot-potato routing is a mechanism employed when there are multiple (equally good) interdomain routes available for a given destination. In this scenario, the Border Gateway Protocol (BGP) selects the interdomain route associated with the closest egress point based upon intradomain path costs. Consequently, intradomain routing changes can impact interdomain routing and cause abrupt swings of external routes, which we call hot-potato disruptions. Recent work has shown that hot-potato disruptions can have a substantial impact on large ISP backbones and thereby jeopardize the network robustness. As a result, there is a need for guidelines and tools to assist in the design of networks that minimize hot-potato disruptions. However, developing these tools is challenging due to the complex and subtle nature of the interactions between exterior and interior routing. In this paper, we address these challenges using an analytic model of hot-potato routing that incorporates metrics to evaluate network sensitivity to hot-potato disruptions. We then present a methodology for computing these metrics using measurements of real ISP networks. We demonstrate the utility of our model by analyzing the sensitivity of a large AS in a tier 1 ISP network.

Traffic engineering and traffic matrix estimation are often treated as separate fields, even though one of the major applications for a traffic matrix is traffic engineering. In cases where a traffic matrix cannot be measured directly, it may still be estimated from indirect data (such as link measurements), but these estimates contain errors. Yet little thought has been given to the effects of inexact traffic estimates on traffic engineering. In this paper we consider how well traffic engineering works with estimated traffic matrices in the context of a specific task; namely that of optimizing network routing to minimize congestion, measured by maximum link-utilization. Our basic question is: how well is the real traffic routed if the routing is only optimized for an estimated traffic matrix? We compare against optimal routing of the real traffic using data derived from an operational tier-1 ISP. We find that the magnitude of errors in the traffic matrix estimate is not, in itself, a good indicator of the performance of that estimate in route optimization. Likewise, the optimal algorithm for traffic engineering given knowledge of the real traffic matrix is no longer the best with only the estimated traffic matrix as input. Our main practical finding is that the combination of a known traffic matrix estimation technique and a known traffic engineering technique can get close to the optimum in avoiding congestion for the real traffic. We even demonstrate stability in the sense that routing optimized on data from one day continued to perform well on subsequent days. This stability is crucial for the practical relevance to off-line traffic engineering, as it can be performed by ISPs today.

In this paper, we adapt the heuristic of Fortz and Thorup for optimizing the weights of Shortest Path First protocols such as Open Shortest Path First (OSPF) or Intermediate System-Intermediate System (IS-IS), in order to take into account failure scenarios.

Abstract—Link failures are part of the day-to-day operation of a network due to many causes such as maintenance, faulty interfaces, and accidental fiber cuts. Commonly deployed link state routing protocols such as OSPF react to link failures through global link state advertisements and routing table recomputations causing significant forwarding discontinuity after a failure. Careful tuning of various parameters to accelerate routing convergence may cause instability when the majority of failures are transient. To enhance failure resiliency without jeopardizing routing stability, we propose a local rerouting based approach called failure insensitive routing. The proposed approach prepares for failures using interface-specific forwarding, and upon a failure, suppresses the link state advertisement and instead triggers local rerouting using a backwarding table. With this approach, when no more than one link failure notification is suppressed, a packet is guaranteed to be forwarded along a loop-free path to its destination if such a path exists. This paper demonstrates the feasibility, reliability, and stability of our approach. Index Terms—Fast rerouting, interface-specific forwarding, transient failures.

There exist a wide variety of network design problems that require a traffic matrix as input in order to carry out performance evaluation. The research community has not had at its disposal any information about how to construct realistic traffic matrices. We introduce here the two basic problems that need to be addressed to construct such matrices. The first is that of synthetically generating traffic volume levels that obey spatial and temporal patterns as observed in realistic traffic matrices. The second is that of assigning a set of numbers (representing traffic levels) to particular node pairs in a given topology. This paper provides an in-depth discussion of the many issues that arise when addressing these problems. Our approach to the first problem is to extract statistical characteristics for such traffic from real data collected inside two large IP backbones. We dispel the myth that uniform distributions can be used to randomly generate numbers for populating a traffic matrix. Instead, we show that the lognormal distribution is better for this purpose as it describes well the mean rates of origin-destination flows. We provide estimates for the mean and variance properties of the traffic matrix flows from our datasets. We explain the second problem and discuss the notion of a traffic matrix being well-matched to a topology. We provide two initial solutions to this problem, one using an ILP formulation that incorporates simple and well formed constraints. Our second solution is a heuristic one that incorporates more challenging constraints coming from carrier practices used to design and evolve topologies.