Jens Milbrandt

Resilience Analysis of Packet-Switched Communication Networks

In the presence of local network outages, restoration and protection switching mechanisms redirect the traffic over alternative paths to mitigate the effect of failures. However, some failure combinations still lead to loss of ingress-egress connectivity within a network or to severe congestion due to rerouted traffic. Congestion may also be caused by unexpected traffic shifts due to changed user behavior or due to changes of interdomain routing. This paper presents a framework for the analysis of ingress-egress unavailability and link congestion due to: 1) failures; 2) changes of user behavior; and 3) changed interdomain routing. It proposes algorithms to find the most probable combinations of 1)-3) according to some models, and to evaluate the connectivity and the relative link load of the network under these conditions. We have implemented this concept in a software tool and its visualization of the results leads to a comprehensive view of the networks resilience. It helps to anticipate potential ingress-egress disconnection and congestion before failures and overload occur or before planned modifications (new infrastructure, new routing, new customers) take effect. Thus, it detects weak points in a network, predicts the effectiveness of potential upgrades, and thereby supports careful bandwidth overprovisioning.

Self-Protecting Multipaths — A Simple and Resource-Efficient Protection Switching Mechanism for MPLS Networks

In this paper we propose the concept of an end-to-end (e2e) Self-Protecting Multi-Path (SPM) as a protection switching mechanism that may be implemented, e.g., in Multiprotocol Label Switching (MPLS) networks. In case of local outages, resilient networks redirect the traffic from a failed link over an e2e backup path to its destination. In this case, Quality of Service (QoS) can only be provided if sufficient extra capacity is available. If backup capacity can be shared among different backup paths, multi-path routing allows for considerable savings regarding this extra capacity. The SPM consists of disjoint paths that carry the traffic both in normal operation mode and during local outages. If a partial path is affected by a network failure, the traffic is just distributed to the remaining working paths. This structure is easy to configure and the switching to failure mode operation is simple since no signalling is required. Based on analytical results, we show that load balancing of the traffic across the disjoint paths can reduce the required backup capacity significantly. The backup performance depends strongly on the network topology, and the SPM outperforms simple Open Shortest Path First (OSPF) rerouting by far.

A performance evaluation framework for network admission control methods

We introduce the notion of link and network admission control (LAC, NAC) and present three fundamentally different budget based NAC methods which categorize most of todays implemented NAC approaches. We propose a performance evaluation framework for their comparison. The required network capacity for each method is dimensioned for a certain flow blocking probability, and the average resource utilization is taken as performance measure. We point out several implementation options and investigate their impact. Based on numerical results, we give recommendations for preferred procedures. Finally, we compare different NAC methods under varying load conditions.

Experience-based admission control (EBAC)

Classical admission control approaches take either descriptor or measurement based information about the traffic into account without relating them to each other. We propose a experience-based AC (EBAC) which uses an empirical percentile of the effective reservation utilization to determine a suitable overbooking factor. In this paper, we show the impact of different measurement time scale resolutions and different quantiles on the performance of the system. We propose aging mechanisms for statistic collection to make the system adaptive to traffic mixes that change over time. We illustrate their effectiveness by simulation results.

Fair assignment of efficient network admission control budgets

In this paper, we review several network admission control (NAC) methods. We explain how the NAC budgets and the required link capacities can be dimensioned based on a traffic matrix, a desired blocking probability, and the routing. The objective of this work is the inversion of that process. Based on a traffic matrix, the routing, and given link capacities, the budgets are to be assigned such that their blocking probabilities are as low as possible. We present an algorithm for fair resource assignment and illustrate its effect on a single link. We extend this mechanism to entire networks, such that it is adaptable to all budget-based NAC approaches. The evaluation of our concept shows that it is most effective in real networking scenarios where heterogeneous traffic patterns occur.

Experience-based admission control with type-specific overbooking

Experience-based admission control (EBAC) is a hybrid approach combining the classical parameter-based and measurement-based admission control schemes. EBAC calculates an appropriate overbooking factor used to overbook link capacities with resource reservations in packet-based networks. This overbooking factor correlates with the average peak-to-mean rate ratio of all admitted traffic flows on the link. So far, a single overbooking factor is calculated for the entire traffic aggregate. In this paper, we propose type-specific EBAC which provides a compound overbooking factor considering different types of traffic that subsume flows with similar peak-to-mean rate ratios. The concept can be well implemented since it does not require type-specific traffic measurements. We give a proof of concept for this extension and compare it with the conventional EBAC approach. We show that EBAC with type-specific overbooking leads to better resource utilization under normal conditions and to faster response times for changing traffic mixes.

Experience-Based Admission Control in the Presence of Traffic Changes

This article investigates the transient behavior of experience-based admission control (EBAC) in case of traffic changes. EBAC is a robust and resource-efficient admission control (AC) mechanism used for reservation overbooking of link capacities in packet-based networks. Recent analyses gave a proof of concept for EBAC and showed its efficiency and robustness through steady state simulation on a single link carrying traffic with constant properties. The contribution of this paper is an examination of the memory from which EBAC gains its experience and which strongly influences the behavior of EBAC in stationary and nonstationary state. For the latter, we investigate the transient behavior of the EBAC mechanism through simulation of strong traffic changes which are characterized by either a sudden decrease or increase of the traffic intensity. Our results show that the transient behavior of EBAC partly depends on its tunable memory and that it copes well with even strongly changing traffic characteristics.

QRP05-2: Time-Exponentially Weighted Moving Histograms (TEWMH) for Application in Adaptive Systems

The distribution of a stationary point process can be sampled by an ordinary histogram. If the distribution of the process varies over time, a static histogram still yields results that are averaged over time since the beginning of the data collection. In this paper, we propose the time-exponentially weighted moving histogram (TEWMH) to derive an estimate for the time-dependent distribution of an instationary point process. The importance of the samples decays exponentially over time such that young samples contribute more to the empirical distribution than old ones. The strength of the decay can be controlled by a simple parameter which determines the memory of the histogram. We present a simple implementation of the TEWMH such that this mechanism can be well applied in practice. The empirical distribution serves for the derivation of other time-dependent statistical measures such as time-dependent percentiles of the observed random variable. These provide useful feedback in adaptive systems. We illustrate the application of the TEWMH for experience-based admission control (EBAC) and show its benefits.

Sensitivity of backup capacity requirements to traffic distribution and resilience constraints

In this work, we present several end-to-end protection switching mechanisms for application in multiprotocol label switching (MPLS). In case of local outages in the network, they deviate the traffic around the failed element over backup paths. They are easy to implement and reduce the additional capacity to maintain the quality of service (QoS) on the backup paths. We study the capacity savings of the presented methods for various protection schemes with different traffic matrices. We further test the influence of different resilience constraints such as the set of protected failure scenarios, bandwidth reuse restrictions due to optical communication, and traffic reduction due to failed border routers.

Impact of routing and traffic distribution on the performance of network admission control

In contrast to link admission control (LAC), which limits the truffle on a single link, network admission control (NAC) methods limit the traffic within a network. In this paper we present four basic budget based NAC approaches that have different complexity. They categorize most resource management schemes from a performance point of view regarding the maximum bandwidth utilization. Our results show that the option of single- or multi-path routing has a significant impact on the NAC performance while it is rather independent of the structure of the traffic matrix.