Algorithms for Fault-Tolerant Placement of Stateful Virtualized Network Functions

Traditional network functions (NFs) such as firewalls are implemented in costly dedicated hardware. By decoupling NFs from physical devices, network function virtualization enables virtual network functions (VNF) to run in virtual machines (VMs). However, VNFs are vulnerable to various faults such as software and hardware failures. To enhance VNF fault tolerance, the deployment of backup VNFs in stand-by VM instances is necessary. In case of stateful VNFs, stand-by instances require constant state updates from active instances during its operation. This will guarantee a correct and seamless handover from failed instances to stand-by instances after failures. Nevertheless, such state updates to stand-by instances could consume significant network bandwidth resources and lead to potential admission failures for VNF requests. In this paper, we study the fault-tolerant VNF placement problem with the optimization objective of admitting as many requests as possible. In particular, the VNF placement of active/stand-by instances, the request routing paths to active instances, and state transfer paths to stand-by instances are jointly considered. We devise an efficient heuristic algorithm to solve this problem, and propose a bi-criteria approximation algorithm with performance guarantees for a special case of the problem. Simulations with realistic settings show that our algorithms can significantly improve the request admission rate compared to conventional approaches

Merlin: A Language for Provisioning Network Resources

This paper presents Merlin, a new framework for managing resources in software-defined networks. With Merlin, administrators express high-level policies using programs in a declarative language. The language includes logical predicates to identify sets of packets, regular expressions to encode forwarding paths, and arithmetic formulas to specify bandwidth constraints. The Merlin compiler uses a combination of advanced techniques to translate these policies into code that can be executed on network elements including a constraint solver that allocates bandwidth using parameterizable heuristics. To facilitate dynamic adaptation, Merlin provides mechanisms for delegating control of sub-policies and for verifying that modifications made to sub-policies do not violate global constraints. Experiments demonstrate the expressiveness and scalability of Merlin on real-world topologies and applications. Overall, Merlin simplifies network administration by providing high-level abstractions for specifying network policies and scalable infrastructure for enforcing them

Fault Tolerant Placement of Stateful VNFs and Dynamic Fault Recovery in Cloud Networks

Traditional network functions such as firewalls are implemented in costly dedicated hardware. By decoupling network functions from physical devices, network function virtualization enables virtual network functions (VNF) to run in virtual machines (VMs). However, VNFs are vulnerable to various faults such as software and hardware failures. To enhance VNF fault tolerance, the deployment of backup VNFs in stand-by VM instances is necessary. In case of stateful VNFs, stand-by instances require constant state updates from active instances during its operation. This will guarantee a correct and seamless handover from failed instances to stand-by instances after failures. Nevertheless, such state updates to stand-by instances could consume significant network bandwidth resources and lead to potential admission failures for VNF requests. In this paper, we study the fault-tolerant VNF placement problem with the optimization objective of admitting as many requests as possible. In particular, the VNF placement of active/stand-by instances, the request routing paths to active instances, and state transfer paths to stand-by instances are jointly considered. We devise an efficient heuristic algorithm to solve this problem. For the fault tolerance problem without computing or bandwidth constraints, we also propose two bicriteria approximation algorithms with performance guarantees for a special case of the problem. Given the placement locations of VNFs, some of them may go faulty. We thus consider the dynamic fault recovery problem, for which we propose an approximation algorithm that dynamically switches traffic processing from faulty VNFs to normal ones. Simulations with realistic settings show that our algorithms can significantly improve the request admission rate compared to conventional approaches

Survey of Consistent Network Updates

Computer networks have become a critical infrastructure. Designing dependable computer networks however is challenging, as such networks should not only meet strict requirements in terms of correctness, availability, and performance, but they should also be flexible enough to support fast updates, e.g., due to a change in the security policy, an increasing traffic demand, or a failure. The advent of Software-Defined Networks (SDNs) promises to provide such flexiblities, allowing to update networks in a fine-grained manner, also enabling a more online traffic engineering. In this paper, we present a structured survey of mechanisms and protocols to update computer networks in a fast and consistent manner. In particular, we identify and discuss the different desirable update consistency properties a network should provide, the algorithmic techniques which are needed to meet these consistency properties, their implications on the speed and costs at which updates can be performed. We also discuss the relationship of consistent network update problems to classic algorithmic optimization problems. While our survey is mainly motivated by the advent of Software-Defined Networks (SDNs), the fundamental underlying problems are not new, and we also provide a historical perspective of the subject

Software defined wireless backhauling for 5G networks

Some of the important elements to guarantee a network?s minimum level of performance are: i) using an efficient routing of the data traffic and, ii) a good resource allocation strategy. This project proposes tools to optimise these elements in an IEEE 802.11ac-based wireless backhaul network considering the constraints derived from an implementation in a software defined network. These tools have been designed using convex optimisation?s theory in order to provide an optimal solution that ensures a circuit mode routing where the impact in higher and lower layers of the network is considered. Additionally, the traffic dynamics of the network is controlled by a sensitivity analysis of the convex problem using the Lagrange multipliers to adapt the solution to the changes produced by the evolution of the traffic. Finally, results obtained using the proposed solutions show an improved performance in bit rate and end-to-end delay with respect to typical routing algorithms for simple and complex network deployments.Algunos elementos importantes para asegurar unos niveles mínimos de rendimiento en una red son: i) utilizar un enrutamiento eficiente del tráfico de datos y, ii) una buena estrategia en la asignación de recursos. Este proyecto propone herramientas para optimizar estos elementos en una red de backhaul inalámbrica basada en el protocolo IEEE 802.11ac considerando las restricciones derivadas de una implementación en una software defined network (red definida por software). Estas herramientas han sido diseñadas utilizando la teoría de optimización convexa para proponer una solución óptima que asegure un enrutamiento en modo circuito en el que se considere el impacto en capas altas y bajas de la red. Además, la dinámica del tráfico de la red se controla mediante un análisis se sensibilidad del problema convexo utilizando los multiplicadores de Lagrange para adaptar la solución a cambios de la red producidos por la evolución del tráfico. Finalmente, los resultados obtenidos a partir de las soluciones propuestas demuestran un mejor rendimiento en bit rate y latencia extremo a extremo respecto a algoritmos de enrutamiento típicos tanto en despliegues de redes sencillas como más complejas.Alguns elements importants per assegurar uns nivells mínims de rendiment en una xarxa són: i) utilitzar un encaminament eficient del trànsit de dades i, ii) una bona estratègia en l'assignació de recursos. Aquest projecte proposa eines per optimitzar aquests elements en una xarxa de backhaul sense fils basada en el protocol IEEE 802.11ac considerant les restriccions derivades d'una implementació en una software defined network (xarxa definida per software). Aquestes eines han estat dissenyades utilitzant la teoria d'optimització convexa per tal de proposar una solució òptima que asseguri un encaminament en mode circuit on es consideri l'impacte en capes altes i baixes de la xarxa. A més, la dinàmica del trànsit de la xarxa es controla mitjançant una anàlisi de sensibilitat del problema convex utilitzant els multiplicadors de Lagrange per adaptar la solució a canvis de la xarxa produïts per l'evolució del trànsit. Finalment, els resultats obtinguts a partir de les solucions proposades demostren un millor rendiment en bit rate i latència extrem a extrem respecte a algoritmes d'encaminament típics tant en desplegaments de xarxes senzilles com més complexes

Single source unsplittable flows with arc-wise lower and upper bounds

In a digraph with a source and several destination nodes with associated demands, an unsplittable flow routes each demand along a single path from the common source to its destination. Given some flow x that is not necessarily unsplittable but satisfies all demands, it is a natural question to ask for an unsplittable flow y that does not deviate from x by too much, i.e., ya≈xa for all arcs a. Twenty years ago, in a landmark paper, Dinitz et al. (Combinatorica 19:17–41, 1999) proved that there exists an unsplittable flow y such that ya≤xa+dmax for all arcs a, where dmax denotes the maximum demand value. Our first contribution is a considerably simpler one-page proof for this classical result, based upon an entirely new approach. Secondly, using a subtle variant of this approach, we obtain a new result: There is an unsplittable flow y such that ya≥xa−dmax for all arcs a. Finally, building upon an iterative rounding technique previously introduced by Kolliopoulos and Stein (SIAM J Comput 31:919–946, 2002) and Skutella (Math Program 91:493–514, 2002), we prove existence of an unsplittable flow that simultaneously satisfies the upper and lower bounds for the special case when demands are integer multiples of each other. For arbitrary demand values, we prove the weaker simultaneous bounds xa/2−dmax≤ya≤2xa+dmax for all arcs a.TU Berlin, Open-Access-Mittel – 202

Time4: Time for SDN

With the rise of Software Defined Networks (SDN), there is growing interest in dynamic and centralized traffic engineering, where decisions about forwarding paths are taken dynamically from a network-wide perspective. Frequent path reconfiguration can significantly improve the network performance, but should be handled with care, so as to minimize disruptions that may occur during network updates. In this paper we introduce Time4, an approach that uses accurate time to coordinate network updates. Time4 is a powerful tool in softwarized environments, that can be used for various network update scenarios. Specifically, we characterize a set of update scenarios called flow swaps, for which Time4 is the optimal update approach, yielding less packet loss than existing update approaches. We define the lossless flow allocation problem, and formally show that in environments with frequent path allocation, scenarios that require simultaneous changes at multiple network devices are inevitable. We present the design, implementation, and evaluation of a Time4-enabled OpenFlow prototype. The prototype is publicly available as open source. Our work includes an extension to the OpenFlow protocol that has been adopted by the Open Networking Foundation (ONF), and is now included in OpenFlow 1.5. Our experimental results show the significant advantages of Time4 compared to other network update approaches, and demonstrate an SDN use case that is infeasible without Time4.Comment: This report is an extended version of "Software Defined Networks: It's About Time", which was accepted to IEEE INFOCOM 2016. A preliminary version of this report was published in arXiv in May, 201

A Branch and Price Algorithm for the k-splittable Maximum Flow Problem

The Maximum Flow Problem with flow width constraints is a NP-hard problem. Two models are proposed: the first model is a compact node-arc model using two flow conservation blocks per path. For each path, one block de?nes the path while the other one send the right amount of flow on it. The second model is an extended arc-path model. It is obtained from the first model after a Dantzig-Wolfe reformulation. It is an extended model as it relies on the set of all the paths between the source and the sink nodes. Some symmetry breaking constraints are used to improve the model. A branch and price algorithm is proposed to solve the problem. The column generation reduces to the computation of a shortest path whose cost depends on weights on the arcs and on the path capacity. A polynomial time algorithm is proposed to solve this subproblem. Computational results are shown on a set of medium-sized instances to show the effectiveness of our approach

Department of Computer Science Activity 1998-2004

This report summarizes much of the research and teaching activity of the Department of Computer Science at Dartmouth College between late 1998 and late 2004. The material for this report was collected as part of the final report for NSF Institutional Infrastructure award EIA-9802068, which funded equipment and technical staff during that six-year period. This equipment and staff supported essentially all of the department\u27s research activity during that period