Rosario Giuseppe Garroppo

A survey on multi-constrained optimal path computation: Exact and approximate algorithms

The paper presents a survey on the techniques to solve the multi-constrained optimal path (MCOP) problem. Computing the MCOP is a task shared by many research areas, from transportation systems to telecommunication networks. In the latter, the MCOP is often related to the issue of Quality of Service (QoS) routing, which consists in finding a route between a couple of nodes that meets a series of QoS requirements such as bounded delay, packet loss, and other parameters. The MCOP problem has been faced by several authors and a plethora of solving methods is now available. In the present work, we draw the state of the art of exact and approximate MCOP computation algorithms, with particular attention to the networking area. We describe and analyse the most representative methods, and for each of them we derive the worst case computational complexity. In addition, we provide the reader with a uniform notation and with the detailed pseudo-code of various algorithms, so that the paper can indeed serve as a workable starting point for further studies on the MCOP problem.

SIP intrusion detection and prevention: recommendations and prototype implementation

As VoIP deployment are expected to grow, intrusion problems similar to those of which data networks experience will become very critical. In the early stages of deployment, the intrusion and security problems have not been seriously considered, although they could have a negative impact on VoIP deployment. In the paper, SIP intrusion detection and prevention requirements are analyzed and an IDS/IPS architecture is proposed. A prototype of the proposed architecture was implemented using as a basis the very popular open-source software Snort, a network-based intrusion detection and prevention system. The prototype of the proposed architecture extends the basic functionality of Snort, making use of the preprocessing feature that permits analyzing protocols of layers above the TCP/UDP one. The preprocessors block is a very powerful one since it permits to implement both knowledge and behavior based intrusion detection and prevention techniques in Snort that basically adopts a network based technique. An important requirement of an IPS is that legitimate traffic should be forwarded to the recipient with no apparent disruption or delay of service. Hence, the performance of the proposed architecture has been evaluated in terms of impact that its operation has on the QoS experienced by the VoIP users.

Design principles of an operator-owned highly distributed content delivery network

Mobile network operators are experiencing a tremendous increase in data traffic due to the growing popularity of bandwidth-intensive video services. This challenge can be faced either by boosting the capacity of the network infrastructure, or by means of offloading traffic from the backhaul and core network and serving contents from distributed cache servers close to the users. Network operators can extend the coverage of traditional CDNs by making usage of caching locations much closer to the users than traditional CDNs. Additionally, network operators can optimize the caching and delivery of contents by exploiting the complete knowledge of their network for designing a cost-effective infrastructure able to achieve both improved user satisfaction and cost savings. This article provides thoroughly justified design principles for a highly distributed operator-owned CDN while focusing on four key aspects: the optimal location of cache servers, mechanisms for request routing, content replica placement, and content outsourcing and retrieval.

Experimental assessment of the coexistence of Wi-Fi, ZigBee, and Bluetooth devices

Many wireless technologies used to build local or personal area networks (WLANs, WPANs) operate into the 2.4 GHz ISM band. Due to the mutual interference, the coexistence of such devices working at the same time in the same area can be troublesome. This paper reports the result of an extensive experimental study on the performance of three popular technologies (ZigBee, Wi-Fi, and Bluetooth), under their mutual disturbance. In addition to assessing the effect of the interference between pairs of technologies, we also analyzed the simultaneous interference among the three systems, a scenario that has never been studied before. Our work partly confirms previous findings, but also reveals some unexpected behaviors, imposing to reconsider some assumptions about the supposed interference-free ZigBee and Bluetooth channels.

Experimental evaluation of two open source solutions for wireless mesh routing at layer two

The paper reports the outcome of an experimental evaluation of two open source solutions for realising a Wireless Mesh Network (WMN). Both works at layer two of the ISO/OSI stack and are transparent to the IP layer, i.e. they allow keeping the existing TCP/IP stack unchanged and avoid dealing with the complex IP configuration and management tasks. The first solution is the upcoming IEEE 802.11s standard, as implemented by the open 80211s project, and the other is the B.A.T.M.A.N. routing protocol, in its layer-2 version. We compared them in a small experimental testbed, with main focus on their behaviour in typical mesh situations. We found that both have strengths and shortcomings, but none can be claimed to be completely mature.

Implementation frameworks for IEEE 802.11s systems

The standardisation process of IEEE 802.11s requires some effort to be devoted to assess the effectiveness of the proposed solutions. Specifically, the path selection and forwarding facilities, embodied by the Hybrid Wireless Mesh Protocol (HWMP), are the major and most delicate components. A working prototype can be profitably used to experimentally evaluate such features, thus returning immediate and significant feedback. In this scenario, the paper offers a threefold contribution. First, we review the current 802.11s implementation efforts, such as the Open 80211s and One Laptop Per Child projects, highlighting the pros and cons of each solution. Then, we describe a prototype IEEE 802.11s Mesh Access Point (MAP) we developed starting from common off-the-shelf hardware and software. The main innovation of this prototype is the support of both modes of operation of HWMP. Conversely to the other projects, which implement only the reactive mode, our device also supports the proactive mode, and runs it in the same way as suggested by the draft standard, thus becoming a more complete evaluation platform. As a final contribution, we provide some simulation results to assess the behaviour of the mixed proactive-reactive mode. From this evaluation, we can confirm that employing the proactive algorithm can bring some benefits to the operation of the 802.11s network, even though a careful approach must be taken to avoid some drawbacks.

Queueing Strategies for Local Overload Control in SIP Server

As other signaling protocols in the past, also SIP suffers of server overload leading to performance collapse. In this framework, recent Internet drafts propose the improvements of overload control mechanisms already presented in SIP and/or a closed loop system model for overload avoidance. In this framework, the paper presents the system simulator developed extending the Network Simulator (ns-2) with logical for local overload control. Furthermore, the paper proposes a new queueing discipline obtained combining the simple First In First Out (FIFO) service discipline with the priority one. Then the paper presents a simulation analysis, aimed at evaluating the impact on system performance of different queueing structures, service disciplines, and buffer sizes. Simulation results clearly show that the proposed queueing discipline produces good system performance with a low complexity increase. Finally, the simulation results point out the weakness of the 503 Service Unavailable message mechanism, which does not introduce significant improvement when combined with the proposed solution.

Providing air-time usage fairness in IEEE 802.11 networks with the deficit transmission time (DTT) scheduler

Wireless systems based on the IEEE 802.11 standard are known to suffer a performance degradation when just a single station in the network experiences bad channel conditions toward the Access Point (AP). This phenomenon, known as the “performance anomaly”, is mainly due to the max-min throughput fairness of the CSMA/CA algorithm of the 802.11 MAC. The simple FIFO scheduling policy usually implemented in the AP also contributes to this problem. In order to overcome the performance anomaly, we propose the Deficit Transmission Time (DTT) scheduler. The aim of DTT is guaranteeing each station a fair medium usage in terms of transmission time. This feature, directly related to the proportional fairness concept, allows to ideally achieve exact isolation among the traffic flows addressed to different stations. DTT achieves this goal taking advantage of measurements of actual frame transmission times. Experiments carried out using a prototype implementation of DTT are compared with analogous tests performed with a classic FIFO queue of a commercial AP and a recently proposed traffic shaping scheme aimed at solving the same 802.11 performance anomaly.

A Prediction-Based Overload Control Algorithm for SIP Servers

Overload is a challenging problem for a SIP server because the built-in overload control mechanism based on generating rejection messages could not prevent the server from collapsing due to congestion. In this scenario, the paper presents an overload mechanism combining a local and a remote solution. The local part of the overload control mechanism is based on the appropriate queueing structure and buffer management of the SIP proxy. The remote overload control mechanism is based on feedback reports provided by the SIP proxy to the upstream neighbors. These reports permit the traffic regulation necessary to avoid the critical condition of overload. The main paper contributions are the design of key components of a remote control mechanism, the proposal of a new approach for dynamic load estimation, and the use of a prediction technique in the remote control loop.

A joint experimental and simulation study of the IEEE 802.11s HWMP protocol and airtime link metric

One of the main features of the upcoming IEEE 802.11s standard is the path selection procedure, which is performed by the Hybrid Wireless Mesh Protocol by means of thetextitairtime link metric (ALM). Through a small experimental testbed, and using the open80211s code, which provides one of the most compliant draft implementations, we analysed the impact that the implementation choices of the developers have on the system performance. The analysis revealed that even small deviations from the specified routines (or different choices in the estimation of the parameters used in the online calculation of the ALM) can lead to noteworthy performance degradations. We thus realized some fixes to improve the performance of the system and verified the effectiveness of such amendments in a realistic simulation environment. We then compared the performance of the modified version to the original one. The outcome of the test highlighted the remarkable advantage achieved by the proposed fixes. Copyright