Virtual machine (VM) replication provides a software solution of for business continuity and disaster recovery through application-agnostic hardware fault tolerance by replicating the state of primary VM (PVM) to secondary VM (SVM) on a different physical node. Unfortunately, current VM replication approaches suffer from excessive overhead, which severely limit their applicability and suitability. In this paper, we leverage the practical effect of networked server-client system that PVM and SVM are considered as in the same state only if they can generate the same response from the clients' point of view, and this is exploited to optimize performance. To this end, we propose a generic and highly efficient non-stop service solution, named as "COLO" (COarse-grained LOck-stepping virtual machine) utilizing on-demand VM replication. COLO monitors the output responses of the PVM and SVM, and rules the SVM as a valid replica of the PVM according to the output similarity between PVM and SVM. If the responses do not match, the commit of network response is withheld until PVM's state has been synchronized to SVM. Hence, we ensure that the system is always capable of failover by SVM. Although non-determinism may mean a different internal state of SVM from that of the PVM, it is equally valid and remains consistent from external observations. Unlike earlier instruction level lock-stepping deterministic execution approaches, COLO can easily support Multi-Processors (MP) involving workloads with the satisfying performance. Results show that COLO significantly outperforms existing approaches, particularly on server-client workloads such as online databases and web server applications.

Wireless wide area networks (WWANs) are becoming ubiquitous across most geographic regions, enabling simultaneous coverage from multiple providers. WWAN channels exhibit both uncorrelated and correlated behaviour on a variety of levels. In this paper we examine the statistical properties of WWAN links, and illustrate the benefits in heterogeneity that can be exploited to improve statistical throughput and multimedia quality. Our results are based on real network measurements. We describe the design and implementation of a high quality multimedia streaming application that implements WWAN streaming optimisations utilising unequal error protection coding techniques, and we evaluate the performance over an operational WWAN network.

We introduce MoB, an infrastructure for collaborative wide-area wireless data services. MoB proposes to change the current model of data services in the following fundamental ways: (1) it decouples infrastructure providers from services providers and enables fine-grained competition, (2) it allows service interactions on arbitrary timescales, and, (3) it promotes flexible composition of these fine-grained service interactions based on user and application needs.At the heart of MoB is an open market architecture in which mobile users can opportunistically trade various services with each other in a flexible manner. In this paper we first describe the overall architecture of MoB including various enablers like user reputation management, incentive management, and accounting services. We next present our experience from both simulations as well as our prototype implementation of MoB in enhancing application performance in multiple different scenarios --- file transfers, web browsing, media streaming, and location-enhanced services.

World over wide-area wireless Global System for Mobile Communication (GSM) networks have been upgraded to support the general packet radio service (GPRS). GPRS brings "always-on" wireless data connectivity at bandwidths comparable to that of conventional fixed-line telephone modems. Unfortunately many users have found the reality to be rather different, experiencing very disappointing performance when, for example, browsing the Web over GPRS. In This work, we show what causes the web and its underlying transport protocol TCP to underperform in a GPRS wide-area wireless environment. We examine why certain GPRS network characteristics interact badly with TCP to yield problems such as: link underutilization for short-lived flows, excess queueing for long-lived flows, ACK compression, poor loss recovery, and gross unfairness between competing flows. We also show that many Web browsers tend to be overly aggressive, and by opening too many simultaneous TCP connections can aggravate matters. We present the design and implementation of a web optimizing proxy system called GPRSWeb that mitigates many of the GPRS link-related performance problems with a simple software update to a mobile device. The update is a link-aware middleware (a local "client proxy") that sits in the mobile device, and communicates with a "server proxy" located at the other end of the wireless link, close to the wired-wireless border. The dual-proxy architecture collectively implements a number of key enhancements-an aggressive caching scheme that employs content-based hash keying to improve hit rates for dynamic content, a preemptive push of Web page support resources to mobile clients, resource adaptation to suit client capabilities, delta encoded data transfer of modified pages, DNS lookup migration, and a UDP-based reliable transport protocol that is specifically optimized for use over GPRS. We show that these enhancements results in significant improvement in web performance over GPRS links.

All over the world Global System for Mobile Communication (GSM) cellular mobile networks have been upgraded to support the "always-on" general packet radio service (GPRS). Despite the apparent availability of levels of bandwidth not dissimilar to that provided by conventional fixed-wire telephone modems, the user experience using GPRS is still considerably poor. In this paper, we examine the performance of protocols such as transmission control protocol (TCP) over GPRS, and show how certain network characteristics interact badly with TCP to yield problems such as: link underutilization for short-lived flows, excess queueing for long-lived flows, acknowledgment bunching, poor loss recovery, and gross unfairness between competing flows. We present the design and implementation of a transparent TCP proxy that mitigates many of these problems without requiring any changes to the TCP implementations in either mobile or fixed-wire end systems. The proxy is interposed in the cellular provider's network, and splits TCP connections transparently into two halves-the wired and wireless sides. Connections destined for the same mobile host are treated as an aggregate due to their statistical dependence. We demonstrate packet scheduling and flow control algorithms that use information shared between the connections to maximize performance of the wireless link, while interworking with unmodified TCP peers. We also demonstrate how fairness between flows and response to loss is improved, and that queueing and, hence, network latency is reduced. We discuss how TCP enhancing proxies could be transparently deployed, and conclude that installing such a proxy into GPRS network would be of significant benefit to users.

We present a comparative performance study of a wide selection of optimization techniques to enhance application performance in the context of wide-area wireless networks (WWANs). Unlike in traditional wired and wireless IP-based networks, applications running over WWAN cellular environments are significantly affected by the vagaries of the cellular wireless medium. Prior research has proposed and analyzed optimizations at individual layers of the protocol stack. In contrast, we introduce the first detailed experiment-based evaluation and comparison of all such optimization techniques in a commercial WWAN testbed. This paper, therefore, summarizes our experience in implementing and deploying an infrastructure to improve WWAN performance.The goals of this paper are: (1) to perform an accurate benchmark of application performance over such commercially deployed WWAN environments, (2) to implement and characterize the impact of various optimization techniques across different layers of the protocol stack, and (3) to quantify their interdependencies in realistic scenarios. Additionally, we also discuss measurement pitfalls that we experienced and provide guidelines that may be useful for future experimentation in WWAN environments.

We introduce MAR, a commuter mobile access router infrastructure that exploits wireless diversity (e.g. channel diversity, network diversity, and technology diversity) to provide improved data performance for wireless data users. Our system design stems from the observation that rather than choosing a single wireless service provider (e.g. Sprint, AT&T, BT, Vodafone), a single technology (e.g. GPRS, UMTS, CDMA, 802.11), or a single wireless channel, users can obtain significant benefits by using the multiplicity of choices available. MAR is a wireless multi-homed device that can be placed in moving vehicles (e.g. car, bus, train) to enable high-speed data access. MAR dynamically instantiates new channels based on traffic demand, aggregates the bandwidth and dynamically shifts load from poor quality to better quality channels. MAR, thus, provides a faster, more stable, and reliable communication channel to mobile users.We have implemented and tested the MAR system in our testbed which spans the networks of three different cellular providers. Through our experiments we have performed a detailed evaluation to quantify the benefits of MAR for different protocols and applications. For example, even in highly mobile environments, MAR, on average, improves the end-user experience of web-browsing and streaming applications by a factor of 2.8 and 4.4 respectively. Our results show that significant benefits can be obtained by exploiting the diversity in coverage offered by many cellular operators, different technology networks (e.g. GPRS, CDMA), and diverse wireless channels.

Most peer-to-peer systems are built upon the assumption that their running environment is homogeneous. However, for Internet-scale applications this assumption could lead to performance limitations. We propose a framework for building peer-to-peer systems which use performance monitoring techniques to adapt to changes in their environment.

Interworking heterogeneous wireless access technologies is an important step towards building the next generation, all-IP wireless access infrastructure. We present an experimental study of inter-network mobility between GPRS Cellular and 802.11b-based WLAN hot-spots, and deeply analyze its impact on active transport TCP flows. Our experiments were conducted over a loosely-coupled, Mobile IPv6-based, GPRS-WLAN experimental testbed. Detailed analysis from packet traces of inter-network (vertical) handovers reveals a number of performance bottlenecks. In particular, the disparity in the round trip time and bandwidth offered by GPRS and WLAN networks, and presence of deep buffers in GPRS, can aggravate performance during vertical handovers. This paper, therefore, summarizes practical experiences and challenges of providing transparent mobility in heterogeneous environments. Based on the observations, we propose a number of network-layer handover optimization techniques, e.g. fast router advertisements (RA), RA caching, binding update (BU) simulcasting and layer-3 based soft handovers that improve performance during vertical handovers. The paper concludes with our experiences of migrating TCP connections, thereby also improving application e.g. FTP, Web performance in this environment.

With the evolution of the QoS-capable third generation wireless networks, the wireless community has been increasingly looking for a framework that can provide an effective, network independent, end-to-end QoS control. In this paper, we first construct such a framework and then describe how dynamic SLA-based control can be used to achieve end-to-end QoS in a wired and wireless (UMTS) environment. The proposed framework, which is an extension of the IST CADENUS project, offers an effective wired-wireless QoS translation, an efficient QoS control and management, and a dynamic SLA policy-based QoS provisioning.

Throughout the world, GSM cellular mobile networks are being upgraded to support the "always-on" General Packet Radio Service (GPRS). Despite the apparent availability of levels of bandwidth not dissimilar to that provided by conventional fixed-wire telephone modems, the user experience using GPRS is currently considerably worse. In this paper we examine the performance of TCP and HTTP over GPRS, and show how certain network characteristics interact badly with TCP to yield problems such as: link under-utilization for short-lived flows, excess queueing for long-lived flows, ACK compression, poor loss recovery, and gross unfairness between competing flows. We present the design and implementation of a transparent TCP proxy that mitigates many of these problems without requiring any changes to the TCP implementations in either mobile or fixed-wire end systems. The proxy transparently splits TCP connections into two halves, the wired and wireless sides. Connections destined for the same mobile host are treated as an aggregate due to their statistical dependence. We demonstrate packet scheduling and flow control algorithms that use information shared between the connections to maximise performance of the wireless link while inter-working with unmodified TCP peers. We also demonstrate how fairness between flows and response to loss is improved, and that queueing and hence network latency is reduced. We conclude that installing such a proxy into GPRS network would be of significant benefit to users.

The General Packet Radio Service (GPRS) is being deployed by GSM network operators world-wide, and promises to offer users "always-on" data access at bandwidths comparable to that of conventional fixed-line telephone modems. Unfortunately, many users have found the reality to be rather different, experiencing very disappointing performance when, for example, browsing the web over GPRS.In this paper we investigate what causes the HTTP protocol and its underlying transport TCP to underperform in a GPRS environment. We examine why certain GPRS network characteristics interact badly with TCP to yield problems such as: link under-utilization for short-lived flows, excess queueing for long-lived flows, ACK compression, poor loss recovery, and gross unfairness between competing flows. We also show that many web browsers tend to be overly aggressive, and by opening too many simultaneous TCP connections can aggravate matters.We present the design and implementation of GPRSWeb - a mobile HTTP proxy system that mitigates many of the performance problems with a simple software update to a GPRS mobile device. The update is a 'client proxy' that sits in the mobile device, and communicates with a 'server proxy' located at the other end of the GPRS link close to the wired-wireless border. The dual proxy architecture collectively implements a number of key enhancements - an aggressive caching scheme that employs content-based hash keying to improve hit rates for dynamic content, a preemptive push of web page support resources to mobile clients, resource adaptation to suit client capabilities, delta encoded data transfers, DNS lookup migration, and a UDP-based reliable transport protocol that is specifically optimized for use over GPRS. We show that these enhancements result in significant improvement in overall WWW performance over GPRS.

Numerous systems have been designed which use virtualization to subdivide the ample resources of a modern computer. Some require specialized hardware, or cannot support commodity operating systems. Some target 100% binary compatibility at the expense of performance. Others sacrifice security or functionality for speed. Few offer resource isolation or performance guarantees; most provide only best-effort provisioning, risking denial of service.This paper presents Xen, an x86 virtual machine monitor which allows multiple commodity operating systems to share conventional hardware in a safe and resource managed fashion, but without sacrificing either performance or functionality. This is achieved by providing an idealized virtual machine abstraction to which operating systems such as Linux, BSD and Windows XP, can be ported with minimal effort.Our design is targeted at hosting up to 100 virtual machine instances simultaneously on a modern server. The virtualization approach taken by Xen is extremely efficient: we allow operating systems such as Linux and Windows XP to be hosted simultaneously for a negligible performance overhead --- at most a few percent compared with the unvirtualized case. We considerably outperform competing commercial and freely available solutions in a range of microbenchmarks and system-wide tests.

We present the results of a series of experiments used to characterise the performance of a GPRS (General Packet Radio Service) wireless data network, hence highlighting issues that software architects should consider when designing applications to run over this soon to be widely-deployed service. In summary, we show that packet round trip time (RTTs) are large (>1000 ms) and can be highly variable, packet losses are relatively rare, and that available bandwidth can be quite variable. These network characteristics do not interact well with current TCP implementations. We show how it takes many seconds before a new TCP connection can expand its congestion window to make use of the full bandwidth available, leading to very poor performance of protocols like HTTP. Beyond the point of full bandwidth utilisation, TCP continues to expand the window needlessly, resulting in excessive queueing at the GPRS router. This leads to greatly inflated RTTs (10s of seconds) and hence poor interactive response and slow recovery should loss occur. We show how a simple transparent proxy interposed between the fixed and GPRS networks can be used to significantly improve TCP connection performance, particularly for activities like Web browsing.

Arsenic is a gigabit Ethernet NIC which exports an extended interface to the operating system and user applications. Unlike conventional adaptors, it implements some of the protection and multiplexing functions traditionally performed by the operating system. This enables applications to be given direct access to their own 'virtual interface', allowing them to send and receive packets without operating system interaction. Packet filters uploaded to the interface card by the operating system are used to demultiplex received packets to their destination application, and to validate packets before transmission. Transmit traffic shaping and scheduling mechanisms enable the bandwidth used by applications to be controlled. These features allow protocol processing to be moved into user-space shared libraries without sacrificing the security and resource management functionality that the operating system normally provides. The paper describes Arsenic's design and implementation, and outlines how it is integrated into the Linux 2.3 operating system. Performance measurements are presented that show Arsenic supports low latency, high bandwidth communication while offering greater CPU efficiency and better quality of service than conventional devices.

We introduce a novel passive network monitoring architecture which enables us to capture and integrate data from different levels of the protocol stack using a probe which can be placed at any arbitrary point in the network. We subject the data gathered to off-line analysis and show how, by modelling the dynamics of the protocols observed, we are able to extract information which would not be available using existing means. We show how the technique can be used to passively measure Web server response latency. Experiments are described which use artificially generated loads to validate the techniques used, and results obtained from the observation of real traffic are presented.

Internet protocols currently use packet-level mechanisms to detect and react to congestion. Although these controls are essential to ensure fair sharing of the available resource between multiple flows, in some cases they are insufficient to ensure overall network stability. We believe that it is also necessary to take account of higher level concepts, such as connections, flows, and sessions when controlling network congestion. This becomes of increasing importance as more real-time traffic is carried on the Internet, since this traffic is less elastic in nature than traditional Web traffic. We argue that, in order to achieve better utility of the network as a whole, higher level congestion controls are required. By way of example, we present a simple connection admission control (CAC) scheme which can significantly improve the overall performance. This paper discusses our motivation for the use of admission control in the Internet, focusing specifically on control for TCP flows. The technique is not TCP specific, and can be applied to any type of flow in a modern IP infrastructure. Simulation results are used to show that it can drastically improve the performance of TCP over bottleneck links. We go on to describe an implementation of our algorithm for a router running the Linux 2.2.9 operating system. We show that by giving routers at bottlenecks the ability to intelligently deny admission to TCP connections, the goodput of existing connections can be significantly increased. Furthermore, the fairness of the resource allocation achieved by TCP is improved.

The desk area network was proposed as an architecture suitable for a multimedia workstation. This paper describes how the architecture has evolved and the demonstration workstation that has been constructed.>