Study on the Performance of TCP over 10Gbps High Speed Networks

Internet traffic is expected to grow phenomenally over the next five to ten years. To cope with such large traffic volumes, high-speed networks are expected to scale to capacities of terabits-per-second and beyond. Increasing the role of optics for packet forwarding and transmission inside the high-speed networks seems to be the most promising way to accomplish this capacity scaling. Unfortunately, unlike electronic memory, it remains a formidable challenge to build even a few dozen packets of integrated all-optical buffers. On the other hand, many high-speed networks depend on the TCP/IP protocol for reliability which is typically implemented in software and is sensitive to buffer size. For example, TCP requires a buffer size of bandwidth delay product in switches/routers to maintain nearly 100\% link utilization. Otherwise, the performance will be much downgraded. But such large buffer will challenge hardware design and power consumption, and will generate queuing delay and jitter which again cause problems. Therefore, improve TCP performance over tiny buffered high-speed networks is a top priority. This dissertation studies the TCP performance in 10Gbps high-speed networks. First, a 10Gbps reconfigurable optical networking testbed is developed as a research environment. Second, a 10Gbps traffic sniffing tool is developed for measuring and analyzing TCP performance. New expressions for evaluating TCP loss synchronization are presented by carefully examining the congestion events of TCP. Based on observation, two basic reasons that cause performance problems are studied. We find that minimize TCP loss synchronization and reduce flow burstiness impact are critical keys to improve TCP performance in tiny buffered networks. Finally, we present a new TCP protocol called Multi-Channel TCP and a new congestion control algorithm called Desynchronized Multi-Channel TCP (DMCTCP). Our algorithm implementation takes advantage of a potential parallelism from the Multi-Path TCP in Linux. Over an emulated 10Gbps network ruled by routers with only a few dozen packets of buffers, our experimental results confirm that bottleneck link utilization can be much better improved by DMCTCP than by many other TCP variants. Our study is a new step towards the deployment of optical packet switching/routing networks

高速無線通信を含む多様なネットワークのための高速TCP/IPデータ転送技術の研究

早大学位記番号:新7791早稲田大

Performance Analytics of Cloud Networks

As the world becomes more inter-connected and dependent on the Internet, networks become ever more pervasive, and the stresses placed upon them more demanding. Similarly, the expectations of networks to maintain a high level of performance have also increased. Network performance is highly important to any business that operates online, depends on web traffic, runs any part of their infrastructure in a cloud environment, or even hosts their own network infrastructure. Depending upon the exact nature of a network, whether it be local or wide-area, 10 or 100 Gigabit, it will have distinct performance characteristics and it is important for a business or individual operating on the network to understand those performance characteristics and how they affect operations. To better understand our networks, it is necessary that we test them to measure their performance capabilities and track these metrics over time. In our work, we provide an in-depth analysis of how best to run cloud benchmarks to increase our network intelligence and how we can use the results of those benchmarks to predict future performance and identify performance anomalies. To achieve this, we explain how to effectively run cloud benchmarks and propose a scheduling algorithm for running large numbers of cloud benchmarks daily. We then use the performance data gathered from this method to conduct a thorough analysis of the performance characteristics of a cloud network, train neural networks to forecast future throughput based on historical results and detect performance anomalies as they occur

Data Movement Challenges and Solutions with Software Defined Networking

With the recent rise in cloud computing, applications are routinely accessing and interacting with data on remote resources. Interaction with such remote resources for the operation of media-rich applications in mobile environments is also on the rise. As a result, the performance of the underlying network infrastructure can have a significant impact on the quality of service experienced by the user. Despite receiving significant attention from both academia and industry, computer networks still face a number of challenges. Users oftentimes report and complain about poor experiences with their devices and applications, which can oftentimes be attributed to network performance when downloading or uploading application data. This dissertation investigates problems that arise with data movement across computer networks and proposes novel solutions to address these issues through software defined networking (SDN). SDN is lauded to be the paradigm of choice for next generation networks. While academia explores use cases in various contexts, industry has focused on data center and wide area networks. There is a significant range of complex and application-specific network services that can potentially benefit from SDN, but introduction and adoption of such solutions remains slow in production networks. One impeding factor is the lack of a simple yet expressive enough framework applicable to all SDN services across production network domains. Without a uniform framework, SDN developers create disjoint solutions, resulting in untenable management and maintenance overhead. The SDN-based solutions developed in this dissertation make use of a common agent-based approach. The architecture facilitates application-oriented SDN design with an abstraction composed of software agents on top of the underlying network. There are three key components modern and future networks require to deliver exceptional data transfer performance to the end user: (1) user and application mobility, (2) high throughput data transfer, and (3) efficient and scalable content distribution. Meeting these key components will not only ensure the network can provide robust and reliable end-to-end connectivity, but also that network resources will be used efficiently. First, mobility support is critical for user applications to maintain connectivity to remote, cloud-based resources. Today\u27s network users are frequently accessing such resources while on the go, transitioning from network to network with the expectation that their applications will continue to operate seamlessly. As users perform handovers between heterogeneous networks or between networks across administrative domains, the application becomes responsible for maintaining or establishing new connections to remote resources. Although application developers often account for such handovers, the result is oftentimes visible to the user through diminished quality of service (e.g. rebuffering in video streaming applications). Many intra-domain handover solutions exist for handovers in WiFi and cellular networks, such as mobile IP, but they are architecturally complex and have not been integrated to form a scalable, inter-domain solution. A scalable framework is proposed that leverages SDN features to implement both horizontal and vertical handovers for heterogeneous wireless networks within and across administrative domains. User devices can select an appropriate network using an on-board virtual SDN implementation that manages available network interfaces. An SDN-based counterpart operates in the network core and edge to handle user migrations as they transition from one edge attachment point to another. The framework was developed and deployed as an extension to the Global Environment for Network Innovations (GENI) testbed; however, the framework can be deployed on any OpenFlow enabled network. Evaluation revealed users can maintain existing application connections without breaking the sockets and requiring the application to recover. Second, high throughput data transfer is essential for user applications to acquire large remote data sets. As data sizes become increasingly large, often combined with their locations being far from the applications, the well known impact of lower Transmission Control Protocol (TCP) throughput over large delay-bandwidth product paths becomes more significant to these applications. While myriads of solutions exist to alleviate the problem, they require specialized software and/or network stacks at both the application host and the remote data server, making it hard to scale up to a large range of applications and execution environments. This results in high throughput data transfer that is available to only a select subset of network users who have access to such specialized software. An SDN based solution called Steroid OpenFlow Service (SOS) has been proposed as a network service that transparently increases the throughput of TCP-based data transfers across large networks. SOS shifts the complexity of high performance data transfer from the end user to the network; users do not need to configure anything on the client and server machines participating in the data transfer. The SOS architecture supports seamless high performance data transfer at scale for multiple users and for high bandwidth connections. Emphasis is placed on the use of SOS as a part of a larger, richer data transfer ecosystem, complementing and compounding the efforts of existing data transfer solutions. Non-TCP-based solutions, such as Aspera, can operate seamlessly alongside an SOS deployment, while those based on TCP, such as wget, curl, and GridFTP, can leverage SOS for throughput improvement beyond what a single TCP connection can provide. Through extensive evaluation in real-world environments, the SOS architecture is proven to be flexibly deployable on a variety of network architectures, from cloud-based, to production networks, to scaled up, high performance data center environments. Evaluation showed that the SOS architecture scales linearly through the addition of SOS “agents†to the SOS deployment, providing data transfer performance improvement to multiple users simultaneously. An individual data transfer enhanced by SOS was shown to have increased throughput nearly forty times the same data transfer without SOS assistance. Third, efficient and scalable video content distribution is imperative as the demand for multimedia content over the Internet increases. Current state of the art solutions consist of vast content distribution networks (CDNs) where content is oftentimes hosted in duplicate at various geographically distributed locations. Although CDNs are useful for the dissemination of static content, they do not provide a clear and scalable model for the on demand production and distribution of live, streaming content. IP multicast is a popular solution to scalable video content distribution; however, it is seldom used due to deployment and operational complexity. Inspired from the distributed design of todays CDNs and the distribution trees used by IP multicast, a SDN based framework called GENI Cinema (GC) is proposed to allow for the distribution of live video content at scale. GC allows for the efficient management and distribution of live video content at scale without the added architectural complexity and inefficiencies inherent to contemporary solutions such as IP multicast. GC has been deployed as an experimental, nation-wide live video distribution service using the GENI network, broadcasting live and prerecorded video streams from conferences for remote attendees, from the classroom for distance education, and for live sporting events. GC clients can easily and efficiently switch back and forth between video streams with improved switching latency latency over cable, satellite, and other live video providers. The real world dep loyments and evaluation of the proposed solutions show how SDN can be used as a novel way to solve current data transfer problems across computer networks. In addition, this dissertation is expected to provide guidance for designing, deploying, and debugging SDN-based applications across a variety of network topologies

Reconfigurable Optically Interconnected Systems

With the immense growth of data consumption in today's data centers and high-performance computing systems driven by the constant influx of new applications, the network infrastructure supporting this demand is under increasing pressure to enable higher bandwidth, latency, and flexibility requirements. Optical interconnects, able to support high bandwidth wavelength division multiplexed signals with extreme energy efficiency, have become the basis for long-haul and metro-scale networks around the world, while photonic components are being rapidly integrated within rack and chip-scale systems. However, optical and photonic interconnects are not a direct replacement for electronic-based components. Rather, the integration of optical interconnects with electronic peripherals allows for unique functionalities that can improve the capacity, compute performance and flexibility of current state-of-the-art computing systems. This requires physical layer methodologies for their integration with electronic components, as well as system level control planes that incorporates the optical layer characteristics. This thesis explores various network architectures and the associated control plane, hardware infrastructure, and other supporting software modules needed to integrate silicon photonics and MEMS based optical switching into conventional datacom network systems ranging from intra-data center and high-performance computing systems to the metro-scale layer networks between data centers. In each of these systems, we demonstrate dynamic bandwidth steering and compute resource allocation capabilities to enable significant performance improvements. The key accomplishments of this thesis are as follows. In Part 1, we present high-performance computing network architectures that integrate silicon photonic switches for optical bandwidth steering, enabling multiple reconfigurable topologies that results in significant system performance improvements. As high-performance systems rely on increased parallelism by scaling up to greater numbers of processor nodes, communication between these nodes grows rapidly and the interconnection network becomes a bottleneck to the overall performance of the system. It has been observed that many scientific applications operating on high-performance computing systems cause highly skewed traffic over the network, congesting only a small percentage of the total available links while other links are underutilized. This mismatch of the traffic and the bandwidth allocation of the physical layer network presents the opportunity to optimize the bandwidth resource utilization of the system by using silicon photonic switches to perform bandwidth steering. This allows the individual processors to perform at their maximum compute potential and thereby improving the overall system performance. We show various testbeds that integrates both microring resonator and Mach-Zehnder based silicon photonic switches within Dragonfly and Fat-Tree topology networks built with conventional equipment, and demonstrate 30-60% reduction in execution time of real high-performance benchmark applications. Part 2 presents a flexible network architecture and control plane that enables autonomous bandwidth steering and IT resource provisioning capabilities between metro-scale geographically distributed data centers. It uses a software-defined control plane to autonomously provision both network and IT resources to support different quality of service requirements and optimizes resource utilization under dynamically changing load variations. By actively monitoring both the bandwidth utilization of the network and CPU or memory resources of the end hosts, the control plane autonomously provisions background or dynamic connections with different levels of quality of service using optical MEMS switching, as well as initializing live migrations of virtual machines to consolidate or distribute workload. Together these functionalities provide flexibility and maximize efficiency in processing and transferring data, and enables energy and cost savings by scaling down the system when resources are not needed. An experimental testbed of three data center nodes was built to demonstrate the feasibility of these capabilities. Part 3 presents Lightbridge, a communications platform specifically designed to provide a more seamless integration between processor nodes and an optically switched network. It addresses some of the crucial issues faced by the works presented in the previous chapters related to optical switching. When optical switches perform switching operations, they change the physical topology of the network, and they lack the capability to buffer packets, resulting in certain optical circuits being unavailable. This prompts the question of whether it is safe to transmit packets by end hosts at any given time. Lightbridge was developed to coordinate switching and routing of optical circuits across the network, by having the processors gain information about the current state of the optical network before transmitting packets, and being able to buffer packets when the optical circuit is not available. This part describes details of Lightbridge which is constituted by a loadable Linux kernel module along with other supporting modifications to the Linux kernel in order to achieve the necessary functionalities

Experimental Evaluation and Comparison of Time-Multiplexed Multi-FPGA Routing Architectures

Emulating large complex designs require multi-FPGA systems (MFS). However, inter-FPGA communication is confronted by the challenge of lack of interconnect capacity due to limited number of FPGA input/output (I/O) pins. Serializing parallel signals onto a single trace effectively addresses the limited I/O pin obstacle. Besides the multiplexing scheme and multiplexing ratio (number of inter-FPGA signals per trace), the choice of the MFS routing architecture also affect the critical path latency. The routing architecture of an MFS is the interconnection pattern of FPGAs, fixed wires and/or programmable interconnect chips. Performance of existing MFS routing architectures is also limited by off-chip interface selection. In this dissertation we proposed novel 2D and 3D latency-optimized time-multiplexed MFS routing architectures. We used rigorous experimental approach and real sequential benchmark circuits to evaluate and compare the proposed and existing MFS routing architectures. This research provides a new insight into the encouraging effects of using off-chip optical interface and three dimensional MFS routing architectures. The vertical stacking results in shorter off-chip links improving the overall system frequency with the additional advantage of smaller footprint area. The proposed 3D architectures employed serialized interconnect between intra-plane and inter-plane FPGAs to address the pin limitation problem. Additionally, all off-chip links are replaced by optical fibers that exhibited latency improvement and resulted in faster MFS. Results indicated that exploiting third dimension provided latency and area improvements as compared to 2D MFS. We also proposed latency-optimized planar 2D MFS architectures in which electrical interconnections are replaced by optical interface in same spatial distribution. Performance evaluation and comparison showed that the proposed architectures have reduced critical path delay and system frequency improvement as compared to conventional MFS. We also experimentally evaluated and compared the system performance of three inter-FPGA communication schemes i.e. Logic Multiplexing, SERDES and MGT in conjunction with two routing architectures i.e. Completely Connected Graph (CCG) and TORUS. Experimental results showed that SERDES attained maximum frequency than the other two schemes. However, for very high multiplexing ratios, the performance of SERDES & MGT became comparable

Performance Optimization and Dynamics Control for Large-scale Data Transfer in Wide-area Networks

Transport control plays an important role in the performance of large-scale scientific and media streaming applications involving transfer of large data sets, media streaming, online computational steering, interactive visualization, and remote instrument control. In general, these applications have two distinctive classes of transport requirements: large-scale scientific applications require high bandwidths to move bulk data across wide-area networks, while media streaming applications require stable bandwidths to ensure smooth media playback. Unfortunately, the widely deployed Transmission Control Protocol is inadequate for such tasks due to its performance limitations. The purpose of this dissertation is to conduct rigorous analytical study of the design and performance of transport solutions, and develop an integrated transport solution in a systematical way to overcome the limitations of current transport methods. One of the primary challenges is to explore and compose a set of feasible route options with multiple constraints. Another challenge essentially arises from the randomness inherent in wide-area networks, particularly the Internet. This randomness must be explicitly accounted for to achieve both goodput maximization and stabilization over the constructed routes by suitably adjusting the source rate in response to both network and host dynamics.The superior and robust performance of the proposed transport solution is extensively evaluated in a simulated environment and further verified through real-life implementations and deployments over both Internet and dedicated connections under disparate network conditions in comparison with existing transport methods

Exploitation from Malicious PCI Express Peripherals

The thesis of this dissertation is that, despite widespread belief in the security community, systems are still vulnerable to attacks from malicious peripherals delivered over the PCI Express (PCIe) protocol. Malicious peripherals can be plugged directly into internal PCIe slots, or connected via an external Thunderbolt connection. To prove this thesis, we designed and built a new PCIe attack platform. We discovered that a simple platform was insufficient to carry out complex attacks, so created the first PCIe attack platform that runs a full, conventional OS. To allows us to conduct attacks against higher-level OS functionality built on PCIe, we made the attack platform emulate in detail the behaviour of an Intel 82574L Network Interface Controller (NIC), by using a device model extracted from the QEMU emulator. We discovered a number of vulnerabilities in the PCIe protocol itself, and with the way that the defence mechanisms it provides are used by modern OSs. The principal defence mechanism provided is the Input/Output Memory Management Unit (IOMMU). The remaps the address space used by peripherals in 4KiB chunks, and can prevent access to areas of address space that a peripheral should not be able to access. We found that, contrary to belief in the security community, the IOMMUs in modern systems were not designed to protect against attacks from malicious peripherals, but to allow virtual machines direct access to real hardware. We discovered that use of the IOMMU is patchy even in modern operating systems. Windows effectively does not use the IOMMU at all; macOS opens windows that are shared by all devices; Linux and FreeBSD map windows into host memory separately for each device, but only if poorly documented boot flags are used. These OSs make no effort to ensure that only data that should be visible to the devices is in the mapped windows. We created novel attacks that subverted control flow and read private data against systems running macOS, Linux and FreeBSD with the highest level of relevant protection enabled. These represent the first use of the relevant exploits in each case. In the final part of this thesis, we evaluate the suitability of a number of proposed general purpose and specific mitigations against DMA attacks, and make a number of recommendations about future directions in IOMMU software and hardware.EPSRC and ARM iCASE Awar

Offset time-emulated architecture for optical burst switching-modelling and performance evaluation

L'evolució de les xarxes publiques de transport de dades destaca per el continu augment de la demanda de tràfic a la que estan sotmeses. La causa és la imparable popularització d'Internet i del seu ús per a tot tipus d'aplicacions. Les xarxes de commutació de ràfegues òptiques (OBS: Optical Bursts Switching) són una solució extraordinàriament prometedora per la pròxima generació de xarxes, tant per la flexibilitat que ofereixen com per el seu alt rendiment fruit de l'explotació de la multiplexació estadística en el domini òptic.Aquesta tesi presenta l'anàlisi, modelització i avaluació de les xarxes de commutació de ràfegues òptiques basades en l'emulació del temps de compensació (emulated offset time: E-OBS). El concepte d'E-OBS defineix una arquitectura de xarxa OBS per al transportar i commutar ràfegues òptiques en una xarxa troncal en la que, al contrari de l'arquitectura convencional (C-OBS) en la que el temps de compensació s'introdueix des dels nodes d'accés, el temps de compensació s'introdueix en cadascun dels nodes de la xarxa per mitjà d'un retardador de fibra addicional. L'arquitectura E-OBS permet superar algunes de les desavantatges inherents a arquitectures C-OBS, però la seva gran virtut és la compatibilitat amb les xarxes de commutació de circuits òptics (OCS: Optical Circuit Switching) actuals i les futures xarxes de commutació de paquets òptics (OPS: Optical Packet Switching), de manera que les xarxes OBS basades en una arquitectura E-OBS) poden facilitat enormement la transició de unes a les altres.A ala vista dels principals requeriments de disseny de les xarxes OBS, que són la resolució de contencions en el domini òptic, la provisió de qualitat de servei (QoS) i l'òptim encaminament de les ràfegues per tal de minimitzar la congestió de la xarxa, . en aquesta tesi es proposa un disseny de l'arquitectura E-OBS basada en (i) un mètode viable per a la provisió del temps de compensació, (ii) una qualitat alta global de servei, i (iii) un mecanisme d'encaminament que minimitzi congestió de xarxa.- La primera part d'aquesta tesi proporciona la informació documental necessària per al disseny d'E-OBS.- La segona part se centra en l'estudi de la funcionalitat i viabilitat de l'arquitectura E-OBS. S'introdueixen els principis d'operació d'E-OBS i s'identifiquen els principals esculls que presenten les arquitectures C-OBS i que deixen de ser-ho en una arquitectura E-OBS. Alguns d'aquests esculls són la dificultat d'utilitzar un algorisme d'encaminament amb rutes alternatives, la complexitat dels algoritmes de reserva de recursos i la seva falta d'equitat, la complexitat en la provisió de la QoS, etc. En aquesta segona part es constata que l'arquitectura E-OBS redueix la complexitat dels de reserva de recursos i es verifica la viabilitat d'operació i de funcionament de la provisió del tremps de compensació en aquesta arquitectura a partir de figures de comportament obtingudes amb retardadors de fibra comercialment disponibles.- La tercera part encara el problema de la provisió de la QoS. Primer s'hi revisen els conceptes bàsics de QoS així com els mecanismes de tractament de la QoS per a xarxes OBS fent-ne una comparació qualitativa i de rendiment de tots ells. Com a resultat s'obté que el mecanisme que presenta un millor comportament és el d'avortament de la transmissió de les ràfegues de més baixa prioritat quan aquestes col·lisionen amb una de prioritat més alta (es l'anomenat Burst Preemption mechanism), el qual en alguns casos presenta un problema de senyalització innecessària. Aquesta tercera part es conclou amb la proposta d'un mecanisme de finestra a afegir al esquema de Burst Preemption que només funciona sobre una arquitectura E-OBS i que soluciona aquest problema.- En la quarta part s'afronta el problema de l'encaminament en xarxes OBS. S'estudia el comportament dels algoritmes d'encaminament adaptatius, els aïllats amb rutes alternatives i els multicamí distribuïts, sobre xarxes E-OBS. A la vista dels resultats no massa satisfactoris que s'obtenen, es planteja una solució alternativa que es basa en model d'optimització no lineal. Es formulen i resolen dos models d'optimització per als algoritmes encaminament de font multicamí que redueixen notablement la congestió en les xarxes OBS.Finalment, aquesta tesi conclou que l'arquitectura E-OBS és factible, que és més eficient que la C-OBS, que proveeix eficaçment QoS, i que és capaç d'operar amb diverses estratègies d'encaminament i de reduir eficaçment la congestió de xarxa.The fact that the Internet is a packet-based connection-less network is the main driver to develop a data-centric transport network. In this context, the optical burst switching (OBS) technology is considered as a promising solution for reducing the gap between transmission and switching speeds in future networks.This thesis presents the analysis, modelling, and evaluation of the OBS network with Emulated offset-time provisioning (E-OBS). E-OBS defines an OBS network architecture to transport and switch optical data bursts in a core network. On the contrary to a conventional offset-time provisioning OBS (C-OBS) architecture, where a transmission offset time is introduced in the edge node, in an E-OBS network the offset time is provided in the core node by means of an additional fibre delay element. The architecture is motivated by several drawbacks inherent to C-OBS architectures. It should be pointed out that the E-OBS has not been studied intensively in the literature and this concept has been considered rather occasionally.Due to the limitations in optical processing and queuing, OBS networks need a special treatment so that they could solve problems typical of data-centric networks. Contention resolution in optical domain together with quality of service (QoS) provisioning for quality demanding services are, among other things, the main designing issues when developing OBS networks. Another important aspect is routing problem, which concerns effective balancing of traffic load so that to reduce burst congestion at overloaded links. Accounting for these requirements, the design objectives for the E-OBS architecture are (i) feasibility of offset-time provisioning, (ii) an overall high quality of service, and (iii) reduction of network congestion. These objectives are achieved by combining selected concepts and strategies, together with appropriate system design as well as network traffic engineering.The contributions in this thesis can be summarized as follows.- At the beginning, we introduce the principles of E-OBS operation and we demonstrate that C-OBS possesses many drawbacks that can be easily avoided in E-OBS. Some of the discussed issues are the problem of unfairness in resources reservation, difficulty with alternative routing, complexity of resources reservation algorithms, efficiency of burst scheduling, and complexity in QoS provisioning. The feasibility of E-OBS operation is investigated as well; in this context, the impact of congestion in control plane on OBS operation is studied. As a result, we confirm the feasibility of E-OBS operation with commercially available fibre delay elements.- Then, we provide both qualitative and quantitative comparison of the selected, most addressed in the literature, QoS mechanisms. As an outcome a burst preemption mechanism, which is characterized by the highest overall performance, is qualified for operating in E-OBS. Since the preemptive mechanism may produce the overbooking of resources in an OBS network we address this issue as well. We propose the preemption window mechanism to solve the problem. An analytical model of the mechanism legitimates correctness of our solution.- Finally, we concern with a routing problem - our routing objective is to help the contention resolution algorithms in the reduction of burst losses. We propose and evaluate two isolated alternative routing algorithms designed for labelled E-OBS networks. Then we study multi-path source routing and we use network optimization theory to improve it. The presented formulae for partial derivatives, to be used in a non-linear optimization problem, are straightforward and very fast to compute. It makes the proposed non-linear optimization method a viable alternative for linear programming formulations based on piecewise linear approximations.Concluding, E-OBS is shown to be a feasible OBS network architecture of profitable functionality, to support efficiently the QoS provisioning, and to be able to operate with different routing strategies and effectively reduce the network congestion