Preventing TCP performance interference on asymmetric links using ACKs-first variable-size queuing

In developing network-enabled embedded systems, developers are often forced to spend a great deal of time and effort analyzing and solving network performance problems. In this paper, we address one such problem: TCP performance interference on an asymmetric link. The upload or download throughput abruptly degrades if there is simultaneously upload and download TCP traffic on the link. While the problem has been addressed by many researchers, their solutions are incomplete as they only improve throughput in one direction, require TCP protocol modifications in end-user devices or are effective for a limited range of network configurations.In order to overcome such limitations, we propose ACKs-first variable-size queuing (AFVQ) for a gateway. In doing so, we have derived an analytic model of the steady-state TCP performance with bidirectional traffic to clearly identify the two sources of the problem: the excessive queuing delay of ACK packets and the excessive number of ACK packets in the queue. Our AFVQ mechanism is designed to directly eliminate the two causes. Specifically, we have based AFVQ on two policies. First, ACKs-first scheduling is used to shorten the queuing delay of ACK packets. Second, the queue size for ACK packets is dynamically adjusted depending on the number of data packets queued in the gateway so that the number of ACK packets is reduced when packets are congested in the gateway. By applying the two policies simultaneously at the uplink and downlink output queue in the gateway, AFVQ achieves balanced TCP throughput improvements in both directions. In this way, it breaks circular dependencies between upload and download traffic.We have implemented AFVQ in our ADSL-based residential gateway using the traffic control module of the Linux kernel. Our gateway yields 95.2% and 93.8% of the maximum download and upload bandwidth, respectively. We have also evaluated the proposed mechanism using the ns-2 simulator over a wide range of network configurations and have shown that AFVQ achieves better upload and download throughput than other representative gateway-based mechanisms such as ACQ, ACKs-first scheduling and ACK Filtering.     

无线局域网中实现TCP流公平的接入点缓存区大小的估计算法

研究了无线局域网的接入点(Access Point, AP)缓存区大小对TCP公平性的影响,并通过分析平均窗口与平均丢包率以及使用窗口与平均丢包率的关系,加上实现TCP公平性时的窗口限制条件,提出一种计算TCP上下行流公平时所需的AP缓存大小的方法。通过NS2仿真,验证了算法的准确性。     

一种实用的嵌入式Web服务器设计

数字化变电站越来越多地采用以太网技术,针对各种保护和控制IED(Integrated Drive Electronics)的参数配置以及状态监测问题,本文通过对Linux平台上的BOA服务器和CGIC的源码进行研究和修改,设计了一种实用的嵌入式Web服务器。它可方便地在支持TCP/IP协议的非Linux的嵌入式系统上运行,在不影响IED原有功能的前提下对用户提供Web服务,同时提出了一种适合嵌入式系统使用的访问权限认证和对Web页面的管理及动态生成机制,并支持Web方式下文件的上传与下载。它为变电站现场嵌入式IED的远端访问与控制提供了一种新的思路和方法。     

GE信道下WLAN节能站点传输TCP流功耗分析模型*

针对目前在无线局域网(WLAN)基础服务集(BSS)节能站点功耗分析模型上没有融入错误信道模型的问题,本文有机地结合了Gilbert-Elliot(GE)错误信道模型和WLAN BSS节能模式站点传输TCP流的功耗模型,提出并建立一种新的混合模型,同时给出了GE信道参数影响节能站点传输TCP流功耗的量化表达式,通过网络模拟器(NS2)仿真验证了新模型的正确性和有效性。     

Resource-aware and quality-fair video-streaming using multiple adaptive TCP connections

In this paper, we present a resource-aware and quality-fair video content sharing system. When a video sharing server has insufficient uplink bandwidth and needs to serve multiple video content sharing services via streaming or downloading to other client peers using TCP transport, each service shares the limited uplink bandwidth equitably, due to the fair sharing characteristics inherent in TCP. However this bandwidth fair sharing cannot always guarantee quality fairness among the services, due to the specific requirements for video-streaming services, such as the playout rate and the size of the playout buffer. In our system, the server uses multiple TCP connections adaptively, depending on the anticipated status of each client playout buffer, to guarantee the bandwidth of each video-streaming session. By guaranteeing the quality of each video-streaming session, without the quality loss of other service sessions, the proposed system can successfully achieve service quality fairness. Simulation results show that our proposed algorithm can dramatically enhance the quality of each streaming session and thus provide service quality fairness among simultaneous multiple heterogeneous video-streaming services and content download services.     

Analysis of two competing TCP/IP connections

Many mathematical models exist for describing the behavior of TCP/IP (TCP: transmission control protocol) under an exogenous loss process that does not depend on the window size. The goal of this paper is to present a mathematical analysis of two asymmetric competing TCP connections where loss probabilities are directly related to their instantaneous window size, and occur when the sum of throughputs attains a given level. We obtain bounds for the stationary throughput of each connection, as well as an exact expression for symmetric connections. This allows us to further study the fairness as a function of the different round trip times. We avoid the simplifying artificial synchronization assumption that has frequently been used in the past to study similar problems, according to which whenever one connection looses a packet, the other one looses a packet as well.     

基于asp.net的文件上传和下载

文件的上传和下载是网络中非常重要的两种应用,本文介绍了使用asp.net控件实现文件上传的基本方法。解决了当上传大文件时出现各种问题,并给出了几种解决方案和技巧。另外,下载文件用二进制读写的方式实现。     

Socket Buffer Auto-Sizing for High-Performance Data Transfers

It is often claimed that TCP is not a suitable transport protocol for data intensive Grid applications in high-performance networks. We argue that this is not necessarily the case. Without changing the TCP protocol, congestion control, or implementation, we show that an appropriately tuned TCP bulk transfer can saturate the available bandwidth of a network path. The proposed technique, called SOBAS, is based on automatic socket buffer sizing at the application layer. In non-congested paths, SOBAS limits the socket buffer size based on direct measurements of the received throughput and of the corresponding round-trip time. The key idea is that the send window should be limited, after the transfer has saturated the available bandwidth in the path, so that the transfer does not cause buffer overflows (self-induced losses). A difference with other socket buffer sizing schemes is that SOBAS does not require prior knowledge of the path characteristics, and it can be performed while the transfer is in progress. Experimental results in several high bandwidth-delay product paths show that SOBAS provides consistently a significant throughput increase (20% to 80%) compared to TCP transfers that use the maximum possible socket buffer size. We expect that SOBAS will be mostly useful for applications such as GridFTP in non-congested wide-area networks.     

DotDFS: A Grid-based high-throughput file transfer system

DotGrid platform is a Grid infrastructure integrated with a set of open and standard protocols recently implemented on the top of Microsoft .NET in Windows and MONO .NET in UNIX/Linux. DotGrid infrastructure along with its proposed protocols provides a right and solid approach to targeting other platforms, e.g., the native C/C++ runtime. In this paper, we propose a new concurrent file transfer protocol called DotDFS as a high-throughput distributed file transfer component for DotGrid. DotDFS introduces some open binary protocols for efficient file transfers on current Grid infrastructures. DotDFS protocol also provides mechanisms for multiple file streams to gain high-throughput file transfer similar to GridFTP protocol, but by proposing and implementing a new parallel TCP connection-oriented paradigm. Almost no research work has been conducted to suggest a concurrent file transfer protocol that simultaneously employs threaded and event-driven models in the protocol level. Due to our knowledge, DotDFS is the first concurrent file transfer protocol that, from this viewpoint, presents a new computing paradigm in the field of data transmission protocols. In our LAN tests, we have achieved better results than Globus GridFTP implementation particularly in multiple TCP streams and directory tree transfers. Our LAN experiences in memory-to-memory tests show that DotDFS accesses to the 94% bottleneck bandwidth while GridFTP is accessing 91%. In LAN disk-to-disk tests, comparing DotDFS protocol with GridFTP protocol unveils a set of interesting and technical problems in GridFTP for both the nature of the protocol and its implementation by Globus. In the WAN experimental studies, we propose a new idea for analytical modeling of file transfer protocols like DotDFS inspired by sampling, experimentation and mathematical interpolation approaches. The cross-platform and open standard-based features of DotDFS provide a substantial framework for unifying data access and resource sharing in real heterogeneous Grid environments.     

分布式视频会议服务器媒体管理系统

由于其方便易用及可靠性,又得益于网络的普及,基于TCP/IP的视频会议已经成为目前市场主流,因此搭建可靠的视频会议服务器也成为了需要解决的主要需求。视频会议服务器不同于普通的流媒体服务器,视频会议服务器需要较高的实时性,而且其接受的上传/下载速度比要比普通的流媒体下载/在线观看服务器高得多。视频会议服务器的媒体服务器主要负责流媒体的接收保存及同步转发。本文借鉴HDFS的最基本原则,提出了一种可靠的分布式结构用于解决视频会议服务器的媒体存储问题。     

窗口限制系统中基于令牌的Fast-TCP的延时性能分析

给出了在高带宽时延乘积网络中无拥塞丢包的TCP缓冲区的占用量的微分方程,然后针对广域网中窗口大小小于网络带宽时延乘积和无线网络中,由于误码丢包而使窗口尺寸无法达到链路带宽时延乘积(如卫星网)的情况,分析了一种基于令牌的Fast-TCP机制,得出基于令牌的ACK延时机制,不仅能够大大减少缓冲区的占用量结论,而且可获得与TCP在无限缓冲区下的同样的吞吐量.还把这个结论推广到多连接的情况,在以前论文中没有涉及.     

A stochastic model for the throughput of non-persistent TCP flows

The general aim of this paper is to analyze the throughput of a HTTP flow. For this, we introduce a simplified model of such a flow which consists of a succession of idle and download periods. The file downloads are subject to a fixed packet loss probability. The same TCP connection is possibly used for the download of a random number of files, for which the effect of the slow start is taken into account. For this stochastic model, we derive a closed form formula for the stationary throughput obtained by a flow. We also derive closed form expressions for the mean time to transfer a file and for the distribution of the throughput. Several laws of file sizes and idle times are considered including heavy tailed distributions. We also briefly discuss how the formulas can be applied to predict bandwidth sharing among competing HTTP flows.     

一种缩短下载时间优先的自适应BitTorrent激励协议

BitTorrent激励机制的目标是保证节点上传和下载之间的公平性,但相比公平性而言,实际应用中的节点更优先考虑的是文件下载时间,据此文中提出了一种缩短文件下载时间优先的自适应BitTorrent激励协议AIPS.文中首先基于Markov模型对BitTorrent现有激励机制的效果给出了定量分析,分析了激励机制下的文件传输结构,并用概率分析方法给出了该传输结构下最小化文件下载时间的条件.应用分析结果文中定义了一个以缩短文件下载时间为效用的博弈,在该博弈达到Nash平衡时各节点采用的策略就是激励协议AIPS.模拟实验表明文中提出的AIPS较现有的BitTorrent激励协议能明显提高文件共享系统性能,提高文件下载效率.     

Towards More Complete Models of TCP Latency and Throughput

Recently, several researchers have developed equations for modeling TCP behaviors, such as the expected throughput or latency, based on Markov chains derived from TCP with additional simplifying assumptions. In this paper, we suggest new directions for Markov chain analyses of TCP. Our first contribution is to closely examine not just the expectation but the entire cumulative distribution function of transfer times under various models. Particularly for short or medium transfers, the distribution is likely to be more useful than the expectation in terms of measuring end-user satisfaction. We find that the shapes of TCP cumulative distribution functions are remarkably robust to small changes in the model. Our results suggest that simplifying Markov analyses can be extended to yield approximations for the entire distribution as well as for the expectation.Our second contribution is to consider correction procedures to enhance these models. A correction procedure is a rule of thumb that allows equations from one model to be used in other situations. As an example, several analyses use a Drop-Tail loss model. We determine correction procedures for the deviation between this model and other natural loss models based on simulations. The existence of a simple correction procedure in this instance suggests that the high-level behavior of TCP is robust against changes in the loss model.     

基于WEB的学生作业评阅系统的设计与实现

基于Browser/Server/Database结构设计和实现了Web方式的学生作业评阅系统。系统详细分析了以文件上传和下载实现作业评阅的流转过程,也解决了支持中文文件名通过Internet传送过程中的问题。     

A Queuing Model for Evaluating the Transfer Latency of Peer-to-Peer Systems

This paper presents a queuing model to evaluate the latency associated with file transfers or replications in peer-to-peer (P2P) computer systems. The main contribution of this paper is a modeling framework for the peers that accounts for the file size distribution, the search time, load distribution at peers, and number of concurrent downloads allowed by a peer. We propose a queuing model that models the nodes or peers in such systems as M/G/1/K processor sharing queues. The model is extended to account for peers which alternate between online and offline states. The proposed queuing model for the peers is combined with a single class open queuing network for the routers interconnecting the peers to obtain the overall file transfer latency. We also show that in scenarios with multipart downloads from different peers, a rate proportional allocation strategy minimizes the download times.     

一种Web 中快速传输大型文件的方法

实现了WEB应用程序中快速传输大型文件的机制．浏览器端采用HTML5标准中文件系统的访问特性,对上载的文件首先进行分片实现并行上传．服务器端采用CGI程序接收浏览器端上传的文件．后台管理服务对上传的分片文件进行管理．下载文件时通过CGI请求后台管理服务,按序发送文件分片．整个过程均不涉及文件分片的重组过程,减少了磁盘IO请求．与其它文件上传下载方式相比,本方法具有上下载文件大小无限制,传输速度高及不需要第三方控件等特点．     

基于PHP技术的网络文件管理系统设计

针对当前网络文件管理系统发展的需要,V2PHP和MySQL技术为重点,设计了一种基于PHP才支术的网络文件管理系统,以提供文件上传、下载等存储服务和文件浏览、更新、分类、分享等管理服务。该系统操作界面友好,处理效率高,并具有良好的安全性设计,可为用户进行文件在线存储、管理以及用户之间的文件共享等提供方便可靠的途径。     

Fair and distributed peer-to-peer allocation of a common, refillable resource

We consider the general problem of distributed and fair peer-to-peer (P2P) allocation of a common, refillable resource. This problem recurs in a number of scenarios, for example grid computing, content distribution, Internet Service Provider service sharing, and distributed file storage over asymmetric channels. We present several distributed schemes for this allocation problem and show that these schemes guarantee two key properties: (i) asymptotic fairness, in that (even maliciously colluding) users are proportionally assigned resources corresponding to what they contribute; (ii) natural incentive to join and cooperate fairly in the system. We demonstrate the practicability of our approaches on a prototype P2P file storage system designed for typical residential Internet connections, in which download capacities often significantly exceed upload capacities. Our implementation shares file data when communications are idle using random linear codes so that, when needed, an end-user can download a file from several sources at a higher data rate than his home computer’s upload capacity. We present experimental results that support our analytical guarantees.