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177.
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TCP is a connection-based protocol, offering error correction and guaranteed delivery of data via what is known as <emphasis>flow control</emphasis>. Flow control determines when the flow of a data stream needs to be stopped, and previously sent data packets should to be re-sent due to problems such as <emphasis>collisions</emphasis>, for example, thus ensuring complete and accurate delivery of the data. TCP is typically used in the exchange of important information such as database transactions.
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TCP 是一个基于连接的协议，提供纠错并通过 <emphasis>流量控制</emphasis> 来传输数据。流量控制决定什么时间一个数据流需要停止，例如在出现诸如 <emphasis>冲突</emphasis> 等问题时重发先前发送的数据包，以确保完整和准确的数据传输。TCP 常用于重要信息的交换，如数据库传输。
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Translated and reviewed by
姚渺波
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TCP 是一个基于连接的协议，提供纠错并通过 <emphasis>流量控制</emphasis> 来确保数据的送达。流量控制决定像什么时候需要停止一个数据流，以及在出现诸如 <emphasis>冲突</emphasis> 等问题时重发先前发送的数据包，以确保完整和准确的数据传输。TCP 常用于重要信息的交换，如数据库事务。
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Suggested by
Ihnus Qcshz
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Located in
serverguide/C/network-config.xml:783(para)
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178.
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The User Datagram Protocol (UDP), on the other hand, is a <emphasis>connectionless</emphasis> protocol which seldom deals with the transmission of important data because it lacks flow control or any other method to ensure reliable delivery of the data. UDP is commonly used in such applications as audio and video streaming, where it is considerably faster than TCP due to the lack of error correction and flow control, and where the loss of a few packets is not generally catastrophic.
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另一方面，用户数据报协议 (UDP) 是一个 <emphasis>无连接</emphasis> 协议，很少用于重要数据的传输，因为缺乏流量控制或其他一些确保可靠数据传输的方法。UDP 常用在如音视频流这样的应用程序，由于它缺少纠错和流控，因此相对于 TCP 来说更快，而且丢失少量包通常也不会造成灾难性的后果。
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Translated and reviewed by
姚渺波
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Located in
serverguide/C/network-config.xml:791(para)
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179.
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ICMP
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ICMP
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Translated and reviewed by
姚渺波
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Located in
serverguide/C/network-config.xml:801(title)
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180.
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The Internet Control Messaging Protocol (ICMP) is an extension to the Internet Protocol (IP) as defined in the Request For Comments (RFC) #792 and supports network packets containing control, error, and informational messages. ICMP is used by such network applications as the <application>ping</application> utility, which can determine the availability of a network host or device. Examples of some error messages returned by ICMP which are useful to both network hosts and devices such as routers, include <emphasis>Destination Unreachable</emphasis> and <emphasis>Time Exceeded</emphasis>.
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Internet 控制消息协议是在Request For Comments (RFC) #792 中定义的，是对网际协议 (IP) 的一个扩充。支持的网络包包括控制、错误和信息的消息。ICMP 常被用在诸如判断一台网络主机或设备可用性的 <application>ping</application> 工具这样的网络应用程序。在网络主机和设备如路由器之间使用 ICMP 所返回的错误消息示例包括 <emphasis>Destination Unreachable</emphasis> 和 <emphasis>Time Exceeded</emphasis>。
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Translated and reviewed by
姚渺波
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Located in
serverguide/C/network-config.xml:802(para)
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181.
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Daemons
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守护程序
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Translated and reviewed by
姚渺波
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Located in
serverguide/C/network-config.xml:812(title)
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182.
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Daemons are special system applications which typically execute continuously in the background and await requests for the functions they provide from other applications. Many daemons are network-centric; that is, a large number of daemons executing in the background on an Ubuntu system may provide network-related functionality. Some examples of such network daemons include the <emphasis>Hyper Text Transport Protocol Daemon</emphasis> (httpd), which provides web server functionality; the <emphasis>Secure SHell Daemon</emphasis> (sshd), which provides secure remote login shell and file transfer capabilities; and the <emphasis>Internet Message Access Protocol Daemon</emphasis> (imapd), which provides E-Mail services.
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守护程序是特殊的系统应用程序，一般常驻在后台并等待来自其他应用程序请求其所提供的功能。许多守护程序都是网络中心；在 Ubuntu 系统后台执行的许多守护程序都可以提供网络的相关功能。这些网络守护程序包括 <emphasis>超文本传输协议守护程序</emphasis> (httpd)，用于提供网站服务器功能；<emphasis>Secure SHell 守护程序</emphasis> (sshd)，用于提供安全远程登录 shell 和文件传输功能；<emphasis>Internet Message Access Protocol 守护程序</emphasis> (imapd)，用于提供 E-Mail 服务。
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Translated and reviewed by
姚渺波
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| In upstream: |
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守护程序是特殊的系统应用程序，一般常驻在后台并等待来自其他应用程序请求其所提供的功能。许多守护程序都是面向网络的；也就是说，在 Ubuntu 系统后台执行的许多守护程序都可以提供网络的相关功能。这些网络守护程序包括 <emphasis>超文本传输协议守护程序</emphasis> (httpd)，用于提供web服务器功能；<emphasis>Secure SHell 守护程序</emphasis> (sshd)，用于提供安全远程登录 shell 和文件传输功能；<emphasis>Internet Message Access Protocol 守护程序</emphasis> (imapd)，用于提供 E-Mail 服务。
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Suggested by
Ihnus Qcshz
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Located in
serverguide/C/network-config.xml:813(para)
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183.
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Firewall Configuration
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防火墙配置
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Translated and reviewed by
姚渺波
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Located in
C/network-applications.xml:374(title)
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184.
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The Linux kernel includes the <emphasis>Netfilter</emphasis> subsystem, which is used to manipulate or decide the fate of network traffic headed into or through your server. All modern Linux firewall solutions use this system for packet filtering.
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Linux 内核包括 <emphasis>Netfilter</emphasis> 子系统，用来处理或决定网络传输头部进入或穿过你的服务器，目前所有的 Linux 防火墙都用该系统来做包过滤。
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Translated and reviewed by
姚渺波
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Located in
serverguide/C/security.xml:355(para)
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185.
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Firewall Introduction
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防火墙介绍
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Translated and reviewed by
姚渺波
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Located in
C/network-applications.xml:381(title)
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186.
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The kernel's packet filtering system would be of little use to administrators without a userspace interface to manage it. This is the purpose of iptables. When a packet reaches your server, it will be handed off to the Netfilter subsystem for acceptance, manipulation, or rejection based on the rules supplied to it from userspace via iptables. Thus, iptables is all you need to manage your firewall if you're familiar with it, but many frontends are available to simplify the task.
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内核的包过滤系统如果没有一个用户态 (userspace) 界面来管理它的话对管理员来说几乎没有用。这正是 iptables 的目的。当一个包到达您的服务器，它从用户态 (userspace) 通过 iptables 传给 Netfilter 子系统，然后基于提供的规则去接受、操作或拒绝。因此，如果你能熟悉它的话，那么 iptables 就是您管理您防火墙所需的全部。
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Translated and reviewed by
姚渺波
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Located in
serverguide/C/security.xml:360(para)
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