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📁 IBM SOCKET 教程。LINUX环境下SOCKET学习的教程。
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>IP、TCP 和 UDP</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 3 页（共4 页）</nobr></font></td>
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<p>正如上一小节所指出的，当您编写 socket 应用程序的时候，您可以在使用 TCP 还是使用 UDP 之间做出选择。它们都有各自的优点和缺点。</p>
<p>TCP 是流协议，而 UDP 是数据报协议。换句话说，TCP 在客户机和服务器之间建立持续的开放连接，在该连接的生命期内，字节可以通过该连接写出（并且保证顺序正确）。然而，通过 TCP 写出的字节没有内置的结构，所以需要高层协议在被传输的字节流内部分隔数据记录和字段。</p>
<p>另一方面，UDP 不需要在客户机和服务器之间建立连接，它只是在地址之间传输报文。UDP 的一个很好特性在于它的包是自分隔的（self-delimiting），也就是一个数据报都准确地指出它的开始和结束位置。然而，UDP 的一个可能的缺点在于，它不保证包将会按顺序到达，甚至根本就不保证。当然，建立在 UDP 之上的高层协议可能会提供握手和确认功能。</p>
<p>对于理解 TCP 和 UDP 之间的区别来说，一个有用的类比就是电话呼叫和邮寄信件之间的区别。在呼叫者用铃声通知接收者，并且接收者拿起听筒之前，电话呼叫不是活动的。只要没有一方挂断，该电话信道就保持活动，但是在通话期间，他们可以自由地想说多少就说多少。来自任何一方的谈话都按临时的顺序发生。另一方面，当你发一封信的时候，邮局在投递时既不对接收方是否存在作任何保证，也不对信件投递将花多长时间做出有力保证。接收方可能按与信件的发送顺序不同的顺序接收不同的信件，并且发送方也可能在他们发送信件是交替地接收邮件。与（理想的）邮政服务不同，无法送达的信件总是被送到死信办公室处理，而不再返回给发送者。
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