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计算机网络概述.html
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$('.post_thumbnail-random').addClass('lazy');</script><p class="article-headline-p">计算机网络概述</p></div><div class="mdl-color-text--grey-700 mdl-card__supporting-text meta"><div id="author-avatar"><img src="/img/avatar.jpg" width="44px" height="44px" alt="Author Avatar"></div><div><strong>freelee</strong> <span>创建于3月 09, 2020</span> <span>更新于4月 19, 2020</span></div><div class="section-spacer"></div><button id="article-functions-viewtags-button" class="mdl-button mdl-js-button mdl-js-ripple-effect mdl-button--icon"><i class="material-icons" role="presentation">bookmark</i> <span class="visuallyhidden">bookmark</span></button><ul class="mdl-menu mdl-menu--bottom-right mdl-js-menu mdl-js-ripple-effect" for="article-functions-viewtags-button"><li class="mdl-menu__item"><a class="post_tag-link" href="/tags/计算机网络/">计算机网络</a></li></ul><button id="article-fuctions-share-button" class="mdl-button mdl-js-button mdl-js-ripple-effect mdl-button--icon"><i class="material-icons" 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href="https://cdn.jsdelivr.net/npm/aplayer/dist/APlayer.min.css"><script src="https://cdn.jsdelivr.net/npm/aplayer/dist/APlayer.min.js"></script><script src="https://cdn.jsdelivr.net/npm/meting@2/dist/Meting.min.js"></script><script src="https://unpkg.com/[email protected]"></script><link rel="stylesheet" href="/css/prism-atom-dark.css" type="text/css"></head><div id="post-content" class="mdl-color-text--grey-700 mdl-card__supporting-text fade out"><p> 两个主机相互连接，不仅需要链路和路由器/交换机，还需要一系列的协议，最重要的一个协议就是TCP/IP协议，否则数据无法正常互相传送。<br><br> 在网络的边缘，有很多接入ISP将无数个主机相连，首先是结构1：一个全球ISP与所有主机相连，但是开销太大难以实现。随后演化出了结构2：有数个区域ISP先与主机相连，再由根ISP将这些区域ISP连接起来，这个时候由于有多个全球ISP存在，出现了竞争现象。这个结构逐渐演化成更多级的类似结构，像树结构一样。在此基础上，为了使这个因特网成为网络的网络，引入了多宿和对等概念，多宿即一个ISP可以与多个ISP提供商相连，增强了网络的健壮性，如果有一个ISP提供商挂了，其他相连的ISP提供商仍然可以继续维持其运行。对等即在同一级别的ISP可以直接相连，在此前结构中，两个低级ISP要相连必须通过上一级ISP直至顶层ISP来实现，而对等则可以使两个同级别ISP直接相连，不仅提高了速度还减少了支付给上级ISP的费用，且对等ISP基本是不会相互结算的。结构5中加入了内容提供商网络，如谷歌，不仅可以与较低层ISP对等，也可与第一层ISP相连。<br><br> 接着是网络核心，主机是通过路由器实现交换报文，而报文可以有两种方式进行转发。一种是分组交换，另一种是电路交换。分组交换就是将报文分块形成分组，然后进行交换，有点类似于流水线。分组交换机中多数使用存储转发机制，即一个分组全部到达交换机后才可以向外转发，因为交换机先缓存分组，这也就会出现一个现象，排队，就是分组的发送速度大于交换机发出的速度。一个分组通过由N条（N-1个交换机）速率均为R的链路组成的路径，所需时间为N<em>（L/R），若有p个分组则需要（N+P-1）</em>（L/R）[分组大小为L比特]。在转发的时候，路由器需要准确的将分组转发到对应的输出链路，这时就要通过转发表来实现，交换机可以根据分组的目的地址索引转发表来找到输出链路，那么如何实现转发表机制呢？路由选择协议来实现这一功能。<br><br> 另一个中报文交换方式为电路交换，分组交换时不需要预留资源和缓存，但是电路交换则必须要预留。电路交换好比打电话，只有两方都接通才能进行通话，这里就会造成一种资源浪费，如果对方接通了但就放在那什么也没说，那么这部分资源不久闲置了嘛，导致了分组交换在很多领域取代了电路交换。以前的宽带都是从电话线接出来的，是因为使用了频分复用FDM，一部分频段分给电话，一部分分给网络链路。对应的还有时分复用方式TDM，即将一段时间分成几段，比如说4段，那么第一段就是给用户1来使用，第二段给用户2来使用，这就会造成很大的浪费，但是未搞懂为什么时分复用中传输时间与链路数量无关？？？总的来说，分组交换更加简单、有效、成本更低。<br><br> 时延除了传输时延，还有处理时延（在交换机、源处理机检查处理报文的时间）、传播时延（报文在链路上传播的时间）、排队时延。在排队时延里引入了一个概念流量强度La/R（报文由L比特组成，a表示分组到达队列的平均速率pkt/s，R是传输速率bps/s）。若流量强度大于1，就会造成排队现象，进一步若路由器的队列已满，之后来的分组都会被丢弃也就是丢包。<br><br> 除了时延和丢包，吞吐量也是评价计算机性能的一个重要指标。吞吐量又分为瞬时吞吐量和平均吞吐量，例如传输文件更关注平均吞吐量，而视频通话则更依赖瞬时吞吐量。如图这种情况则会出现瓶颈链路的概念，即吞吐量为min{Rc，Rs}<br><img src="/images/Compute Network/genelization/cs.png" alt=""><br> 然后是分层协议，分层有一个好处，就是该层使用下一层提供的服务，并向上一层提供服务，具有结构化和概念化的优点，这种模块化方式可以在修改一层情况下不需要修改其他层。各层通过协议与对应的层进行联系，各层的所有协议被成为协议栈。协议栈有两种形式：五层协议栈和七层ISO OSI参考模型。五层协议栈中分别为应用层、链路层、网络层、运输层、物理层。其中应用层是网络应用程序及其协议所在的地方，例如HTTP、FTP、SMTP、DNS。运输层在应用程序之间传送应用层报文，有两个协议为TCP和UDP，其中TCP提供了面向连接的服务，运输层报文成为报文段。网络层的IP协议定义了数据报中各个字段以及端系统和路由器如何作用于这些字段，网络层还具有路由选择协议。为了将分组从一个结点移动到下一个结点，需使用链路层的服务，传到下一个节点后，链路层将数据报上传给网络层，并且在链路层的分组称为帧。物理层的任务是将该帧中的一个一个比特移动到下一个结点，与该链路的实际传输媒体相关。<br><img src="/images/Compute Network/genelization/osi.png" alt=""><br> OSI七层协议栈，在应用层下添加了表示层和会话层。表示层的作用是使通信的应用程序能够解释交换数据的含义，这些服务包括数据压缩、加密、描述。会话层提供了数据交换定界和同步功能，包括了建立检查点和恢复方案的方法。而后的五层协议减少的这两层留给了应用程序开发者来处理，若服务重要则在应用程序中构建该服务。<br><img src="/images/Compute Network/genelization/protocol_stack.png" alt=""><br> 还有一个重要的概念：封装。上一层的数据信息向下传作为有效载荷字段和首部字段组成下一层的数据信息。从应用层到物理层分别为：应用层报文-》运输层报文段-》网络数据报-》链路层帧。</p></div><script language="javascript">function siteTime(){window.setTimeout("siteTime()",1e3);var e=36e5,t=24*e,o=new 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