无线客户端通过开放无线接入点上网全过程

802.11是现在部署非常广泛的也是支持的客户端最多的无线上网协议。

支持802.11协议的客户端想要通过无线路由上网时，首先必须要知道有哪些无线接入点存在，这就想你想通过有线上网必须要

先找到网口一样。在找到无线接入点之后，客户端要和接入点进行绑定，并在得到接入点的认可之后，才能获得

权限访问网络。整个过程涉及到很多内容，包括客户端和接入点之间协商数据传输速度，协商认证方式等。

为了搞清楚整个客户端通过接入点连接网络的整个过程，我用wireshark工具抓了笔记本和一个开放接入点之间

的网络包，简单分析后，使用mscgen工具描绘了大概的交互过程。

抓包的过程是这样的：首先打开wireshark指定从wlan0抓包，然后在浏览器中输入www.163.com. 当然上图只是从大量的网络包中提取

出来的我认为能够描述主要过程的包。实际上802.11协议需要大量的控制包来协助无线客户端和接入点进行通信，同时你的无线网卡

也会收到大量的其他无线节点发过来的协议包，因为空中的协议包是对任何人开放的。

下面是我画这个交互图的mscgen描述语言。

msc { arcgradient = 10 ; a [label = " PC " ],b [label = " AP " ]; a => b [label = " 12468 probe request " , textcolour = " blue " , linecolour = " blue " ]; a => b [label = " 12487 authentication " , textcolour = " blue " , linecolour = " blue " ]; b => a [label = " 12490 authentication " , textcolour = " red " , linecolour = " red " ]; a => b [label = " 12492 associate request " , textcolour = " blue " , linecolour = " blue " ]; b => a [label = " 12494 associate response " , textcolour = " red " , linecolour = " red " ]; a => b [label = " 12678 dhcp request " , textcolour = " blue " , linecolour = " blue " ]; b => a [label = " 12687 dhcp ack " , textcolour = " red " , linecolour = " red " ]; a => b [label = " 12698 igmp/ipv4 mcast " , textcolour = " blue " , linecolour = " blue " ]; a => b [label = " 15195 dns query www.163.com " , textcolour = " blue " , linecolour = " blue " ]; b => a [label = " 15217 dns response " , textcolour = " red " , linecolour = " red " ]; a => b [label = " 15223 tcp [syn] " , textcolour = " blue " , linecolour = " blue " ]; b => a [label = " 15226 tcp [syn,ack] " , textcolour = " red " , linecolour = " red " ]; a => b [label = " 15229 tcp [ack] " , textcolour = " blue " , linecolour = " blue " ]; a => b [label = " 15232 http get " , textcolour = " blue " , linecolour = " blue " ]; }

我之前用过visio，dia等图形化工具来画这种交互图，但是感觉还是很麻烦，比如摆放箭头，调整颜色

之类的，在发现mscgen这个工具后，我觉得可以放弃visio和dia了。mscgen的作者是受了graphviz的启发才开始开发这个工具的。

graphviz的dot语言也是画各种结构图的一件利器