Internet History, Technology and Security (Week 7)

Week 7

Technology: Application Protocols

Welcome to Week 7 of IHTS. This week has less material than other weeks. I like to think of it as a "seventh inning stretch" - where you get a little breather. This week wraps up the three weeks of "Technology" (i.e packets and stuff). After this week we have two weeks on Security and the final exam and then you are done. Thanks for sticking with it. You are almost there.
This week, we’ll be covering application protocols. With reliable “pipes” available from the Transport layer, we can build applications like web browsers, file transfer applications, or email clients and servers.

Layer 4: Applications

Application Layer

现在讲Application Layer.

The services of the TCP layer are basically to give us a reliable, sequenced end to end stream, that can start in one application in one computer and end in an application in a different computer, and have a two way communication.

TCP give us a reliable pipe.当关注某一层级时，我们就可以忽略其他层级的实现细节。

端口（port）：

像电话话号码的扩展；
具体到某一个应用（application）。

关于应用层的两个问题:

哪个应用得到data；
什么规则和应用进行通话。

One is, which application gets the data. And this is done using a mechanism called ports, and ports allow a IP address, or a single computer, or a single server, to serve up multiple services, and then for a client to be able to dial up much like a telephone extension, and pick the service that they are interested in.

一句话来说：IP地址决定你连接哪台电脑，端口决定你访问这台电脑之后，会与哪个应用通信。

下图展示了各个不同的Ports:

常用的端口

既然TCP给我们提供了一个可靠的通信管道，那我们可以用这个管道做什么呢？什么问题又需要解决呢?

So, TCP gives us this reliable connection, we now can connect to the server that we desire to connect to by using ports, and the question is, what are we going to to say across that connection, and what we say across that connection, who talks first, what do you send, what comes back, depends on the kind of server that you are talking to。

即万维网服务器。万维网中的客户端和服务端使用的是名为HTTP的协议进行通信。网址URL中的HTTP://dada.dada.dada就是。
下图是 HTTP Request/Response Cycle.点击一个运行在你电脑中的客户端client的一个link，浏览器就会和web server 连接并发送一个请求，web server看到了就会返回请求，返回的document就会出现在客户端的桌面上。