基于自顶向下互联网特色计算机网络-CDIO.PPT

2: Application Layer * Reading-works 2.1 WEB数据传输安全 我们发现HTTP协议在GET、POST及其响应数据都是明文传输的，这在电子商务、电子政务应用中会出现商业利益和个人隐私问题，请通过动手实验或网络有哪些信誉好的足球投注网站调研： 1 研究你所使用的WEB电子邮局是否存在账户、密码安全问题？ 2 目前有没有具有安全性更好的HTTP协议标准？ 提示：HTTPS和SSL 2: Application Layer * DIY!now 2.4带有内嵌对象的HTML文档 访问一个带两个图片的网页； 观察和分析首页、两个图片的下载过程； 2: Application Layer * DIY!now 2.5 多重对象的HTML文档 再次访问google首页； 观察后续多个图片的下载过程； 2: Application Layer * Nonpersistent HTTP issues: requires 2 RTTs per object OS overhead for each TCP connection browsers often open parallel TCP connections to fetch referenced objects Default in HTTP/1.0 Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server sent over open connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects 2: Application Layer * Nonpersistent HTTP Suppose user enters URL 1a. HTTP client initiates TCP connection to HTTP server (process) at on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object index.html 1b. HTTP server at host waiting for TCP connection at port 80. “accepts” connection, then notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time (contains 5 text files, 1 css file, 31 gif images) 2: Application Layer * Nonpersistent HTTP (cont.) 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 31 referenced jpeg objects and other files 6. Steps 1-5 repeated for each of 31 jpeg objects and other files 4. HTTP server closes TCP connection. time 2: Application Layer * NonPersistent HTTP Nonpersiste