The Role of the Application layer
The Application layer is the interface between the network and its users
› It contains network services (e.g. DNS)
› It contains user applications (e.g. email, web browsing）
Domain Name System (DNS)
› The Domain Name System (DNS) is the phonebook of the Internet. Humans access information online through domain names, like nytimes.com or espn.com.
› Web browsers interact through Internet Protocol(IP) addresses. DNS translates domain names to IP addresses so browsers can load Internet resources.
›域名系统（DNS）是互联网的电话簿。人们通过域名访问在线信息，如nytimes.com或espn.com。
› Web浏览器通过互联网协议（IP）地址进行交互。DNS将域名转换为IP地址，以便浏览器可以加载Internet资源。
DNS names
› DNS name space is hierarchical staring from the root (often omitted)
› Name space is divided into zones. At least one DNS name server exists in each zone
› A DNS name server knows about its children (all hosts in its zone) and its parent (higher level DNS server).
› Top level domains are in the highest hieratical level after root
› DNS名称空间是从根开始的层次结构（通常省略）
›命名空间被划分为多个区域。每个区域中至少存在一个DNS名称服务器
DNS名称服务器知道它的子服务器（它所在区域中的所有主机）和它的父服务器（更高级别的DNS服务器）。
›顶级域名位于根之后的最高层次
› Finding the IP address for a given hostname is called resolution and is done with the DNS protocol
› E.g. Host A (flits.cs.vu.nl) wants the IP address of Host B(robot.cs.washington.edu)
›查找给定主机名的IP地址称为解析，通过DNS协议完成
›例如，主机A（flits.cs.vu.nl）想要主机B（robot.cs.washington.edu）的IP地址
1. Originating host contacts its local DNS name server
2. Local name server asks the root name server
3. Root returns the name server for a lower zone
4. Continue down zones until name server completes resolution and returns the final answer
› The query can be cache for future uses (including partial answers)
1.发起主机联系其本地DNS名称服务器
2.本地名称服务器询问根名称服务器
3. Root返回较低区域的名称服务器
4.继续向下分区，直到名称服务器完成解析并返回最终答案
›查询可以缓存以备将来使用（包括部分答案）
Email (Electronic mail)
› User agents: allow people to read and send email (e.g. MS Outlook).
› Message transfer agents: move the messages from the source to the destination (e.g. MS Exchange).
› Mail is transferred from source host to destination mail server by a message transfer agent using SMTP (Simple Mail Transfer Protocol).
› Mail is transferred from destination mail server to destination host by a user agent using POP3 (Post Office Protocol,version3) or IMAP (Internet Message Access Protocol)
›邮件由邮件传输代理使用SMTP（简单邮件传输协议）从源主机传输到目标邮件服务器。
›邮件由用户代理使用POP3（邮局协议，版本3）或IMAP（Internet消息访问协议）从目标邮件服务器传输到目标主机
Message format (text)
› Message sent by the user agent must be placed in a standard form (RFC 822) to be handled by the message transfer agent
›用户代理发送的消息必须以标准格式（RFC 822）放置，以便由消息传输代理处理
SMTP (a “push” protocol)
Example of the transfer of a message from source host elinor@abcd.com ©
to destination mailbox carolyn@xyz.com (S):
- Source contacts destination
- Mailbox says it is ready
- Source identifies itself
- Mailbox accepts
- Source identifies recipient
- Mailbox recognizes recipient
- Negotiate any parameters
- Source sends email
- Mailbox accepts email
- Source terminates conversation
POP3 ( a “pull” protocol)
Example of retrieving messages from destination mailbox (S) to destination
host ©:
- Destination identifies user
- Mailbox recognizes user
- Destination supplies password
- Mailbox accepts password
- Destination requests message list
- Mailbox sends message list
- Destination requests message 1
- Mailbox sends message 1
- Destination deletes message 1 from ISP mailbox (optional)
…
… - Destination terminates conversation
World Wide Web
› The Web is an architectural framework for accessing linked content (Web pages) spread out over millions of machines all over the Internet
› Most pages are written in HTML. They are viewed with a program called a browser (e.g. MS Explorer or Chrome)
› Page may contain links to other pages called hypertext
› A piece of text, icon, image associated with another page is called a hyperlink
› HTTP (HyperText Transfer Protocol) is used to fetch pages. It is a simple text-based protocol that runs over TCP
Web是一个架构框架，用于访问分布在Internet上数百万台机器上的链接内容（Web页面）。
大多数页面都是用HTML编写的。它们是通过一个名为浏览器的程序（例如MS Explorer或Chrome）查看的
›页面可能包含指向其他页面的链接，称为超文本
›与另一个页面相关联的文本、图标、图像称为超链接
› HTTP（超文本传输协议）用于获取页面。它是一个简单的基于文本的协议，在TCP上运行
› Pages are named with URLs (Uniform Resource Locators) (e.g. http://www.cs.washington.edu/index.html)
› This URL consists of three parts:
• Part before 😕/ specifies application/protocol (e.g. http)
• Part after 😕/ specifies the DNS name of the target host (e.g.www.cs.washington.edu)
• and the path (file) name (e.g. index.html)
›页面以URL（统一资源定位器）命名（例如http：//www.cs.washington.edu/index.html）
›此URL由三部分组成：
· Part before：//指定应用程序/协议（例如http）
· Part after：//指定目标主机的DNS名称（e.g.www.cs.washington.edu）
·路径（文件）名称（例如index.html）
› What happens when you click a link?
Client (browser)
› Browser understands html (hypertext mark-up language)
› Steps a client (browser) takes to follow a hyper link:
- Determine the URL (and protocol used).
- Ask DNS for the IP address of the server
- Make a TCP connection to server
- Send an HTTP request for the page /index.html, server sends the page as an HTTP response,
- Fetch other URLs as needed to display the page.
- Close idle TCP connections
›浏览器理解html（超文本标记语言）
›客户端（浏览器）跟踪超链接的步骤：
1.确定URL（和使用的协议）。
2.向DNS请求服务器的IP地址
3.建立到服务器的TCP连接
4.发送对页面/index. html的HTTP请求，服务器将页面作为HTTP响应发送，
5.根据需要获取其他URL以显示页面。
6.关闭空闲的TCP连接
› Browser uses plug-ins or helper applications to handle specific file types referenced by html web pages e.g. audio, video
›浏览器使用插件或助手应用程序来处理HTML网页引用的特定文件类型，例如音频，视频
Server
› Main steps a server takes to serve pages (Modern Web servers have more features):
- Accept TCP connection from a client (browser)
- Get the path to the page, which is the name of the file requested
- Get the resource (e.g. file from disk)
- Send contents of the resource to the client
- Release the TCP connection
› Web servers can use caching, multiple threads, and a front end to scale performance
›服务器提供页面服务的主要步骤（现代Web服务器具有更多功能）：
1.接受来自客户端（浏览器）的TCP连接
2.获取页面的路径，即所请求文件的名称
3.获取资源（例如从磁盘获取文件）
4.将资源的内容发送到客户端
5.释放TCP连接
› Web服务器可以使用缓存、多线程和前端来扩展性能
HTTP protocol
› HTTP has several request methods
› And reply (Response) codes
HTTP message header
Cookies
Used to store state information so a returning user can resume a session
› First-party cookies are directly stored by the website you are on (e.g. login IDs,passwords, credit card numbers)
› Third-party cookies are transmitted to websites other than the one you are on(e.g. tracking Internet usage for marketing purposes)
用于存储状态信息，以便返回的用户可以继续会话
›第一方Cookie直接存储在您访问的网站上（例如登录ID、密码、信用卡号）
›第三方Cookie传输到您所在网站以外的网站（例如，出于营销目的跟踪互联网使用情况）
Static web pages
› Static Web pages are simply files. Have the same contents for each viewing
› Can be visually rich and interactive nonetheless (e.g. HTML that mixes text and images, forms that gather user input)
› Html pages describe both content and formatting. Makes it difficult to achieve consistent look and feel across a large website (CSS introduced style sheets to the Web with HTML 4.0)
静态网页只是文件。每次观看的内容相同
›可以是视觉丰富和交互式的（例如，混合文本和图像的HTML，收集用户输入的表单）
HTML页面描述内容和格式。很难在一个大型网站上实现一致的外观和感觉（CSS通过HTML 4.0将样式表引入Web）
Dynamic web pages
› Dynamic pages are generated dynamically by programs running at the server (with a database) and the client
› E.g. PHP at server, JavaScript at client
› Pages vary each time like using an application
动态页面由运行在服务器（带有数据库）和客户端的程序动态生成
›例如服务器端的PHP，客户端的JavaScript
›页面每次都不同，就像使用应用程序一样
Caching
› Copies of frequently used web pages stored on client, local proxy host or ISP proxy host. Caching improves performance but content could be old (big issue with dynamic pages)
› How to determine if a local copy can be re-used or not?
• Locally determine copy is still valid (e.g. “expires” header)
• Revalidate copy with remote server (e.g. “last-modified” header)
›存储在客户端、本地代理主机或ISP代理主机上的常用网页副本。缓存提高了性能，但内容可能是旧的（动态页面的大问题）
›如何确定本地副本是否可以重复使用？
·本地确定副本仍然有效（例如，“expires”报头）
·使用远程服务器重新验证副本（例如，“last-modified”报头）
