Chapter 1

Talk about :

• Web客户端与服务器是如何通信的
• （表示Web内容的）资源来自何方
• Web事务是怎样工作的
• HTTP通信所使用的报文格式
• 底层TCP网络传输
• 不同的HTTP协议变体
• 因特网上安装的大量HTTP架构组件中的一部分

How web clients and servers communicate?

Web content lives onweb servers. Web servers speak the HTTP protocol, so they are often called HTTP servers. These HTTP servers store the Internet's data and provide the data when it is requested by HTTP clients. The clients send HTTP requests to servers, and servers return the requested data in HTTP responses, as sketched in Figure 1-1. Together, HTTP clients and HTTP servers make up the basic components of the World Wide Web.

Figure 1-1

Where resources(web content) come from?

Web servers host web resources . A web resource is the source of web content. The simplest kind of web resource is a static file on the web server's filesystem. These files can contain anything: they might be text files, HTML files, Microsoft Word files, Adobe Acrobat files, JPEG image files, AVI movie files, or any other format you can think of.
However, resources don't have to be static files. Resources can also be software programs that generate content on demand. These dynamic content resources can generate content based on your identity, on what information you've requested, or on the time of day. They can show you a live image from a camera, or let you trade stocks, search real estate databases, or buy gifts from online stores (see Figure 1-2).

Figure 1-2

In summary, a resource is any kind of content source. A file containing your company's sales forecast spreadsheet is a resource. A web gateway to scan your local public library's shelves is a resource. An Internet search engine is a resource.

How web transactions work?

Let's look in more detail how clients use HTTP to transact with web servers and their resources. An HTTP transaction consists of a request command (sent from client to server), and a response result (sent from the server back to the client). This communication happens with formatted blocks of data called HTTP messages , as illustrated in Figure 1-5.

Figure 1-5

The format of the messages used for HTTP communication

HTTP messages sent from web clients to web servers are called request messages . Messages from servers to clients are called response messages . There are no other kinds of HTTP messages. The formats of HTTP request and response messages are very similar.
HTTP messages consist of three parts:

• Start line
  The first line of the message is the start line, indicating what to do for a request or what happened for a response.

• Header fields
  Zero or more header fields follow the start line. Each header field consists of a name and a value, separated by a colon (:) for easy parsing. The headers end with a blank line. Adding a header field is as easy as adding another line.

• Body
  After the blank line is an optional message body containing any kind of data. Request bodies carry data to the web server; response bodies carry data back to the client. Unlike the start lines and headers, which are textual and structured, the body can contain arbitrary binary data (e.g., images, videos, audio tracks, software applications). Of course, the body can also contain text.

The underlying TCP network transport

HTTP is an application layer protocol. HTTP doesn't worry about the nitty-gritty details of network communication; instead, it leaves the details of networking to TCP/IP, the popular reliable Internet transport protocol.
TCP provides:

• Error-free data transportation
• In-order delivery (data will always arrive in the order in which it was sent)
• Unsegmented data stream (can dribble out data in any size at any time)

The Internet itself is based on TCP/IP, a popular layered set of packet-switched network protocols spoken by computers and network devices around the world. TCP/IP hides the peculiarities and foibles of individual networks and hardware, letting computers and networks of any type talk together reliably.
Once a TCP connection is established, messages exchanged between the client and server computers will never be lost, damaged, or received out of order.

The different variations of the HTTP protocol

HTTP/0.9

• The 1991 prototype version of HTTP is known as HTTP/0.9. This protocol contains many serious design flaws and should be used only to interoperate with legacy clients. HTTP/0.9 supports only the GET method, and it does not support MIME typing of multimedia content, HTTP headers, or version numbers. HTTP/0.9 was originally defined to fetch simple HTML objects. It was soon replaced with HTTP/1.0.

HTTP/1.0

• 1.0 was the first version of HTTP that was widely deployed. HTTP/1.0 added version numbers, HTTP headers, additional methods, and multimedia object handling. HTTP/1.0 made it practical to support graphically appealing web pages and interactive forms, which helped promote the wide-scale adoption of the World Wide Web. This specification was never well specified. It represented a collection of best practices in a time of rapid commercial and academic evolution of the protocol.

HTTP/1.0+

• Many popular web clients and servers rapidly added features to HTTP in the mid-1990s to meet the demands of a rapidly expanding, commercially successful World Wide Web. Many of these features, including long-lasting "keep-alive" connections, virtual hosting support, and proxy connection support, were added to HTTP and became unofficial, de facto standards. This informal, extended version of HTTP is often referred to as HTTP/1.0+.

HTTP/1.1

• HTTP/1.1 focused on correcting architectural flaws in the design of HTTP, specifying semantics, introducing significant performance optimizations, and removing mis-features. HTTP/1.1 also included support for the more sophisticated web applications and deployments that were under way in the late 1990s. HTTP/1.1 is the current version of HTTP.

HTTP-NG (a.k.a. HTTP/2.0)

• HTTP-NG is a prototype proposal for an architectural successor to HTTP/1.1 that focuses on significant performance optimizations and a more powerful framework for remote execution of server logic. The HTTP-NG research effort concluded in 1998, and at the time of this writing, there are no plans to advance this proposal as a replacement for HTTP/1.1. See Chapter 10 for more information.

Some of the many HTTP architectural components installed around the Internet

Proxies

• HTTP intermediaries that sit between clients and servers

Caches

• HTTP storehouses that keep copies of popular web pages close to clients

Gateways

• Special web servers that connect to other applications

Tunnels

• Special proxies that blindly forward HTTP communications

Agents

• Semi-intelligent web clients that make automated HTTP requests