Protocol Versions

1.7 Protocol Versions

There are several versions of the HTTP protocol in use today. HTTP applications need to work hard to robustly handle different variations of the HTTP protocol. The versions in use are:

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.

1.8 Architectural Components of the Web

In this overview chapter, we've focused on how two web applications (web browsers and web servers) send messages back and forth to implement basic transactions. There are many other web applications that you interact with on the Internet. In this section, we'll outline several other important applications, including:

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

1.8.1 Proxies

Let's start by looking at HTTP proxy servers , important building blocks for web security, application integration, and performance optimization.

As shown in Figure 1-11 , a proxy sits between a client and a server, receiving all of the client's HTTP requests and relaying the requests to the server (perhaps after modifying the requests). These applications act as a proxy for the user, accessing the server on the user 's behalf .

Figure 1-11. Proxies relay traffic between client and server

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Proxies are often used for security, acting as trusted intermediaries through which all web traffic flows. Proxies can also filter requests and responses; for example, to detect application viruses in corporate downloads or to filter adult content away from elementary-school students. We'll talk about proxies in detail in Chapter 6 .

1.8.2 Caches

A web cache or caching proxy is a special type of HTTP proxy server that keeps copies of popular documents that pass through the proxy. The next client requesting the same document can be served from the cache's personal copy (see Figure 1-12 ).

Figure 1-12. Caching proxies keep local copies of popular documents to improve performance

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A client may be able to download a document much more quickly from a nearby cache than from a distant web server. HTTP defines many facilities to make caching more effective and to regulate the freshness and privacy of cached content. We cover caching technology in Chapter 7 .

1.8.3 Gateways

Gateways are special servers that act as intermediaries for other servers. They are often used to convert HTTP traffic to another protocol. A gateway always receives requests as if it was the origin server for the resource. The client may not be aware it is communicating with a gateway.

For example, an HTTP/FTP gateway receives requests for FTP URIs via HTTP requests but fetches the documents using the FTP protocol (see Figure 1-13 ). The resulting document is packed into an HTTP message and sent to the client. We discuss gateways in Chapter 8 .

Figure 1-13. HTTP/FTP gateway

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1.8.4 Tunnels

Tunnels are HTTP applications that, after setup, blindly relay raw data between two connections. HTTP tunnels are often used to transport non-HTTP data over one or more HTTP connections, without looking at the data.

One popular use of HTTP tunnels is to carry encrypted Secure Sockets Layer (SSL) traffic through an HTTP connection, allowing SSL traffic through corporate firewalls that permit only web traffic. As sketched in Figure 1-14 , an HTTP/SSL tunnel receives an HTTP request to establish an outgoing connection to a destination address and port, then proceeds to tunnel the encrypted SSL traffic over the HTTP channel so that it can be blindly relayed to the destination server.

Figure 1-14. Tunnels forward data across non-HTTP networks (HTTP/SSL tunnel shown)

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1.8.5 Agents

User agents (or just agents ) are client programs that make HTTP requests on the user's behalf. Any application that issues web requests is an HTTP agent. So far, we've talked about only one kind of HTTP agent: web browsers. But there are many other kinds of user agents.

For example, there are machine-automated user agents that autonomously wander the Web, issuing HTTP transactions and fetching content, without human supervision. These automated agents often have colorful names , such as "spiders" or "web robots" (see Figure 1-15 ). Spiders wander the Web to build useful archives of web content, such as a search engine's database or a product catalog for a comparison-shopping robot. See Chapter 9 for more information.