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All HTTP messages fall into two types: request messages and response messages.Request messages request an action from a web server. Response messages carry results of a request back to a client. Both request and response messages have the same basic message structure. Figure 3-4 shows request and response messages to get a GIF image.
Figure 3-4. An HTTP transaction has request and response messages
Here's the format for a request message:
<method> <request-URL> <version>
<headers>
<entity-body>Here's the format for a response message (note that the syntax differs only in the start line):
<version> <status> <reason-phrase>
<headers>
<entity-body>Here's a quick description of the various parts:
HTTP/<major>.<minor>:), followed by optional whitespace, followed by a value, followed by a CRLF. The headers are terminated by a blank line (CRLF), marking the end of the list of headers and the beginning of the entity body. Some versions of HTTP, such as HTTP/1.1, require certain headers to be present for the request or response message to be valid. The various HTTP headers are covered later in this chapter.Figure 3-5 demonstrates hypothetical request and response messages.
Figure 3-5. Example request and response messages
Note that a set of HTTP headers should always end in a blank line (bare CRLF), even if there are no headers and even if there is no entity body. Historically, however, many clients and servers (mistakenly) omitted the final CRLF if there was no entity body. To interoperate with these popular but noncompliant implementations, clients and servers should accept messages that end without the final CRLF.
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Created: January 13, 2003
Revised: January 13, 2003
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