HTTP: The Definitive Guide

Messages Are Crates, Entities Are Cargo

If you think of HTTP messages as the crates of the Internet shipping system, then HTTP entities are the actual cargo of the messages. Figure 15-1 shows a simple entity, carried inside an HTTP response message.

Figure 15-1. Message entity is made up of entity headers and entity body

The entity headers indicate a plaintext document (Content-Type: text/plain) that is a mere 18 characters long (Content-Length: 18). As always, a blank line (CRLF) separates the header fields from the start of the body.

HTTP entity headers (covered in Chapter 3) describe the contents of an HTTP message. HTTP/1.1 defines 10 primary entity header fields:

Content-Type

The kind of object carried by the entity.

Content-Length

The length or size of the message being sent.

Content-Language

The human language that best matches the object being sent.

Content-Encoding

Any transformation (compression, etc.) performed on the object data.

Content-Location

An alternate location for the object at the time of the request.

Content-Range

If this is a partial entity, this header defines which pieces of the whole are included.

Content-MD5

A checksum of the contents of the entity body.

Last-Modified

The date on which this particular content was created or modified at the server.

Expires

The date and time at which this entity data will become stale.

Allow

What request methods are legal on this resource; ...