Messages Are Crates, Entities Are Cargo

15.1 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; e.g., GET and HEAD.

ETag

A unique validator for this particular instance ^[1] of the document. The ETag header is not defined formally as an entity header, but it is an important header for many operations involving entities.

^[1] Instances are described later in this chapter, in Section 15.7 .

Cache-Control

Directives on how this document can be cached. The Cache-Control header, like the ETag header, is not defined formally as an entity header.

15.1.1 Entity Bodies

The entity body just contains the raw cargo. ^[2] Any other descriptive information is contained in the headers. Because the entity body cargo is just raw data, the entity headers are needed to describe the meaning of that data. For example, the Content-Type entity header tells us how to interpret the data (image, text, etc.), and the Content-Encoding entity header tells us if the data was compressed or otherwise recoded. We talk about all of this and more in upcoming sections.

^[2] If there is a Content-Encoding header, the content already has been encoded by the content-encoding algorithm, and the first byte of the entity is the first byte of the encoded (e.g., compressed) cargo.

The raw content begins immediately after the blank CRLF line that marks the end of the header fields. Whatever the content istext or binary, document or image, compressed or uncompressed, English or French or Japaneseit is placed right after the CRLF.

Figure 15-2 shows two examples of real HTTP messages, one carrying a text entity, the other carrying an image entity. The hexadecimal values show the exact contents of the message:

                In Figure 15-2 a, the entity body begins at byte number 65, right after the end-of-headers CRLF. The entity body contains the ASCII characters for "Hi! I'm a message."

                In Figure 15-2 b, the entity body begins at byte number 67. The entity body contains the binary contents of the GIF image. GIF files begin with 6-byte version signature, a 16-bit width, and a 16-bit height. You can see all three of these directly in the entity body.

Figure 15-2. Hex dumps of real message content (raw message content follows blank CRLF)