ch15: Entities and Encodings

15.实体和编码

1. Messages Are Crates, Entities Are Cargo

Headers about entities:

Header	Description
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 obejct being sent.
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.
ETag	A unique validator for this particular instance 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.
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.

2. Content-Length: The Entity's Size

Content-Lenght is needed to detect premature message truncation when servers crash and to properly segment messages that share a persistent connection.

2.1 Content-Length and Persistent Connections

The client know where one message ends and the next begins on persistent connections with correct Content-Length.

2.2 Content Encoding

If the body has been content-encoded, the Content-Length header specifies the length, in bytes, of the encoded body, not the length of the original, unencoded body.

3. Media Type and Charset

The Content-Type header field describes the MIME type of the entity body, the original entity body, before encoding.

3.1 Multipart Media Types

Each component is self-contained, with its own set of headers describing its content, the different components are concatenated together and delimited by a string.

3.2 Multipart Form Submissions

Requests:

Content-Type: multipart/form-data; boundary=[abcdefghijklmnopqrstuvwxyz]

4. Content Encoding

5. Chunked Encoding

6. Range Requests

Example:

GET /bigfile.html HTTP/1.1
Host: www.joes-hardware.com
Range: bytes=4000-
User-Agent: Mozilla/4.61 [en] (WinNT; I)
...

7. Delta Encoding

Rather than sending the whole document, the client would get the page faster if the server sent just t he changes to the client's copy of the page.