HTTP message

HTTP message is composed by:

1. start line

2. header

3. CRLF (empty line)

4. body

Start line

Request line

It is composed by:

Method URL Version

GET / HTTP/1.1

Response line

Version code reason

HTTP/1.1 200 OK

or

HTTP/1.1 404 Not Found

Header

Header contains the info in key-value format. There are some rule.

• Key is insensible to the case. Host is equal to host.
• _ and \s is not allowed in the key. - is ok.
• There is no space before :.
• The order of header is meaningless.

fields

There are 4 classes of header.

• common fields

  It exists in all kinds of http message.

  • Data: the creation time of http message.

• request fields

  • Host: It's only the required request fields in HTTP/1.1.

  • User-Agent: the client who sends the request.

• response fields

  • Server:

• body fields:

  It's a part of the common fields.

  • Content-Length: the length of the body. If this header is not set, the body is sent separately. We need to add Transfer-Encoding:chuncked.