1. Position in the TCP/IP protocol suite
Located at the application layer of the four-layer protocol. Implementation of TCP protocol based on the transport layer.
2. Request message structure
Including the message header, blank line, and message body 3 parts.
Message header: the
first line: request line, request method, request path, HTTP version.
Subsequent headers: including request header fields, general header fields, and entity header fields
Blank line:
Message body:
the data sent to the server. Such as the various parameters in the get request. The parameters in the post request.
3. Response message structure
It also includes three parts: message header, blank line, and message body.
Message header: the
first line: the status line, including the HTTP version status code reason phrase.
Followed by the header fields: response header field, general header field, entity header field
Message body: the response body returned by the server. Such as the HTTM page.
4. Common status codes
（1） 2xx
The status code at the beginning of 2 indicates success
200 OK
Processed normally and returned
204 No Content
It was processed normally, but there is no body in the response.
It is used when data needs to be sent from the client to the server but no response content is required.
206 Partial Content
The client made a range request, and the server returned normally. Specify the range through Content-Range when requesting.
（2）3xx
Redirect related
301 Moved Permanently
Permanent redirection. Indicates that the requested resource has been permanently assigned a new URI, and the new URI should be used in the future.
Use the Location header field to indicate the new URI address. The browser will request the URI again.
302 Found
Temporary redirect, hope that the user will use the newly assigned URI this time.
Very similar to 301, the browser will also re-request based on the Location field.
It is often used for page jump in actual development.
303 See Other
It has the same function as 302, except that it clearly indicates that the client should use a get request.
304 Not Modified
It has nothing to do with redirection. Indicates that the resource has not changed, and the client's unexpired cache can be used directly. It is possible to return this status code when requesting with conditions.
（4）4xx
Client error
400 Bad Request
There is a syntax error in the request. Such as the problem of parameter splicing.
401 Unauthorized
not certified
403
No Access
404 Not Found
(5)5xx
Server Error
500
Server internal error
503
service is not available
5. The role of the host header
Distinguish between different hosts. Some servers run multiple websites, each with a different domain name. If you do not specify a domain name when receiving a request, you cannot know which website is required to respond.