HTTP
Hypertext Transfer Protocol is an application protocol for distributed information systems. HTTP is the foundation of data communication for the Internet. It is a request-response protocol in which the client sends a request to the server. The server then either provides a resource or performs a function on behalf of the client and sends a response message back to the client. The response contains a status code and possibly a body containing the requested resource or other information.
Sessions
An HTTP session is a sequence of network request-response transactions. An HTTP client initiates a request by establishing a TCP connection to a particular port on a server (usually 80).
Persistent Connections
In HTTP/0.9 and 1.0, the connection is closed after a single request/response pair. A keep-alive mechanism was introduced in HTTP/1.1, where a connection could be reused for more than one request. This reduced request latency because the client does not need to re-negotiate the TCP 3-way handshake connection after the first request has been sent.
Request Methods
HTTP defines methods to indicate the desired action to be performed on the resource.
| Method | Description |
|---|---|
| GET | Retrieve data |
| HEAD | Same as GET but without the body |
| POST | Send data |
| PUT | Send data and modify the server’s copy |
| DELETE | Delete a resource |
| TRACE | Echo the received request |
| OPTIONS | Return HTTP methods that the server supports |
| CONNECT | Convert the request connection to a TCP/IP tunnel |
| PATCH | Apply partial modifications to a resource |
Status Codes
All HTTP responses include a status code to indicate the success of the request. In addition to the code, a status message accompanies it for humans to read. There are 5 main groups of status codes:
| Code | Description |
|---|---|
1XX | Informational - the request was received, continuing process |
2XX | Successful - the request was successfully received, understood and accepted |
3XX | Redirection - further action needs to be taken in order to complete the request |
4XX | Client Error - the request contains bad syntax or cannot be fulfilled |
5XX | Server Error - the server failed to fulfill an apparently valid request |
Reference: List of HTTP Status Codes
HTTPS
HTTPS is an extension of HTTP by securing the communication protocol with TLS (previously SSL).