HTTP Protocol Analysis: Request Methods, Status Codes, and Header Information

# Chapter 2.1 GET Method

The GET method is one of the most commonly used HTTP methods, designed for retrieving resources from the server. Its syntax format is as follows:

GET /path/to/resource HTTP/1.1
Host: ***

GET requests have an empty body, with parameters passed through the URL query string in the format `?key1=value1&key2=value2`. For example, the following request retrieves the `/users` resource and specifies the `id` parameter as `1`:

GET /users?id=1 HTTP/1.1
Host: ***

GET methods are typically used for fetching static resources such as HTML pages, images, or files, or for querying data such as database records. Since their request bodies are empty, they do not modify resources on the server.

# Chapter 2. HTTP Request Methods

### 2.1 GET Method

The GET method is used to retrieve resources from the server. It is the most frequently used HTTP request method for fetching data from the server, such as web pages, images, or files. GET requests send data through the URL query string, starting with a question mark (?), followed by parameter-value pairs, separated by the ampersand (&) symbol.

GET /index.html?name=John&age=30 HTTP/1.1

**Parameter Description:**

* `name`: User's name
* `age`: User's age

**Logical Flow:**

1. The client sends a GET request to the server, specifying the resource to be retrieved (`/index.html`).
2. The server receives the request and retrieves data based on the parameters in the query string (`name` and `age`).
3. The server returns the data as the response body to the client.

### 2.2 POST Method

The POST method is used to submit data to the server. It is commonly used for creating or updating resources, such as submitting form data or uploading files. POST requests send data in the request body, which is typically specified with the `Content-Type` header.

POST /submit-form HTTP/1.1
Content-Type: application/x-www-form-urlencoded

name=John&age=30

**Parameter Description:**

* `name`: User's name
* `age`: User's age

**Logical Flow:**

1. The client sends a POST request to the server, specifying the URL to which the data is to be submitted (`/submit-form`).
2. The client provides data (`name` and `age`) in the request body.
3. The server processes the data in the request body.
4. The server returns a response, usually 200 (Success) or 400 (Bad Request), based on the result.

### 2.3 PUT Method

The PUT method is used for updating or creating a resource on the server. It replaces the data of an existing resource on the server with the data in the request body. If the resource does not exist, it is created.

PUT /update-user HTTP/1.1
Content-Type: application/json

{"id": 1, "name": "John", "age": 30}

**Parameter Description:**

* `id`: User's ID
* `name`: User's name
* `age`: User's age

**Logical Flow:**

1. The client sends a PUT request to the server, specifying the URL of the resource to be updated or created (`/update-user`).
2. The client provides the data to be updated or created (`id`, `name`, and `age`) in the request body.
3. The server processes the data in the request body.
4. The server returns a response, usually 200 (Success) or 404 (Resource Not Found), based on the result.

### 2.4 DELETE Method

The DELETE method is used to remove a resource from the server. It deletes the specified resource on the server.

DELETE /delete-user/1 HTTP/1.1

**Parameter Description:**

* `id`: User's ID to be deleted

**Logical Flow:**

1. The client sends a DELETE request to the server, specifying the URL of the resource to be deleted (`/delete-user/1`).
2. The server deletes the specified resource.
3. The server returns a response, usually 200 (Success) or 404 (Resource Not Found).

# Chapter 3. HTTP Status Codes

HTTP status codes are three-digit codes included in HTTP responses that indicate the server's response status to the client's request. Status codes are categorized into five classes, each with its own meaning:

### 3.1 1xx Informational Response

1xx status codes indicate that the server has received the request and is processing it. The client should continue to wait for the final response from the server.

| Status Code | Meaning |
|---|---|
| 100 Continue | The server has received the request headers and the client should proceed to send the request body. |
| 101 Switching Protocols | The server has switched to the protocol requested by the client. |

### 3.2 2xx Success Response

2xx status codes indicate that the server has successfully processed the request.

| Status Code | Meaning |
|---|---|
| 200 OK | The request has been successfully processed, and the server has returned the requested resource. |
| 201 Created | The request has been successfully processed, and the server has created a new resource. |
| 202 Accepted | The request has been successfully processed, but the server has not completed the operation yet. |
| 204 No Content | The request has been successfully processed, but the server did not return any content. |

### 3.3 3xx Redirection Response

3xx status codes indicate that the client needs to take further action to complete the request.

| Status Code | Meaning |
|---|---|
| 301 Moved Permanently | The requested resource has been permanently moved to a new location. |
| 302 Found | The requested resource has been temporarily moved to a new location. |
| 303 See Other | The requested resource is located elsewhere; the client should use the GET method to retrieve the resource. |
| 304 Not Modified | The requested resource has not been modified, and the client can use the cached version. |

### 3.4 4xx Client Error Response

4xx status codes indicate that the client has made an error in the request.

| Status Code | Meaning |
|---|---|
| 400 Bad Request | The request syntax is incorrect, and the server cannot understand it. |
| 401 Unauthorized | The client is not authorized to access the requested resource. |
| 403 Forbidden | The client does not have permission to access the requested resource. |
| 404 Not Found | The requested resource does not exist. |

### 3.5 5xx Server Error Response

5xx status codes indicate that the server encountered an error while processing the request.

| Status Code | Meaning |
|---|---|
| 500 Internal Server Error | The server encountered an internal error while processing the request. |
| 502 Bad Gateway | The server, acting as a gateway or proxy, received an invalid response from an upstream server. |
| 503 Service Unavailable | The server is temporarily unable to handle the request, please try again later. |

### 3.6 Status Code Lookup Table

The table below lists all HTTP status codes and their meanings:

| Status Code | Meaning |
|---|---|
| 100 Continue | The server has received the request headers, and the client should continue sending the request body. |
| 101 Switching Protocols | The server has switched to the protocol requested by the client. |
| 200 OK | The request has been successfully processed, and the server has returned the requested resource. |
| 201 Created | The request has been successfully processed, and the server has created a new resource. |
| 202 Accepted | The request has been successfully processed, but the server has not completed the operation yet. |
| 204 No Content | The request has been successfully processed, but the server did not return any content. |
| 301 Moved Permanently | The requested resource has been permanently moved to a new location. |
| 302 Found | The requested resource has been temporarily moved to a new location. |
| 303 See Other | The requested resource is located elsewhere; the client should use the GET method to retrieve the resource. |
| 304 Not Modified | The requested resource has not been modified, and the client can use the cached version. |
| 400 Bad Request | The request syntax is incorrect, and the server cannot understand it. |
| 401 Unauthorized | The client is not authorized to access the requested resource. |
| 403 Forbidden | The client does not have permission to access the requested resource. |
| 404 Not Found | The requested resource does not exist. |
| 500 Internal Server Error | The server encountered an internal error while processing the request. |
| 502 Bad Gateway | The server, acting as a gateway or proxy, received an invalid response from an upstream server. |
| 503 Service Unavailable | The server is temporarily unable to handle the request, please try again later. |

# Chapter 4. HTTP Header Information

HTTP header information is metadata appended to HTTP requests or responses, providing additional information about the request or response. Header information is divided into request header information and response header information.

### 4.1 Request Header Information

Request header information contains information about the client and the request itself. Some commonly used request header information includes:

#### 4.1.1 Host

The `Host` header specifies the target host and port number for the request. It is crucial for the server to determine the destination of the request.

**Example:**

Host: ***:80

#### 4.1.2 User-Agent

The `User-Agent` header identifies the client that is sending the request. It contains detailed information about the client's software and operating system.

**Example:**

User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36

#### 4.1.3 Accept

The `Accept` header specifies the content types of responses that the client can accept. The server will choose the most appropriate content type for the response based on this information.

**Example:**

Accept: text/html,application/json

### 4.2 Response Header Information

Response header information contains information about the server and the response itself. Some commonly used response header information includes:

#### 4.2.1 Content-Type

The `Content-Type` header specifies the media type of the response. It tells the client how to interpret the content of the response.

**Example:**

Content-Type: text/html

#### 4.2.2 Content-Length

The `Content-Length` header specifies the number of bytes in the response body. It allows the client to pre-allocate memory to receive the response content.

**Example:**

Content-Length: 1234

#### 4.2.3 Date

The `Date` header specifies the time the response was generated. It is essential for caching responses and tracking the timing of request-response interactions.

**Example:**

Date: Tue, 15 Nov 2022 12:00:00 GMT

### 4.2.4 Other Header Information

In addition to the headers listed above, there are many other HTTP headers that can be used to provide additional information about requests or responses. These headers include:

- `Cache-Control`: Controls the caching behavior of the response.
- `Connection`: Specifies the type and duration of the connection.
- `Cookie`: Stores information about the client's state.
- `Referer`: Specifies the source page of the request.
- `Server`: Identifies the server software processing the request.

### 4.2.5 Table: Common HTTP Headers

| Header Information | Description |
|---|---|
| Host | Target host and port number |
| User-Agent | Client sending the request |
| Accept | Content types of responses the client can accept |
| Content-Type | Media type of the response |
| Content-Length | Number of bytes in the response body |
| Date | Time the response was generated |

### 4.2.6 Diagram: HTTP Header Information Processing

sequenceDiagram
participant Client
participant Server
Client->Server: Send Request with Headers
Server->Client: Send Response with Headers
Client: Parse Headers
Server: Parse Headers

# Chapter 5. Practical Application of HTTP Protocol

### 5.1 Using cURL to Send HTTP Requests

cURL is a command-line tool for sending HTTP requests and receiving responses. It supports various HTTP methods, headers, and parameters.

# Send GET request
curl ***

***
*** "key1=value1&key2=value2"

# Send PUT request
curl -X PUT *** "new_data"

# Send DELETE request
curl -X DELETE ***

### 5.2 Using Python to Parse HTTP Responses

The `requests` library in Python makes it easy to send and parse HTTP requests.

import requests

# Send GET request
response = requests.get("***")

# Get response status code
print(response.status_code)

# Get response headers
print(response.headers)

# Get response body
print(response.text)

### 5.3 Application of HTTP Protocol in Web Development

The HTTP protocol is fundamental to Web development. It is used to transfer data between the client and the server, including HTML, CSS, JavaScript, and images.

The HTTP request-response model allows clients to send requests to the server, which then returns responses. The request contains headers that specify the method, target resource, and client information. The response contains headers that specify the status, content type, and size of the content.

With the use of the HTTP protocol, Web servers can provide dynamic content to clients, such as data retrieved from databases. Clients can use HTTP requests to control resources on the server, such as creating, updating, or deleting data.