HTTP Features

• Request/response model: all communication starts with a client request, and the server responds to that request.
• Stateless: the server does not store client state. Each request is independent.
• Connectionless: after a client and server complete one request-response cycle, the connection is closed. But HTTP/1.1 introduced Keep-Alive, allowing multiple requests and responses over the same connection to reduce connection setup overhead.

Connection: keep-aliveKeep-Alive: timeout=30, max=100

HTTP Message

HTTP messages are divided into request messages and response messages. Their structures are similar. As shown below, both can be divided into three parts: the start line, headers, and an optional body.

HTTP Request Message

Request Line

The first line of the message contains:

• Method: GET, POST, PUT, DELETE, HEAD, OPTIONS, PATCH
• Request Target: /resource/user?id=10
• Version: HTTP/1.1

Method

Request Headers

Request metadata. Each header occupies one line, with the key and value separated by a colon. Common headers include:

• Host: the request target, such as example.com, a.com:8080, or localhost:4000. Combining Host with the Request Target determines the requested resource location, such as example.com/index.html.

• User-Agent: client software information, such as browser type and version.
• Referer: the URL of the source page.

• Content-Type: the data type of the request body, such as application/json.
• Content-Length: the request body length in bytes.

• Authorization: authentication information, such as Bearer token or Basic base64encodedcredentials.
• Cookie: cookie data stored on the client.

Request Body

An optional part containing data sent from the client to the server. Its format depends on the type specified by the Content-Type header.

{	"username": "example",	"password": "password123"}

username=example&password=password123

HTTP Response Message

Status Line

The first line of the message contains the HTTP version, status code, and reason phrase, for example HTTP/1.1 200 OK.

HTTP Status Code

See more details in HTTP Common Status Code Summary (Application Layer) | JavaGuide

Response Headers

Response metadata. curl can use the -I option to view only response headers, for example curl -I vluv.space.

Common headers include:

• Server: server software information, such as Apache/2.4.41 (Ubuntu).

• Cache-Control^[1]: tells the client how to cache the response, for example public, private, no-cache, or max-age=3600.
• Etag: a unique resource identifier used for cache validation. The server returns the ETag in the response. In later requests, the client sends that ETag through the If-None-Match header. The server uses the ETag to determine whether the resource has changed. If it has not changed, the server can avoid sending the response body and return 304 Not Modified, saving bandwidth.

• Content-Type & Content-Length: the response body data type and length.

Response Body

An optional part containing the actual data returned by the server. Its format depends on the type specified by the Content-Type header.