HTTP is the application layer protocol of TCP/IP protocol family

preface

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HTTP

HTTP full name “HyperText Transfer Protocol”, English name: “HyperText Transfer Protocol”, is a distributed, collaborative and hypermedia information system application layer Protocol. HTTP is the basis for data communication on the World Wide Web.

In plain English, it is a set of conventions designed to publish and receive HTML pages (HTTP/0.9).

Then HTTP went viral, and the update (HTTP/1.0) got even better, allowing you to send anything (which laid the foundation for the Internet). Besides basic GET commands, POST and HEAD commands are added, which makes it possible to not only read the contents of the server through HTTP, but also modify the contents of the server! These commands are detailed below.

Because there is so much more to transfer, you need to identify the format of the transfer, so the content of the HTTP/1.0 request response is not just the data part. Each communication must include a header (HTTP header) that describes some metadata. Other new features include Status Code, multi-character set support, multi-part Type, authorization, cache, and Content encoding.

HTTP/1.0, while good, was designed to be “connectionless”, meaning that only one request could be sent per TCP connection. Once the data is sent, the connection is closed, and if additional resources are requested, a new connection must be created.

This is because of the high cost of establishing TCP connections, the need for a three-way handshake between the client and the server, and slow start. As a result, HTTP 1.0 has poor performance. The more external resources a web page loads, the more this problem becomes.

Great things are great by iterating on themselves, and HTTP/1.1 was released in January 1997, just six months after 1.0. It further refined the HTTP protocol, which is still in use 20 years later and is still the most popular version.

The biggest change in HTTP/1.1 is the introduction of persistent connections, which means TCP connections are not closed by default and can be reused by multiple requests without declaring connection: keep-alive (the browser adds it to you by default).

The client and server can close the connection if they find the other side inactive for a period of time. However, it is standard practice for the client to send Connection: close on its last request, explicitly asking the server to close the TCP Connection.

Currently, most browsers allow up to six persistent connections to the same domain name.

HTTP/1.1 also introduced pipelining, which allows clients to send multiple requests simultaneously over the same TCP connection. This further improves the efficiency of the HTTP protocol. However, most browsers turn this feature off by default (because it doesn’t solve real problems well and can cause performance problems).

HTTP/2, based on Google’s experimental project SPDY, became a standard in May 2015. The major change in HTTP/2 over HTTP/1.1 is in performance. Key words: binary frame layer, data stream, message and frame, request and response multiplexing, data stream priority, flow control, server push, header compression

Although HTTP/2 has been around for some time, and HTTP/3 is on its way, HTTP/1.1 is by far the most widely used and compatible version of the Internet.

HTTP/1.1 Request and response packets

The browser initiates the HTTP request process

1. Build the request (Build the request line information)
2. Lookup cache (browser cache is a technique that saves a copy locally for immediate use on the next request)
3. The IP address and port number are available
4. TCP queue (A maximum of six TCP connections can be established for a domain name at the same time. If 10 requests occur for a domain name at the same time, four of the requests are queued.)
5. Establishing a TCP Connection
6. Sending an HTTP request

HTTP requests and responses have a similar structure, consisting of the following parts

• The starting line describes the request to be executed, or its corresponding status, success or failure. The starting line is always a single line.
• An optional set of HTTP headers indicates the request or description message body.
• A blank line indicates that all metadata about the request has been sent.
• An optional body containing data related to the request, such as HTML form content, or a document related to the response. The size of the body is specified by the HTTP header of the starting line.
• The starting line and the HTTP headers in the HTTP message are collectively referred to as the request headers, and the payload is referred to as the message body.

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The HTTP request

The request line

An HTTP request is a message sent by a client to cause the server to perform an action. The start line contains three elements:

An HTTP method, a verb (like GET, PUT, or POST) or a noun (like HEAD or OPTIONS), describes the action to be performed. For example, GET means to fetch a resource, and POST means to push data to the server (to create or modify a resource, or to produce a temporary file to return).

A request target, usually a URL or an absolute path to a protocol, port, or domain name, is typically characterized by the requested environment. The format of the request varies with different HTTP methods. It could be:

An absolute path followed by a ‘? ‘and the query string. This is the most common form, called the Origin form, and is used by the GET, POST, HEAD and OPTIONS methods.

POST / HTTP / 1.1
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GET /background.png HTTP / 1.0
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HEAD /test.html? query=alibaba HTTP / 1.1
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OPTIONS /anypage.html HTTP / 1.0
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A complete URL, called absolute form, is used primarily when connecting to a proxy using the GET method.

GET http://developer.mozilla.org/en-US/docs/Web/HTTP/Messages HTTP / 1.1
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The Authority Component of a URL that consists of a domain name and an optional port prefixed with a ‘:’ is called the Authority Form. Used only when using CONNECT to establish an HTTP tunnel.

CONNECT developer.mozilla.org:80 HTTP / 1.1
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The asterisk form, a simple asterisk (‘*’) used with the OPTIONS method, represents the entire server.

OPTIONS * HTTP / 1.1
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HTTP version, which defines the structure of the remaining message as an indicator of the expected response version.

Request header

HTTP Headers from the request follows the same basic structure as HTTP headers: a case-insensitive string followed by a colon (‘:’), and a structure that depends on the value of the header. The entire header (including the value) consists of a single line, which can be quite long.

There are many request headers available, which can be grouped into several groups:

General headers, such as Via, applies to the entire packet.

Request headers, such as user-agent or accept-type, are defined further (such as accept-language) or given a context (such as Referer). Or modify the request with a conditional restriction (such as if-None).

Entity Headers, such as Content-Length, applies to the body of the request. Obviously, if there is no body in the request, no such header file will be sent.

Example:

Request body

The last part of the request is its body. Not all requests have a body: for example, resource requests, GET, HEAD, DELETE, and OPTIONS, usually do not require a body. Some requests send data to the server to update it: a common case is a POST request (containing HTML form data).

Request bodies can be broadly divided into two categories:

Single-resource bodies, consisting of a Single file. The body Type is defined by two headers: content-type and Content-Length.

Multiple-resource bodies, consisting of Multiple body parts, each containing different bits of information. It is usually associated with HTML Forms.

The HTTP response

Response line

The starting line of the HTTP response is called a status line and contains the following information:

Protocol version, usually HTTP/1.1. Status code: indicates whether the request succeeded or failed. Common status codes are 200,404, or 302. Status text. A short, pure piece of information that helps people understand the HTTP message through a textual description of the status code. A typical status line would look like this: HTTP/1.1 404 Not Found.

Response headers

The HTTP headers for the response follows the same structure as any other header: a case-insensitive string followed by a colon (‘:’), and a structure that depends on the value of the header type. The entire header, including its value, appears as a single line.

There are many response headers available, which can be grouped into several groups:

General headers, such as Via, applies to the entire packet.

Response Headers, such as Vary and Accept-ranges, provide other information about the server that does not match the status line.

Entity Headers, such as Content-Length, applies to the body of the request. Obviously, if there is no body in the request, no such header file will be sent.

Example:

Response body

The final part of the response is the body. Not all responses have a body: Responses with a status code (such as 201 or 204) usually do not.

The response body can be roughly divided into three categories:

Single-resource bodies, consisting of Single files of known length. The body Type is defined by two headers: content-type and Content-Length.

Single-resource bodies, consisting of Single files of unknown length, use chunk-encoding by setting transfer-Encoding to chunked.

Multiple-resource bodies, consisting of Multiple body parts, each containing different information segments. But this is relatively rare.

Common HTTP request modes

HTTP response code

HTTP status code Classification The HTTP status code consists of three decimal digits. The first decimal digit defines the type of the status code, and the last two digits do not classify the status code. HTTP status codes are classified into five types:

HTTP status code classification

HTTP status code list: