Every front-end needs to know some network knowledge

1. OSI reference model and TCP/IP five-layer model

It is sufficient for us to focus mainly on the application layer and the transport layer of the TCP/IP five-layer model.

Application layer:

• Function: Provide services for applications.
• Common protocols: HTTP, HTTPS, FTP, POP3, SMTP, etc.

Transport layer:

• Function: To realize data transmission between applications.
• Protocol: UDP, TCP

UDP and TCP

1. UDP

UDP is a connectionless protocol, which only transmits data to the receiving end, but does not care whether the receiving end has received the data correctly, so UDP is sometimes considered an unreliable datagram protocol. However, this feature is suitable for multicast, real-time video and audio transmission.

advantage:

• No connection establishment required (reduced latency)
• Simple implementation (high efficiency)
• Small header overhead (8 bytes)
• No congestion control (better control over sending time and rate)

shortcoming:

• No connection established (data can be sent as it pleases, unreliable)
• No congestion control (poor network conditions may cause packet loss)

2. TCP

TCP is a connection-oriented protocol. Before using the TCP protocol to transmit data, a connection must be established between the sender and the receiver. To establish a connection, three handshakes are required. To disconnect, four handshakes are required.

(1) TCP three-way handshake to establish a connection

First handshake: The client sends a SYN (Seq=X) packet to the server, and the client enters the SYN-SENT state, waiting for the server's ACK (Ack=X+1) reply.

ps:: Seq is the sequence number, Ack is the confirmation number.

Second handshake: After receiving the SYN (Seq=X) packet sent by the client, the server returns an ACK (Ack=X+1) and SYN (Seq=Y) packet to the client. The server enters the SYN-RECIVED state and waits for the client's ACK (Ack=Y+1) reply.

Third handshake: After receiving ACK(X+1), the client enters the ESTABLISHED state and returns an ACK(Y+1) packet to the server according to the SYN(Y) sent by the server.

After receiving ACK(Y+1), the server enters the ESTABLISHED state. At this time, the connection is successfully established.

This process can be vividly expressed in the following three sentences:

• (Client): I want to establish a connection, are you ready, server?
• (Server): I'm ready, are you ready?
• (Client): I'm ready too, let's get started~

(2) TCP closes the connection four times

This process can be vividly expressed in the following four sentences:

• (Client): I want to close the connection.
• (Server): I understand.
• (Server): I am going to close the connection now, ok?
• (Client): OK, you can close it.

3. Differences between UDP and TCP

• The UDP protocol is connectionless and cannot guarantee that data is transmitted to the other end in order and without loss, but UDP is more efficient than TCP.
• The TCP protocol is connection-oriented. A handshake is required to establish and disconnect a connection. During data transmission, the sliding window (flow control) and congestion handling (slow start, congestion avoidance, fast retransmission, and fast recovery) can correctly handle packet loss problems and ensure that the receiver can receive data while effectively utilizing network bandwidth.

HTTP

HTTP (HyperText Transfer Protocol) Hypertext Transfer Protocol is an application layer protocol based on TCP (transport layer) and is the standard for requests and responses between clients and servers.

Key Features:

• Simple and fast: When a client requests a service from a server, it only needs to request the method and path.
• Stateless: When the client requests service from the server again, the server does not know whether the client has requested it before.
• Connectionless: A TCP connection is established for each request, and the connection is disconnected after the request is processed.

HTTP message:

(1) Request line: GET https://www.baidu.com/ HTTP/1.1, consisting of request method, URL, and protocol version

(2) Response line:

• HTTP/1.1 200 OK
• Protocol version, status code, and status information

HTTP request method:

There are many types of request methods, the most commonly used ones are GET and POST. Although there are many request methods, they are mostly for conveying semantics. For more semantic descriptions of methods, you can read the documentation.

4. The difference between GET and POST

GET:

• Can cache, request length limit, and history
• GET is mostly used in scenarios with no side effects (no modification of resources) and idempotence (the number of requests is independent of the resource).

POST:

• POST is slightly safer than GET because the data sent by a GET request is contained in the URL.

A detailed comparison between the two:

 ![GET and POST](https://inknight.cn/pic/note/... 
 
 )

Status code: The status code indicates the status of the response, which allows us to know whether the request was successful or failed, and if it failed, what caused it.

2XX Success:

• 200 OK, the request is successful and data is returned
• 204 No Content, success but no content
• 206 Partial Content, range request

3XX Redirects:

• 301 *** Redirect, indicating that the resource has been assigned a new URL
• 302 Temporary redirection, the resource is temporarily assigned a new URL
• 304 The resource has not been modified, you can use the cache

4XX Client Error:

• 400 Request Syntax Error
• 401 Authentication Required
• 403 Request rejected by the server
• 404 Resource Not Found

5XX Server Errors:

• 500 Server Error
• 503 The server is overloaded or down for maintenance

HTTPS

More secure network transmission protocol:

• Need to install certificate (public key)
• Encrypted by SSL/TLS protocol, the transmitted content is encrypted
• Use port 443

HTTP/2

• Multiplexing: Transmit all request data on the same TCP connection to avoid head-of-line blocking (browsers limit the number of connections under the same domain name)
• Header compression: HPACK compression format is used to encode the transmitted header, reducing the size of the header. An index table is maintained at both ends to record the headers that have appeared to avoid repeated header transmission.
• Binary transmission: In previous HTTP versions, we transmitted data in text format. A new encoding mechanism is introduced in HTTP/2, and all transmitted data will be segmented and encoded in binary format.
• Server push: After a client makes a request, the server can proactively push resources that other clients will use later. This eliminates the need for clients to make repeated requests and reduces latency.

References:

• https://juejin.im/post/5c64d15d6fb9a049d37f9c20#heading-49
• https://mp.weixin.qq.com/s/GICbiyJpINrHZ41u_4zT-A?
• http://www.alloyteam.com/2016/07/httphttp2-0spdyhttps-reading-this-is-enough/
• https://juejin.im/book/5bdc715fe51d454e755f75ef/section/5bdc72b151882516f039fce3