Many misunderstandings of UDP communication

Problem Description

UDP protocol does not need to establish a link, then I can link to other people to send data at will?

If udp is connectionless, can I send data to anyone? If he only writes the receiving function, isn't it just casual communication?

There is also how the receiving end of the communication message knows when the message was sent and how it was received in real time. Is it monitoring? Or are there other ways?

This is a topic that everyone thinks they will, but there are often many misunderstandings.

What does UDP connectionless mean?

• Communication does not require any foreplay, but goes straight to topic communication. Each message carries user data.

• The UDP transport layer, IP network layer, Ethernet link layer, hardware physical layer, and the Internet will not copy messages. The subtext is that if there is a loss of UDP messages, they do not have the message copy , So they will not be retransmitted.

• Only the application has a copy of the data, and only the application has the possibility of data retransmission.

Does the statelessness of UDP mean that the entire basic network architecture will not maintain any state of UDP?

Before the invention of NAT, it can be said that since the birth of NAT, it cannot be said that. NAT actually regards UDP communication as a TCP-like connection, so it will maintain the connection status. However, NAT also does not store data copy.

UDP is similar to sending a letter to the other party. In general, the recipient accepts all people's letters by default. In the past when the United States was fighting terrorism, Bush Jr. often received dangerous emails from terrorist organizations. UDP communications without a whitelist or blacklist system were the most vulnerable.

But there is a premise here, the recipient must be alive, so that it has the meaning of communication. Corresponding to the world of UDP, it means that the receiver of UDP packets must be in the listening state.

What does listening state mean?

The DNS resolution program responsible for the domain name query is to listen on UDP port 53 for query requests from any corner of the world.

Message notification mode

The DNS resolver registered a message with the TCP / IP protocol stack: "I use port 53. If there is a message sent to port 53, please let me know. My process number is 12345"

After a while, a packet with a UDP destination port = 53 arrived. The guardian manager immediately sent a message to the process number = "12345", "There is mail, cache". The DNS parser heard that it was taken away immediately. This is the message notification mode.

Callback mode

The DNS program is an otaku. I feel that the efficiency of running in the communication room is too low. I say hello (registration) to the guard. Mr. Wang, there will be a message sent to port 53 in the future. Thank you!

This is the situation on the server side, what about the client?

Xiaoming writes a love letter to Xiaomei, can Xiaoming know when Xiaomei will reply to himself? No!

Query mode

Xiao Ming is very anxious again, what should I do? Xiaoming ran the communication room three times a day to see if there was a reply from Xiaomei. This is a UDP query mode.

Message notification mode

The guard door king can't stand it anymore, Xiaoming, come back quickly, Xiaomei's letter has arrived, I will call you to get it.

The UDP non-connected communication mode provides convenience for DoS attacks on forged messages. To deal with this challenge, applications usually use cookies to filter out all forged source hosts.

In addition, UDP can use a whitelist filtering list, only the source IP hosts that are explicitly allowed to communicate can be filtered out, and others are filtered out.

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