Recently I am studying P2P technology, but there are not many related materials. I figured it out myself, share some principles of learning P2P, and how to create a P2P chat application.

P2P here refers topeer to peer, Point-to-point technology, each client is a server, there is no central server, not websocket push messages for a connection.

Technical points

• udp protocol
• Establishment, connection and broadcasting between nodes
• Intranet penetration, how can two nodes located in the intranet discover each other's existence and establish communication

principle

The first solution is the problem of intranet penetration. The common underlying protocols tcp and udp have their own advantages and disadvantages. Briefly explain. tcp: Need to deal with sticky packets, duplex flow channel, is a reliable link. udp: Each time it sends a data packet, there is no sticky packet problem, but the connection is unreliable and only a small amount of data can be transmitted

More detailed please Google

Choose udp protocol here, simpler.

The next step is intranet penetration, first to say the conclusion:Two nodes in different internal networks can never discover the mutual existence between them, Even if you want to hit him along the network cable.

All intranet penetration principles require nothing more than an intermediary server with a public IP, including virtual currencies like bitcoin, etc., so firstGenesis Node

In NodeJS, creating udp service is also very simple

const dgram = require("dgram");
const udp = dgram.createSocket("udp4");
udp.bind(1090, callback)

If the service is deployed on the public network, all other nodes can access it. Through the transit server, two nodes can establish a connection

We are going to build such a P2P network

If there are only 3 nodes now:Genesis Node, Node B, Node C, Genesis node has public IP

In the form of a dialogue, I elaborate on the process of establishing a link:

Node B: hey, the genesis node, I want to join the P2P network and tell other brothers, I have come to the genesis node: Brothers, just now a node called B has joined the network, you also go to tell other nodes other nodes : I just received a notification from the "Genesis Node" that a fresh meet has joined the network, called "B"

… At this point, everyone knows that Node B has joined the network, which records the relevant information of Node B, including the IP address, including the udp port number

At this point, node C also wants to join the network and wants to talk to node B:

Node C: hey, the genesis node, I want to join the P2P network, and I want to talk to the genesis node with B: Brothers, just now a node called B has joined the network, you also go to tell other nodes, by the way See if there are other nodes in the B node: I just received a notification from the "Genesis node", there is a fresh meet to join the network, called "C", you also see if there are other nodes in the B node 2: received the notification , I heard that a node called C is looking for a node B, and I have its information here, ip is xxx.xxxx.xxx.xxxx, port 10086 B node: There is a C guy (ip: xxxx.xxxx.xxxx. xxxx, port 1000)

At this point, B got C's information, including IP and port, and C got B's information.

Thus, the two of them can establish communication. Message flow: B <----> C. No server in the middle

Use a picture to describe:

to sum up

In the design, the function of each node is the same. If you need to join the network, you do n’t have to link to the genesis node

Suppose there are already existing nodes: the genesis node, nodes A, B, and C. At this time, a node D wants to join the network.

Then node D does not necessarily have to be linked to the genesis node, it can be linked to any one of nodes A, B, and C, and then the node broadcasts to other nodes saying "Hey, there is a new person called D who joined the network".

In this way, everyone knows that there is a node called D, you can communicate with it, and at the same time, D node and the existing node will be synchronized. In this way, node D also knows the existence of other nodes.