Torrent Peer-to-Peer Connection

Question

I used to think that my understanding of the TCP and UDP protocols, although limited, is correct. Although recently, when I realized that peers with a common torrent can connect to each other via TCP or UDP without the real need for port forwarding, I got confused. How does the router know which machine on the LAN to forward packets to? Any help in resolving this issue would be greatly appreciated. Schemes of torrent protocols and articles on the Internet are greatly simplified and, therefore, do not contain any information that could help.

Answer 1

A router (running NAT) monitors all outgoing packets and then allows incoming packets that are responses to these outgoing packets.

So, if you make an outbound TCP connection with google.com:80, then it will return the packets back (in response) with google.com:80. If two internal nodes connect to the same port, they can distinguish between them because the local port is different, for example:

1) Host A establishes a connection with Google, and the router uses its own local port 10001 for a TCP connection.

2) Host B makes a similar connection, and the router uses its own local port 10002 for a TCP connection.

If the packet comes from google.com:80 and its address for port 10001 on the WAN IP router, then the router knows to send it to host A. If its router addressed to port 10002 knows to send it to Host B.

If you have two peers behind routers (two NATs), then there is no way to establish a connection, except that if there is something to transmit information about each other's IP addresses (that is, a server that they can use to exchange information), they may try to guess which port the router will choose as the local port, and then start sending data to each other on that port, to WAN IP routers. Since both routers see OUT data, they establish a rule to allow IN data to be entered. If the ports are guessed correctly, packets on each side can go through because both routers have a configured rule. This is called UDP / TCP Hole Punching.

http://en.wikipedia.org/wiki/UDP_hole_punching

I believe Skype is an example of an application that uses UDP and punch holes.

Answer 2

Strictly speaking, since you did not qualify your router of terms with "NAT", the answer is that the router uses ARP to determine the MAC address of the target host, and then sends and sends an Ethernet address with this MAC address as the address goals.

But I think that was not what you had in mind.

You mean, how should a NAT router know where to forward incoming packets?

The answer is that the router maintains a list of active “connections” so that it can translate addresses. It uses the external port number to match the internal host address and port number. In the case of TCP, the concept of “connection” is simply the presence of a TCP connection (although usually with a timeout to stop the leak). In the case of UDP, this is more complicated because there is no UDP connection as such, so this is usually timeout tracking.