NAT traversal: STUN, TURN, ICE, what do they actually do?
When searching for nat traversal I found all these protocols but no one can tell me what do they essentially do to traverse nat, surely not by magic, but what? Turns out it’s conceptually very simple.
What nat traversal does is not really “punching holes” on the nat, or delivering message through some tunnel, or some arcane magic, but to simply find the public address:port that can can reach me.
If I’m behind a nat or even multiple nat’s, what happens is that my packets get relayed by my nat’s and they appears on the public Internet at the out-most nat’s address with a port assigned to me. And reply packets going to that address:port are relayed back to me. So essentially I got a public address:port that can reach me on the public Internet. The purpose of nat traversal is to find that public address:port. That’s basically what the initial/classic stun (rfc 3489) does.
The truly host-to-host traversal is just that, finding the public address:port. Sometimes having that address:port is not enough, because nat poses extra restrictions1. Then we have to resort to having a public-visible relay server in the middle, which is what turn (rfc 5766) does. But any host-to-host traversal is just finding public address:port. There is no extra magic there.
How does the classic stun works is
quite simple. You send a stun server a
message, the stun server looks at the
source IP address and port of the IP packet, and reply that back to you.
Voilà, you know you public
That leaves us with ice, what does it do? Basically, people realize that there are so many network environments that are wildly different, so there is no one single method that guarantees to work everywhere. It is best to try a bunch of ways and pick the one that works and works best.
ice does basically that. It gathers a
address:port’s that possibly works (through
stun messages with stun servers) and tries
them one-by-one according to some algorithm, and reports to you the best
one. If none works, it tries to establish a relay through turn.
And here is where the new stun comes
in. People threw away the algorithm for finding
in classic stun, and kept and extended the
stun message format. Now ice runs a more thorough algorithm that uses
stun messages to communicate with
stun servers. And the new stun (rfc 5389) just
defines the stun message format. There is
a even newer version (rfc 8489) that
5389 slightly, but with no fundamental changes.
Similarly, turn is updated in rfc 8656 and now is a method used by ice rather than a standalone solution.