What is NAT traversal?

When two people are on a video call, their voices usually travel straight to each other. Direct, no middleman. Except sometimes they cannot, and the call quietly reroutes through a server in another city, adding lag, without anyone knowing why. The reason is the same thing that hides your devices from the internet: your home router.

Two people who only have the switchboard number

Think of it like two people who only have each other's office switchboard number, not a direct line. Both are behind a router that shares one public address across every device in the house. From outside, your laptop has no findable address of its own. Neither does theirs.

So how do two hidden devices ever form a direct line? Usually, cleverly. Sometimes, not at all. This whole problem, and the tricks used to solve it, is called NAT traversal.

The trick: hole punching

Both devices poke outward at the same moment, and if the timing and router types cooperate, they meet in the middle and lock in a direct path. It works surprisingly often. This is exactly what ICE is doing behind the scenes during that "connecting..." moment.

But some routers are stubborn, the strict, symmetric kind, and simply refuse to let two hidden devices find each other directly, no matter how many times they try.

When it fails: the relay

When the direct path fails, the call does not drop. It falls back to a relay. A server in the middle catches both streams and passes them along. The call still works, but your voice now takes a detour, maybe through another country, adding delay every time you speak.

That subtle lag on some calls but not others? Often this, silently happening in the background. The server doing the relaying is a TURN server, and the whole direct-vs-relay decision is exactly what STUN and TURN, working together, are for.