Quick answer: A TURN relay is a server that passes data between two devices that can't connect to each other directly, for example because a firewall or mobile network blocks it.
TURN relay (Traversal Using Relays around NAT) explained
Browser-to-browser transfers and video calls (WebRTC) try to connect the two devices directly. That usually works, but some company, hotel, school and mobile networks block direct connections between devices. TURN (Traversal Using Relays around NAT) is the standard fallback: both devices connect out to a relay server, and the relay passes the data from one to the other.
The data stays encrypted end to end on the way, because WebRTC encrypts it between the two browsers before it reaches the relay. The relay only forwards packets it cannot read, and it does not store the file. The cost is speed: every byte travels to the relay and back, so a relayed transfer is often slower than a direct one.
On this site, Send via Wi-Fi uses a relay automatically when two devices can't reach each other directly; the transfer then shows that it goes via our relay. Relayed transfers have a size cap (5 GB by default), while direct transfers have no size limit.
Direct vs relayed
| Direct (peer to peer) | Through a TURN relay | |
|---|---|---|
| Path | Device to device | Device to relay to device |
| Encryption | End to end | Still end to end; the relay can't read it |
| Speed | As fast as the slower connection allows | Usually slower |
| When it is used | Whenever the networks allow it | Only when a direct connection fails |
Frequently asked questions
Can the relay server see my files?
No. WebRTC encrypts the data between the two browsers, so the relay only forwards encrypted packets and keeps nothing.
Why is my transfer going through a relay?
One of the networks blocks direct connections between devices, which is common on company, hotel and mobile networks. Putting both devices on the same Wi-Fi usually allows a direct connection.
What is the difference between STUN and TURN?
STUN only helps each device find its public address so they can try to connect directly. TURN carries the data itself when a direct connection is impossible.
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