A plain-language explanation of the technology behind private, serverless file transfer.
No plugins, no installs, no accounts. Just two browser tabs and a link.
A unique peer ID is registered with the signaling server. A share link containing that ID is shown to the sender. The sender adds up to 10 files to the queue.
The receiver's browser connects to the signaling server, locates the sender by peer ID, and exchanges ICE candidates to find the best network path.
The WebRTC DataChannel opens. The sender offers the file, receiver accepts, and bytes flow in 64 KB chunks — encrypted, peer-to-peer.
The receiver always sees what's coming before a single byte is transferred.
Both sides see a "Connection Established" confirmation before anything happens.
Sender proposes each file with its name, size, and type. Receiver sees this before accepting.
Receiver clicks Accept to start the download, or Decline to skip that file. Sender is notified either way.
Only after acceptance does the sender start streaming. No surprise downloads.
Most devices sit behind NAT routers that hide their real IP address. WebRTC uses ICE to solve this.
Each browser contacts a STUN server to discover its public IP address. Both sides exchange these addresses and attempt a direct connection (hole-punch). This succeeds for around 90% of home WiFi and broadband pairs worldwide.
If direct fails (symmetric NAT, CGNAT, strict firewall), the app automatically offers a relay button to the receiver. When clicked, all data flows through a TURN relay server — still encrypted — until it reaches the other browser.
FileCourier tries three methods in order, falling back if the browser doesn't support the previous one.
| Tier | Method | Browsers | Size limit |
|---|---|---|---|
| 1 | StreamSaver.js — service worker streams directly to disk | Chrome, Edge | Unlimited |
| 2 | File System Access API — native browser write stream | Chrome, Edge | Unlimited |
| 3 | In-memory Blob — accumulate in RAM, save at end | All browsers | 512 MB max |
PeerJS cloud (0.peerjs.com) handles only peer discovery — peer IDs and ICE candidates. After the WebRTC channel opens, the signaling server is never contacted again. File data never passes through it.
Files are split into 64 KB chunks. If the DataChannel's send buffer exceeds 8 MB, sending pauses until the buffer drains. This prevents out-of-memory errors on the sender for very large files.
If the receiver switches networks (WiFi to mobile or vice versa), the app detects the change and automatically retries the connection up to three times with a short delay.
WebRTC mandates DTLS encryption on every DataChannel. There is no opt-out. All file chunks are encrypted in transit, even when relayed through a TURN server.
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