Peer-to-peer file sharing: how it works and which kind to use

Peer-to-peer (P2P) file sharing means the file moves directly from one device to another rather than being uploaded to a company's server and downloaded later. That one idea covers three quite different kinds of tool: browser-based P2P for sending a file to one person right now, local-network apps such as LocalSend or PairDrop for moving files between devices in the same building, and BitTorrent for distributing the same file to many people at once. Pick by situation: browser P2P for a one-off send across the internet, a LAN app for your own devices at home or in the office, and BitTorrent when many people need the same large file.

What "peer-to-peer" actually means

In a classic cloud transfer (WeTransfer, Google Drive, Dropbox), the sender uploads a complete copy to the provider's storage, and the recipient downloads it from there. The file exists on a third party's servers, at least for a while, and it crosses the internet twice.

In a P2P transfer, the two devices exchange the data with each other. Almost every P2P system still uses a server for something, though. It is worth being precise about what:

• Discovery and introduction. Two devices on the internet cannot find each other without help. A signaling server, a tracker or a local broadcast tells each peer how to reach the other.
• Relaying when a direct path is blocked. Some home routers, mobile carriers and corporate firewalls prevent incoming connections. Browser P2P then falls back to a relay (a TURN server) that forwards encrypted packets without storing them.

So the honest definition is not "no server is involved" but "no server keeps a copy of your file". That is the property that matters for privacy and for avoiding storage limits.

The three families at a glance

Browser P2P: WebRTC data channels

Modern browsers ship with WebRTC, the same technology that carries the audio and video in browser-based calls. Besides media, WebRTC can open a "data channel" that carries arbitrary bytes, which makes it a natural fit for file transfer.

A typical session works like this. The sender opens a page and picks a file. The page registers with a signaling server and produces a link or code. The receiver opens the link, the two browsers swap connection details through the signaling server, and then they try to connect directly using STUN (a service that tells each browser its public address). If that works, data flows browser to browser. If it does not, the session falls back to a TURN relay. Either way, WebRTC encrypts the data channel with DTLS, which is mandatory in the standard, so a relay forwards ciphertext it cannot read.

Strengths: nothing to install, works across platforms (iPhone to Windows, Android to Mac), no account, no storage quota, and the file is not uploaded to anyone's cloud.

Weaknesses: both people must be online with the page open for the whole transfer. Most browser tools cannot resume a broken transfer. Phones are aggressive about suspending background tabs, so the screen usually has to stay on. And some browsers hold the incoming file in memory before saving it, which can make multi-gigabyte transfers to phones unreliable.

Our own tool, TransferFiles.net, sits in this family: you get a link and a short code, the receiver connects, and the file streams across under DTLS encryption without being stored on a server. If a strict network forces the relay path, the encrypted data passes through a relay but is not kept there. How it works goes into the details.

Local-network apps: LocalSend, PairDrop and Snapdrop

When both devices are on the same Wi-Fi or Ethernet network, the data does not need to touch the internet at all. Local apps discover nearby devices and send directly across your router, which is usually the fastest option you have.

LocalSend

LocalSend is a free, open-source app (Apache 2.0 licence) for Windows, macOS, Linux, Android and iOS. Its site says it needs no internet connection and no account; devices on the same network find each other automatically, and transfers are encrypted over HTTPS, with optional PIN verification. It is ad-free and funded by donations. Because it is an installed app rather than a browser tab, it tends to cope better with large batches of files. The catch is that both devices must be on the same local network, and some guest or corporate Wi-Fi networks isolate devices from one another, which stops discovery.

Snapdrop and PairDrop

Snapdrop was a popular browser-based "AirDrop for everyone": open the page on two devices on the same network and they appear to each other. According to the project's GitHub README, Snapdrop has since been acquired by LimeWire, and the original code remains available for self-hosting.

PairDrop is an open-source fork of Snapdrop (GPL-3.0) that keeps the original idea and adds features. Devices on the same network still discover each other automatically, but you can also pair devices with a six-digit code or QR code so they find each other from different networks, or open a temporary public room with a short code. When devices sit behind NAT, PairDrop connects them through its TURN server. It runs in the browser using WebRTC and WebSockets, so nothing needs installing.

BitTorrent: P2P for many recipients

BitTorrent solves a different problem: getting one large file to lots of people without the original sender needing enormous bandwidth. The protocol specification (BEP 3) describes how it works. Files are split into fixed-size pieces, and the torrent's metadata stores a SHA-1 hash for every piece. Downloaders fetch pieces from many peers at once, check each piece against its hash, and re-download any piece that fails. A tracker (or, in modern clients, a distributed hash table) helps peers find each other. Every downloader also uploads the pieces it already has, so the more popular a file is, the more sources there are.

That design gives BitTorrent two things browser P2P lacks: built-in integrity checking and the ability to resume indefinitely, piece by piece. It is why Linux distributions, game patch systems and research datasets have used it for years.

It is the wrong tool for sending a private file to one colleague. You would need to create a torrent, share the .torrent file or magnet link, keep a client seeding, and accept that in a public swarm your IP address is visible to every peer. For one-to-one transfers, browser P2P or a LAN app is far simpler.

Is P2P file sharing legal?

The technology is legal. P2P is a way of moving data, the same as email or HTTP downloads, and it has many legitimate uses: sending your own videos to a client, moving photos between your devices, distributing open-source software.

What is not legal in most countries is sharing copyrighted material without permission, whatever tool you use. BitTorrent gets singled out because public swarms make infringement easy to see: rights holders can join a swarm and record the IP addresses of peers uploading a film, and those addresses can lead back to internet subscribers. In the United States, the Supreme Court's 2005 decision in MGM Studios v. Grokster also held that a company can be liable if it promotes its P2P software as a tool for infringement. That case concerned how the software was marketed, not P2P technology itself.

This is general information, not legal advice. Copyright rules differ by country.

Privacy and safety: what P2P does and does not protect

• No stored copy. The biggest privacy gain over cloud services is that no provider ends up holding your file, so there is nothing to leak later, scan, or forget to delete.
• Encryption in transit. WebRTC encrypts by default. LocalSend uses HTTPS. BitTorrent's optional protocol encryption is mainly about disguising traffic from network operators, not about keeping contents secret from other peers, who receive the data by design.
• IP visibility. In a direct connection the other side can generally see your IP address. Among friends and colleagues this rarely matters.
• The file itself. P2P does not make a file safe to open. Treat received files as you would any download; our guide on whether it is safe to receive files covers the checks worth doing.

Choosing: a quick decision list

1. Is the recipient going to download later, while you are offline? Use a cloud link (Google Drive, WeTransfer, Dropbox). Live P2P cannot do this.
2. Are both devices on the same network? Use LocalSend or PairDrop. It is the fastest route and does not consume internet bandwidth.
3. Is it one recipient in another place, right now? Use browser P2P. Nothing to install on either side.
4. Do many people need the same big public file? Use BitTorrent, or host it on a proper download server.
5. Is the network locked down (school, office, hotel)? Browser P2P with a TURN relay usually still works; if not, see our notes on transfers blocked on a network.

Sources

• LocalSend official site
• PairDrop on GitHub
• Snapdrop README (GitHub)
• BEP 3: The BitTorrent Protocol Specification
• MGM Studios v. Grokster, 545 U.S. 913 (2005), Cornell LII