149 lines
7.4 KiB
Markdown
149 lines
7.4 KiB
Markdown
# qft
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QFT is a small command line application for Quick (and really reliable) Peer-To-Peer UDP file
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transfer.
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## If a friend sent you here...
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...look at the "Releases" section on the sidebar. You should see a link titled vX.Y.Z. Click on
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that, and then choose the right file for your OS: `qft` for Linux, `qft-mac` for Mac, and `qft.exe`
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for Windows. Download this file, make it executable in case of Linux or Mac, and then follow your
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friend's instructions on how to receive the file they wanted to send you.
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## Usage:
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- Find a public QFT helper (for example tudbut.de:4277)
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- On the sender PC, enter `qft sender <helper> <shared-phrase> <filename>`.
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- On the receiver PC, enter `qft receiver <helper> <shared-phrase> <filename>`.
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- Both PCs should start transferring after a short while. If they don't, try again.
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### Arguments:
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```
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qft helper <bind-port>
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qft sender <helper-address>:<helper-port> <phrase> <filename> [bitrate] [skip]
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qft receiver <helper-address>:<helper-port> <phrase> <filename> [bitrate] [skip]
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```
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## What helpers do
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Helpers are NOT relays for data, they are only used to ESTABLISH the connection.
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Helpers are there to help with holepunching.
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- P1 connects\* to helper
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- P1 sends the phrase to the helper
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- P1 waits for a response
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- Some time passes
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- P2 connects\* to the same helper
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- P2 sends the phrase to the helper
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- P2 gets P1's public IP and port
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- P1 gets P2's public IP and port
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- P1 and P2 disconnect\* from the helper
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- P1 and P2 start a loop (slightly simplified):
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- fire a packet at eachother multiple times
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- try to receive as many packets from the other one
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- if none are received, loop again
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- if one is received, exit the loop
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- Connection between P1 and P2 is established.
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\*UDP is a connection-less protocol, there are no handshakes. The word "connection" is used here as
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an indicator that data will be exchanged between the "connected" parties. The word "disconnect" is used
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here as an indicator that no more data will be exchanged between the "previously connected" parties.
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## Debunking some myths about P2P networking
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- "True P2P is only possible without a NAT" - [Both my experiments and wikipedia would like to have
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a word about hole punching.](https://en.wikipedia.org/wiki/UDP_hole_punching) The only issue I
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have found are *some* german mobile data providers, but sending large files over mobile data is
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rarely something you'd want to do - and if so, use something like croc or the magic wormhole,
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which aren't purely true P2P.
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- "Croc is P2P as well, why does this exist?" - Croc is not Peer-to-Peer. Croc uses a relay server
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to exchange data between the two clients (unless one of the client has a port-forward set up,
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which is almost never the case). That is Client-to-Server-to-Client, which is *not* really
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Peer-to-Peer. Peer-to-Peer means two clients sending their data directly to eachother, without a
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server. "Peers make a portion of their resources, such as processing power, disk storage or
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network bandwidth, directly available to other network participants, without the need for central
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coordination by servers or stable hosts." -
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[Wikipedia](https://en.wikipedia.org/wiki/Peer-to-peer)
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## Tips 'n Tricks
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- You can add a number to the end of both of your commands (after the filename) to
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boost transfer speeds (higher = faster), but a too large number might cause unreliability
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due to local network conditions or VPNs. The maximum possible is 65532 (65535 - 3).
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- You can run a helper yourself, as the "helper" mode argument suggests. This helper should simply
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be run on a server which is reachable from all over the web (a cheap VPS will definitely do).
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- Helpers don't **have to** be run on a public server, they work in LAN too, but that way, only
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computers in the same LAN will be able to use them.
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- You can allow streaming (for example when you want to transmit from /dev/stdin) by setting
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the `QFT_STREAM` environmental variable.
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- To use qfts and qftr aliases on linux or mac, run (replacing `(shell)` with your shell name,
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usually bash or zsh):
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```sh
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echo 'alias qftr="qft receiver tudbut.de:4277"' >> ~/.(shell)rc
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echo 'alias qfts="qft sender tudbut.de:4277"' >> ~/.(shell)rc
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source ~/.(shell)rc
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```
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## Cool stuff
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- Files are transferred over UDP, but qft has additional reliability measures in place to avoid
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broken files.
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- Unreliable internet connection? No problem! QFT will simply pause transmission until the
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connection is back! Doesn't work? Check out the "Resume a fully stopped transfer" section!
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- Did you know you can hibernate or suspend your computer while it's transferring and it'll continue
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where it left of, even when only one side is suspended? (Unless your router blocked the port, read
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the "Resume a fully stopped transfer" section in that case)
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- QFT can withstand heavy ~~weather~~ network conditions: 1000ms ping is just as fast as 10ms ping,
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packet loss/reorder rates of over 10% are tolerated (but can slow speeds down, especially when
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ping is high).
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- It's written in *100% pure Rust*.
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## Resume a fully stopped transfer
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You most likely won't need this unless the transfer completely died due to a VERY long pause or a
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computer restart, but if you do:
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Stop qft on both ends and start it again with the [skip] parameter in place (if you didn't specify a
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bitrate before, the default is 256). It will skip those bytes and continue where you specified.
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## Troubleshooting
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### It says `Connecting...` but doesn't connect
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One of your ends didn't correctly connect to the helper. Stop the transfer on both ends
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and try again.
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## Croc
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Many people have mentioned how this is like croc. It isn't, because croc uses a relay that all your
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data is sent through. This is a bottleneck and also means that the relay admins are responsible for
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the content that is sent. The relay also buffers a lot of data, meaning its RAM might fill up if the
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sender's connection is much faster than the receiver's. Croc being tagged "peer-to-peer" is
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misleading at best because it rarely uses the P2P capabilities (it requires a port-forward to do
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P2P, which is rarely done). Read the previous section about P2P myths if you think Croc is always
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peer-to-peer.
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## [Relevant XKCD](https://xkcd.com/949)
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## FAQ
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#### What is a helper?
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As explained above, it is used to establish the connection between the two partners.
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#### Why is a helper needed?
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Your router uses a thing called **N**etwork **A**ddress **T**ranslation. It is required because
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otherwise, there would be way too many IP addresses in use and the internet would cease to work
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during busy times. This NAT is also a problem however, because it is a layer between your PC and the
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open internet. When there is a new incoming connection, the NAT won't know which PC in your LAN to
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forward the connection to, so the connection is simply rejected. Any Peer-to-Peer software therefore
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needs a helper server (also called "STUN" server) which both peers will ask for the other's IP
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address and port. Both peers can then send a bunch of outgoing connections to eachother. If
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everything goes well, both peers have sent an outgoing with the right timing, causing both NATs to
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*think* they are outgoing connections, when actually, they are a sort of combination of incoming and
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outgoing ones.
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TL;DR: P2P networking is impossible without a helper server, because of Routers. Port-forwarding
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would be required otherwise, which can be hard to set up.
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#### How to make a public/private helper?
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Read the 2nd bullet point in the Tips 'n Tricks section.
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