qft/README.md

# qft
QFT is a small command line application for Quick Peer-To-Peer UDP file transfer.

## Usage:
- Find a public QFT helper (for example tudbut.de:4277)
- On the sender PC, enter `qft sender <helper> <shared-phrase> <filename>`.
- On the receiver PC, enter `qft receiver <helper> <shared-phrase> <filename>`.
- Both PCs should start transferring after a short while. If they don't, try again.

### Arguments:
```
qft helper   <bind-port>
qft sender   <helper-address>:<helper-port> <phrase> <filename> [bitrate] [skip]
qft receiver <helper-address>:<helper-port> <phrase> <filename> [bitrate] [skip]
```

## What helpers do

Helpers are NOT relays for data, they are only used to ESTABLISH the connection.

Helpers are there to help with holepunching.
- P1 connects\* to helper
- P1 sends the phrase to the helper
- P1 waits for a response
- Some time passes
- P2 connects\* to the same helper
- P2 sends the phrase to the helper
- P2 gets P1's public IP and port
- P1 gets P2's public IP and port
- P1 and P2 disconnect\* from the helper
- P1 and P2 start a loop:
  - wait until their system clock is at .0 or .5 of a second
  - fire a packet at eachother at the same time
  - try to receive the packet from the other one
  - if none is received, loop again
  - if one is received, exit the loop
- Connection between P1 and P2 is established.

\*UDP is a connection-less protocol, there are no handshakes. The word "connection" is used here as
an indicator that data will be exchanged between the "connected" parties. The word "disconnect" is used
here as an indicator that no more data will be exchanged between the "previously connected" parties.

## Tips 'n Tricks
- You can add a number to the end of both of your commands (after the filename) to
  boost transfer speeds (higher = faster), but a too large number might cause unreliability
  due to local network conditions or VPNs. The maximum possible is 65533 (65535 - 2).
- Helpers don't **have to** be run on a public server, they work in LAN too, but that way, only
  computers in the same LAN will be able to use them.
- To use qfts and qftr aliases on linux or mac, run (replacing `(shell)` with your shell name,
  usually bash or zsh):
```sh
echo 'alias qftr="qft receiver tudbut.de:4277"' >> ~/.(shell)rc
echo 'alias qfts="qft sender tudbut.de:4277"' >> ~/.(shell)rc
source ~/.(shell)rc
```

## Cool stuff
- Files are transferred over UDP, but qft has additional reliability measures in place to avoid
  broken files.
- Unreliable internet connection? No problem! QFT will simply pause transmission until the
  connection is back! Doesn't work? Check out the "Resume a fully stopped transfer" section!
- Did you know you can hibernate or suspend your computer while it's transferring and it'll continue
  where it left of, even when only one side is suspended (unless your router blocked the port, read 
  the "Resume a fully stopped transfer" section in that case)?
- It's written in *100% pure Rust*.

## Resume a fully stopped transfer
You most likely won't need this unless the transfer completely died due to a VERY long pause or a
computer restart, but if you do:

Stop qft on both ends and start it again with the [skip] parameter in place (if you didn't specify a
bitrate before, the default is 256). It will skip those bytes and continue where you specified.

## Troubleshooting

### It constantly says `CONNECTING`
One of your ends didn't correctly connect to the helper. Stop the transfer on both ends
and try again. If it still doesn't work, make SURE the time and date on both ends are within an
error of <0.1 seconds! Holepunching strongly relies on the time and date matching. (If you have any
suggestion on how I can mitigate this reliance on time, please open an issue!)

## [Relevant XKCD](https://xkcd.com/949)

![Relevant XKCD Image](https://imgs.xkcd.com/comics/file_transfer.png)