After many long months of planning and work, I have completed another peer-to-peer downloader for Debian. If you've been keeping track, that makes 2 now. This one is called apt-p2p, and as of yesterday it is available in unstable.

The functionality is very similar to the first one I wrote, DebTorrent, so if you've used that one you should feel very comfortable. After installation you just add a localhost:9977 to the start of your sources.list entries (see the man page). The only difference is the port number (DebTorrent uses 9988 by default) and that you can use it on all your sources.list entries, whether they be deb, deb-src, official Debian archive, or any other archive. Then an apt-get update gets it started, and you can begin installing packages. Point your web browser to http://localhost:9977 to see what's going on.

IMPORTANT: as with any P2P program, it works much better if you open a port through your NAT or firewall. Without this crucial step, you won't be able to share with any peers, and your lookup of peers to download from may take longer. Make sure to forward both TCP and UDP ports 9977, or whichever port you set in the config file. For more details, see the port forwarding section of the FAQ.

The similarities with DebTorrent are all external, so let's look at how it differs from DebTorrent internally:

• it's very general, it doesn't matter what you're trying to download, a source package or a Packages file, from constantly-updated unstable or a year-old stable, for i386 or hppa architectures (DebTorrent only works for .deb files, is only supported for the official archive, and breaks downloaders into groups by architectures)
• it doesn't require anything other than what's available to apt (DebTorrent uses piece hashes of large files, and ordering information, both of which are downloaded separately)
• it can be very fast when downloading from mirrors (mirror downloads with DebTorrent are not so fast)
• the code is simple, and makes use of available code, such as Twisted, Khashmir, and python-apt, which should make future enhancements and maintenance easier (DebTorrent is large and monolithic)
• requires less memory and CPU power (50% to 75% less memory than DebTorrent)

Here's some technical details for those interested:

• makes use of hashes to uniquely identify files
• uses a Distributed Hash Table (DHT) to find peers
• also stores piece hashes in the DHT for efficient downloading
• uses HTTP/1.1 requests to download from peers
• no peers available causes it to fallback to a download from the mirror

Though I know it can be fast, I'm not yet sure if the peer lookup in the DHT will be quick enough to keep up with the downloading. All my tests so far show that it is, but until there are a number of peers out there trying it, I can't be sure. Also, I have some improvements in mind to enhance the speed, in particular the wait for a timeout to occur, so this may improve in the future. If you see a delay in downloading where apt seems to be stalled saying 'Waiting for headers', for now be patient and see what happens. It may be that apt-p2p is downloading in the background (it does this sometimes), or that it's waiting for a lookup in the DHT to complete. If it hangs for more than a minute, or there's errors in the log file, please file a bug so that I can look into it.

Finally, for you DebTorrent fans don't worry, I haven't given up on it. Stay tuned for more info on it coming soon.