libtorrent blog

bittorrent development from the trenches

libtorrent alert queue

The main mechanism libtorrent uses to report events and errors to the client is via alerts. Alerts are messages as c++ objects with additional information depending on the type of message. Periodically clients poll for new alerts from a session object. In the next major release of libtorrent detailed peer logging will be available as . . .

bdecode parsers

I have recently revisited the bdecoder in libtorrent, and ended up implementing a new bdecoder that is two orders of magnitude faster than the original (naive) parser. This is the 3rd decoder in libtorrent’s history, and I would like to cover its evolution of parsing bencoded data.

a bittorrent filesystem

One of the main bottlenecks when downloading and seeding content over bittorrent is accessing the disk. This post explores the option to bypass traditional filesystems and use a block device as storage for torrents, in order to improve download performance. bittorrent protocol BitTorrent downloads conceptually divide up content into pieces, which are downloaded in rarest-first order. . . .

filenames

A .torrent file is very flexible in what it allows a path or filename to contain. Each directory name in a path is a length-prefixed utf-8 string. It can be an empty string, and it can have any character imaginable in it. The way a .torrent file represents filenames and paths is also agnostic to . . .

VirtualAlloc pitfall

When allocating blocks in the disk cache, libtorrent uses valloc(), to allocate page-aligned 16kiB blocks. On windows, the natural couterpart to valloc() is VirtualAlloc(). Having these blocks page aligned may provide performance improvements when reading and writing files that are aligned to the block boundaries. The 16kiB allocation size is derived from the bittorrent protocol . . .

DHT routing table maintenance

I’ve been working on performance improvements of the DHT recently that I would like to cover in this post. NICE routing table One of the proposed improvements from the sub-seconds lookups paper is referred to as NICE. It proposes replacing the method of maintaining the routing table buckets (see kademlia paper) with directly pinging the nodes, the most . . .

running averages

Many aspects of bittorrent requires maintaining an estimate of some kinds of samples. uTP keeps a running estimate of round-trip times for each connection. When streaming torrents, it is useful to keep an estimate of download time per piece (to know what a reasonable timeout is). This post takes a closer look at how to . . .

memory cache optimizations

When optimizing memory access, and memory cache misses in particular, there are surprisingly few tools to help you. valgrind’s cachegrind tool is the closest one I’ve found. It gives you a lot of information on cache misses, but not necessarily in the form you need it. About a week ago I started looking into lowering . . .

swarm connectivity

In bittorrent it is important to keep the swarm as evenly connected as possible. Clustering of peers may create bottlenecks for piece distribution and create a skewed market for trading pieces. Keep in mind that local piece availability is used as an approximation for global piece availability in the rarest-first piece picking algorithm. This post . . .

DHT security

One of the vulnerabilities of typical DHTs, in particular the bittorrent DHT, is the fact that participants can choose their own node ID. This enables an attacker to deliberately place themselves at a locaton in the DHT where they know they will be responsible for storing some specific data. At that point, there are a few naughty things . . .