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Sawadee: A simple bittorrent client in OCaml/Async

Sawadee is an OCaml implementation of the bittorrent protocol, based on the async concurrency library.

It is still in development and not ready for end-users although it is possible to download torrents and magnets from the command line.

The code is divided into several libraries and executables.

  • bittorrent is the main library allowing the user to download torrents and magnets,
  • dht is the distributed hashtable library,
  • common are modules common to both libraries as well as client modules,
  • sawadee is a simple executable to download a torrent/magnet from the command-line,
  • sawadee_repl is a read-eval-print-loop for interactive torrent/magnet downloading,
  • sawadee_rpc provides the same operations as sawadee_repl but as an rpc server.

Installation

Sawadee requires ocaml >= 4.05.0 and opam >= 1.2.2. It can be installed with:

opam pin add https://github.com/phlalx/sawadee

This installs all dependencies, then builds and installs the application. The executables sawadee, sawadee_repl and sawadee_rpc should be available from the command-line.

Alternatively, If you want to install everything (including ocaml and opam) from scratch on Ubuntu, you can use the travis script .travis-ci.sh.

Quick reminder and terminlogy

A torrent is a file that contains the information needed to download a file (or set of files) from other peers (abusively, we sometimes call torrent the actual data the user wishes to download). The torrent file contains:

  • The URL of one or several trackers.
  • The metadata (the info section of the torrent file) that contains notably the files names and sizes, and the piece size.

The torrent is uniquely identified by the 20-bytes SHA1 code of the info section. A magnet is an URI that contains this code (and possibly other parameters).

There are two ways to download a torrent depending on the availability of the torrent file.

From a torrent file.

Historically, this was the only way to download torrents. The user downloads a torrent file from an indexing website. The bittorrent client queries the tracker to obtain peers. Then, using bittorrent peer wire protocol, peers send eachother pieces of the files.

From a magnet

In the case of magnets, all is known is the SHA identifier of the torrent. Neither the peers nor the metadata are known (the metadata can be retrieved from a previous session though). Peers are obtained from the DHT. Metadata is obtained from the peers (if they support the metadata extension). When both metadata and peers are available, downloading works just as with a torrent file.

Usage

sawadee

> sawadee -h
Download torrent/magnet

sawadee URI/FILE

=== flags ===

-l             set port
-p             set download path
[-v]           verbose (level = 1 or 2)
[-build-info]  print info about this build and exit
[-version]     print the version of this build and exit
[-help]        print this help text and exit
                (alias: -?)

The anonymous parameter is either a local torrent file, or the URI of a magnet. Download path specifies where to store downloaded files and any other persistent data the client wishes to save (e.g. metadata, logs, DHT table). Files to be downloaded are created with their final size with the name specified in the torrent info section.

There are two types of metadata.

  • The info section, saved as sha.torrent where sha is the hexadecimal representation of the SHA1 identifier,
  • and a bitfield, saved as sha.bitfield, which specifies which pieces are already downloaded.

The logs (log and dht_log) are actived by the verbose option (-v 1 for fewer info and -v 2 for more).

The DHT library needs a table of DHT nodes addresses. It dynamically adds new nodes when running and saves them across sessions. By default, the table is called routing. To bootstrap the system, a table is available in tests/. It is not needed for simple torrent files (peers can be found from the tracker), but necessary for magnets.

The port is used for UDP (for DHT nodes) and TCP connections (for bittorrent peers).

Example:

# set up download directory and DHT table
> mkdir download
> cp tests/routing download/

# download a torrent
> sawadee -p download -l 7000 -v 2 tests/torrents/NuTyX_x86_64-20170625.torrent 

2017-09-17 16:01:50.217451+02:00 Info Bittorrent: peer-id:GXJHHP/sawadee
2017-09-17 16:01:50.226483+02:00 Info Dht: RAWSZJ created port 7000
2017-09-17 16:01:50.232091+02:00 Info Node_server: RAWSZJ 7000 started
2017-09-17 16:01:50.226267+02:00 Info Server: started on port 7000
2017-09-17 16:01:50.226486+02:00 Info Bittorrent: trying to read dht table download/routing
2017-09-17 16:01:50.263041+02:00 Info Dht: RAWSZJ added node DFPDCQ
2017-09-17 16:01:50.263722+02:00 Info Dht: RAWSZJ added node RZMAFE
...
2017-09-17 16:01:55.238837+02:00 Info Bittorrent: added 59 nodes to DHT
2017-09-17 16:01:55.242300+02:00 Info Bittorrent: add torrent ((info_hash f148b63816d7e3c162000835b5d9241b6929820f)(announce((scheme http)(host linuxtracker.org)(port 2710)(path(/ 00000000000000000000000000000000 / announce))(query())))(announce_list())(tinfo((name NuTyX_x86_64-20170625.iso)(piece_length 131072)(pieces_hash <opaque>)(files_info((NuTyX_x86_64-20170625.iso 258998272)))(total_length 258998272)(num_pieces 1976)(num_files 1)(priv(1)))))
...
2017-09-17 16:02:01.310679+02:00 Info Shared_meta: writing bitfield to file download/f148b63816d7e3c162000835b5d9241b6929820f.bitset
2017-09-17 16:02:01.310731+02:00 Info Shared_meta: written 23/1976 pieces (1%)
2017-09-17 16:02:01.311015+02:00 Info Pers: closing all files
2017-09-17 16:02:01.311025+02:00 Info Bittorrent: trying to write dht table download/routing
2017-09-17 16:02:01.311313+02:00 Info Bittorrent: writing download/routing

sawadee_repl

The REPL uses the same options, but allows the user to launch and follow several downloads in parallel.

> sawadee_repl -p download -l 7000 
# add tests/torrents/ubuntu-17.04-desktop-amd64.iso.torrent
Added 59066769b9ad42da2e508611c33d7c4480b3857b.
# status 59066769b9ad42da2e508611c33d7c4480b3857b
ubuntu-17.04-desktop-amd64.iso
piece_length:    512kB
total_length:   1534MB
num_pieces: 3069
downloaded = 285
2.234.173.5      transmission dl/ul    128kB       0B, dl/ul speed    6.6kB/s     0.0B/s
5.12.212.252     transmission dl/ul    144kB       0B, dl/ul speed    7.4kB/s     0.0B/s
5.145.100.93     deluge       dl/ul     16kB       0B, dl/ul speed   819.2B/s     0.0B/s
24.17.122.116    transmission dl/ul       0B       0B, dl/ul speed     0.0B/s     0.0B/s
37.48.81.84      libtorrent   dl/ul      1MB       0B, dl/ul speed   75.4kB/s     0.0B/s
37.187.6.193     deluge       dl/ul      3MB       0B, dl/ul speed  167.9kB/s     0.0B/s
...
# add tests/torrents/NuTyX_x86_64-20170625.torrent 
Added f148b63816d7e3c162000835b5d9241b6929820f.
# list 
f148b63816d7e3c162000835b5d9241b6929820f
59066769b9ad42da2e508611c33d7c4480b3857b
# status f148b63816d7e3c162000835b5d9241b6929820f
NuTyX_x86_64-20170625.iso
piece_length:    128kB
total_length:    247MB
num_pieces: 1976
downloaded = 17
31.162.242.94    transmission dl/ul    144kB       0B, dl/ul speed    7.4kB/s     0.0B/s
71.202.96.160    qBittorrent  dl/ul    464kB       0B, dl/ul speed   23.8kB/s     0.0B/s
185.21.216.195   libtorrent   dl/ul      1MB       0B, dl/ul speed   81.9kB/s     0.0B/s

Implementation

The system can be built with jbuilder using:

jbuilder build @install

Executables are installed in _build/install/default/bin.

In what follows, we give some hindsight on the global architecture of the application. See the ocaml doc for more details.

Common

The common library defines datatypes used by the other librairies and client modules.

Hash_id defines generic SHA1-based IDs that are specialized in three modules:

  • Node_id for DHT node identifiers,
  • Peer_id for bittorrent peer identifiers,
  • Bt_hash for torrent/magnet identifiers.

The bencode binary format is used extensively for data serialization in bittorrent (e.g. encoding of bittorrent messages, extension messages, torrent metadata, tracker messages). Encoding/decoding functions are defined in Bencode_ext, based on the bencode library.

DHT

The dht library implements distributed hash tables as specified by BEP 5. It is a service used by bittorrent to found peers. The interface used by the local peer is given in Dht.mli. DHT nodes communicate with eachother using a simple UDP-based RPC-protocol.

Bittorrent

This is the main library. The library interface is given by module Bittorrent. It provides the end-user operations such as downloading a torrent/magnet, getting information on a download (completion, metadata, peers...).

The following modules are singletons:

  • Bittorrent library interface,
  • Dht interface to the DHT,
  • Server TCP server waiting for peer connections,
  • Torrent_table, keeps track of torrents being downloaded (maps Bt_hash.t to Swarm.t).

Swarm

Several torrents can be downloaded concurrently. For each of them, a Swarm.t is created. It controls a set of Peer.t. A Peer.t is a worker whose job is to interact with a remote peer. The swarm has two main functions:

  • it controls the workers with high-level command such as start or stop,
  • it controls when peers can join, by consuming peers from a Peer_producer.t.

Peers notify the swarm with asynchronous events (a piece has been downloaded, the peer is leaving, metadata has been found...).

At any given time, the Swarm and the peers can be in one of two states:

  • metadata isn't known, in that case they request it from the remote peers,
  • metadata is known, and they try to download pieces of the torrent

Once in the second state, all the peers and the swarm share a Shared_meta.t value which contains:

  • information about the files (what has been downloaded, in-memory representation of the file)
  • pieces already requested

This is summarized by the following figure:

alt text

Peer

A peer worker is subdivided into more specialized modules:

  • Peer_comm.t is the lower layer interface responsible to send/receive messages (Message.t and Extension.t) to the remote peer. It also implements the handshaking protocol.
  • All extension messages (BEP 9 and 10) are processed by a Peer_ext.t, which notifies back the controlling peer using a pipe. We only deal with hanshake and metadata extension messages.
  • Block_producer.t and Block_consumer.t are used to control the flow of blocks requested.

Let's try to clarify the last item. Bittorrent peers advertise pieces, but the unit of transmission (a block) is smaller. (typically, a piece is made of dozens of blocks). For efficiency (following the implementation guidelines), we aim at keeping a constant number of pending block requests (currently less than 10, but requires tweaking). The block consumer generates the block requests when the number of pending blocks get below the treshold. It signals the producer via push back on a pipe. The producer generates blocks based on the pieces available at the remote peer.

alt text

Front-ends and RPCs

The front-ends sawadee, sawdee_repl and sawadee_rpc are simple wrappers around the bittorrent library. sawadee_rpc implements an rpc server using async rpc modules.

Tests

Testing is a difficult aspect of this project. One can use wild torrents but most torrents are illegal and swarm of peers are very dynamic which make tests not reproductible. Legal torrents usually have few seeders and even fewer leeches and don't make useful test cases. Moreover, torrent files with old-fashioned trackers are harder to find as people favor magnets. On the other hand, local system testing is tedious as it requires more scripting and tools (especially if one wants to test diverse environment with different clients).

We provide the following tests as a starting point but they should be extended and improved.

Unit and integration tests

  • common/test/test_bitfield.ml
  • dht/test/test_krpc_packet.ml
  • dht/test/test_dht.ml

The first two are simple Ounit tests for the corresponding module. The last one is an integration test for the DHT, it tests store and retrieve operations in a clique of nodes.

They are bound to the runtest directive in jbuilder. They can be run with jbuilder runtest.

System tests

Test scripts are in the scripts/ directory. scripts/bt_test.sh tests the downloading of a torrent file. It runs a number of clients and a tracker. Each client is initialized with partially downloaded files (as specified by a random bitfield) in its download directory. It registers itself on the tracker and gets addresses of peers registered before itself. Test passes if all peers download all pieces. Bittorrent clients are run as rpc servers using sawadee_rpc. They are commanded by an rpc client (test_rpc/test_simple_torrent.ml).

Number of clients and ports are paramterized scripts/test_env.sh. Here is a sample execution with 5 clients:

> bt_test.sh 
cleaning up
rm -rf download0
rm -rf download1
rm -rf download2
rm -rf download3
rm -rf download4
rm -rf download5
pkill -f sawadee
pkill -f test_dht
pkill -f test_simple_torrent
pkill -f sawadee_rpc
pkill -f bt_tracker
setting up tests
setting up dir download0
scripts/gen_bitset.py download0/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download1
scripts/gen_bitset.py download1/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download2
scripts/gen_bitset.py download2/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download3
scripts/gen_bitset.py download3/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download4
scripts/gen_bitset.py download4/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download5
scripts/gen_bitset.py download5/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
starting tracker on port 6969
starting rpc server on port -l 6000 -r 7000
starting rpc server on port -l 6001 -r 7001
starting rpc server on port -l 6002 -r 7002
starting rpc server on port -l 6003 -r 7003
starting rpc server on port -l 6004 -r 7004
starting rpc server on port -l 6005 -r 7005
_build/install/default/bin/test_simple_torrent -r 7000 -n 6 -s 25 tests/torrents/multifiles_localhost_6969.torrent
Peer 7001: 66/66
Peer 7004: 66/66
Peer 7000: 66/66
Peer 7002: 66/66
Peer 7003: 66/66
Peer 7005: 66/66
Test OK

The torrent file multifile_localhost_6969.torrent has been generated beforehand using the folder tests/multifiles.

The other test is scripts/bt_test_dht.sh. It is similar to the previous one and uses the same test_env.sh parameters. It relies on DHT to find peers instead of the tracker. One of the bittorrent client is asked to seed a file. It will announce itself on the DHT which let the other peers find it.

> bt_test_dht.sh 
cleaning up
rm -rf download0
rm -rf download1
rm -rf download2
rm -rf download3
rm -rf download4
rm -rf download5
pkill -f sawadee
pkill -f test_dht
pkill -f test_simple_torrent
pkill -f sawadee_rpc
pkill -f bt_tracker
setting up tests
setting up dir download0
scripts/gen_bitset.py download0/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download1
scripts/gen_bitset.py download1/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download2
scripts/gen_bitset.py download2/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download3
scripts/gen_bitset.py download3/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download4
scripts/gen_bitset.py download4/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
setting up dir download5
scripts/gen_bitset.py download5/e816b060122c95fa171e41227b12b4a8f30c4e20.bitset 
starting tracker on port 6969
starting rpc server on port -l 6000 -r 7000
starting rpc server on port -l 6001 -r 7001
starting rpc server on port -l 6002 -r 7002
starting rpc server on port -l 6003 -r 7003
starting rpc server on port -l 6004 -r 7004
starting rpc server on port -l 6005 -r 7005
_build/install/default/bin/test_dht -r 7000 -n 6 -s 60 tests/multifiles/11318.jpg
Peer 7004: 3/3
Peer 7003: 3/3
Peer 7002: 3/3
Peer 7001: 3/3
Peer 7005: 3/3
Test OK

Travis

.travis-ci.sh installs sawadee on a clean Ubuntu Trusty and runs all the tests.

In test_env.sh, there are time parameters that may need adjustement for the tests to pass. Depending on the number of clients and application settings (e.g. DHT querying timers), it may take more or less time for peers to be found.

Roadmap

At the current stage, one can download torrents and magnets using a simple interface. The next milestone is to test and fully implement the following BEPs.

They are partly implemented. A few things are missing:

  • An unchoking strategy. Currently, we unchoke every interested peers, but we should limit their number and pick them according to some merit metric,
  • dealing with end of download (request the same blocks to different peers).
  • correctly implementing the DHT table (in particular the bucket and token mechanism).

Before anything else, testing should be improved and target better different parts of the system.

  • the DHT,
  • the peer wire protocol (with simple torrents),
  • the metadata extension.

The code can certainly be improved.

  • many TODOs in the code...
  • improve traces. Need more consistency in the trace messages.
  • improve error management. Need more consistency in error handling depending on the sources of errors ( badly behaved remote peers, fatal errors, assert that could be recovered)
  • review possible fd and memory leaks, race conditions with pipe closing.
  • make better use of buffers

Resources

Documentation and specifications

Libraries