This document discusses using BitTorrent on iOS. It provides an overview of BitTorrent, including its history, usage statistics, and technical details. It then discusses challenges with using BitTorrent on iOS and potential solutions, including using the libtorrent C++ library. It covers how to build and configure libtorrent for iOS, and how to interface it with Swift. It also describes how to implement an event loop to fetch torrent updates and progress. Some caveats mentioned are the need for fine-tuning to avoid heavy battery usage and lack of background session support on iOS.

1. BitTorrent on iOS
Michael Caylus / Simone Civetta
12/01/2017

2. Michael Simone

3. Quiz

4. ‘How many of you need to
sync large quantity of files
periodically?’

5. BitTorrent

8. Designed: April 2001
First released: July 2001 Bram Cohen
A
Peer-To-Peer Protocol
#1 usage:
Sharing Large Media Files

9. 170M active users* 40% of all Internet traffic**
10 popular clients
uTorrent, Xunlei, Transmission, Vuze, qBittorent,
Deluge, BitComet and Tixatit
* Wikipedia
CompaniesPiracy
** BitTorrent Inc.

10. Facilitates transfer of large/multiple files
Works on unreliable network connections
Downloads can be resumed any time
✔
✔
✔

11. In Detail

12. Basics
Files are
shared among
the users
Files to be shared
are split in
multiple chunks
Peers
contribute

13. Torrent (metadata)
Pieces / Segment
Swarm
Peers / Leechers
Seeds
Trackers
Stakeholders

14. Metainfo file (bencoded dictionary)
Torrent File
ANNOUNCE
Announce URL of the tracker
PIECE LENGTH
INFO
Dictionary describing the files’ chunks
PIECES (20-byte SHA1 Hash Values)
LENGTH
NAME
PIECE LENGTH
PIECES (20-byte SHA1 Hash Values)
NAME
FILES
LENGTH
PATH

15. Always-available seed
Firewall-friendly stream
Easy-to-manage service
Our Challenges
!
!
!

16. Torrent (metadata)
Pieces / Segment
Swarm
Peers / Leechers
Seeds
Trackers
WebSeed

17. 2
WebSeed
Similar to HTTP direct download
Guarantees availability of resources
Independent of swarm sanity
Extension on bitTorrent protocol
Introduced in 2006

18. Quiz

19. ‘How many of you have
already developed a
BitTorrent client?’

20. libtorrent

21. BitTorrent OSS Implementation
Developed by Arvid Norberg, since 2003
Written in C++, Cross Platform
Memory/CPU efficient, feature-packed
✔
✔
✔
✔
libtorrent

22. Pros
Easily configurable
Supports trackerless mode, with
DHT
Supports Web Seeds (i.e. a seed can
be a HTTP host), and SSL Torrents
Supports Local Service Discovery
Pros and Cons
Cons
No CocoaPods/Carthage distribution
Depends on other libraries
Not based on URLConnection
Does not support iOS background
sessions

23. Alternatives

24. MultiPeer
Connectivity
Couchbase Realm
Alternatives

25. Alternatives: Pros
MultiPeer
Connectivity
Couchbase
It’s a real DB
Sync based on
NSURLSession
Supports MPC
Realm
It’s a real DB
Sync based on
NSURLSession
Native
Supports
Bluetooth

26. Alternatives: Cons
MultiPeer
Connectivity
Couchbase
Depends on a
proprietary stack
Doesn’t work
with chunks
Realm
Depends on a
proprietary stack
No real P2P
8 devices per
session
Manual handling
of chunks

27. How to Build

28. OpenSSL
Boost C++ TLS/SSL and crypto C library
Requirements

29. How to Build
https://github.com/xebia-france/libtorrent-ios-resources
Option A: Pre-configured Xcode Project
- https://github.com/chublix/libtorrent-ios
- Adapt for OpenSSL
Option B: Qt Creator
- Build OpenSSL and Boost separately
- Build libtorrent, for all the platforms (e.g. iphoneos and iphonesimulator)
- lipo (i.e., merge the slices) of the different platforms
Option C: Bash Script
- cf. xebia-france/libtorrent-ios-resources/build-script

30. Sources
Popcorn Time iOS
https://github.com/danylokostyshyn/popcorntime-ios
Popcorn Time TV:
https://github.com/PopcornTimeTV/PopcornTimeTV
Popcorn Torrent:
https://github.com/PopcornTimeTV/PopcornTorrent (GCD WebServer / Torrent client for iOS)

31. Configuration

32. Configuration
session_settings settings = ses.settings();
settings.ssl_listen = 4443;
settings.active_loaded_limit = 1;
settings.max_peerlist_size = 10;
settings.min_announce_interval = 30;
settings.urlseed_wait_retry = 10;
ses.set_settings(settings);
// http://libtorrent.org/reference-Settings.html

33. settings.local_service_announce_interval = 30;
settings.active_lsd_limit = 1;
// ...
ses.start_lsd();
// ...
ses.stop_lsd();
Configuration (Local Service Discovery)

34. How Incremental
Download Works

35. How Incremental Download Works
No modification in the content: No Download
Modification in the content referenced changed: Download Starts
The SHA1 of each chunk is compared to the version on disk:

36. Implementation

37. C++ / Swift Bridging
03
04
01
02 Add wrapper class’s header file into bridging header
Implement a wrapper class in .mm file (Obj C++ )
Use wrapper class in Swift03

38. C++ / Swift Bridging
02 Implement a wrapper class
Wrapper Class (Header)
#import <Foundation/Foundation.h>
@interface LibTorrentWrapper : NSObject {
- (void)initSession:(NSInteger)fromPort toPort:(NSInteger)toPort;
- (void)addNewTorrentFromFile:(NSString *)filePath;
}

39. C++ / Swift Bridging
02 Implement a wrapper class
Wrapper Class (Implementation)
@interface LibtorrentWrapper()
@property libtorrent::session *session;
@end
@implementation LibtorrentWrapper
- (void)initSession:(NSInteger)fromPort toPort:(NSInteger)toPort {
self.session = new libtorrent::session();
session_settings settings = self.session->settings();
// Apply Settings here...
self.session->set_settings(settings);
libtorrent::error_code ec;
self.session->listen_on(std::make_pair(fromPort, toPort), ec)
}

40. C++ / Swift Bridging
02 Implement a wrapper class
Wrapper Class (Implementation)
//...
- (void)addNewTorrentWithData:(NSData *)data {
libtorrent::add_torrent_params params;
params.save_path = // Some dir...
libtorrent::error_code ec;
params.ti = new libtorrent::torrent_info((const char*)data.bytes, data.length, ec);
self.session->add_torrent(params, ec);
}
@end

41. C++ / Swift Bridging
02 Implement a wrapper class
Invoke Wrapper Class in Swift
func startTorrentSession() {
var torrentWrapper = LibtorrentWrapper()
torrentWrapper.initSession(6881, toPort: 6889)
torrentWrapper.addNewTorrentWithData(/* Some Data */)
}

42. Event loop
03
04
01
02 Pop alert events each time callback is invoked
Set-up a timer that triggers a callback periodically
Feed torrent progress info03

43. Fetch alerts
- (void)fetchAlerts {
std::deque<alert *> deque;
_session->pop_alerts(&deque);
for (std::deque<alert *>::iterator it=deque.begin(); it != deque.end(); ++it) {
std::unique_ptr<alert> alert(*it);
switch (alert->type()) {
case metadata_received_alert::alert_type:
//**
case piece_finished_alert::alert_type:
//** You can feed progress info here
case torrent_finished_alert::alert_type:
//**
default: break;
}
}
deque.clear();
}

44. Fetch torrent progress info
- (dl_info)getCurrentTorrentInfo:(piece_finished_alert *)alert {
dl_info result;
torrent_handle t = alert->handle;
try {
t->get_torrent_info();
} catch (libtorrent_exception &e) {
return result;
}
torrent_status ts = t->status(torrent_handle::query_pieces);
result.seeders = ts.num_seeds;
result.peers = ts.num_peers;
result.downloadRateBs = ts.download_rate;
result.uploadRateBs = ts.upload_rate;
torrent_info ti = t->get_torrent_info();
result.downloaded = ts.total_done;
result.progress = ts.progress_ppm;
result.completed_time = ts.completed_time;
return result
}

45. Caveats

46. Caveats
1 Needs fine tuning, i.e. for avoiding heavy
battery consumption
No background sessions
Almost no iOS help resource available
System proxy support to be manually
implemented (via
CFNetworkCopySystemProxySettings)
2
3
4