typedef asio::ip::address address;
typedef asio::ip::address_v4 address_v4;
typedef asio::ip::address_v6 address_v6;
using asio::ip::tcp;
using asio::ip::udp;

tcp::endpoint
udp::endpoint

is_listening() listen_port() listen_on()

bool is_listening() const;
unsigned short listen_port() const;

enum {
        listen_reuse_address = 1,
        listen_no_system_port = 2
};

void listen_on(
        std::pair<int, int> const& port_range
        , error_code& ec
        , char const* interface = 0
        , int flags = 0);

is_listening() will tell you whether or not the session has successfully opened a listening port. If it hasn't, this function will return false, and then you can use listen_on() to make another attempt.

listen_port() returns the port we ended up listening on. Since you just pass a port-range to the constructor and to listen_on(), to know which port it ended up using, you have to ask the session using this function.

listen_on() will change the listen port and/or the listen interface. If the session is already listening on a port, this socket will be closed and a new socket will be opened with these new settings. The port range is the ports it will try to listen on, if the first port fails, it will continue trying the next port within the range and so on. The interface parameter can be left as 0, in that case the os will decide which interface to listen on, otherwise it should be the ip-address of the interface you want the listener socket bound to. listen_on() returns the error code of the operation in ec. If this indicates success, the session is listening on a port within the specified range. If it fails, it will also generate an appropriate alert (listen_failed_alert).

If all ports in the specified range fails to be opened for listening, libtorrent will try to use port 0 (which tells the operating system to pick a port that's free). If that still fails you may see a listen_failed_alert with port 0 even if you didn't ask to listen on it.

It is possible to prevent libtorrent from binding to port 0 by passing in the flag session::no_system_port in the flags argument.

The interface parameter can also be a hostname that will resolve to the device you want to listen on. If you don't specify an interface, libtorrent may attempt to listen on multiple interfaces (typically 0.0.0.0 and ::). This means that if your IPv6 interface doesn't work, you may still see a listen_failed_alert, even though the IPv4 port succeeded.

The flags parameter can either be 0 or session::listen_reuse_address, which will set the reuse address socket option on the listen socket(s). By default, the listen socket does not use reuse address. If you're running a service that needs to run on a specific port no matter if it's in use, set this flag.

If you're also starting the DHT, it is a good idea to do that after you've called listen_on(), since the default listen port for the DHT is the same as the tcp listen socket. If you start the DHT first, it will assume the tcp port is free and open the udp socket on that port, then later, when listen_on() is called, it may turn out that the tcp port is in use. That results in the DHT and the bittorrent socket listening on different ports. If the DHT is active when listen_on is called, the udp port will be rebound to the new port, if it was configured to use the same port as the tcp socket, and if the listen_on call failed to bind to the same port that the udp uses.

If you want the OS to pick a port for you, pass in 0 as both first and second.

The reason why it's a good idea to run the DHT and the bittorrent socket on the same port is because that is an assumption that may be used to increase performance. One way to accelerate the connecting of peers on windows may be to first ping all peers with a DHT ping packet, and connect to those that responds first. On windows one can only connect to a few peers at a time because of a built in limitation (in XP Service pack 2).

add_feed()

feed_handle add_feed(feed_settings const& feed);

This adds an RSS feed to the session. The feed will be refreshed regularly and optionally add all torrents from the feed, as they appear. The feed is defined by the feed_settings object:

struct feed_settings
{
        feed_settings();

std::string url;
        bool auto_download;
        bool auto_map_handles;
        int default_ttl;
        add_torrent_params add_args;
};

By default auto_download is true, which means all torrents in the feed will be downloaded. Set this to false in order to manually add torrents to the session. You may react to the rss_alert when a feed has been updated to poll it for the new items in the feed when adding torrents manually. When torrents are added automatically, an add_torrent_alert is posted which includes the torrent handle as well as the error code if it failed to be added. You may also call session::get_torrents() to get the handles to the new torrents.

Before adding the feed, you must set the url field to the feed's url. It may point to an RSS or an atom feed.

auto_map_handles defaults to true and determines whether or not to set the handle field in the feed_item, returned as the feed status. If auto-download is enabled, this setting is ignored. If auto-download is not set, setting this to false will save one pass through all the feed items trying to find corresponding torrents in the session.

The default_ttl is the default interval for refreshing a feed. This may be overridden by the feed itself (by specifying the <ttl> tag) and defaults to 30 minutes. The field specifies the number of minutes between refreshes.

If torrents are added automatically, you may want to set the add_args to appropriate values for download directory etc. This object is used as a template for adding torrents from feeds, but some torrent specific fields will be overridden by the individual torrent being added. For more information on the add_torrent_params, see `async_add_torrent() add_torrent()`_.

The returned feed_handle is a handle which is used to interact with the feed, things like forcing a refresh or querying for information about the items in the feed. For more information, see feed_handle.

remove_feed()

void remove_feed(feed_handle h);

Removes a feed from being watched by the session. When this call returns, the feed handle is invalid and won't refer to any feed.

get_feeds()

void get_feeds(std::vector<feed_handle>& f) const;

Returns a list of all RSS feeds that are being watched by the session.

add_extension()

void add_extension(boost::function<
        boost::shared_ptr<torrent_plugin>(torrent*, void*)> ext);

This function adds an extension to this session. The argument is a function object that is called with a torrent* and which should return a boost::shared_ptr<torrent_plugin>. To write custom plugins, see libtorrent plugins. For the typical bittorrent client all of these extensions should be added. The main plugins implemented in libtorrent are:

metadata extension

Allows peers to download the metadata (.torren files) from the swarm directly. Makes it possible to join a swarm with just a tracker and info-hash.

#include <libtorrent/extensions/metadata_transfer.hpp>
ses.add_extension(&libtorrent::create_metadata_plugin);

uTorrent metadata

Same as metadata extension but compatible with uTorrent.

#include <libtorrent/extensions/ut_metadata.hpp>
ses.add_extension(&libtorrent::create_ut_metadata_plugin);

uTorrent peer exchange

Exchanges peers between clients.

#include <libtorrent/extensions/ut_pex.hpp>
ses.add_extension(&libtorrent::create_ut_pex_plugin);

smart ban plugin

A plugin that, with a small overhead, can ban peers that sends bad data with very high accuracy. Should eliminate most problems on poisoned torrents.

#include <libtorrent/extensions/smart_ban.hpp>
ses.add_extension(&libtorrent::create_smart_ban_plugin);

set_settings() set_pe_settings()

void set_settings(session_settings const& settings);
void set_pe_settings(pe_settings const& settings);

Sets the session settings and the packet encryption settings respectively. See session_settings and pe_settings for more information on available options.

set_proxy() proxy()

void set_proxy(proxy_settings const& s);
proxy_setting proxy() const;

These functions sets and queries the proxy settings to be used for the session.

For more information on what settings are available for proxies, see proxy_settings.

set_i2p_proxy() i2p_proxy()

void set_i2p_proxy(proxy_settings const&);
proxy_settings const& i2p_proxy();

set_i2p_proxy sets the i2p proxy, and tries to open a persistant connection to it. The only used fields in the proxy settings structs are hostname and port.

i2p_proxy returns the current i2p proxy in use.

start_natpmp() stop_natpmp()

natpmp* start_natpmp();
void stop_natpmp();

Starts and stops the NAT-PMP service. When started, the listen port and the DHT port are attempted to be forwarded on the router through NAT-PMP.

The natpmp object returned by start_natpmp() can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until stop_natpmp() is called. See UPnP and NAT-PMP.

It is off by default.

class entry
{
public:

        typedef std::map<std::string, entry> dictionary_type;
        typedef std::string string_type;
        typedef std::list<entry> list_type;
        typedef size_type integer_type;

        enum data_type
        {
                int_t,
                string_t,
                list_t,
                dictionary_t,
                undefined_t
        };

        data_type type() const;

        entry(dictionary_type const&);
        entry(string_type const&);
        entry(list_type const&);
        entry(integer_type const&);

        entry();
        entry(data_type t);
        entry(entry const& e);
        ~entry();

        void operator=(entry const& e);
        void operator=(dictionary_type const&);
        void operator=(string_type const&);
        void operator=(list_type const&);
        void operator=(integer_type const&);

        integer_type& integer();
        integer_type const& integer() const;
        string_type& string();
        string_type const& string() const;
        list_type& list();
        list_type const& list() const;
        dictionary_type& dict();
        dictionary_type const& dict() const;

        // these functions requires that the entry
        // is a dictionary, otherwise they will throw
        entry& operator[](char const* key);
        entry& operator[](std::string const& key);
        entry const& operator[](char const* key) const;
        entry const& operator[](std::string const& key) const;
        entry* find_key(char const* key);
        entry const* find_key(char const* key) const;

        void print(std::ostream& os, int indent = 0) const;
};

integer() string() list() dict() type()

integer_type& integer();
integer_type const& integer() const;
string_type& string();
string_type const& string() const;
list_type& list();
list_type const& list() const;
dictionary_type& dict();
dictionary_type const& dict() const;

The integer(), string(), list() and dict() functions are accessors that return the respective type. If the entry object isn't of the type you request, the accessor will throw libtorrent_exception (which derives from std::runtime_error). You can ask an entry for its type through the type() function.

The print() function is there for debug purposes only.

If you want to create an entry you give it the type you want it to have in its constructor, and then use one of the non-const accessors to get a reference which you then can assign the value you want it to have.

The typical code to get info from a torrent file will then look like this:

entry torrent_file;
// ...

// throws if this is not a dictionary
entry::dictionary_type const& dict = torrent_file.dict();
entry::dictionary_type::const_iterator i;
i = dict.find("announce");
if (i != dict.end())
{
        std::string tracker_url = i->second.string();
        std::cout << tracker_url << "\n";
}

The following code is equivalent, but a little bit shorter:

entry torrent_file;
// ...

// throws if this is not a dictionary
if (entry* i = torrent_file.find_key("announce"))
{
        std::string tracker_url = i->string();
        std::cout << tracker_url << "\n";
}

To make it easier to extract information from a torrent file, the class torrent_info exists.

operator[]

entry& operator[](char const* key);
entry& operator[](std::string const& key);
entry const& operator[](char const* key) const;
entry const& operator[](std::string const& key) const;

All of these functions requires the entry to be a dictionary, if it isn't they will throw libtorrent::type_error.

The non-const versions of the operator[] will return a reference to either the existing element at the given key or, if there is no element with the given key, a reference to a newly inserted element at that key.

The const version of operator[] will only return a reference to an existing element at the given key. If the key is not found, it will throw libtorrent::type_error.

find_key()

entry* find_key(char const* key);
entry const* find_key(char const* key) const;

These functions requires the entry to be a dictionary, if it isn't they will throw libtorrent::type_error.

They will look for an element at the given key in the dictionary, if the element cannot be found, they will return 0. If an element with the given key is found, the return a pointer to it.

class torrent_info
{
public:

        // these constructors throws exceptions on error
        torrent_info(sha1_hash const& info_hash, int flags = 0);
        torrent_info(lazy_entry const& torrent_file, int flags = 0);
        torrent_info(char const* buffer, int size, int flags = 0);
        torrent_info(std::string const& filename, int flags = 0);
        torrent_info(std::wstring const& filename, int flags = 0);

        // these constructors sets the error code on error
        torrent_info(sha1_hash const& info_hash, error_code& ec, int flags = 0);
        torrent_info(lazy_entry const& torrent_file, error_code& ec, int flags = 0);
        torrent_info(char const* buffer, int size, error_code& ec, int flags = 0);
        torrent_info(fs::path const& filename, error_code& ec, int flags = 0);
        torrent_info(fs::wpath const& filename, error_code& ec, int flags = 0);

        void add_tracker(std::string const& url, int tier = 0);
        std::vector<announce_entry> const& trackers() const;

        file_storage const& files() const;
        file_storage const& orig_files() const;

        void remap_files(file_storage const& f);

        void rename_file(int index, std::string const& new_filename);
        void rename_file(int index, std::wstring const& new_filename);

        typedef file_storage::iterator file_iterator;
        typedef file_storage::reverse_iterator reverse_file_iterator;

        file_iterator begin_files() const;
        file_iterator end_files() const;
        reverse_file_iterator rbegin_files() const;
        reverse_file_iterator rend_files() const;

        int num_files() const;
        file_entry const& file_at(int index) const;

        std::vector<file_slice> map_block(int piece, size_type offset
                , int size) const;
        peer_request map_file(int file_index, size_type file_offset
                , int size) const;

        bool priv() const;

        void add_url_seed(std::string const& url);
        void add_http_seed(std::string const& url);
        std::vector<web_seed_entry> const& web_seeds() const;

        size_type total_size() const;
        int piece_length() const;
        int num_pieces() const;
        sha1_hash const& info_hash() const;
        std::string const& name() const;
        std::string const& comment() const;
        std::string const& creator() const;

        std::vector<std::pair<std::string, int> > const& nodes() const;
        void add_node(std::pair<std::string, int> const& node);

        boost::optional<time_t> creation_date() const;

        int piece_size(unsigned int index) const;
        sha1_hash const& hash_for_piece(unsigned int index) const;
        char const* hash_for_piece_ptr(unsigned int index) const;

        std::vector<sha1_hash> const& merkle_tree() const;
        void set_merkle_tree(std::vector<sha1_hash>& h);

        boost::shared_array<char> metadata() const;
        int metadata_size() const;
};

torrent_info()

torrent_info(sha1_hash const& info_hash, int flags = 0);
torrent_info(lazy_entry const& torrent_file, int flags = 0);
torrent_info(char const* buffer, int size, int flags = 0);
torrent_info(std::string const& filename, int flags = 0);
torrent_info(std::wstring const& filename, int flags = 0);

torrent_info(sha1_hash const& info_hash, error_code& ec, int flags = 0);
torrent_info(lazy_entry const& torrent_file, error_code& ec, int flags = 0);
torrent_info(char const* buffer, int size, error_code& ec, int flags = 0);
torrent_info(fs::path const& filename, error_code& ec, int flags = 0);
torrent_info(fs::wpath const& filename, error_code& ec, int flags = 0);

The constructor that takes an info-hash will initialize the info-hash to the given value, but leave all other fields empty. This is used internally when downloading torrents without the metadata. The metadata will be created by libtorrent as soon as it has been downloaded from the swarm.

The constructor that takes a lazy_entry will create a torrent_info object from the information found in the given torrent_file. The lazy_entry represents a tree node in an bencoded file. To load an ordinary .torrent file into a lazy_entry, use lazy_bdecode().

The version that takes a buffer pointer and a size will decode it as a .torrent file and initialize the torrent_info object for you.

The version that takes a filename will simply load the torrent file and decode it inside the constructor, for convenience. This might not be the most suitable for applications that want to be able to report detailed errors on what might go wrong.

The overloads that takes an error_code const& never throws if an error occur, they will simply set the error code to describe what went wrong and not fully initialize the torrent_info object. The overloads that do not take the extra error_code parameter will always throw if an error occurs. These overloads are not available when building without exception support.

The flags argument is currently unused.

add_tracker()

void add_tracker(std::string const& url, int tier = 0);

add_tracker() adds a tracker to the announce-list. The tier determines the order in which the trackers are to be tried. For more information see trackers().

files() orig_files()

file_storage const& files() const;
file_storage const& orig_files() const;

The file_storage object contains the information on how to map the pieces to files. It is separated from the torrent_info object because when creating torrents a storage object needs to be created without having a torrent file. When renaming files in a storage, the storage needs to make its own copy of the file_storage in order to make its mapping differ from the one in the torrent file.

orig_files() returns the original (unmodified) file storage for this torrent. This is used by the web server connection, which needs to request files with the original names. Filename may be chaged using torrent_info::rename_file().

For more information on the file_storage object, see the separate document on how to create torrents.

remap_files()

void remap_files(file_storage const& f);

Remaps the file storage to a new file layout. This can be used to, for instance, download all data in a torrent to a single file, or to a number of fixed size sector aligned files, regardless of the number and sizes of the files in the torrent.

The new specified file_storage must have the exact same size as the current one.

rename_file()

void rename_file(int index, std::string const& new_filename);
void rename_file(int index, std::wstring const& new_filename);

Renames a the file with the specified index to the new name. The new filename is reflected by the file_storage returned by files() but not by the one returned by orig_files().

If you want to rename the base name of the torrent (for a multifile torrent), you can copy the file_storage (see files() orig_files()), change the name, and then use remap_files().

The new_filename can both be a relative path, in which case the file name is relative to the save_path of the torrent. If the new_filename is an absolute path (i.e. is_complete(new_filename) == true), then the file is detached from the save_path of the torrent. In this case the file is not moved when move_storage_ is invoked.

begin_files() end_files() rbegin_files() rend_files()

file_iterator begin_files() const;
file_iterator end_files() const;
reverse_file_iterator rbegin_files() const;
reverse_file_iterator rend_files() const;

This class will need some explanation. First of all, to get a list of all files in the torrent, you can use begin_files(), end_files(), rbegin_files() and rend_files(). These will give you standard vector iterators with the type internal_file_entry, which is an internal type.

You can resolve it into the public representation of a file (file_entry) using the file_storage::at function, which takes an index and an iterator;

struct file_entry
{
        std::string path;
        size_type offset;
        size_type size;
        size_type file_base;
        time_t mtime;
        sha1_hash filehash;
        bool pad_file:1;
        bool hidden_attribute:1;
        bool executable_attribute:1;
        bool symlink_attribute:1;
};

The path is the full path of this file. The paths are unicode strings encoded in UTF-8.

size is the size of the file (in bytes) and offset is the byte offset of the file within the torrent. i.e. the sum of all the sizes of the files before it in the list.

file_base is the offset in the file where the storage should start. The normal case is to have this set to 0, so that the storage starts saving data at the start if the file. In cases where multiple files are mapped into the same file though, the file_base should be set to an offset so that the different regions do not overlap. This is used when mapping "unselected" files into a so-called part file.

mtime is the modification time of this file specified in posix time.

symlink_path is the path which this is a symlink to, or empty if this is not a symlink. This field is only used if the symlink_attribute is set.

filehash is a sha-1 hash of the content of the file, or zeroes, if no file hash was present in the torrent file. It can be used to potentially find alternative sources for the file.

pad_file is set to true for files that are not part of the data of the torrent. They are just there to make sure the next file is aligned to a particular byte offset or piece boundry. These files should typically be hidden from an end user. They are not written to disk.

hidden_attribute is true if the file was marked as hidden (on windows).

executable_attribute is true if the file was marked as executable (posix)

symlink_attribute is true if the file was a symlink. If this is the case the symlink_index refers to a string which specifies the original location where the data for this file was found.

num_files() file_at()

int num_files() const;
file_entry const& file_at(int index) const;

If you need index-access to files you can use the num_files() and file_at() to access files using indices.

map_block()

std::vector<file_slice> map_block(int piece, size_type offset
        , int size) const;

This function will map a piece index, a byte offset within that piece and a size (in bytes) into the corresponding files with offsets where that data for that piece is supposed to be stored.

The file slice struct looks like this:

struct file_slice
{
        int file_index;
        size_type offset;
        size_type size;
};

The file_index refers to the index of the file (in the torrent_info). To get the path and filename, use file_at() and give the file_index as argument. The offset is the byte offset in the file where the range starts, and size is the number of bytes this range is. The size + offset will never be greater than the file size.

map_file()

peer_request map_file(int file_index, size_type file_offset
        , int size) const;

This function will map a range in a specific file into a range in the torrent. The file_offset parameter is the offset in the file, given in bytes, where 0 is the start of the file. The peer_request structure looks like this:

struct peer_request
{
        int piece;
        int start;
        int length;
        bool operator==(peer_request const& r) const;
};

piece is the index of the piece in which the range starts. start is the offset within that piece where the range starts. length is the size of the range, in bytes.

The input range is assumed to be valid within the torrent. file_offset + size is not allowed to be greater than the file size. file_index must refer to a valid file, i.e. it cannot be >= num_files().

add_url_seed() add_http_seed()

void add_url_seed(std::string const& url
        , std::string const& extern_auth = std::string()
        , web_seed_entry::headers_t const& extra_headers = web_seed_entry::headers_t());
void add_http_seed(std::string const& url
        , std::string const& extern_auth = std::string()
        , web_seed_entry::headers_t const& extra_headers = web_seed_entry::headers_t());
std::vector<web_seed_entry> const& web_seeds() const;

web_seeds() returns all url seeds and http seeds in the torrent. Each entry is a web_seed_entry and may refer to either a url seed or http seed.

add_url_seed() and add_http_seed() adds one url to the list of url/http seeds. Currently, the only transport protocol supported for the url is http.

The extern_auth argument can be used for other athorization schemese than basic HTTP authorization. If set, it will override any username and password found in the URL itself. The string will be sent as the HTTP authorization header's value (without specifying "Basic").

The extra_headers argument defaults to an empty list, but can be used to insert custom HTTP headers in the requests to a specific web seed.

See HTTP seeding for more information.

The web_seed_entry has the following members:

struct web_seed_entry
{
        enum type_t { url_seed, http_seed };

        typedef std::vector<std::pair<std::string, std::string> > headers_t;

        web_seed_entry(std::string const& url_, type_t type_
                , std::string const& auth_ = std::string()
                , headers_t const& extra_headers_ = headers_t());

        bool operator==(web_seed_entry const& e) const;
        bool operator<(web_seed_entry const& e) const;

        std::string url;
        type_t type;
        std::string auth;
        headers_t extra_headers;

        // ...
};

trackers()

std::vector<announce_entry> const& trackers() const;

The trackers() function will return a sorted vector of announce_entry. Each announce entry contains a string, which is the tracker url, and a tier index. The tier index is the high-level priority. No matter which trackers that works or not, the ones with lower tier will always be tried before the one with higher tier number.

struct announce_entry
{
        announce_entry(std::string const& url);
        std::string url;

        int next_announce_in() const;
        int min_announce_in() const;

        int scrape_incomplete;
        int scrape_complete;
        int scrape_downloaded;

        error_code last_error;

        std::string message;

        boost::uint8_t tier;
        boost::uint8_t fail_limit;
        boost::uint8_t fails;

        enum tracker_source
        {
                source_torrent = 1,
                source_client = 2,
                source_magnet_link = 4,
                source_tex = 8
        };
        boost::uint8_t source;

        bool verified:1;
        bool updating:1;
        bool start_sent:1;
        bool complete_sent:1;
};

next_announce_in() returns the number of seconds to the next announce on this tracker. min_announce_in() returns the number of seconds until we are allowed to force another tracker update with this tracker.

If the last time this tracker was contacted failed, last_error is the error code describing what error occurred.

scrape_incomplete, scrape_complete and scrape_downloaded are either -1 or the scrape information this tracker last responded with. incomplete is the current number of downloaders in the swarm, complete is the current number of seeds in the swarm and downloaded is the cumulative number of completed downloads of this torrent, since the beginning of time (from this tracker's point of view).

If the last time this tracker was contacted, the tracker returned a warning or error message, message contains that message.

fail_limit is the max number of failures to announce to this tracker in a row, before this tracker is not used anymore.

fails is the number of times in a row we have failed to announce to this tracker.

source is a bitmask specifying which sources we got this tracker from.

verified is set to true the first time we receive a valid response from this tracker.

updating is true while we're waiting for a response from the tracker.

start_sent is set to true when we get a valid response from an announce with event=started. If it is set, we won't send start in the subsequent announces.

complete_sent is set to true when we send a event=completed.

total_size() piece_length() piece_size() num_pieces()

size_type total_size() const;
int piece_length() const;
int piece_size(unsigned int index) const;
int num_pieces() const;

total_size(), piece_length() and num_pieces() returns the total number of bytes the torrent-file represents (all the files in it), the number of byte for each piece and the total number of pieces, respectively. The difference between piece_size() and piece_length() is that piece_size() takes the piece index as argument and gives you the exact size of that piece. It will always be the same as piece_length() except in the case of the last piece, which may be smaller.

hash_for_piece() hash_for_piece_ptr() info_hash()

size_type piece_size(unsigned int index) const;
sha1_hash const& hash_for_piece(unsigned int index) const;
char const* hash_for_piece_ptr(unsigned int index) const;

hash_for_piece() takes a piece-index and returns the 20-bytes sha1-hash for that piece and info_hash() returns the 20-bytes sha1-hash for the info-section of the torrent file. For more information on the sha1_hash, see the big_number class. hash_for_piece_ptr() returns a pointer to the 20 byte sha1 digest for the piece. Note that the string is not null-terminated.

merkle_tree() set_merkle_tree()

std::vector<sha1_hash> const& merkle_tree() const;
void set_merkle_tree(std::vector<sha1_hash>& h);

merkle_tree() returns a reference to the merkle tree for this torrent, if any.

set_merkle_tree() moves the passed in merkle tree into the torrent_info object. i.e. h will not be identical after the call. You need to set the merkle tree for a torrent that you've just created (as a merkle torrent). The merkle tree is retrieved from the create_torrent::merkle_tree() function, and need to be saved separately from the torrent file itself. Once it's added to libtorrent, the merkle tree will be persisted in the resume data.

name() comment() creation_date() creator()

std::string const& name() const;
std::string const& comment() const;
std::string const& creator() const;
boost::optional<time_t> creation_date() const;

name() returns the name of the torrent.

comment() returns the comment associated with the torrent. If there's no comment, it will return an empty string. creation_date() returns the creation date of the torrent as time_t (posix time). If there's no time stamp in the torrent file, the optional object will be uninitialized.

Both the name and the comment is UTF-8 encoded strings.

creator() returns the creator string in the torrent. If there is no creator string it will return an empty string.

priv()

bool priv() const;

priv() returns true if this torrent is private. i.e., it should not be distributed on the trackerless network (the kademlia DHT).

nodes()

std::vector<std::pair<std::string, int> > const& nodes() const;

If this torrent contains any DHT nodes, they are put in this vector in their original form (host name and port number).

add_node()

void add_node(std::pair<std::string, int> const& node);

This is used when creating torrent. Use this to add a known DHT node. It may be used, by the client, to bootstrap into the DHT network.

metadata() metadata_size()

boost::shared_array<char> metadata() const;
int metadata_size() const;

metadata() returns a the raw info section of the torrent file. The size of the metadata is returned by metadata_size().

struct torrent_handle
{
        torrent_handle();

        enum status_flags_t
        {
                query_distributed_copies = 1,
                query_accurate_download_counters = 2,
                query_last_seen_complete = 4,
                query_pieces = 8,
                query_verified_pieces = 16,
                query_torrent_file = 32,
                query_name = 64,
                query_save_path = 128,
        };

        torrent_status status(boost::uint32_t flags = 0xffffffff);
        void file_progress(std::vector<size_type>& fp, int flags = 0);
        void get_download_queue(std::vector<partial_piece_info>& queue) const;
        void get_peer_info(std::vector<peer_info>& v) const;
        boost::intrusive_ptr<torrent_info> torrent_file() const;
        bool is_valid() const;

        enum save_resume_flags_t { flush_disk_cache = 1, save_info_dict = 2 };
        void save_resume_data(int flags = 0) const;
        bool need_save_resume_data() const;
        void force_reannounce() const;
        void force_dht_announce() const;
        void force_reannounce(boost::posix_time::time_duration) const;
        void scrape_tracker() const;
        void connect_peer(asio::ip::tcp::endpoint const& adr, int source = 0) const;

        void set_tracker_login(std::string const& username
                , std::string const& password) const;

        std::vector<announce_entry> trackers() const;
        void replace_trackers(std::vector<announce_entry> const&);
        void add_tracker(announce_entry const& url);

        void add_url_seed(std::string const& url);
        void remove_url_seed(std::string const& url);
        std::set<std::string> url_seeds() const;

        void add_http_seed(std::string const& url);
        void remove_http_seed(std::string const& url);
        std::set<std::string> http_seeds() const;

        int max_uploads() const;
        void set_max_uploads(int max_uploads) const;
        void set_max_connections(int max_connections) const;
        int max_connections() const;
        void set_upload_limit(int limit) const;
        int upload_limit() const;
        void set_download_limit(int limit) const;
        int download_limit() const;
        void set_sequential_download(bool sd) const;
        bool is_sequential_download() const;

        int queue_position() const;
        void queue_position_up() const;
        void queue_position_down() const;
        void queue_position_top() const;
        void queue_position_bottom() const;

        void set_priority(int prio) const;

        void use_interface(char const* net_interface) const;

        enum pause_flags_t { graceful_pause = 1 };
        void pause(int flags = 0) const;
        void resume() const;
        bool is_seed() const;
        void force_recheck() const;
        void clear_error() const;
        void set_upload_mode(bool m) const;
        void set_share_mode(bool m) const;

        void apply_ip_filter(bool b) const;

        void flush_cache() const;

        void resolve_countries(bool r);
        bool resolve_countries() const;

        enum deadline_flags { alert_when_available = 1 };
        void set_piece_deadline(int index, int deadline, int flags = 0) const;
        void reset_piece_deadline(int index) const;

        void piece_availability(std::vector<int>& avail) const;
        void piece_priority(int index, int priority) const;
        int piece_priority(int index) const;
        void prioritize_pieces(std::vector<int> const& pieces) const;
        std::vector<int> piece_priorities() const;

        void file_priority(int index, int priority) const;
        int file_priority(int index) const;
        void prioritize_files(std::vector<int> const& files) const;
        std::vector<int> file_priorities() const;

        void auto_managed(bool m) const;

        bool set_metadata(char const* buf, int size) const;

        void move_storage(std::string const& save_path, int flags = 0) const;
        void move_storage(std::wstring const& save_path, int flags = 0) const;
        void rename_file(int index, std::string) const;
        void rename_file(int index, std::wstring) const;
        storage_interface* get_storage_impl() const;

        void super_seeding(bool on) const;

        enum flags_t { overwrite_existing = 1 };
        void add_piece(int piece, char const* data, int flags = 0) const;
        void read_piece(int piece) const;
        bool have_piece(int piece) const;

        sha1_hash info_hash() const;

        void set_ssl_certificate(std::string const& cert
                , std::string const& private_key
                , std::string const& dh_params
                , std::string const& passphrase = "");

        bool operator==(torrent_handle const&) const;
        bool operator!=(torrent_handle const&) const;
        bool operator<(torrent_handle const&) const;

        boost::shared_ptr<torrent> native_handle() const;
};

set_piece_deadline() reset_piece_deadline()

enum deadline_flags { alert_when_available = 1 };
void set_piece_deadline(int index, int deadline, int flags = 0) const;
void reset_piece_deadline(int index) const;

This function sets or resets the deadline associated with a specific piece index (index). libtorrent will attempt to download this entire piece before the deadline expires. This is not necessarily possible, but pieces with a more recent deadline will always be prioritized over pieces with a deadline further ahead in time. The deadline (and flags) of a piece can be changed by calling this function again.

The flags parameter can be used to ask libtorrent to send an alert once the piece has been downloaded, by passing alert_when_available. When set, the read_piece_alert alert will be delivered, with the piece data, when it's downloaded.

If the piece is already downloaded when this call is made, nothing happens, unless the alert_when_available flag is set, in which case it will do the same thing as calling read_piece() for index.

deadline is the number of milliseconds until this piece should be completed.

reset_piece_deadline removes the deadline from the piece. If it hasn't already been downloaded, it will no longer be considered a priority.

piece_availability()

void piece_availability(std::vector<int>& avail) const;

Fills the specified std::vector<int> with the availability for each piece in this torrent. libtorrent does not keep track of availability for seeds, so if the torrent is seeding the availability for all pieces is reported as 0.

The piece availability is the number of peers that we are connected that has advertized having a particular piece. This is the information that libtorrent uses in order to prefer picking rare pieces.

piece_priority() prioritize_pieces() piece_priorities()

void piece_priority(int index, int priority) const;
int piece_priority(int index) const;
void prioritize_pieces(std::vector<int> const& pieces) const;
std::vector<int> piece_priorities() const;

These functions are used to set and get the prioritiy of individual pieces. By default all pieces have priority 1. That means that the random rarest first algorithm is effectively active for all pieces. You may however change the priority of individual pieces. There are 8 different priority levels:

0. piece is not downloaded at all
1. normal priority. Download order is dependent on availability
2. higher than normal priority. Pieces are preferred over pieces with the same availability, but not over pieces with lower availability
3. pieces are as likely to be picked as partial pieces.
4. pieces are preferred over partial pieces, but not over pieces with lower availability
5. currently the same as 4
6. piece is as likely to be picked as any piece with availability 1
7. maximum priority, availability is disregarded, the piece is preferred over any other piece with lower priority

The exact definitions of these priorities are implementation details, and subject to change. The interface guarantees that higher number means higher priority, and that 0 means do not download.

piece_priority sets or gets the priority for an individual piece, specified by index.

prioritize_pieces takes a vector of integers, one integer per piece in the torrent. All the piece priorities will be updated with the priorities in the vector.

piece_priorities returns a vector with one element for each piece in the torrent. Each element is the current priority of that piece.

file_priority() prioritize_files() file_priorities()

void file_priority(int index, int priority) const;
int file_priority(int index) const;
void prioritize_files(std::vector<int> const& files) const;
std::vector<int> file_priorities() const;

index must be in the range [0, number_of_files).

file_priority queries or sets the priority of file index.

prioritize_files takes a vector that has at as many elements as there are files in the torrent. Each entry is the priority of that file. The function sets the priorities of all the pieces in the torrent based on the vector.

file_priorities returns a vector with the priorities of all files.

The priority values are the same as for piece_priority.

Whenever a file priority is changed, all other piece priorities are reset to match the file priorities. In order to maintain sepcial priorities for particular pieces, piece_priority has to be called again for those pieces.

You cannot set the file priorities on a torrent that does not yet have metadata or a torrent that is a seed. file_priority(int, int) and prioritize_files() are both no-ops for such torrents.

file_progress()

void file_progress(std::vector<size_type>& fp, int flags = 0);

This function fills in the supplied vector with the the number of bytes downloaded of each file in this torrent. The progress values are ordered the same as the files in the torrent_info. This operation is not very cheap. Its complexity is O(n + mj). Where n is the number of files, m is the number of downloading pieces and j is the number of blocks in a piece.

The flags parameter can be used to specify the granularity of the file progress. If left at the default value of 0, the progress will be as accurate as possible, but also more expensive to calculate. If torrent_handle::piece_granularity is specified, the progress will be specified in piece granularity. i.e. only pieces that have been fully downloaded and passed the hash check count. When specifying piece granularity, the operation is a lot cheaper, since libtorrent already keeps track of this internally and no calculation is required.

move_storage()

void move_storage(std::string const& save_path, int flags = 0) const;
void move_storage(std::wstring const& save_path, int flags = 0) const;

Moves the file(s) that this torrent are currently seeding from or downloading to. If the given save_path is not located on the same drive as the original save path, the files will be copied to the new drive and removed from their original location. This will block all other disk IO, and other torrents download and upload rates may drop while copying the file.

Since disk IO is performed in a separate thread, this operation is also asynchronous. Once the operation completes, the storage_moved_alert is generated, with the new path as the message. If the move fails for some reason, storage_moved_failed_alert is generated instead, containing the error message.

The flags argument determines the behavior of the copying/moving of the files in the torrent. They are defined in include/libtorrent/storage.hpp:

• always_replace_files = 0
• fail_if_exist = 1
• dont_replace = 2

always_replace_files is the default and replaces any file that exist in both the source directory and the target directory.

fail_if_exist first check to see that none of the copy operations would cause an overwrite. If it would, it will fail. Otherwise it will proceed as if it was in always_replace_files mode. Note that there is an inherent race condition here. If the files in the target directory appear after the check but before the copy or move completes, they will be overwritten. When failing because of files already existing in the target path, the error of move_storage_failed_alert is set to boost::system::errc::file_exists.

The intention is that a client may use this as a probe, and if it fails, ask the user which mode to use. The client may then re-issue the move_storage call with one of the other modes.

dont_replace always takes the existing file in the target directory, if there is one. The source files will still be removed in that case.

Files that have been renamed to have absolute pahts are not moved by this function. Keep in mind that files that don't belong to the torrent but are stored in the torrent's directory may be moved as well. This goes for files that have been renamed to absolute paths that still end up inside the save path.

rename_file()

void rename_file(int index, std::string) const;
void rename_file(int index, std::wstring) const;

Renames the file with the given index asynchronously. The rename operation is complete when either a file_renamed_alert or file_rename_failed_alert is posted.

get_storage_impl()

storage_interface* get_storage_impl() const;

Returns the storage implementation for this torrent. This depends on the storage contructor function that was passed to session::add_torrent.

super_seeding()

void super_seeding(bool on) const;

Enables or disabled super seeding/initial seeding for this torrent. The torrent needs to be a seed for this to take effect.

add_piece()

enum flags_t { overwrite_existing = 1 };
void add_piece(int piece, char const* data, int flags = 0) const;

This function will write data to the storage as piece piece, as if it had been downloaded from a peer. data is expected to point to a buffer of as many bytes as the size of the specified piece. The data in the buffer is copied and passed on to the disk IO thread to be written at a later point.

By default, data that's already been downloaded is not overwritten by this buffer. If you trust this data to be correct (and pass the piece hash check) you may pass the overwrite_existing flag. This will instruct libtorrent to overwrite any data that may already have been downloaded with this data.

Since the data is written asynchronously, you may know that is passed or failed the hash check by waiting for piece_finished_alert or has_failed_alert.

read_piece()

void read_piece(int piece) const;

This function starts an asynchronous read operation of the specified piece from this torrent. You must have completed the download of the specified piece before calling this function.

When the read operation is completed, it is passed back through an alert, read_piece_alert. Since this alert is a reponse to an explicit call, it will always be posted, regardless of the alert mask.

Note that if you read multiple pieces, the read operations are not guaranteed to finish in the same order as you initiated them.

have_piece()

bool have_piece(int piece) const;

Returns true if this piece has been completely downloaded, and false otherwise.

force_reannounce() force_dht_announce()

void force_reannounce() const;
void force_reannounce(boost::posix_time::time_duration) const;
void force_dht_announce() const;

force_reannounce() will force this torrent to do another tracker request, to receive new peers. The second overload of force_reannounce that takes a time_duration as argument will schedule a reannounce in that amount of time from now.

If the tracker's min_interval has not passed since the last announce, the forced announce will be scheduled to happen immediately as the min_interval expires. This is to honor trackers minimum re-announce interval settings.

force_dht_announce will announce the torrent to the DHT immediately.

scrape_tracker()

void scrape_tracker() const;

scrape_tracker() will send a scrape request to the tracker. A scrape request queries the tracker for statistics such as total number of incomplete peers, complete peers, number of downloads etc.

This request will specifically update the num_complete and num_incomplete fields in the torrent_status struct once it completes. When it completes, it will generate a scrape_reply_alert. If it fails, it will generate a scrape_failed_alert.

connect_peer()

void connect_peer(asio::ip::tcp::endpoint const& adr, int source = 0) const;

connect_peer() is a way to manually connect to peers that one believe is a part of the torrent. If the peer does not respond, or is not a member of this torrent, it will simply be disconnected. No harm can be done by using this other than an unnecessary connection attempt is made. If the torrent is uninitialized or in queued or checking mode, this will throw libtorrent_exception. The second (optional) argument will be bitwised ORed into the source mask of this peer. Typically this is one of the source flags in peer_info. i.e. tracker, pex, dht etc.

set_upload_limit() set_download_limit() upload_limit() download_limit()

void set_upload_limit(int limit) const;
void set_download_limit(int limit) const;
int upload_limit() const;
int download_limit() const;

set_upload_limit will limit the upload bandwidth used by this particular torrent to the limit you set. It is given as the number of bytes per second the torrent is allowed to upload. set_download_limit works the same way but for download bandwidth instead of upload bandwidth. Note that setting a higher limit on a torrent then the global limit (session_settings::upload_rate_limit) will not override the global rate limit. The torrent can never upload more than the global rate limit.

upload_limit and download_limit will return the current limit setting, for upload and download, respectively.

set_sequential_download()

void set_sequential_download(bool sd);

set_sequential_download() enables or disables sequential download. When enabled, the piece picker will pick pieces in sequence instead of rarest first.

Enabling sequential download will affect the piece distribution negatively in the swarm. It should be used sparingly.

pause() resume()

enum pause_flags_t { graceful_pause = 1 };
void pause(int flags) const;
void resume() const;

pause(), and resume() will disconnect all peers and reconnect all peers respectively. When a torrent is paused, it will however remember all share ratios to all peers and remember all potential (not connected) peers. Torrents may be paused automatically if there is a file error (e.g. disk full) or something similar. See file_error_alert.

To know if a torrent is paused or not, call torrent_handle::status() and inspect torrent_status::paused.

The flags argument to pause can be set to torrent_handle::graceful_pause which will delay the disconnect of peers that we're still downloading outstanding requests from. The torrent will not accept any more requests and will disconnect all idle peers. As soon as a peer is done transferring the blocks that were requested from it, it is disconnected. This is a graceful shut down of the torrent in the sense that no downloaded bytes are wasted.

torrents that are auto-managed may be automatically resumed again. It does not make sense to pause an auto-managed torrent without making it not automanaged first. Torrents are auto-managed by default when added to the session. For more information, see queuing.

flush_cache()

void flush_cache() const;

Instructs libtorrent to flush all the disk caches for this torrent and close all file handles. This is done asynchronously and you will be notified that it's complete through cache_flushed_alert.

Note that by the time you get the alert, libtorrent may have cached more data for the torrent, but you are guaranteed that whatever cached data libtorrent had by the time you called torrent_handle::flush_cache() has been written to disk.

force_recheck()

void force_recheck() const;

force_recheck puts the torrent back in a state where it assumes to have no resume data. All peers will be disconnected and the torrent will stop announcing to the tracker. The torrent will be added to the checking queue, and will be checked (all the files will be read and compared to the piece hashes). Once the check is complete, the torrent will start connecting to peers again, as normal.

clear_error()

void clear_error() const;

If the torrent is in an error state (i.e. torrent_status::error is non-empty), this will clear the error and start the torrent again.

set_upload_mode()

void set_upload_mode(bool m) const;

Explicitly sets the upload mode of the torrent. In upload mode, the torrent will not request any pieces. If the torrent is auto managed, it will automatically be taken out of upload mode periodically (see session_settings::optimistic_disk_retry). Torrents are automatically put in upload mode whenever they encounter a disk write error.

m should be true to enter upload mode, and false to leave it.

To test if a torrent is in upload mode, call torrent_handle::status() and inspect torrent_status::upload_mode.

set_share_mode()

void set_share_mode(bool m) const;

Enable or disable share mode for this torrent. When in share mode, the torrent will not necessarily be downloaded, especially not the whole of it. Only parts that are likely to be distributed to more than 2 other peers are downloaded, and only if the previous prediction was correct.

apply_ip_filter()

void apply_ip_filter(bool b) const;

Set to true to apply the session global IP filter to this torrent (which is the default). Set to false to make this torrent ignore the IP filter.

resolve_countries()

void resolve_countries(bool r);
bool resolve_countries() const;

Sets or gets the flag that derermines if countries should be resolved for the peers of this torrent. It defaults to false. If it is set to true, the peer_info structure for the peers in this torrent will have their country member set. See peer_info for more information on how to interpret this field.

is_seed()

bool is_seed() const;

Returns true if the torrent is in seed mode (i.e. if it has finished downloading).

auto_managed()

void auto_managed(bool m) const;

auto_managed() changes whether the torrent is auto managed or not. For more info, see queuing.

set_metadata()

bool set_metadata(char const* buf, int size) const;

set_metadata expects the info section of metadata. i.e. The buffer passed in will be hashed and verified against the info-hash. If it fails, a metadata_failed_alert will be generated. If it passes, a metadata_received_alert is generated. The function returns true if the metadata is successfully set on the torrent, and false otherwise. If the torrent already has metadata, this function will not affect the torrent, and false will be returned.

set_tracker_login()

void set_tracker_login(std::string const& username
        , std::string const& password) const;

set_tracker_login() sets a username and password that will be sent along in the HTTP-request of the tracker announce. Set this if the tracker requires authorization.

trackers() replace_trackers() add_tracker()

std::vector<announce_entry> trackers() const;
void replace_trackers(std::vector<announce_entry> const&) const;
void add_tracker(announc_entry const& url);

trackers() will return the list of trackers for this torrent. The announce entry contains both a string url which specify the announce url for the tracker as well as an int tier, which is specifies the order in which this tracker is tried. If you want libtorrent to use another list of trackers for this torrent, you can use replace_trackers() which takes a list of the same form as the one returned from trackers() and will replace it. If you want an immediate effect, you have to call force_reannounce() force_dht_announce(). See trackers() for the definition of announce_entry.

add_tracker() will look if the specified tracker is already in the set. If it is, it doesn't do anything. If it's not in the current set of trackers, it will insert it in the tier specified in the announce_entry.

The updated set of trackers will be saved in the resume data, and when a torrent is started with resume data, the trackers from the resume data will replace the original ones.

add_url_seed() remove_url_seed() url_seeds()

void add_url_seed(std::string const& url);
void remove_url_seed(std::string const& url);
std::set<std::string> url_seeds() const;

add_url_seed() adds another url to the torrent's list of url seeds. If the given url already exists in that list, the call has no effect. The torrent will connect to the server and try to download pieces from it, unless it's paused, queued, checking or seeding. remove_url_seed() removes the given url if it exists already. url_seeds() return a set of the url seeds currently in this torrent. Note that urls that fails may be removed automatically from the list.

See HTTP seeding for more information.

add_http_seed() remove_http_seed() http_seeds()

void add_http_seed(std::string const& url);
void remove_http_seed(std::string const& url);
std::set<std::string> http_seeds() const;

These functions are identical as the *_url_seed() variants, but they operate on BEP 17 web seeds instead of BEP 19.

See HTTP seeding for more information.

queue_position() queue_position_up() queue_position_down() queue_position_top() queue_position_bottom()

int queue_position() const;
void queue_position_up() const;
void queue_position_down() const;
void queue_position_top() const;
void queue_position_bottom() const;

Every torrent that is added is assigned a queue position exactly one greater than the greatest queue position of all existing torrents. Torrents that are being seeded have -1 as their queue position, since they're no longer in line to be downloaded.

When a torrent is removed or turns into a seed, all torrents with greater queue positions have their positions decreased to fill in the space in the sequence.

queue_position() returns the torrent's position in the download queue. The torrents with the smallest numbers are the ones that are being downloaded. The smaller number, the closer the torrent is to the front of the line to be started.

The queue position is also available in the torrent_status.

The queue_position_*() functions adjust the torrents position in the queue. Up means closer to the front and down means closer to the back of the queue. Top and bottom refers to the front and the back of the queue respectively.

set_priority()

void set_priority(int prio) const;

This sets the bandwidth priority of this torrent. The priority of a torrent determines how much bandwidth its peers are assigned when distributing upload and download rate quotas. A high number gives more bandwidth. The priority must be within the range [0, 255].

The default priority is 0, which is the lowest priority.

To query the priority of a torrent, use the torrent_handle::status() call.

Torrents with higher priority will not nececcarily get as much bandwidth as they can consume, even if there's is more quota. Other peers will still be weighed in when bandwidth is being distributed. With other words, bandwidth is not distributed strictly in order of priority, but the priority is used as a weight.

Peers whose Torrent has a higher priority will take precedence when distributing unchoke slots. This is a strict prioritization where every interested peer on a high priority torrent will be unchoked before any other, lower priority, torrents have any peers unchoked.

use_interface()

void use_interface(char const* net_interface) const;

use_interface() sets the network interface this torrent will use when it opens outgoing connections. By default, it uses the same interface as the session_ uses to listen on. The parameter must be a string containing one or more, comma separated, ip-address (either an IPv4 or IPv6 address). When specifying multiple interfaces, the torrent will round-robin which interface to use for each outgoing conneciton. This is useful for clients that are multi-homed.

info_hash()

sha1_hash info_hash() const;

info_hash() returns the info-hash for the torrent.

set_max_uploads() max_uploads()

void set_max_uploads(int max_uploads) const;
int max_uploads() const;

set_max_uploads() sets the maximum number of peers that's unchoked at the same time on this torrent. If you set this to -1, there will be no limit. This defaults to infinite. The primary setting controlling this is the global unchoke slots limit, set by unchoke_slots_limit in session_settings.

max_uploads() returns the current settings.

set_max_connections() max_connections()

void set_max_connections(int max_connections) const;
int max_connections() const;

set_max_connections() sets the maximum number of connection this torrent will open. If all connections are used up, incoming connections may be refused or poor connections may be closed. This must be at least 2. The default is unlimited number of connections. If -1 is given to the function, it means unlimited. There is also a global limit of the number of connections, set by connections_limit in session_settings.

max_connections() returns the current settings.

save_resume_data()

enum save_resume_flags_t { flush_disk_cache = 1, save_info_dict = 2 };
void save_resume_data(int flags = 0) const;

save_resume_data() generates fast-resume data and returns it as an entry. This entry is suitable for being bencoded. For more information about how fast-resume works, see fast resume.

The flags argument is a bitmask of flags ORed together. If the flag torrent_handle::flush_cache is set, the disk cache will be flushed before creating the resume data. This avoids a problem with file timestamps in the resume data in case the cache hasn't been flushed yet.

If the flag torrent_handle::save_info_dict is set, the resume data will contain the metadata from the torrent file as well. This is default for any torrent that's added without a torrent file (such as a magnet link or a URL).

This operation is asynchronous, save_resume_data will return immediately. The resume data is delivered when it's done through an save_resume_data_alert.

The fast resume data will be empty in the following cases:

1. The torrent handle is invalid.
2. The torrent is checking (or is queued for checking) its storage, it will obviously not be ready to write resume data.
3. The torrent hasn't received valid metadata and was started without metadata (see libtorrent's metadata from peers extension)

Note that by the time you receive the fast resume data, it may already be invalid if the torrent is still downloading! The recommended practice is to first pause the session, then generate the fast resume data, and then close it down. Make sure to not `remove_torrent()`_ before you receive the save_resume_data_alert though. There's no need to pause when saving intermittent resume data.

It also means that if the resume data is out dated, libtorrent will not re-check the files, but assume that it is fairly recent. The assumption is that it's better to loose a little bit than to re-check the entire file.

It is still a good idea to save resume data periodically during download as well as when closing down.

Example code to pause and save resume data for all torrents and wait for the alerts:

extern int outstanding_resume_data; // global counter of outstanding resume data
std::vector<torrent_handle> handles = ses.get_torrents();
ses.pause();
for (std::vector<torrent_handle>::iterator i = handles.begin();
        i != handles.end(); ++i)
{
        torrent_handle& h = *i;
        if (!h.is_valid()) continue;
        torrent_status s = h.status();
        if (!s.has_metadata) continue;
        if (!s.need_save_resume_data()) continue;

        h.save_resume_data();
        ++outstanding_resume_data;
}

while (outstanding_resume_data > 0)
{
        alert const* a = ses.wait_for_alert(seconds(10));

        // if we don't get an alert within 10 seconds, abort
        if (a == 0) break;

        std::auto_ptr<alert> holder = ses.pop_alert();

        if (alert_cast<save_resume_data_failed_alert>(a))
        {
                process_alert(a);
                --outstanding_resume_data;
                continue;
        }

        save_resume_data_alert const* rd = alert_cast<save_resume_data_alert>(a);
        if (rd == 0)
        {
                process_alert(a);
                continue;
        }

        torrent_handle h = rd->handle;
        torrent_status st = h.status(torrent_handle::query_save_path | torrent_handle::query_name);
        std::ofstream out((st.save_path
                + "/" + st.name + ".fastresume").c_str()
                , std::ios_base::binary);
        out.unsetf(std::ios_base::skipws);
        bencode(std::ostream_iterator<char>(out), *rd->resume_data);
        --outstanding_resume_data;
}

need_save_resume_data()

bool need_save_resume_data() const;

This function returns true if any whole chunk has been downloaded since the torrent was first loaded or since the last time the resume data was saved. When saving resume data periodically, it makes sense to skip any torrent which hasn't downloaded anything since the last time.

status()

torrent_status status(boost::uint32_t flags = 0xffffffff) const;

status() will return a structure with information about the status of this torrent. If the torrent_handle is invalid, it will throw libtorrent_exception exception. See torrent_status. The flags argument filters what information is returned in the torrent_status. Some information in there is relatively expensive to calculate, and if you're not interested in it (and see performance issues), you can filter them out.

By default everything is included. The flags you can use to decide what to include are:

• query_distributed_copies

  calculates distributed_copies, distributed_full_copies and distributed_fraction.

• query_accurate_download_counters

  includes partial downloaded blocks in total_done and total_wanted_done.

• query_last_seen_complete

  includes last_seen_complete.

• query_pieces

  includes pieces.

• query_verified_pieces

  includes verified_pieces (only applies to torrents in seed mode).

• query_torrent_file

  includes torrent_file, which is all the static information from the .torrent file.

• query_name

  includes name, the name of the torrent. This is either derived from the .torrent file, or from the &dn= magnet link argument or possibly some other source. If the name of the torrent is not known, this is an empty string.

• query_save_path

  includes save_path, the path to the directory the files of the torrent are saved to.

get_download_queue()

void get_download_queue(std::vector<partial_piece_info>& queue) const;

get_download_queue() takes a non-const reference to a vector which it will fill with information about pieces that are partially downloaded or not downloaded at all but partially requested. The entry in the vector (partial_piece_info) looks like this:

struct partial_piece_info
{
        int piece_index;
        int blocks_in_piece;
        enum state_t { none, slow, medium, fast };
        state_t piece_state;
        block_info* blocks;
};

piece_index is the index of the piece in question. blocks_in_piece is the number of blocks in this particular piece. This number will be the same for most pieces, but the last piece may have fewer blocks than the standard pieces.

piece_state is set to either fast, medium, slow or none. It tells which download rate category the peers downloading this piece falls into. none means that no peer is currently downloading any part of the piece. Peers prefer picking pieces from the same category as themselves. The reason for this is to keep the number of partially downloaded pieces down. Pieces set to none can be converted into any of fast, medium or slow as soon as a peer want to download from it.

struct block_info
{
        enum block_state_t
        { none, requested, writing, finished };

        void set_peer(tcp::endpoint const& ep);
        tcp::endpoint peer() const;

        unsigned bytes_progress:15;
        unsigned block_size:15;
        unsigned state:2;
        unsigned num_peers:14;
};

The blocks field points to an array of blocks_in_piece elements. This pointer is only valid until the next call to get_download_queue() for any torrent in the same session. They all share the storaga for the block arrays in their session object.

The block_info array contains data for each individual block in the piece. Each block has a state (state) which is any of:

• none - This block has not been downloaded or requested form any peer.
• requested - The block has been requested, but not completely downloaded yet.
• writing - The block has been downloaded and is currently queued for being written to disk.
• finished - The block has been written to disk.

The peer field is the ip address of the peer this block was downloaded from. num_peers is the number of peers that is currently requesting this block. Typically this is 0 or 1, but at the end of the torrent blocks may be requested by more peers in parallel to speed things up. bytes_progress is the number of bytes that have been received for this block, and block_size is the total number of bytes in this block.

get_peer_info()

void get_peer_info(std::vector<peer_info>&) const;

get_peer_info() takes a reference to a vector that will be cleared and filled with one entry for each peer connected to this torrent, given the handle is valid. If the torrent_handle is invalid, it will throw libtorrent_exception exception. Each entry in the vector contains information about that particular peer. See peer_info.

torrent_file()

boost::intrusive_ptr<torrent_info> torrent_file() const;

Returns a pointer to the torrent_info object associated with this torrent. The torrent_info object is a copy of the internal object. If the torrent doesn't have metadata, the object being returned will not be fully filled in. The torrent may be in a state without metadata only if it was started without a .torrent file, e.g. by using the libtorrent extension of just supplying a tracker and info-hash.

is_valid()

bool is_valid() const;

Returns true if this handle refers to a valid torrent and false if it hasn't been initialized or if the torrent it refers to has been aborted. Note that a handle may become invalid after it has been added to the session. Usually this is because the storage for the torrent is somehow invalid or if the filenames are not allowed (and hence cannot be opened/created) on your filesystem. If such an error occurs, a file_error_alert is generated and all handles that refers to that torrent will become invalid.

set_ssl_certificate()

void set_ssl_certificate(std::string const& cert, std::string const& private_key
        , std::string const& dh_params, std::string const& passphrase = "");

For SSL torrents, use this to specify a path to a .pem file to use as this client's certificate. The certificate must be signed by the certificate in the .torrent file to be valid.

cert is a path to the (signed) certificate in .pem format corresponding to this torrent.

private_key is a path to the private key for the specified certificate. This must be in .pem format.

dh_params is a path to the Diffie-Hellman parameter file, which needs to be in .pem format. You can generate this file using the openssl command like this: openssl dhparam -outform PEM -out dhparams.pem 512.

passphrase may be specified if the private key is encrypted and requires a passphrase to be decrypted.

Note that when a torrent first starts up, and it needs a certificate, it will suspend connecting to any peers until it has one. It's typically desirable to resume the torrent after setting the ssl certificate.

If you receive a torrent_need_cert_alert, you need to call this to provide a valid cert. If you don't have a cert you won't be allowed to connect to any peers.

native_handle()

boost::shared_ptr<torrent> native_handle() const;

This function is intended only for use by plugins and the alert dispatch function. Any code that runs in libtorrent's network thread may not use the public API of torrent_handle. Doing so results in a dead-lock. For such routines, the native_handle gives access to the underlying type representing the torrent. This type does not have a stable API and should be relied on as little as possible.

struct feed_handle
{
        feed_handle();
        void update_feed();
        feed_status get_feed_status() const;
        void set_settings(feed_settings const& s);
        feed_settings settings() const;
};

update_feed()

void update_feed();

Forces an update/refresh of the feed. Regular updates of the feed is managed by libtorrent, be careful to not call this too frequently since it may overload the RSS server.

get_feed_status()

feed_status get_feed_status() const;

Queries the RSS feed for information, including all the items in the feed. The feed_status object has the following fields:

struct feed_status
{
        std::string url;
        std::string title;
        std::string description;
        time_t last_update;
        int next_update;
        bool updating;
        std::vector<feed_item> items;
        error_code error;
        int ttl;
};

url is the URL of the feed.

title is the name of the feed (as specified by the feed itself). This may be empty if we have not recevied a response from the RSS server yet, or if the feed does not specify a title.

description is the feed description (as specified by the feed itself). This may be empty if we have not received a response from the RSS server yet, or if the feed does not specify a description.

last_update is the posix time of the last successful response from the feed.

next_update is the number of seconds, from now, when the feed will be updated again.

updating is true if the feed is currently being updated (i.e. waiting for DNS resolution, connecting to the server or waiting for the response to the HTTP request, or receiving the response).

items is a vector of all items that we have received from the feed. See feed_item for more information.

error is set to the appropriate error code if the feed encountered an error.

ttl is the current refresh time (in minutes). It's either the configured default ttl, or the ttl specified by the feed.

set_settings() settings()

void set_settings(feed_settings const& s);
feed_settings settings() const;

Sets and gets settings for this feed. For more information on the available settings, see add_feed().

presets

The default values of the session settings are set for a regular bittorrent client running on a desktop system. There are functions that can set the session settings to pre set settings for other environments. These can be used for the basis, and should be tweaked to fit your needs better.

session_settings min_memory_usage();
session_settings high_performance_seed();

min_memory_usage returns settings that will use the minimal amount of RAM, at the potential expense of upload and download performance. It adjusts the socket buffer sizes, disables the disk cache, lowers the send buffer watermarks so that each connection only has at most one block in use at any one time. It lowers the outstanding blocks send to the disk I/O thread so that connections only have one block waiting to be flushed to disk at any given time. It lowers the max number of peers in the peer list for torrents. It performs multiple smaller reads when it hashes pieces, instead of reading it all into memory before hashing.

This configuration is inteded to be the starting point for embedded devices. It will significantly reduce memory usage.

high_performance_seed returns settings optimized for a seed box, serving many peers and that doesn't do any downloading. It has a 128 MB disk cache and has a limit of 400 files in its file pool. It support fast upload rates by allowing large send buffers.

session_settings

struct session_settings
{
        session_settings();
        int version;
        std::string user_agent;
        int tracker_completion_timeout;
        int tracker_receive_timeout;
        int stop_tracker_timeout;
        int tracker_maximum_response_length;

        int piece_timeout;
        float request_queue_time;
        int max_allowed_in_request_queue;
        int max_out_request_queue;
        int whole_pieces_threshold;
        int peer_timeout;
        int urlseed_timeout;
        int urlseed_pipeline_size;
        int file_pool_size;
        bool allow_multiple_connections_per_ip;
        int max_failcount;
        int min_reconnect_time;
        int peer_connect_timeout;
        bool ignore_limits_on_local_network;
        int connection_speed;
        bool send_redundant_have;
        bool lazy_bitfields;
        int inactivity_timeout;
        int unchoke_interval;
        int optimistic_unchoke_interval;
        std::string announce_ip;
        int num_want;
        int initial_picker_threshold;
        int allowed_fast_set_size;

        enum { no_piece_suggestions = 0, suggest_read_cache = 1 };
        int suggest_mode;
        int max_queued_disk_bytes;
        int handshake_timeout;
        bool use_dht_as_fallback;
        bool free_torrent_hashes;
        bool upnp_ignore_nonrouters;
        int send_buffer_watermark;
        int send_buffer_watermark_factor;

#ifndef TORRENT_NO_DEPRECATE
        bool auto_upload_slots;
        bool auto_upload_slots_rate_based;
#endif

        enum choking_algorithm_t
        {
                fixed_slots_choker,
                auto_expand_choker,
                rate_based_choker,
                bittyrant_choker
        };

        int choking_algorithm;

        enum seed_choking_algorithm_t
        {
                round_robin,
                fastest_upload,
                anti_leech
        };

        int seed_choking_algorithm;

        bool use_parole_mode;
        int cache_size;
        int cache_buffer_chunk_size;
        int cache_expiry;
        bool use_read_cache;
        bool explicit_read_cache;
        int explicit_cache_interval;

        enum io_buffer_mode_t
        {
                enable_os_cache = 0,
                disable_os_cache_for_aligned_files = 1,
                disable_os_cache = 2
        };
        int disk_io_write_mode;
        int disk_io_read_mode;

        std::pair<int, int> outgoing_ports;
        char peer_tos;

        int active_downloads;
        int active_seeds;
        int active_dht_limit;
        int active_tracker_limit;
        int active_limit;
        bool auto_manage_prefer_seeds;
        bool dont_count_slow_torrents;
        int auto_manage_interval;
        float share_ratio_limit;
        float seed_time_ratio_limit;
        int seed_time_limit;
        int peer_turnover_interval;
        float peer_turnover;
        float peer_turnover_cutoff;
        bool close_redundant_connections;

        int auto_scrape_interval;
        int auto_scrape_min_interval;

        int max_peerlist_size;

        int min_announce_interval;

        bool prioritize_partial_pieces;
        int auto_manage_startup;

        bool rate_limit_ip_overhead;

        bool announce_to_all_trackers;
        bool announce_to_all_tiers;

        bool prefer_udp_trackers;
        bool strict_super_seeding;

        int seeding_piece_quota;

        int max_sparse_regions;

        bool lock_disk_cache;

        int max_rejects;

        int recv_socket_buffer_size;
        int send_socket_buffer_size;

        bool optimize_hashing_for_speed;

        int file_checks_delay_per_block;

        enum disk_cache_algo_t
        { lru, largest_contiguous, avoid_readback };

        disk_cache_algo_t disk_cache_algorithm;

        int read_cache_line_size;
        int write_cache_line_size;

        int optimistic_disk_retry;
        bool disable_hash_checks;

        int max_suggest_pieces;

        bool drop_skipped_requests;

        bool low_prio_disk;
        int local_service_announce_interval;
        int dht_announce_interval;

        int udp_tracker_token_expiry;
        bool volatile_read_cache;
        bool guided_read_cache;
        bool default_cache_min_age;

        int num_optimistic_unchoke_slots;
        bool no_atime_storage;
        int default_est_reciprocation_rate;
        int increase_est_reciprocation_rate;
        int decrease_est_reciprocation_rate;
        bool incoming_starts_queued_torrents;
        bool report_true_downloaded;
        bool strict_end_game_mode;

        bool broadcast_lsd;

        bool enable_outgoing_utp;
        bool enable_incoming_utp;
        bool enable_outgoing_tcp;
        bool enable_incoming_tcp;
        int max_pex_peers;
        bool ignore_resume_timestamps;
        bool no_recheck_incomplete_resume;
        bool anonymous_mode;
        bool force_proxy;
        int tick_interval;
        int share_mode_target;

        int upload_rate_limit;
        int download_rate_limit;
        int local_upload_rate_limit;
        int local_download_rate_limit;
        int dht_upload_rate_limit;
        int unchoke_slots_limit;
        int half_open_limit;
        int connections_limit;

        int utp_target_delay;
        int utp_gain_factor;
        int utp_min_timeout;
        int utp_syn_resends;
        int utp_num_resends;
        int utp_connect_timeout;
        bool utp_dynamic_sock_buf;
        int utp_loss_multiplier;

        enum bandwidth_mixed_algo_t
        {
                prefer_tcp = 0,
                peer_proportional = 1

        };
        int mixed_mode_algorithm;
        bool rate_limit_utp;

        int listen_queue_size;

        bool announce_double_nat;

        int torrent_connect_boost;
        bool seeding_outgoing_connections;

        bool no_connect_privileged_ports;
        int alert_queue_size;
        int max_metadata_size;
        bool smooth_connects;
        bool always_send_user_agent;
        bool apply_ip_filter_to_trackers;
        int read_job_every;
        bool use_disk_read_ahead;
        bool lock_files;

        int ssl_listen;

        int tracker_backoff;

        bool ban_web_seeds;
        int max_http_recv_buffer_size;

        bool support_share_mode;
        bool support_merkle_torrents;
        bool report_redundant_bytes;
        std::string handshake_client_version;
        bool use_disk_cache_pool;
};

version is automatically set to the libtorrent version you're using in order to be forward binary compatible. This field should not be changed.

user_agent this is the client identification to the tracker. The recommended format of this string is: "ClientName/ClientVersion libtorrent/libtorrentVersion". This name will not only be used when making HTTP requests, but also when sending extended headers to peers that support that extension.

tracker_completion_timeout is the number of seconds the tracker connection will wait from when it sent the request until it considers the tracker to have timed-out. Default value is 60 seconds.

tracker_receive_timeout is the number of seconds to wait to receive any data from the tracker. If no data is received for this number of seconds, the tracker will be considered as having timed out. If a tracker is down, this is the kind of timeout that will occur. The default value is 20 seconds.

stop_tracker_timeout is the time to wait for tracker responses when shutting down the session object. This is given in seconds. Default is 10 seconds.

tracker_maximum_response_length is the maximum number of bytes in a tracker response. If a response size passes this number it will be rejected and the connection will be closed. On gzipped responses this size is measured on the uncompressed data. So, if you get 20 bytes of gzip response that'll expand to 2 megs, it will be interrupted before the entire response has been uncompressed (given your limit is lower than 2 megs). Default limit is 1 megabyte.

piece_timeout controls the number of seconds from a request is sent until it times out if no piece response is returned.

request_queue_time is the length of the request queue given in the number of seconds it should take for the other end to send all the pieces. i.e. the actual number of requests depends on the download rate and this number.

max_allowed_in_request_queue is the number of outstanding block requests a peer is allowed to queue up in the client. If a peer sends more requests than this (before the first one has been handled) the last request will be dropped. The higher this is, the faster upload speeds the client can get to a single peer.

max_out_request_queue is the maximum number of outstanding requests to send to a peer. This limit takes precedence over request_queue_time. i.e. no matter the download speed, the number of outstanding requests will never exceed this limit.

whole_pieces_threshold is a limit in seconds. if a whole piece can be downloaded in at least this number of seconds from a specific peer, the peer_connection will prefer requesting whole pieces at a time from this peer. The benefit of this is to better utilize disk caches by doing localized accesses and also to make it easier to identify bad peers if a piece fails the hash check.

peer_timeout is the number of seconds the peer connection should wait (for any activity on the peer connection) before closing it due to time out. This defaults to 120 seconds, since that's what's specified in the protocol specification. After half the time out, a keep alive message is sent.

urlseed_timeout is the same as peer_timeout but applies only to url seeds. This value defaults to 20 seconds.

urlseed_pipeline_size controls the pipelining with the web server. When using persistent connections to HTTP 1.1 servers, the client is allowed to send more requests before the first response is received. This number controls the number of outstanding requests to use with url-seeds. Default is 5.

file_pool_size is the the upper limit on the total number of files this session will keep open. The reason why files are left open at all is that some anti virus software hooks on every file close, and scans the file for viruses. deferring the closing of the files will be the difference between a usable system and a completely hogged down system. Most operating systems also has a limit on the total number of file descriptors a process may have open. It is usually a good idea to find this limit and set the number of connections and the number of files limits so their sum is slightly below it.

allow_multiple_connections_per_ip determines if connections from the same IP address as existing connections should be rejected or not. Multiple connections from the same IP address is not allowed by default, to prevent abusive behavior by peers. It may be useful to allow such connections in cases where simulations are run on the same machie, and all peers in a swarm has the same IP address.

max_failcount is the maximum times we try to connect to a peer before stop connecting again. If a peer succeeds, the failcounter is reset. If a peer is retrieved from a peer source (other than DHT) the failcount is decremented by one, allowing another try.

min_reconnect_time is the time to wait between connection attempts. If the peer fails, the time is multiplied by fail counter.

peer_connect_timeout the number of seconds to wait after a connection attempt is initiated to a peer until it is considered as having timed out. The default is 10 seconds. This setting is especially important in case the number of half-open connections are limited, since stale half-open connection may delay the connection of other peers considerably.

ignore_limits_on_local_network, if set to true, upload, download and unchoke limits are ignored for peers on the local network.

connection_speed is the number of connection attempts that are made per second. If a number < 0 is specified, it will default to 200 connections per second. If 0 is specified, it means don't make outgoing connections at all.

send_redundant_have controls if have messages will be sent to peers that already have the piece. This is typically not necessary, but it might be necessary for collecting statistics in some cases. Default is false.

lazy_bitfields prevents outgoing bitfields from being full. If the client is seed, a few bits will be set to 0, and later filled in with have-messages. This is to prevent certain ISPs from stopping people from seeding.

inactivity_timeout, if a peer is uninteresting and uninterested for longer than this number of seconds, it will be disconnected. Default is 10 minutes

unchoke_interval is the number of seconds between chokes/unchokes. On this interval, peers are re-evaluated for being choked/unchoked. This is defined as 30 seconds in the protocol, and it should be significantly longer than what it takes for TCP to ramp up to it's max rate.

optimistic_unchoke_interval is the number of seconds between each optimistic unchoke. On this timer, the currently optimistically unchoked peer will change.

announce_ip is the ip address passed along to trackers as the &ip= parameter. If left as the default (an empty string), that parameter is omitted.

num_want is the number of peers we want from each tracker request. It defines what is sent as the &num_want= parameter to the tracker.

initial_picker_threshold specifies the number of pieces we need before we switch to rarest first picking. This defaults to 4, which means the 4 first pieces in any torrent are picked at random, the following pieces are picked in rarest first order.

allowed_fast_set_size is the number of pieces we allow peers to download from us without being unchoked.

suggest_mode controls whether or not libtorrent will send out suggest messages to create a bias of its peers to request certain pieces. The modes are:

• no_piece_suggestsions which is the default and will not send out suggest messages.
• suggest_read_cache which will send out suggest messages for the most recent pieces that are in the read cache.

max_queued_disk_bytes is the number maximum number of bytes, to be written to disk, that can wait in the disk I/O thread queue. This queue is only for waiting for the disk I/O thread to receive the job and either write it to disk or insert it in the write cache. When this limit is reached, the peer connections will stop reading data from their sockets, until the disk thread catches up. Setting this too low will severly limit your download rate.

handshake_timeout specifies the number of seconds we allow a peer to delay responding to a protocol handshake. If no response is received within this time, the connection is closed.

use_dht_as_fallback determines how the DHT is used. If this is true, the DHT will only be used for torrents where all trackers in its tracker list has failed. Either by an explicit error message or a time out. This is false by default, which means the DHT is used by default regardless of if the trackers fail or not.

free_torrent_hashes determines whether or not the torrent's piece hashes are kept in memory after the torrent becomes a seed or not. If it is set to true the hashes are freed once the torrent is a seed (they're not needed anymore since the torrent won't download anything more). If it's set to false they are not freed. If they are freed, the torrent_info returned by get_torrent_info() will return an object that may be incomplete, that cannot be passed back to `async_add_torrent() add_torrent()`_ for instance.

upnp_ignore_nonrouters indicates whether or not the UPnP implementation should ignore any broadcast response from a device whose address is not the configured router for this machine. i.e. it's a way to not talk to other people's routers by mistake.

send_buffer_watermark is the upper limit of the send buffer low-watermark. if the send buffer has fewer bytes than this, we'll read another 16kB block onto it. If set too small, upload rate capacity will suffer. If set too high, memory will be wasted. The actual watermark may be lower than this in case the upload rate is low, this is the upper limit.

send_buffer_watermark_factor is multiplied to the peer's upload rate to determine the low-watermark for the peer. It is specified as a percentage, which means 100 represents a factor of 1. The low-watermark is still clamped to not exceed the send_buffer_watermark upper limit. This defaults to 50. For high capacity connections, setting this higher can improve upload performance and disk throughput. Setting it too high may waste RAM and create a bias towards read jobs over write jobs.

auto_upload_slots defaults to true. When true, if there is a global upload limit set and the current upload rate is less than 90% of that, another upload slot is opened. If the upload rate has been saturated for an extended period of time, on upload slot is closed. The number of upload slots will never be less than what has been set by session::set_max_uploads(). To query the current number of upload slots, see session_status::allowed_upload_slots.

When auto_upload_slots_rate_based is set, and auto_upload_slots is set, the max upload slots setting is used as a minimum number of unchoked slots. This algorithm is designed to prevent the peer from spreading its upload capacity too thin, but still open more slots in order to utilize the full capacity.

choking_algorithm specifies which algorithm to use to determine which peers to unchoke. This setting replaces the deprecated settings auto_upload_slots and auto_upload_slots_rate_based.

The options for choking algorithms are:

• fixed_slots_choker is the traditional choker with a fixed number of unchoke slots (as specified by session::set_max_uploads()).
• auto_expand_choker opens at least the number of slots as specified by session::set_max_uploads() but opens up more slots if the upload capacity is not saturated. This unchoker will work just like the fixed_slot_choker if there's no global upload rate limit set.
• rate_based_choker opens up unchoke slots based on the upload rate achieved to peers. The more slots that are opened, the marginal upload rate required to open up another slot increases.
• bittyrant_choker attempts to optimize download rate by finding the reciprocation rate of each peer individually and prefers peers that gives the highest return on investment. It still allocates all upload capacity, but shuffles it around to the best peers first. For this choker to be efficient, you need to set a global upload rate limit (session_settings::upload_rate_limit). For more information about this choker, see the paper.

seed_choking_algorithm controls the seeding unchoke behavior. The available options are:

• round_robin which round-robins the peers that are unchoked when seeding. This distributes the upload bandwidht uniformly and fairly. It minimizes the ability for a peer to download everything without redistributing it.
• fastest_upload unchokes the peers we can send to the fastest. This might be a bit more reliable in utilizing all available capacity.
• anti_leech prioritizes peers who have just started or are just about to finish the download. The intention is to force peers in the middle of the download to trade with each other.

use_parole_mode specifies if parole mode should be used. Parole mode means that peers that participate in pieces that fail the hash check are put in a mode where they are only allowed to download whole pieces. If the whole piece a peer in parole mode fails the hash check, it is banned. If a peer participates in a piece that passes the hash check, it is taken out of parole mode.

cache_size is the disk write and read cache. It is specified in units of 16 KiB blocks. Buffers that are part of a peer's send or receive buffer also count against this limit. Send and receive buffers will never be denied to be allocated, but they will cause the actual cached blocks to be flushed or evicted. If this is set to -1, the cache size is automatically set to the amount of physical RAM available in the machine divided by 8. If the amount of physical RAM cannot be determined, it's set to 1024 (= 16 MiB).

Disk buffers are allocated using a pool allocator, the number of blocks that are allocated at a time when the pool needs to grow can be specified in cache_buffer_chunk_size. This defaults to 16 blocks. Lower numbers saves memory at the expense of more heap allocations. It must be at least 1.

cache_expiry is the number of seconds from the last cached write to a piece in the write cache, to when it's forcefully flushed to disk. Default is 60 second.

use_read_cache, is set to true (default), the disk cache is also used to cache pieces read from disk. Blocks for writing pieces takes presedence.

explicit_read_cache defaults to 0. If set to something greater than 0, the disk read cache will not be evicted by cache misses and will explicitly be controlled based on the rarity of pieces. Rare pieces are more likely to be cached. This would typically be used together with suggest_mode set to suggest_read_cache. The value is the number of pieces to keep in the read cache. If the actual read cache can't fit as many, it will essentially be clamped.

explicit_cache_interval is the number of seconds in between each refresh of a part of the explicit read cache. Torrents take turns in refreshing and this is the time in between each torrent refresh. Refreshing a torrent's explicit read cache means scanning all pieces and picking a random set of the rarest ones. There is an affinity to pick pieces that are already in the cache, so that subsequent refreshes only swaps in pieces that are rarer than whatever is in the cache at the time.

disk_io_write_mode and disk_io_read_mode determines how files are opened when they're in read only mode versus read and write mode. The options are:

• enable_os_cache

  This is the default and files are opened normally, with the OS caching reads and writes.

• disable_os_cache_for_aligned_files

  This will open files in unbuffered mode for files where every read and write would be sector aligned. Using aligned disk offsets is a requirement on some operating systems.

• disable_os_cache

  This opens all files in unbuffered mode (if allowed by the operating system). Linux and Windows, for instance, require disk offsets to be sector aligned, and in those cases, this option is the same as disable_os_caches_for_aligned_files.

One reason to disable caching is that it may help the operating system from growing its file cache indefinitely. Since some OSes only allow aligned files to be opened in unbuffered mode, It is recommended to make the largest file in a torrent the first file (with offset 0) or use pad files to align all files to piece boundries.

outgoing_ports, if set to something other than (0, 0) is a range of ports used to bind outgoing sockets to. This may be useful for users whose router allows them to assign QoS classes to traffic based on its local port. It is a range instead of a single port because of the problems with failing to reconnect to peers if a previous socket to that peer and port is in TIME_WAIT state.

peer_tos determines the TOS byte set in the IP header of every packet sent to peers (including web seeds). The default value for this is 0x0 (no marking). One potentially useful TOS mark is 0x20, this represents the QBone scavenger service. For more details, see QBSS.

active_downloads and active_seeds controls how many active seeding and downloading torrents the queuing mechanism allows. The target number of active torrents is min(active_downloads + active_seeds, active_limit). active_downloads and active_seeds are upper limits on the number of downloading torrents and seeding torrents respectively. Setting the value to -1 means unlimited.

For example if there are 10 seeding torrents and 10 downloading torrents, and active_downloads is 4 and active_seeds is 4, there will be 4 seeds active and 4 downloading torrents. If the settings are active_downloads = 2 and active_seeds = 4, then there will be 2 downloading torrents and 4 seeding torrents active. Torrents that are not auto managed are also counted against these limits. If there are non-auto managed torrents that use up all the slots, no auto managed torrent will be activated.

auto_manage_prefer_seeds specifies if libtorrent should prefer giving seeds active slots or downloading torrents. The default is false.

if dont_count_slow_torrents is true, torrents without any payload transfers are not subject to the active_seeds and active_downloads limits. This is intended to make it more likely to utilize all available bandwidth, and avoid having torrents that don't transfer anything block the active slots.

active_limit is a hard limit on the number of active torrents. This applies even to slow torrents.

active_dht_limit is the max number of torrents to announce to the DHT. By default this is set to 88, which is no more than one DHT announce every 10 seconds.

active_tracker_limit is the max number of torrents to announce to their trackers. By default this is 360, which is no more than one announce every 5 seconds.

active_lsd_limit is the max number of torrents to announce to the local network over the local service discovery protocol. By default this is 80, which is no more than one announce every 5 seconds (assuming the default announce interval of 5 minutes).

You can have more torrents active, even though they are not announced to the DHT, lsd or their tracker. If some peer knows about you for any reason and tries to connect, it will still be accepted, unless the torrent is paused, which means it won't accept any connections.

auto_manage_interval is the number of seconds between the torrent queue is updated, and rotated.

share_ratio_limit is the upload / download ratio limit for considering a seeding torrent have met the seed limit criteria. See queuing.

seed_time_ratio_limit is the seeding time / downloading time ratio limit for considering a seeding torrent to have met the seed limit criteria. See queuing.

seed_time_limit is the limit on the time a torrent has been an active seed (specified in seconds) before it is considered having met the seed limit criteria. See queuing.

peer_turnover_interval controls a feature where libtorrent periodically can disconnect the least useful peers in the hope of connecting to better ones. This settings controls the interval of this optimistic disconnect. It defaults to every 5 minutes, and is specified in seconds.

peer_turnover Is the fraction of the peers that are disconnected. This is a float where 1.f represents all peers an 0 represents no peers. It defaults to 4% (i.e. 0.04f)

peer_turnover_cutoff is the cut off trigger for optimistic unchokes. If a torrent has more than this fraction of its connection limit, the optimistic unchoke is triggered. This defaults to 90% (i.e. 0.9f).

close_redundant_connections specifies whether libtorrent should close connections where both ends have no utility in keeping the connection open. For instance if both ends have completed their downloads, there's no point in keeping it open. This defaults to true.

auto_scrape_interval is the number of seconds between scrapes of queued torrents (auto managed and paused torrents). Auto managed torrents that are paused, are scraped regularly in order to keep track of their downloader/seed ratio. This ratio is used to determine which torrents to seed and which to pause.

auto_scrape_min_interval is the minimum number of seconds between any automatic scrape (regardless of torrent). In case there are a large number of paused auto managed torrents, this puts a limit on how often a scrape request is sent.

max_peerlist_size is the maximum number of peers in the list of known peers. These peers are not necessarily connected, so this number should be much greater than the maximum number of connected peers. Peers are evicted from the cache when the list grows passed 90% of this limit, and once the size hits the limit, peers are no longer added to the list. If this limit is set to 0, there is no limit on how many peers we'll keep in the peer list.

max_paused_peerlist_size is the max peer list size used for torrents that are paused. This default to the same as max_peerlist_size, but can be used to save memory for paused torrents, since it's not as important for them to keep a large peer list.

min_announce_interval is the minimum allowed announce interval for a tracker. This is specified in seconds, defaults to 5 minutes and is used as a sanity check on what is returned from a tracker. It mitigates hammering misconfigured trackers.

If prioritize_partial_pieces is true, partial pieces are picked before pieces that are more rare. If false, rare pieces are always prioritized, unless the number of partial pieces is growing out of proportion.

auto_manage_startup is the number of seconds a torrent is considered active after it was started, regardless of upload and download speed. This is so that newly started torrents are not considered inactive until they have a fair chance to start downloading.

If rate_limit_ip_overhead is set to true, the estimated TCP/IP overhead is drained from the rate limiters, to avoid exceeding the limits with the total traffic

announce_to_all_trackers controls how multi tracker torrents are treated. If this is set to true, all trackers in the same tier are announced to in parallel. If all trackers in tier 0 fails, all trackers in tier 1 are announced as well. If it's set to false, the behavior is as defined by the multi tracker specification. It defaults to false, which is the same behavior previous versions of libtorrent has had as well.

announce_to_all_tiers also controls how multi tracker torrents are treated. When this is set to true, one tracker from each tier is announced to. This is the uTorrent behavior. This is false by default in order to comply with the multi-tracker specification.

prefer_udp_trackers is true by default. It means that trackers may be rearranged in a way that udp trackers are always tried before http trackers for the same hostname. Setting this to fails means that the trackers' tier is respected and there's no preference of one protocol over another.

strict_super_seeding when this is set to true, a piece has to have been forwarded to a third peer before another one is handed out. This is the traditional definition of super seeding.

seeding_piece_quota is the number of pieces to send to a peer, when seeding, before rotating in another peer to the unchoke set. It defaults to 3 pieces, which means that when seeding, any peer we've sent more than this number of pieces to will be unchoked in favour of a choked peer.

max_sparse_regions is a limit of the number of sparse regions in a torrent. A sparse region is defined as a hole of pieces we have not yet downloaded, in between pieces that have been downloaded. This is used as a hack for windows vista which has a bug where you cannot write files with more than a certain number of sparse regions. This limit is not hard, it will be exceeded. Once it's exceeded, pieces that will maintain or decrease the number of sparse regions are prioritized. To disable this functionality, set this to 0. It defaults to 0 on all platforms except windows.

lock_disk_cache if lock disk cache is set to true the disk cache that's in use, will be locked in physical memory, preventing it from being swapped out.

max_rejects is the number of piece requests we will reject in a row while a peer is choked before the peer is considered abusive and is disconnected.

recv_socket_buffer_size and send_socket_buffer_size specifies the buffer sizes set on peer sockets. 0 (which is the default) means the OS default (i.e. don't change the buffer sizes). The socket buffer sizes are changed using setsockopt() with SOL_SOCKET/SO_RCVBUF and SO_SNDBUFFER.

optimize_hashing_for_speed chooses between two ways of reading back piece data from disk when its complete and needs to be verified against the piece hash. This happens if some blocks were flushed to the disk out of order. Everything that is flushed in order is hashed as it goes along. Optimizing for speed will allocate space to fit all the the remaingin, unhashed, part of the piece, reads the data into it in a single call and hashes it. This is the default. If optimizing_hashing_for_speed is false, a single block will be allocated (16 kB), and the unhashed parts of the piece are read, one at a time, and hashed in this single block. This is appropriate on systems that are memory constrained.

file_checks_delay_per_block is the number of milliseconds to sleep in between disk read operations when checking torrents. This defaults to 0, but can be set to higher numbers to slow down the rate at which data is read from the disk while checking. This may be useful for background tasks that doesn't matter if they take a bit longer, as long as they leave disk I/O time for other processes.

disk_cache_algorithm tells the disk I/O thread which cache flush algorithm to use. The default algorithm is largest_contiguous. This flushes the entire piece, in the write cache, that was least recently written to. This is specified by the session_settings::lru enum value. session_settings::largest_contiguous will flush the largest sequences of contiguous blocks from the write cache, regarless of the piece's last use time. session_settings::avoid_readback will prioritize flushing blocks that will avoid having to read them back in to verify the hash of the piece once it's done. This is especially useful for high throughput setups, where reading from the disk is especially expensive.

read_cache_line_size is the number of blocks to read into the read cache when a read cache miss occurs. Setting this to 0 is essentially the same thing as disabling read cache. The number of blocks read into the read cache is always capped by the piece boundry.

When a piece in the write cache has write_cache_line_size contiguous blocks in it, they will be flushed. Setting this to 1 effectively disables the write cache.

optimistic_disk_retry is the number of seconds from a disk write errors occur on a torrent until libtorrent will take it out of the upload mode, to test if the error condition has been fixed.

libtorrent will only do this automatically for auto managed torrents.

You can explicitly take a torrent out of upload only mode using set_upload_mode().

disable_hash_checks controls if downloaded pieces are verified against the piece hashes in the torrent file or not. The default is false, i.e. to verify all downloaded data. It may be useful to turn this off for performance profiling and simulation scenarios. Do not disable the hash check for regular bittorrent clients.

max_suggest_pieces is the max number of suggested piece indices received from a peer that's remembered. If a peer floods suggest messages, this limit prevents libtorrent from using too much RAM. It defaults to 10.

If drop_skipped_requests is set to true (it defaults to false), piece requests that have been skipped enough times when piece messages are received, will be considered lost. Requests are considered skipped when the returned piece messages are re-ordered compared to the order of the requests. This was an attempt to get out of dead-locks caused by BitComet peers silently ignoring some requests. It may cause problems at high rates, and high level of reordering in the uploading peer, that's why it's disabled by default.

low_prio_disk determines if the disk I/O should use a normal or low priority policy. This defaults to true, which means that it's low priority by default. Other processes doing disk I/O will normally take priority in this mode. This is meant to improve the overall responsiveness of the system while downloading in the background. For high-performance server setups, this might not be desirable.

local_service_announce_interval is the time between local network announces for a torrent. By default, when local service discovery is enabled a torrent announces itself every 5 minutes. This interval is specified in seconds.

dht_announce_interval is the number of seconds between announcing torrents to the distributed hash table (DHT). This is specified to be 15 minutes which is its default.

dht_max_torrents is the max number of torrents we will track in the DHT.

udp_tracker_token_expiry is the number of seconds libtorrent will keep UDP tracker connection tokens around for. This is specified to be 60 seconds, and defaults to that. The higher this value is, the fewer packets have to be sent to the UDP tracker. In order for higher values to work, the tracker needs to be configured to match the expiration time for tokens.

volatile_read_cache, if this is set to true, read cache blocks that are hit by peer read requests are removed from the disk cache to free up more space. This is useful if you don't expect the disk cache to create any cache hits from other peers than the one who triggered the cache line to be read into the cache in the first place.

guided_read_cache enables the disk cache to adjust the size of a cache line generated by peers to depend on the upload rate you are sending to that peer. The intention is to optimize the RAM usage of the cache, to read ahead further for peers that you're sending faster to.

default_cache_min_age is the minimum number of seconds any read cache line is kept in the cache. This defaults to one second but may be greater if guided_read_cache is enabled. Having a lower bound on the time a cache line stays in the cache is an attempt to avoid swapping the same pieces in and out of the cache in case there is a shortage of spare cache space.

num_optimistic_unchoke_slots is the number of optimistic unchoke slots to use. It defaults to 0, which means automatic. Having a higher number of optimistic unchoke slots mean you will find the good peers faster but with the trade-off to use up more bandwidth. When this is set to 0, libtorrent opens up 20% of your allowed upload slots as optimistic unchoke slots.

no_atime_storage this is a linux-only option and passes in the O_NOATIME to open() when opening files. This may lead to some disk performance improvements.

default_est_reciprocation_rate is the assumed reciprocation rate from peers when using the BitTyrant choker. This defaults to 14 kiB/s. If set too high, you will over-estimate your peers and be more altruistic while finding the true reciprocation rate, if it's set too low, you'll be too stingy and waste finding the true reciprocation rate.

increase_est_reciprocation_rate specifies how many percent the extimated reciprocation rate should be increased by each unchoke interval a peer is still choking us back. This defaults to 20%. This only applies to the BitTyrant choker.

decrease_est_reciprocation_rate specifies how many percent the estimated reciprocation rate should be decreased by each unchoke interval a peer unchokes us. This default to 3%. This only applies to the BitTyrant choker.

incoming_starts_queued_torrents defaults to false. If a torrent has been paused by the auto managed feature in libtorrent, i.e. the torrent is paused and auto managed, this feature affects whether or not it is automatically started on an incoming connection. The main reason to queue torrents, is not to make them unavailable, but to save on the overhead of announcing to the trackers, the DHT and to avoid spreading one's unchoke slots too thin. If a peer managed to find us, even though we're no in the torrent anymore, this setting can make us start the torrent and serve it.

When report_true_downloaded is true, the &downloaded= argument sent to trackers will include redundant downloaded bytes. It defaults to false, which means redundant bytes are not reported to the tracker.

strict_end_game_mode defaults to true, and controls when a block may be requested twice. If this is true, a block may only be requested twice when there's ay least one request to every piece that's left to download in the torrent. This may slow down progress on some pieces sometimes, but it may also avoid downloading a lot of redundant bytes. If this is false, libtorrent attempts to use each peer connection to its max, by always requesting something, even if it means requesting something that has been requested from another peer already.

if broadcast_lsd is set to true, the local peer discovery (or Local Service Discovery) will not only use IP multicast, but also broadcast its messages. This can be useful when running on networks that don't support multicast. Since broadcast messages might be expensive and disruptive on networks, only every 8th announce uses broadcast.

enable_outgoing_utp, enable_incoming_utp, enable_outgoing_tcp, enable_incoming_tcp all determines if libtorrent should attempt to make outgoing connections of the specific type, or allow incoming connection. By default all of them are enabled.

ignore_resume_timestamps determines if the storage, when loading resume data files, should verify that the file modification time with the timestamps in the resume data. This defaults to false, which means timestamps are taken into account, and resume data is less likely to accepted (torrents are more likely to be fully checked when loaded). It might be useful to set this to true if your network is faster than your disk, and it would be faster to redownload potentially missed pieces than to go through the whole storage to look for them.

no_recheck_incomplete_resume determines if the storage should check the whole files when resume data is incomplete or missing or whether it should simply assume we don't have any of the data. By default, this is determined by the existance of any of the files. By setting this setting to true, the files won't be checked, but will go straight to download mode.

anonymous_mode defaults to false. When set to true, the client tries to hide its identity to a certain degree. The peer-ID will no longer include the client's fingerprint. The user-agent will be reset to an empty string.

If you're using I2P, it might make sense to enable anonymous mode.

force_proxy disables any communication that's not going over a proxy. Enabling this requires a proxy to be configured as well, see set_proxy_settings. The listen sockets are closed, and incoming connections will only be accepted through a SOCKS5 or I2P proxy (if a peer proxy is set up and is run on the same machine as the tracker proxy). This setting also disabled peer country lookups, since those are done via DNS lookups that aren't supported by proxies.

tick_interval specifies the number of milliseconds between internal ticks. This is the frequency with which bandwidth quota is distributed to peers. It should not be more than one second (i.e. 1000 ms). Setting this to a low value (around 100) means higher resolution bandwidth quota distribution, setting it to a higher value saves CPU cycles.

share_mode_target specifies the target share ratio for share mode torrents. This defaults to 3, meaning we'll try to upload 3 times as much as we download. Setting this very high, will make it very conservative and you might end up not downloading anything ever (and not affecting your share ratio). It does not make any sense to set this any lower than 2. For instance, if only 3 peers need to download the rarest piece, it's impossible to download a single piece and upload it more than 3 times. If the share_mode_target is set to more than 3, nothing is downloaded.

upload_rate_limit, download_rate_limit, local_upload_rate_limit and local_download_rate_limit sets the session-global limits of upload and download rate limits, in bytes per second. The local rates refer to peers on the local network. By default peers on the local network are not rate limited.

These rate limits are only used for local peers (peers within the same subnet as the client itself) and it is only used when session_settings::ignore_limits_on_local_network is set to true (which it is by default). These rate limits default to unthrottled, but can be useful in case you want to treat local peers preferentially, but not quite unthrottled.

A value of 0 means unlimited.

dht_upload_rate_limit sets the rate limit on the DHT. This is specified in bytes per second and defaults to 4000. For busy boxes with lots of torrents that requires more DHT traffic, this should be raised.

unchoke_slots_limit is the max number of unchoked peers in the session. The number of unchoke slots may be ignored depending on what choking_algorithm is set to. A value of -1 means infinite.

half_open_limit sets the maximum number of half-open connections libtorrent will have when connecting to peers. A half-open connection is one where connect() has been called, but the connection still hasn't been established (nor failed). Windows XP Service Pack 2 sets a default, system wide, limit of the number of half-open connections to 10. So, this limit can be used to work nicer together with other network applications on that system. The default is to have no limit, and passing -1 as the limit, means to have no limit. When limiting the number of simultaneous connection attempts, peers will be put in a queue waiting for their turn to get connected.

connections_limit sets a global limit on the number of connections opened. The number of connections is set to a hard minimum of at least two per torrent, so if you set a too low connections limit, and open too many torrents, the limit will not be met.

utp_target_delay is the target delay for uTP sockets in milliseconds. A high value will make uTP connections more aggressive and cause longer queues in the upload bottleneck. It cannot be too low, since the noise in the measurements would cause it to send too slow. The default is 50 milliseconds.

utp_gain_factor is the number of bytes the uTP congestion window can increase at the most in one RTT. This defaults to 300 bytes. If this is set too high, the congestion controller reacts too hard to noise and will not be stable, if it's set too low, it will react slow to congestion and not back off as fast.

utp_min_timeout is the shortest allowed uTP socket timeout, specified in milliseconds. This defaults to 500 milliseconds. The timeout depends on the RTT of the connection, but is never smaller than this value. A connection times out when every packet in a window is lost, or when a packet is lost twice in a row (i.e. the resent packet is lost as well).

The shorter the timeout is, the faster the connection will recover from this situation, assuming the RTT is low enough.

utp_syn_resends is the number of SYN packets that are sent (and timed out) before giving up and closing the socket.

utp_num_resends is the number of times a packet is sent (and lossed or timed out) before giving up and closing the connection.

utp_connect_timeout is the number of milliseconds of timeout for the initial SYN packet for uTP connections. For each timed out packet (in a row), the timeout is doubled.

utp_dynamic_sock_buf controls if the uTP socket manager is allowed to increase the socket buffer if a network interface with a large MTU is used (such as loopback or ethernet jumbo frames). This defaults to true and might improve uTP throughput. For RAM constrained systems, disabling this typically saves around 30kB in user space and probably around 400kB in kernel socket buffers (it adjusts the send and receive buffer size on the kernel socket, both for IPv4 and IPv6).

utp_loss_multiplier controls how the congestion window is changed when a packet loss is experienced. It's specified as a percentage multiplier for cwnd. By default it's set to 50 (i.e. cut in half). Do not change this value unless you know what you're doing. Never set it higher than 100.

The mixed_mode_algorithm determines how to treat TCP connections when there are uTP connections. Since uTP is designed to yield to TCP, there's an inherent problem when using swarms that have both TCP and uTP connections. If nothing is done, uTP connections would often be starved out for bandwidth by the TCP connections. This mode is prefer_tcp. The peer_proportional mode simply looks at the current throughput and rate limits all TCP connections to their proportional share based on how many of the connections are TCP. This works best if uTP connections are not rate limited by the global rate limiter (which they aren't by default).

rate_limit_utp determines if uTP connections should be throttled by the global rate limiter or not. By default they are.

listen_queue_size is the value passed in to listen() for the listen socket. It is the number of outstanding incoming connections to queue up while we're not actively waiting for a connection to be accepted. The default is 5 which should be sufficient for any normal client. If this is a high performance server which expects to receive a lot of connections, or used in a simulator or test, it might make sense to raise this number. It will not take affect until listen_on() is called again (or for the first time).

if announce_double_nat is true, the &ip= argument in tracker requests (unless otherwise specified) will be set to the intermediate IP address, if the user is double NATed. If ther user is not double NATed, this option has no affect.

torrent_connect_boost is the number of peers to try to connect to immediately when the first tracker response is received for a torrent. This is a boost to given to new torrents to accelerate them starting up. The normal connect scheduler is run once every second, this allows peers to be connected immediately instead of waiting for the session tick to trigger connections.

seeding_outgoing_connections determines if seeding (and finished) torrents should attempt to make outgoing connections or not. By default this is true. It may be set to false in very specific applications where the cost of making outgoing connections is high, and there are no or small benefits of doing so. For instance, if no nodes are behind a firewall or a NAT, seeds don't need to make outgoing connections.

if no_connect_privileged_ports is true (which is the default), libtorrent will not connect to any peers on priviliged ports (<= 1023). This can mitigate using bittorrent swarms for certain DDoS attacks.

alert_queue_size is the maximum number of alerts queued up internally. If alerts are not popped, the queue will eventually fill up to this level. This defaults to 1000.

max_metadata_size is the maximum allowed size (in bytes) to be received by the metadata extension, i.e. magnet links. It defaults to 1 MiB.

smooth_connects is true by default, which means the number of connection attempts per second may be limited to below the connection_speed, in case we're close to bump up against the limit of number of connections. The intention of this setting is to more evenly distribute our connection attempts over time, instead of attempting to connectin in batches, and timing them out in batches.

always_send_user_agent defaults to false. When set to true, web connections will include a user-agent with every request, as opposed to just the first request in a connection.

apply_ip_filter_to_trackers defaults to true. It determines whether the IP filter applies to trackers as well as peers. If this is set to false, trackers are exempt from the IP filter (if there is one). If no IP filter is set, this setting is irrelevant.

read_job_every is used to avoid starvation of read jobs in the disk I/O thread. By default, read jobs are deferred, sorted by physical disk location and serviced once all write jobs have been issued. In scenarios where the download rate is enough to saturate the disk, there's a risk the read jobs will never be serviced. With this setting, every x write job, issued in a row, will instead pick one read job off of the sorted queue, where x is read_job_every.

use_disk_read_ahead defaults to true and will attempt to optimize disk reads by giving the operating system heads up of disk read requests as they are queued in the disk job queue. This gives a significant performance boost for seeding.

lock_files determines whether or not to lock files which libtorrent is downloading to or seeding from. This is implemented using fcntl(F_SETLK) on unix systems and by not passing in SHARE_READ and SHARE_WRITE on windows. This might prevent 3rd party processes from corrupting the files under libtorrent's feet.

ssl_listen sets the listen port for SSL connections. If this is set to 0, no SSL listen port is opened. Otherwise a socket is opened on this port. This setting is only taken into account when opening the regular listen port, and won't re-open the listen socket simply by changing this setting.

It defaults to port 4433.

tracker_backoff determines how aggressively to back off from retrying failing trackers. This value determines x in the following formula, determining the number of seconds to wait until the next retry:

delay = 5 + 5 * x / 100 * fails^2

It defaults to 250.

This setting may be useful to make libtorrent more or less aggressive in hitting trackers.

ban_web_seeds enables banning web seeds. By default, web seeds that send corrupt data are banned.

max_http_recv_buffer_size specifies the max number of bytes to receive into RAM buffers when downloading stuff over HTTP. Specifically when specifying a URL to a .torrent file when adding a torrent or when announcing to an HTTP tracker. The default is 2 MiB.

support_share_mode enables or disables the share mode extension. This is enabled by default.

support_merkle_torrents enables or disables the merkle tree torrent support. This is enabled by default.

report_redundant_bytes enables or disables reporting redundant bytes to the tracker. This is enabled by default.

handshake_client_version is the client name advertized in the peer handshake. If set to an empty string, the user_agent string is used.

use_disk_cache_pool enables using a pool allocator for disk cache blocks. This is disabled by default. Enabling it makes the cache perform better at high throughput. It also makes the cache less likely and slower at returning memory back to the system once allocated.

ip_filter()

ip_filter()

Creates a default filter that doesn't filter any address.

postcondition: access(x) == 0 for every x

add_rule()

void add_rule(address first, address last, int flags);

Adds a rule to the filter. first and last defines a range of ip addresses that will be marked with the given flags. The flags can currently be 0, which means allowed, or ip_filter::blocked, which means disallowed.

precondition: first.is_v4() == last.is_v4() && first.is_v6() == last.is_v6()

postcondition: access(x) == flags for every x in the range [first, last]

This means that in a case of overlapping ranges, the last one applied takes precedence.

access()

int access(address const& addr) const;

Returns the access permissions for the given address (addr). The permission can currently be 0 or ip_filter::blocked. The complexity of this operation is O(log n), where n is the minimum number of non-overlapping ranges to describe the current filter.

export_filter()

boost::tuple<std::vector<ip_range<address_v4> >
        , std::vector<ip_range<address_v6> > > export_filter() const;

This function will return the current state of the filter in the minimum number of ranges possible. They are sorted from ranges in low addresses to high addresses. Each entry in the returned vector is a range with the access control specified in its flags field.

The return value is a tuple containing two range-lists. One for IPv4 addresses and one for IPv6 addresses.

class upnp
{
public:

        enum protocol_type { none = 0, udp = 1, tcp = 2 };
        int add_mapping(protocol_type p, int external_port, int local_port);
        void delete_mapping(int mapping_index);

        void discover_device();
        void close();

        std::string router_model();
};

class natpmp
{
public:

        enum protocol_type { none = 0, udp = 1, tcp = 2 };
        int add_mapping(protocol_type p, int external_port, int local_port);
        void delete_mapping(int mapping_index);

        void close();
        void rebind(address const& listen_interface);
};

add_mapping()

int add_mapping(protocol_type p, int external_port, int local_port);

Attempts to add a port mapping for the specified protocol. Valid protocols are upnp::tcp and upnp::udp for the UPnP class and natpmp::tcp and natpmp::udp for the NAT-PMP class.

external_port is the port on the external address that will be mapped. This is a hint, you are not guaranteed that this port will be available, and it may end up being something else. In the portmap_alert notification, the actual external port is reported.

local_port is the port in the local machine that the mapping should forward to.

The return value is an index that identifies this port mapping. This is used to refer to mappings that fails or succeeds in the portmap_error_alert and portmap_alert respectively. If The mapping fails immediately, the return value is -1, which means failure. There will not be any error alert notification for mappings that fail with a -1 return value.

delete_mapping()

void delete_mapping(int mapping_index);

This function removes a port mapping. mapping_index is the index that refers to the mapping you want to remove, which was returned from add_mapping().

router_model()

std::string router_model();

This is only available for UPnP routers. If the model is advertized by the router, it can be queried through this function.

identify_client()

std::string identify_client(peer_id const& id);

This function is declared in the header <libtorrent/identify_client.hpp>. It can can be used to extract a string describing a client version from its peer-id. It will recognize most clients that have this kind of identification in the peer-id.

client_fingerprint()

boost::optional<fingerprint> client_fingerprint(peer_id const& p);

Returns an optional fingerprint if any can be identified from the peer id. This can be used to automate the identification of clients. It will not be able to identify peers with non- standard encodings. Only Azureus style, Shadow's style and Mainline style. This function is declared in the header <libtorrent/identify_client.hpp>.

lazy_bdecode()

int lazy_bdecode(char const* start, char const* end, lazy_entry& ret
        , error_code& ec, int* error_pos = 0, int depth_limit = 1000
        , int item_limit = 1000000);

This function decodes bencoded data.

Whenever possible, lazy_bdecode() should be preferred over bdecode(). It is more efficient and more secure. It supports having constraints on the amount of memory is consumed by the parser.

lazy refers to the fact that it doesn't copy any actual data out of the bencoded buffer. It builds a tree of lazy_entry which has pointers into the bencoded buffer. This makes it very fast and efficient. On top of that, it is not recursive, which saves a lot of stack space when parsing deeply nested trees. However, in order to protect against potential attacks, the depth_limit and item_limit control how many levels deep the tree is allowed to get. With recursive parser, a few thousand levels would be enough to exhaust the threads stack and terminate the process. The item_limit protects against very large structures, not necessarily deep. Each bencoded item in the structure causes the parser to allocate some amount of memory, this memory is constant regardless of how much data actually is stored in the item. One potential attack is to create a bencoded list of hundreds of thousands empty strings, which would cause the parser to allocate a significant amount of memory, perhaps more than is available on the machine, and effectively provide a denial of service. The default item limit is set as a reasonable upper limit for desktop computers. Very few torrents have more items in them. The limit corresponds to about 25 MB, which might be a bit much for embedded systems.

start and end defines the bencoded buffer to be decoded. ret is the lazy_entry which is filled in with the whole decoded tree. ec is a reference to an error_code which is set to describe the error encountered in case the function fails. error_pos is an optional pointer to an int, which will be set to the byte offset into the buffer where an error occurred, in case the function fails.

bdecode() bencode()

template<class InIt> entry bdecode(InIt start, InIt end);
template<class OutIt> void bencode(OutIt out, const entry& e);

These functions will encode data to bencoded or decode bencoded data.

If possible, lazy_bdecode() should be preferred over bdecode().

The entry class is the internal representation of the bencoded data and it can be used to retrieve information, an entry can also be build by the program and given to bencode() to encode it into the OutIt iterator.

The OutIt and InIt are iterators (InputIterator and OutputIterator respectively). They are templates and are usually instantiated as ostream_iterator, back_insert_iterator or istream_iterator. These functions will assume that the iterator refers to a character (char). So, if you want to encode entry e into a buffer in memory, you can do it like this:

std::vector<char> buffer;
bencode(std::back_inserter(buf), e);

If you want to decode a torrent file from a buffer in memory, you can do it like this:

std::vector<char> buffer;
// ...
entry e = bdecode(buf.begin(), buf.end());

Or, if you have a raw char buffer:

const char* buf;
// ...
entry e = bdecode(buf, buf + data_size);

Now we just need to know how to retrieve information from the entry.

If bdecode() encounters invalid encoded data in the range given to it it will throw libtorrent_exception.

add_magnet_uri()

deprecated

torrent_handle add_magnet_uri(session& ses, std::string const& uri
        add_torrent_params p);
torrent_handle add_magnet_uri(session& ses, std::string const& uri
        add_torrent_params p, error_code& ec);

This function parses the magnet URI (uri) as a bittorrent magnet link, and adds the torrent to the specified session (ses). It returns the handle to the newly added torrent, or an invalid handle in case parsing failed. To control some initial settings of the torrent, sepcify those in the add_torrent_params, p. See `async_add_torrent() add_torrent()`_.

The overload that does not take an error_code throws an exception on error and is not available when building without exception support.

A simpler way to add a magnet link to a session is to pass in the link through add_torrent_params::url argument to session::add_torrent().

For more information about magnet links, see magnet links.

parse_magnet_uri()

void parse_magnet_uri(std::string const& uri, add_torrent_params& p, error_code& ec);

This function parses out information from the magnet link and populates the add_torrent_params object.

make_magnet_uri()

std::string make_magnet_uri(torrent_handle const& handle);

Generates a magnet URI from the specified torrent. If the torrent handle is invalid, an empty string is returned.

For more information about magnet links, see magnet links.

template <T> T* alert_cast(alert* a);
template <T> T const* alert_cast(alert const* a);

class alert
{
public:

        enum category_t
        {
                error_notification = implementation defined,
                peer_notification = implementation defined,
                port_mapping_notification = implementation defined,
                storage_notification = implementation defined,
                tracker_notification = implementation defined,
                debug_notification = implementation defined,
                status_notification = implementation defined,
                progress_notification = implementation defined,
                ip_block_notification = implementation defined,
                performance_warning = implementation defined,
                dht_notification = implementation defined,
                stats_notification = implementation defined,
                rss_notification = implementation defined,

                all_categories = implementation defined
        };

        ptime timestamp() const;

        virtual ~alert();

        virtual int type() const = 0;
        virtual std::string message() const = 0;
        virtual char const* what() const = 0;
        virtual int category() const = 0;
        virtual bool discardable() const;
        virtual std::auto_ptr<alert> clone() const = 0;
};

std::auto_ptr<alert> a = ses.pop_alert();
switch (a->type())
{
        case read_piece_alert::alert_type:
        {
                read_piece_alert* p = (read_piece_alert*)a.get();
                if (p->ec) {
                        // read_piece failed
                        break;
                }
                // use p
                break;
        }
        case file_renamed_alert::alert_type:
        {
                // etc...
        }
}

struct torrent_alert: alert
{
        // ...
        torrent_handle handle;
};

struct tracker_alert: torrent_alert
{
        // ...
        std::string url;
};

torrent_added_alert

The torrent_added_alert is posted once every time a torrent is successfully added. It doesn't contain any members of its own, but inherits the torrent handle from its base class. It's posted when the status_notification bit is set in the alert mask.

struct torrent_added_alert: torrent_alert
{
        // ...
};

add_torrent_alert

This alert is always posted when a torrent was attempted to be added and contains the return status of the add operation. The torrent handle of the new torrent can be found in the base class' handle member. If adding the torrent failed, error contains the error code.

struct add_torrent_alert: torrent_alert
{
        // ...
        add_torrent_params params;
        error_code error;
};

params is a copy of the parameters used when adding the torrent, it can be used to identify which invocation to async_add_torrent() caused this alert.

error is set to the error, if one occurred while adding the torrent.

torrent_removed_alert

The torrent_removed_alert is posted whenever a torrent is removed. Since the torrent handle in its baseclass will always be invalid (since the torrent is already removed) it has the info hash as a member, to identify it. It's posted when the status_notification bit is set in the alert mask.

Even though the handle member doesn't point to an existing torrent anymore, it is still useful for comparing to other handles, which may also no longer point to existing torrents, but to the same non-existing torrents.

The torrent_handle acts as a weak_ptr, even though its object no longer exists, it can still compare equal to another weak pointer which points to the same non-existent object.

struct torrent_removed_alert: torrent_alert
{
        // ...
        sha1_hash info_hash;
};

read_piece_alert

This alert is posted when the asynchronous read operation initiated by a call to read_piece() is completed. If the read failed, the torrent is paused and an error state is set and the buffer member of the alert is 0. If successful, buffer points to a buffer containing all the data of the piece. piece is the piece index that was read. size is the number of bytes that was read.

If the operation fails, ec will indicat what went wrong.

struct read_piece_alert: torrent_alert
{
        // ...
        error_code ec;
        boost::shared_ptr<char> buffer;
        int piece;
        int size;
};

external_ip_alert

Whenever libtorrent learns about the machines external IP, this alert is generated. The external IP address can be acquired from the tracker (if it supports that) or from peers that supports the extension protocol. The address can be accessed through the external_address member.

struct external_ip_alert: alert
{
        // ...
        address external_address;
};

listen_failed_alert

This alert is generated when none of the ports, given in the port range, to session_ can be opened for listening. The endpoint member is the interface and port that failed, error is the error code describing the failure.

libtorrent may sometimes try to listen on port 0, if all other ports failed. Port 0 asks the operating system to pick a port that's free). If that fails you may see a listen_failed_alert with port 0 even if you didn't ask to listen on it.

struct listen_failed_alert: alert
{
        // ...
        tcp::endpoint endpoint;
        error_code error;
};

listen_succeeded_alert

This alert is posted when the listen port succeeds to be opened on a particular interface. endpoint is the endpoint that successfully was opened for listening.

struct listen_succeeded_alert: alert
{
        // ...
        tcp::endpoint endpoint;
};

portmap_error_alert

This alert is generated when a NAT router was successfully found but some part of the port mapping request failed. It contains a text message that may help the user figure out what is wrong. This alert is not generated in case it appears the client is not running on a NAT:ed network or if it appears there is no NAT router that can be remote controlled to add port mappings.

mapping refers to the mapping index of the port map that failed, i.e. the index returned from add_mapping().

map_type is 0 for NAT-PMP and 1 for UPnP.

error tells you what failed.

struct portmap_error_alert: alert
{
        // ...
        int mapping;
        int type;
        error_code error;
};

portmap_alert

This alert is generated when a NAT router was successfully found and a port was successfully mapped on it. On a NAT:ed network with a NAT-PMP capable router, this is typically generated once when mapping the TCP port and, if DHT is enabled, when the UDP port is mapped.

mapping refers to the mapping index of the port map that failed, i.e. the index returned from add_mapping().

external_port is the external port allocated for the mapping.

type is 0 for NAT-PMP and 1 for UPnP.

struct portmap_alert: alert
{
        // ...
        int mapping;
        int external_port;
        int map_type;
};

portmap_log_alert

This alert is generated to log informational events related to either UPnP or NAT-PMP. They contain a log line and the type (0 = NAT-PMP and 1 = UPnP). Displaying these messages to an end user is only useful for debugging the UPnP or NAT-PMP implementation.

struct portmap_log_alert: alert
{
        //...
        int map_type;
        std::string msg;
};

file_error_alert

If the storage fails to read or write files that it needs access to, this alert is generated and the torrent is paused.

file is the path to the file that was accessed when the error occurred.

error is the error code describing the error.

struct file_error_alert: torrent_alert
{
        // ...
        std::string file;
        error_code error;
};

torrent_error_alert

This is posted whenever a torrent is transitioned into the error state.

struct torrent_error_alert: torrent_alert
{
        // ...
        error_code error;
};

The error specifies which error the torrent encountered.

file_renamed_alert

This is posted as a response to a torrent_handle::rename_file call, if the rename operation succeeds.

struct file_renamed_alert: torrent_alert
{
        // ...
        std::string name;
        int index;
};

The index member refers to the index of the file that was renamed, name is the new name of the file.

file_rename_failed_alert

This is posted as a response to a torrent_handle::rename_file call, if the rename operation failed.

struct file_rename_failed_alert: torrent_alert
{
        // ...
        int index;
        error_code error;
};

The index member refers to the index of the file that was supposed to be renamed, error is the error code returned from the filesystem.

tracker_announce_alert

This alert is generated each time a tracker announce is sent (or attempted to be sent). There are no extra data members in this alert. The url can be found in the base class however.

struct tracker_announce_alert: tracker_alert
{
        // ...
        int event;
};

Event specifies what event was sent to the tracker. It is defined as:

0. None
1. Completed
2. Started
3. Stopped

tracker_error_alert

This alert is generated on tracker time outs, premature disconnects, invalid response or a HTTP response other than "200 OK". From the alert you can get the handle to the torrent the tracker belongs to.

The times_in_row member says how many times in a row this tracker has failed. status_code is the code returned from the HTTP server. 401 means the tracker needs authentication, 404 means not found etc. If the tracker timed out, the code will be set to 0.

struct tracker_error_alert: tracker_alert
{
        // ...
        int times_in_row;
        int status_code;
};

tracker_reply_alert

This alert is only for informational purpose. It is generated when a tracker announce succeeds. It is generated regardless what kind of tracker was used, be it UDP, HTTP or the DHT.

struct tracker_reply_alert: tracker_alert
{
        // ...
        int num_peers;
};

The num_peers tells how many peers the tracker returned in this response. This is not expected to be more thant the num_want settings. These are not necessarily all new peers, some of them may already be connected.

tracker_warning_alert

This alert is triggered if the tracker reply contains a warning field. Usually this means that the tracker announce was successful, but the tracker has a message to the client. The msg string in the alert contains the warning message from the tracker.

struct tracker_warning_alert: tracker_alert
{
        // ...
        std::string msg;
};

scrape_reply_alert

This alert is generated when a scrape request succeeds. incomplete and complete is the data returned in the scrape response. These numbers may be -1 if the reponse was malformed.

struct scrape_reply_alert: tracker_alert
{
        // ...
        int incomplete;
        int complete;
};

scrape_failed_alert

If a scrape request fails, this alert is generated. This might be due to the tracker timing out, refusing connection or returning an http response code indicating an error. msg contains a message describing the error.

struct scrape_failed_alert: tracker_alert
{
        // ...
        std::string msg;
};

url_seed_alert

This alert is generated when a HTTP seed name lookup fails.

It contains url to the HTTP seed that failed along with an error message.

struct url_seed_alert: torrent_alert
{
        // ...
        std::string url;
};

hash_failed_alert

This alert is generated when a finished piece fails its hash check. You can get the handle to the torrent which got the failed piece and the index of the piece itself from the alert.

struct hash_failed_alert: torrent_alert
{
        // ...
        int piece_index;
};

peer_alert

The peer alert is a base class for alerts that refer to a specific peer. It includes all the information to identify the peer. i.e. ip and peer-id.

struct peer_alert: torrent_alert
{
        // ...
        tcp::endpoint ip;
        peer_id pid;
};

peer_connect_alert

This alert is posted every time an outgoing peer connect attempts succeeds.

struct peer_connect_alert: peer_alert
{
        // ...
};

peer_ban_alert

This alert is generated when a peer is banned because it has sent too many corrupt pieces to us. ip is the endpoint to the peer that was banned.

struct peer_ban_alert: peer_alert
{
        // ...
};

peer_snubbed_alert

This alert is generated when a peer is snubbed, when it stops sending data when we request it.

struct peer_snubbed_alert: peer_alert
{
        // ...
};

peer_unsnubbed_alert

This alert is generated when a peer is unsnubbed. Essentially when it was snubbed for stalling sending data, and now it started sending data again.

struct peer_unsnubbed_alert: peer_alert
{
        // ...
};

peer_error_alert

This alert is generated when a peer sends invalid data over the peer-peer protocol. The peer will be disconnected, but you get its ip address from the alert, to identify it.

The error_code tells you what error caused this alert.

struct peer_error_alert: peer_alert
{
        // ...
        error_code error;
};

peer_connected_alert

This alert is generated when a peer is connected.

struct peer_connected_alert: peer_alert
{
        // ...
};

peer_disconnected_alert

This alert is generated when a peer is disconnected for any reason (other than the ones covered by peer_error_alert).

The error_code tells you what error caused peer to disconnect.

struct peer_disconnected_alert: peer_alert
{
        // ...
        error_code error;
};

invalid_request_alert

This is a debug alert that is generated by an incoming invalid piece request. ip is the address of the peer and the request is the actual incoming request from the peer.

struct invalid_request_alert: peer_alert
{
        // ...
        peer_request request;
};


struct peer_request
{
        int piece;
        int start;
        int length;
        bool operator==(peer_request const& r) const;
};

The peer_request contains the values the client sent in its request message. piece is the index of the piece it want data from, start is the offset within the piece where the data should be read, and length is the amount of data it wants.

request_dropped_alert

This alert is generated when a peer rejects or ignores a piece request.

struct request_dropped_alert: peer_alert
{
        // ...
        int block_index;
        int piece_index;
};

block_timeout_alert

This alert is generated when a block request times out.

struct block_timeout_alert: peer_alert
{
        // ...
        int block_index;
        int piece_index;
};

block_finished_alert

This alert is generated when a block request receives a response.

struct block_finished_alert: peer_alert
{
        // ...
        int block_index;
        int piece_index;
};

lsd_peer_alert

This alert is generated when we receive a local service discovery message from a peer for a torrent we're currently participating in.

struct lsd_peer_alert: peer_alert
{
        // ...
};

file_completed_alert

This is posted whenever an individual file completes its download. i.e. All pieces overlapping this file have passed their hash check.

struct file_completed_alert: torrent_alert
{
        // ...
        int index;
};

The index member refers to the index of the file that completed.

block_downloading_alert

This alert is generated when a block request is sent to a peer.

struct block_downloading_alert: peer_alert
{
        // ...
        int block_index;
        int piece_index;
};

unwanted_block_alert

This alert is generated when a block is received that was not requested or whose request timed out.

struct unwanted_block_alert: peer_alert
{
        // ...
        int block_index;
        int piece_index;
};

torrent_delete_failed_alert

This alert is generated when a request to delete the files of a torrent fails.

The error_code tells you why it failed.

struct torrent_delete_failed_alert: torrent_alert
{
        // ...
        error_code error;
};

torrent_deleted_alert

This alert is generated when a request to delete the files of a torrent complete.

The info_hash is the info-hash of the torrent that was just deleted. Most of the time the torrent_handle in the torrent_alert will be invalid by the time this alert arrives, since the torrent is being deleted. The info_hash member is hence the main way of identifying which torrent just completed the delete.

This alert is posted in the storage_notification category, and that bit needs to be set in the alert mask.

struct torrent_deleted_alert: torrent_alert
{
        // ...
        sha1_hash info_hash;
};

torrent_finished_alert

This alert is generated when a torrent switches from being a downloader to a seed. It will only be generated once per torrent. It contains a torrent_handle to the torrent in question.

There are no additional data members in this alert.

performance_alert

This alert is generated when a limit is reached that might have a negative impact on upload or download rate performance.

struct performance_alert: torrent_alert
{
        // ...

        enum performance_warning_t
        {
                outstanding_disk_buffer_limit_reached,
                outstanding_request_limit_reached,
                upload_limit_too_low,
                download_limit_too_low,
                send_buffer_watermark_too_low,
                too_many_optimistic_unchoke_slots,
                too_high_disk_queue_limit,
                too_few_outgoing_ports
        };

        performance_warning_t warning_code;
};

outstanding_disk_buffer_limit_reached

This warning means that the number of bytes queued to be written to disk exceeds the max disk byte queue setting (session_settings::max_queued_disk_bytes). This might restrict the download rate, by not queuing up enough write jobs to the disk I/O thread. When this alert is posted, peer connections are temporarily stopped from downloading, until the queued disk bytes have fallen below the limit again. Unless your max_queued_disk_bytes setting is already high, you might want to increase it to get better performance.

outstanding_request_limit_reached

This is posted when libtorrent would like to send more requests to a peer, but it's limited by session_settings::max_out_request_queue. The queue length libtorrent is trying to achieve is determined by the download rate and the assumed round-trip-time (session_settings::request_queue_time). The assumed rount-trip-time is not limited to just the network RTT, but also the remote disk access time and message handling time. It defaults to 3 seconds. The target number of outstanding requests is set to fill the bandwidth-delay product (assumed RTT times download rate divided by number of bytes per request). When this alert is posted, there is a risk that the number of outstanding requests is too low and limits the download rate. You might want to increase the max_out_request_queue setting.

upload_limit_too_low

This warning is posted when the amount of TCP/IP overhead is greater than the upload rate limit. When this happens, the TCP/IP overhead is caused by a much faster download rate, triggering TCP ACK packets. These packets eat into the rate limit specified to libtorrent. When the overhead traffic is greater than the rate limit, libtorrent will not be able to send any actual payload, such as piece requests. This means the download rate will suffer, and new requests can be sent again. There will be an equilibrium where the download rate, on average, is about 20 times the upload rate limit. If you want to maximize the download rate, increase the upload rate limit above 5% of your download capacity.

download_limit_too_low

This is the same warning as upload_limit_too_low but referring to the download limit instead of upload. This suggests that your download rate limit is mcuh lower than your upload capacity. Your upload rate will suffer. To maximize upload rate, make sure your download rate limit is above 5% of your upload capacity.

send_buffer_watermark_too_low

We're stalled on the disk. We want to write to the socket, and we can write but our send buffer is empty, waiting to be refilled from the disk. This either means the disk is slower than the network connection or that our send buffer watermark is too small, because we can send it all before the disk gets back to us. The number of bytes that we keep outstanding, requested from the disk, is calculated as follows:

min(512, max(upload_rate * send_buffer_watermark_factor / 100, send_buffer_watermark))

If you receive this alert, you migth want to either increase your send_buffer_watermark or send_buffer_watermark_factor.

too_many_optimistic_unchoke_slots

If the half (or more) of all upload slots are set as optimistic unchoke slots, this warning is issued. You probably want more regular (rate based) unchoke slots.

too_high_disk_queue_limit

If the disk write queue ever grows larger than half of the cache size, this warning is posted. The disk write queue eats into the total disk cache and leaves very little left for the actual cache. This causes the disk cache to oscillate in evicting large portions of the cache before allowing peers to download any more, onto the disk write queue. Either lower max_queued_disk_bytes or increase cache_size.

too_few_outgoing_ports

This is generated if outgoing peer connections are failing because of address in use errors, indicating that session_settings::outgoing_ports is set and is too small of a range. Consider not using the outgoing_ports setting at all, or widen the range to include more ports.

state_changed_alert

Generated whenever a torrent changes its state.

struct state_changed_alert: torrent_alert
{
        // ...

        torrent_status::state_t state;
        torrent_status::state_t prev_state;
};

state is the new state of the torrent. prev_state is the previous state.

metadata_failed_alert

This alert is generated when the metadata has been completely received and the info-hash failed to match it. i.e. the metadata that was received was corrupt. libtorrent will automatically retry to fetch it in this case. This is only relevant when running a torrent-less download, with the metadata extension provided by libtorrent.

There are no additional data members in this alert.

metadata_received_alert

This alert is generated when the metadata has been completely received and the torrent can start downloading. It is not generated on torrents that are started with metadata, but only those that needs to download it from peers (when utilizing the libtorrent extension).

There are no additional data members in this alert.

Typically, when receiving this alert, you would want to save the torrent file in order to load it back up again when the session is restarted. Here's an example snippet of code to do that:

torrent_handle h = alert->handle();
if (h.is_valid()) {
        boost::intrusive_ptr<torrent_info const> ti = h.torrent_file();
        create_torrent ct(*ti);
        entry te = ct.generate();
        std::vector<char> buffer;
        bencode(std::back_inserter(buffer), te);
        FILE* f = fopen((to_hex(ti->info_hash().to_string()) + ".torrent").c_str(), "wb+");
        if (f) {
                fwrite(&buffer[0], 1, buffer.size(), f);
                fclose(f);
        }
}

fastresume_rejected_alert

This alert is generated when a fastresume file has been passed to add_torrent but the files on disk did not match the fastresume file. The error_code explains the reason why the resume file was rejected.

struct fastresume_rejected_alert: torrent_alert
{
        // ...
        error_code error;
};

peer_blocked_alert

This alert is posted when an incoming peer connection, or a peer that's about to be added to our peer list, is blocked for some reason. This could be any of:

• the IP filter
• i2p mixed mode restrictions (a normal peer is not allowed on an i2p swarm)
• the port filter
• the peer has a low port and no_connect_privileged_ports is enabled
• the protocol of the peer is blocked (uTP/TCP blocking)

The ip member is the address that was blocked.

struct peer_blocked_alert: torrent_alert
{
        // ...
        address ip;
};

storage_moved_alert

The storage_moved_alert is generated when all the disk IO has completed and the files have been moved, as an effect of a call to torrent_handle::move_storage. This is useful to synchronize with the actual disk. The path member is the new path of the storage.

struct storage_moved_alert: torrent_alert
{
        // ...
        std::string path;
};

storage_moved_failed_alert

The storage_moved_failed_alert is generated when an attempt to move the storage (via torrent_handle::move_storage()) fails.

struct storage_moved_failed_alert: torrent_alert
{
        // ...
        error_code error;
};

torrent_paused_alert

This alert is generated as a response to a torrent_handle::pause request. It is generated once all disk IO is complete and the files in the torrent have been closed. This is useful for synchronizing with the disk.

There are no additional data members in this alert.

torrent_resumed_alert

This alert is generated as a response to a torrent_handle::resume request. It is generated when a torrent goes from a paused state to an active state.

There are no additional data members in this alert.

save_resume_data_alert

This alert is generated as a response to a torrent_handle::save_resume_data request. It is generated once the disk IO thread is done writing the state for this torrent. The resume_data member points to the resume data.

struct save_resume_data_alert: torrent_alert
{
        // ...
        boost::shared_ptr<entry> resume_data;
};

save_resume_data_failed_alert

This alert is generated instead of save_resume_data_alert if there was an error generating the resume data. error describes what went wrong.

struct save_resume_data_failed_alert: torrent_alert
{
        // ...
        error_code error;
};

stats_alert

This alert is posted approximately once every second, and it contains byte counters of most statistics that's tracked for torrents. Each active torrent posts these alerts regularly.

struct stats_alert: torrent_alert
{
        // ...
        enum stats_channel
        {
                upload_payload,
                upload_protocol,
                upload_ip_protocol,
                upload_dht_protocol,
                upload_tracker_protocol,
                download_payload,
                download_protocol,
                download_ip_protocol,
                download_dht_protocol,
                download_tracker_protocol,
                num_channels
        };

        int transferred[num_channels];
        int interval;
};

transferred this is an array of samples. The enum describes what each sample is a measurement of. All of these are raw, and not smoothing is performed.

interval the number of milliseconds during which these stats were collected. This is typically just above 1000, but if CPU is limited, it may be higher than that.

cache_flushed_alert

This alert is posted when the disk cache has been flushed for a specific torrent as a result of a call to flush_cache(). This alert belongs to the storage_notification category, which must be enabled to let this alert through. The alert is also posted when removing a torrent from the session, once the outstanding cache flush is complete and the torrent does no longer have any files open.

struct flush_cached_alert: torrent_alert
{
        // ...
};

torrent_need_cert_alert

This is always posted for SSL torrents. This is a reminder to the client that the torrent won't work unless torrent_handle::set_ssl_certificate() is called with a valid certificate. Valid certificates MUST be signed by the SSL certificate in the .torrent file.

struct torrent_need_cert_alert: tracker_alert
{
        // ...
};

dht_announce_alert

This alert is generated when a DHT node announces to an info-hash on our DHT node. It belongs to the dht_notification category.

struct dht_announce_alert: alert
{
        // ...
        address ip;
        int port;
        sha1_hash info_hash;
};

dht_get_peers_alert

This alert is generated when a DHT node sends a get_peers message to our DHT node. It belongs to the dht_notification category.

struct dht_get_peers_alert: alert
{
        // ...
        sha1_hash info_hash;
};

dht_reply_alert

This alert is generated each time the DHT receives peers from a node. num_peers is the number of peers we received in this packet. Typically these packets are received from multiple DHT nodes, and so the alerts are typically generated a few at a time.

struct dht_reply_alert: tracker_alert
{
        // ...
        int num_peers;
};

dht_bootstrap_alert

This alert is posted when the initial DHT bootstrap is done. There's no any other relevant information associated with this alert.

struct dht_bootstrap_alert: alert
{
        // ...
};

anonymous_mode_alert

This alert is posted when a bittorrent feature is blocked because of the anonymous mode. For instance, if the tracker proxy is not set up, no trackers will be used, because trackers can only be used through proxies when in anonymous mode.

struct anonymous_mode_alert: tracker_alert
{
        // ...
        enum kind_t
        {
                tracker_not_anonymous = 1
        };
        int kind;
        std::string str;
};

kind specifies what error this is, it's one of:

tracker_not_anonymous means that there's no proxy set up for tracker communication and the tracker will not be contacted. The tracker which this failed for is specified in the str member.

rss_alert

This alert is posted on RSS feed events such as start of RSS feed updates, successful completed updates and errors during updates.

This alert is only posted if the rss_notifications category is enabled in the alert mask.

struct rss_alert: alert
{
        // ..
        virtual std::string message() const;

        enum state_t
        {
                state_updating, state_updated, state_error
        };

        feed_handle handle;
        std::string url;
        int state;
        error_code error;
};

handle is the handle to the feed which generated this alert.

url is a short cut to access the url of the feed, without having to call get_settings().

state is one of:

rss_alert::state_updating

An update of this feed was just initiated, it will either succeed or fail soon.

rss_alert::state_updated

The feed just completed a successful update, there may be new items in it. If you're adding torrents manually, you may want to request the feed status of the feed and look through the items vector.

rss_akert::state_error

An error just occurred. See the error field for information on what went wrong.

error is an error code used for when an error occurs on the feed.

rss_item_alert

This alert is posted every time a new RSS item (i.e. torrent) is received from an RSS feed.

It is only posted if the rss_notifications category is enabled in the alert mask.

struct rss_alert : alert
{
        // ...
        virtual std::string message() const;

        feed_handle handle;
        feed_item item;
};

incoming_connection_alert

The incoming connection alert is posted every time we successfully accept an incoming connection, through any mean. The most straigh-forward ways of accepting incoming connections are through the TCP listen socket and the UDP listen socket for uTP sockets. However, connections may also be accepted ofer a Socks5 or i2p listen socket, or via a torrent specific listen socket for SSL torrents.

struct incoming_connection_alert: alert
{
        // ...
        virtual std::string message() const;

        int socket_type;
        tcp::endpoint ip;
};

socket_type tells you what kind of socket the connection was accepted as:

value	type
0	none (no socket instantiated)
1	TCP
2	Socks5
3	HTTP
4	uTP
5	i2p
6	SSL/TCP
7	SSL/Socks5
8	HTTPS (SSL/HTTP)
9	SSL/uTP

ip is the IP address and port the connection came from.

state_update_alert

This alert is only posted when requested by the user, by calling `post_torrent_updates()`_ on the session. It contains the torrent status of all torrents that changed since last time this message was posted. Its category is status_notification, but it's not subject to filtering, since it's only manually posted anyway.

struct state_update_alert: alert
{
        // ...
        std::vector<torrent_status> status;
};

status contains the torrent status of all torrents that changed since last time this message was posted. Note that you can map a torrent status to a specific torrent via its handle member. The receiving end is suggested to have all torrents sorted by the torrent_handle or hashed by it, for efficient updates.

torrent_update_alert

When a torrent changes its info-hash, this alert is posted. This only happens in very specific cases. For instance, when a torrent is downloaded from a URL, the true info hash is not known immediately. First the .torrent file must be downloaded and parsed.

Once this download completes, the torrent_update_alert is posted to notify the client of the info-hash changing.

struct torrent_update_alert: torrent_alert
{
        // ...
        sha1_hash old_ih;
        sha1_hash new_ih;
};

old_ih and new_ih are the previous and new info-hash for the torrent, respectively.

libtorrent_exception

struct libtorrent_exception: std::exception
{
        libtorrent_exception(error_code const& s);
        virtual const char* what() const throw();
        virtual ~libtorrent_exception() throw() {}
        boost::system::error_code error() const;
};

translating error codes

The error_code::message() function will typically return a localized error string, for system errors. That is, errors that belong to the generic or system category.

Errors that belong to the libtorrent error category are not localized however, they are only available in english. In order to translate libtorrent errors, compare the error category of the error_code object against libtorrent::get_libtorrent_category(), and if matches, you know the error code refers to the list above. You can provide your own mapping from error code to string, which is localized. In this case, you cannot rely on error_code::message() to generate your strings.

The numeric values of the errors are part of the API and will stay the same, although new error codes may be appended at the end.

Here's a simple example of how to translate error codes:

std::string error_code_to_string(boost::system::error_code const& ec)
{
        if (ec.category() != libtorrent::get_libtorrent_category())
        {
                return ec.message();
        }
        // the error is a libtorrent error

        int code = ec.value();
        static const char const* swedish[] =
        {
                "inget fel",
                "en fil i torrenten kolliderar med en fil fran en annan torrent",
                "hash check misslyckades",
                "torrent filen ar inte en dictionary",
                "'info'-nyckeln saknas eller ar korrupt i torrentfilen",
                "'info'-faltet ar inte en dictionary",
                "'piece length' faltet saknas eller ar korrupt i torrentfilen",
                "torrentfilen saknar namnfaltet",
                "ogiltigt namn i torrentfilen (kan vara en attack)",
                // ... more strings here
        };

        // use the default error string in case we don't have it
        // in our translated list
        if (code < 0 || code >= sizeof(swedish)/sizeof(swedish[0]))
                return ec.message();

        return swedish[code];
}

struct storage_interface
{
        virtual bool initialize(bool allocate_files) = 0;
        virtual bool has_any_file() = 0;
        virtual void hint_read(int slot, int offset, int len);
        virtual int readv(file::iovec_t const* bufs, int slot, int offset, int num_bufs) = 0;
        virtual int writev(file::iovec_t const* bufs, int slot, int offset, int num_bufs) = 0;
        virtual int sparse_end(int start) const;
        virtual bool move_storage(fs::path save_path) = 0;
        virtual bool verify_resume_data(lazy_entry const& rd, error_code& error) = 0;
        virtual bool write_resume_data(entry& rd) const = 0;
        virtual bool move_slot(int src_slot, int dst_slot) = 0;
        virtual bool swap_slots(int slot1, int slot2) = 0;
        virtual bool swap_slots3(int slot1, int slot2, int slot3) = 0;
        virtual bool rename_file(int file, std::string const& new_name) = 0;
        virtual bool release_files() = 0;
        virtual bool delete_files() = 0;
        virtual void finalize_file(int index) {}
        virtual ~storage_interface() {}

        // non virtual functions

        disk_buffer_pool* disk_pool();
        void set_error(std::string const& file, error_code const& ec) const;
        error_code const& error() const;
        std::string const& error_file() const;
        void clear_error();
};

initialize()

bool initialize(bool allocate_files) = 0;

This function is called when the storage is to be initialized. The default storage will create directories and empty files at this point. If allocate_files is true, it will also ftruncate all files to their target size.

Returning true indicates an error occurred.

has_any_file()

virtual bool has_any_file() = 0;

This function is called when first checking (or re-checking) the storage for a torrent. It should return true if any of the files that is used in this storage exists on disk. If so, the storage will be checked for existing pieces before starting the download.

hint_read()

void hint_read(int slot, int offset, int len);

This function is called when a read job is queued. It gives the storage wrapper an opportunity to hint the operating system about this coming read. For instance, the storage may call posix_fadvise(POSIX_FADV_WILLNEED) or fcntl(F_RDADVISE).

readv() writev()

int readv(file::iovec_t const* buf, int slot, int offset, int num_bufs) = 0;
int write(const char* buf, int slot, int offset, int size) = 0;

These functions should read or write the data in or to the given slot at the given offset. It should read or write num_bufs buffers sequentially, where the size of each buffer is specified in the buffer array bufs. The file::iovec_t type has the following members:

struct iovec_t
{
        void* iov_base;
        size_t iov_len;
};

The return value is the number of bytes actually read or written, or -1 on failure. If it returns -1, the error code is expected to be set to

Every buffer in bufs can be assumed to be page aligned and be of a page aligned size, except for the last buffer of the torrent. The allocated buffer can be assumed to fit a fully page aligned number of bytes though. This is useful when reading and writing the last piece of a file in unbuffered mode.

The offset is aligned to 16 kiB boundries most of the time, but there are rare exceptions when it's not. Specifically if the read cache is disabled/or full and a client requests unaligned data, or the file itself is not aligned in the torrent. Most clients request aligned data.

sparse_end()

int sparse_end(int start) const;

This function is optional. It is supposed to return the first piece, starting at start that is fully contained within a data-region on disk (i.e. non-sparse region). The purpose of this is to skip parts of files that can be known to contain zeros when checking files.

move_storage()

bool move_storage(fs::path save_path) = 0;

This function should move all the files belonging to the storage to the new save_path. The default storage moves the single file or the directory of the torrent.

Before moving the files, any open file handles may have to be closed, like release_files().

Returning false indicates an error occurred.

verify_resume_data()

bool verify_resume_data(lazy_entry const& rd, error_code& error) = 0;

This function should verify the resume data rd with the files on disk. If the resume data seems to be up-to-date, return true. If not, set error to a description of what mismatched and return false.

The default storage may compare file sizes and time stamps of the files.

Returning false indicates an error occurred.

write_resume_data()

bool write_resume_data(entry& rd) const = 0;

This function should fill in resume data, the current state of the storage, in rd. The default storage adds file timestamps and sizes.

Returning true indicates an error occurred.

move_slot()

bool move_slot(int src_slot, int dst_slot) = 0;

This function should copy or move the data in slot src_slot to the slot dst_slot. This is only used in compact mode.

If the storage caches slots, this could be implemented more efficient than reading and writing the data.

Returning true indicates an error occurred.

swap_slots()

bool swap_slots(int slot1, int slot2) = 0;

This function should swap the data in slot1 and slot2. The default storage uses a scratch buffer to read the data into, then moving the other slot and finally writing back the temporary slot's data

This is only used in compact mode.

Returning true indicates an error occurred.

swap_slots3()

bool swap_slots3(int slot1, int slot2, int slot3) = 0;

This function should do a 3-way swap, or shift of the slots. slot1 should move to slot2, which should be moved to slot3 which in turn should be moved to slot1.

This is only used in compact mode.

Returning true indicates an error occurred.

rename_file()

bool rename_file(int file, std::string const& new_name) = 0;

Rename file with index file to the thame new_name. If there is an error, true should be returned.

release_files()

bool release_files() = 0;

This function should release all the file handles that it keeps open to files belonging to this storage. The default implementation just calls file_pool::release_files(this).

Returning true indicates an error occurred.

delete_files()

bool delete_files() = 0;

This function should delete all files and directories belonging to this storage.

Returning true indicates an error occurred.

The disk_buffer_pool is used to allocate and free disk buffers. It has the following members:

struct disk_buffer_pool : boost::noncopyable
{
        char* allocate_buffer(char const* category);
        void free_buffer(char* buf);

        char* allocate_buffers(int blocks, char const* category);
        void free_buffers(char* buf, int blocks);

        int block_size() const { return m_block_size; }

        void release_memory();
};

finalize_file()

virtual void finalize_file(int index);

This function is called each time a file is completely downloaded. The storage implementation can perform last operations on a file. The file will not be opened for writing after this.

index is the index of the file that completed.

On windows the default storage implementation clears the sparse file flag on the specified file.

example

This is an example storage implementation that stores all pieces in a std::map, i.e. in RAM. It's not necessarily very useful in practice, but illustrates the basics of implementing a custom storage.

struct temp_storage : storage_interface
{
        temp_storage(file_storage const& fs) : m_files(fs) {}
        virtual bool initialize(bool allocate_files) { return false; }
        virtual bool has_any_file() { return false; }
        virtual int read(char* buf, int slot, int offset, int size)
        {
                std::map<int, std::vector<char> >::const_iterator i = m_file_data.find(slot);
                if (i == m_file_data.end()) return 0;
                int available = i->second.size() - offset;
                if (available <= 0) return 0;
                if (available > size) available = size;
                memcpy(buf, &i->second[offset], available);
                return available;
        }
        virtual int write(const char* buf, int slot, int offset, int size)
        {
                std::vector<char>& data = m_file_data[slot];
                if (data.size() < offset + size) data.resize(offset + size);
                std::memcpy(&data[offset], buf, size);
                return size;
        }
        virtual bool rename_file(int file, std::string const& new_name)
        { assert(false); return false; }
        virtual bool move_storage(std::string const& save_path) { return false; }
        virtual bool verify_resume_data(lazy_entry const& rd, error_code& error) { return false; }
        virtual bool write_resume_data(entry& rd) const { return false; }
        virtual bool move_slot(int src_slot, int dst_slot) { assert(false); return false; }
        virtual bool swap_slots(int slot1, int slot2) { assert(false); return false; }
        virtual bool swap_slots3(int slot1, int slot2, int slot3) { assert(false); return false; }
        virtual size_type physical_offset(int slot, int offset)
        { return slot * m_files.piece_length() + offset; };
        virtual sha1_hash hash_for_slot(int slot, partial_hash& ph, int piece_size)
        {
                int left = piece_size - ph.offset;
                assert(left >= 0);
                if (left > 0)
                {
                        std::vector<char>& data = m_file_data[slot];
                        // if there are padding files, those blocks will be considered
                        // completed even though they haven't been written to the storage.
                        // in this case, just extend the piece buffer to its full size
                        // and fill it with zeroes.
                        if (data.size() < piece_size) data.resize(piece_size, 0);
                        ph.h.update(&data[ph.offset], left);
                }
                return ph.h.final();
        }
        virtual bool release_files() { return false; }
        virtual bool delete_files() { return false; }

        std::map<int, std::vector<char> > m_file_data;
        file_storage m_files;
};

storage_interface* temp_storage_constructor(
        file_storage const& fs, file_storage const* mapped
        , std::string const& path, file_pool& fp
        , std::vector<boost::uint8_t> const& prio)
{
        return new temp_storage(fs);
}

downloading

Torrents that are currently being downloaded or incomplete (with bytes still to download) are queued. The torrents in the front of the queue are started to be actively downloaded and the rest are ordered with regards to their queue position. Any newly added torrent is placed at the end of the queue. Once a torrent is removed or turns into a seed, its queue position is -1 and all torrents that used to be after it in the queue, decreases their position in order to fill the gap.

The queue positions are always in a sequence without any gaps.

Lower queue position means closer to the front of the queue, and will be started sooner than torrents with higher queue positions.

To query a torrent for its position in the queue, or change its position, see: queue_position() queue_position_up() queue_position_down() queue_position_top() queue_position_bottom().

seeding

Auto managed seeding torrents are rotated, so that all of them are allocated a fair amount of seeding. Torrents with fewer completed seed cycles are prioritized for seeding. A seed cycle is completed when a torrent meets either the share ratio limit (uploaded bytes / downloaded bytes), the share time ratio (time seeding / time downloaing) or seed time limit (time seeded).

The relevant settings to control these limits are share_ratio_limit, seed_time_ratio_limit and seed_time_limit in session_settings.

file format

The file format is a bencoded dictionary containing the following fields:

file-format	string: "libtorrent resume file"
file-version	integer: 1
info-hash	string, the info hash of the torrent this data is saved for.
blocks per piece	integer, the number of blocks per piece. Must be: piece_size / (16 * 1024). Clamped to be within the range [1, 256]. It is the number of blocks per (normal sized) piece. Usually each block is 16 * 1024 bytes in size. But if piece size is greater than 4 megabytes, the block size will increase.
pieces	A string with piece flags, one character per piece. Bit 1 means we have that piece. Bit 2 means we have verified that this piece is correct. This only applies when the torrent is in seed_mode.
slots	list of integers. The list maps slots to piece indices. It tells which piece is on which slot. If piece index is -2 it means it is free, that there's no piece there. If it is -1, means the slot isn't allocated on disk yet. The pieces have to meet the following requirement: If there's a slot at the position of the piece index, the piece must be located in that slot.
total_uploaded	integer. The number of bytes that have been uploaded in total for this torrent.
total_downloaded	integer. The number of bytes that have been downloaded in total for this torrent.
active_time	integer. The number of seconds this torrent has been active. i.e. not paused.
seeding_time	integer. The number of seconds this torrent has been active and seeding.
num_seeds	integer. An estimate of the number of seeds on this torrent when the resume data was saved. This is scrape data or based on the peer list if scrape data is unavailable.
num_downloaders	integer. An estimate of the number of downloaders on this torrent when the resume data was last saved. This is used as an initial estimate until we acquire up-to-date scrape info.
upload_rate_limit	integer. In case this torrent has a per-torrent upload rate limit, this is that limit. In bytes per second.
download_rate_limit	integer. The download rate limit for this torrent in case one is set, in bytes per second.
max_connections	integer. The max number of peer connections this torrent may have, if a limit is set.
max_uploads	integer. The max number of unchoked peers this torrent may have, if a limit is set.
seed_mode	integer. 1 if the torrent is in seed mode, 0 otherwise.
file_priority	list of integers. One entry per file in the torrent. Each entry is the priority of the file with the same index.
piece_priority	string of bytes. Each byte is interpreted as an integer and is the priority of that piece.
auto_managed	integer. 1 if the torrent is auto managed, otherwise 0.
sequential_download	integer. 1 if the torrent is in sequential download mode, 0 otherwise.
paused	integer. 1 if the torrent is paused, 0 otherwise.
trackers	list of lists of strings. The top level list lists all tracker tiers. Each second level list is one tier of trackers.
mapped_files	list of strings. If any file in the torrent has been renamed, this entry contains a list of all the filenames. In the same order as in the torrent file.
url-list	list of strings. List of url-seed URLs used by this torrent. The urls are expected to be properly encoded and not contain any illegal url characters.
httpseeds	list of strings. List of httpseed URLs used by this torrent. The urls are expected to be properly encoded and not contain any illegal url characters.
merkle tree	string. In case this torrent is a merkle torrent, this is a string containing the entire merkle tree, all nodes, including the root and all leaves. The tree is not necessarily complete, but complete enough to be able to send any piece that we have, indicated by the have bitmask.
peers	list of dictionaries. Each dictionary has the following layout: ip string, the ip address of the peer. This is not a binary representation of the ip address, but the string representation. It may be an IPv6 string or an IPv4 string. port integer, the listen port of the peer These are the local peers we were connected to when this fast-resume data was saved.
unfinished	list of dictionaries. Each dictionary represents an piece, and has the following layout: piece integer, the index of the piece this entry refers to. bitmask string, a binary bitmask representing the blocks that have been downloaded in this piece. adler32 The adler32 checksum of the data in the blocks specified by bitmask.
file sizes	list where each entry corresponds to a file in the file list in the metadata. Each entry has a list of two values, the first value is the size of the file in bytes, the second is the time stamp when the last time someone wrote to it. This information is used to compare with the files on disk. All the files must match exactly this information in order to consider the resume data as current. Otherwise a full re-check is issued.
allocation	The allocation mode for the storage. Can be either full or compact. If this is full, the file sizes and timestamps are disregarded. Pieces are assumed not to have moved around even if the files have been modified after the last resume data checkpoint.

sparse allocation

On filesystems that supports sparse files, this allocation mode will only use as much space as has been downloaded.

• It does not require an allocation pass on startup.
• It supports skipping files (setting prioirty to 0 to not download).
• Fast resume data will remain valid even when file time stamps are out of date.

full allocation

When a torrent is started in full allocation mode, the disk-io thread will make sure that the entire storage is allocated, and fill any gaps with zeros. This will be skipped if the filesystem supports sparse files or automatic zero filling. It will of course still check for existing pieces and fast resume data. The main drawbacks of this mode are:

• It may take longer to start the torrent, since it will need to fill the files with zeros on some systems. This delay is linearly dependent on the size of the download.
• The download may occupy unnecessary disk space between download sessions. In case sparse files are not supported.
• Disk caches usually perform extremely poorly with random access to large files and may slow down a download considerably.

The benefits of this mode are:

• Downloaded pieces are written directly to their final place in the files and the total number of disk operations will be fewer and may also play nicer to filesystems' file allocation, and reduce fragmentation.
• No risk of a download failing because of a full disk during download. Unless sparse files are being used.
• The fast resume data will be more likely to be usable, regardless of crashes or out of date data, since pieces won't move around.
• Can be used with prioritizing files to 0.

compact allocation

Note that support for compact allocation is deprecated in libttorrent, and will be removed in future versions.

The compact allocation will only allocate as much storage as it needs to keep the pieces downloaded so far. This means that pieces will be moved around to be placed at their final position in the files while downloading (to make sure the completed download has all its pieces in the correct place). So, the main drawbacks are:

• More disk operations while downloading since pieces are moved around.
• Potentially more fragmentation in the filesystem.
• Cannot be used while having files with priority 0.

The benefits though, are:

• No startup delay, since the files don't need allocating.
• The download will not use unnecessary disk space.
• Disk caches perform much better than in full allocation and raises the download speed limit imposed by the disk.
• Works well on filesystems that don't support sparse files.

The algorithm that is used when allocating pieces and slots isn't very complicated. For the interested, a description follows.

storing a piece:

1. let A be a newly downloaded piece, with index n.
2. let s be the number of slots allocated in the file we're downloading to. (the number of pieces it has room for).
3. if n >= s then allocate a new slot and put the piece there.
4. if n < s then allocate a new slot, move the data at slot n to the new slot and put A in slot n.

allocating a new slot:

1. if there's an unassigned slot (a slot that doesn't contain any piece), return that slot index.
2. append the new slot at the end of the file (or find an unused slot).
3. let i be the index of newly allocated slot
4. if we have downloaded piece index i already (to slot j) then
   1. move the data at slot j to slot i.
   2. return slot index j as the newly allocated free slot.
5. return i as the newly allocated slot.

metadata from peers

Extension name: "LT_metadata"

This extension is deprecated in favor of the more widely supported ut_metadata extension, see BEP 9. The point with this extension is that you don't have to distribute the metadata (.torrent-file) separately. The metadata can be distributed through the bittorrent swarm. The only thing you need to download such a torrent is the tracker url and the info-hash of the torrent.

It works by assuming that the initial seeder has the metadata and that the metadata will propagate through the network as more peers join.

There are three kinds of messages in the metadata extension. These packets are put as payload to the extension message. The three packets are:

• request metadata
• metadata
• don't have metadata

request metadata:

size	name	description
uint8_t	msg_type	Determines the kind of message this is 0 means 'request metadata'
uint8_t	start	The start of the metadata block that is requested. It is given in 256:ths of the total size of the metadata, since the requesting client don't know the size of the metadata.
uint8_t	size	The size of the metadata block that is requested. This is also given in 256:ths of the total size of the metadata. The size is given as size-1. That means that if this field is set 0, the request wants one 256:th of the metadata.

metadata:

size	name	description
uint8_t	msg_type	1 means 'metadata'
int32_t	total_size	The total size of the metadata, given in number of bytes.
int32_t	offset	The offset of where the metadata block in this message belongs in the final metadata. This is given in bytes.
uint8_t[]	metadata	The actual metadata block. The size of this part is given implicit by the length prefix in the bittorrent protocol packet.

Don't have metadata:

size	name	description
uint8_t	msg_type	2 means 'I don't have metadata'. This message is sent as a reply to a metadata request if the the client doesn't have any metadata.

dont_have

Extension name: "lt_dont_have"

The dont_have extension message is used to tell peers that the client no longer has a specific piece. The extension message should be advertised in the m dictionary as lt_dont_have. The message format mimics the regular HAVE bittorrent message.

Just like all extension messages, the first 2 bytes in the mssage itself are 20 (the bittorrent extension message) and the message ID assigned to this extension in the m dictionary in the handshake.

size	name	description
uint32_t	piece	index of the piece the peer no longer has.

The length of this message (including the extension message prefix) is 6 bytes, i.e. one byte longer than the normal HAVE message, because of the extension message wrapping.

HTTP seeding

There are two kinds of HTTP seeding. One with that assumes a smart (and polite) client and one that assumes a smart server. These are specified in BEP 19 and BEP 17 respectively.

libtorrent supports both. In the libtorrent source code and API, BEP 19 urls are typically referred to as url seeds and BEP 17 urls are typically referred to as HTTP seeds.

The libtorrent implementation of BEP 19 assumes that, if the URL ends with a slash ('/'), the filename should be appended to it in order to request pieces from that file. The way this works is that if the torrent is a single-file torrent, only that filename is appended. If the torrent is a multi-file torrent, the torrent's name '/' the file name is appended. This is the same directory structure that libtorrent will download torrents into.

internal representation

It is optimized by, at all times, keeping a list of pieces ordered by rarity, randomly shuffled within each rarity class. This list is organized as a single vector of contigous memory in RAM, for optimal memory locality and to eliminate heap allocations and frees when updating rarity of pieces.

Expensive events, like a peer joining or leaving, are evaluated lazily, since it's cheaper to rebuild the whole list rather than updating every single piece in it. This means as long as no blocks are picked, peers joining and leaving is no more costly than a single peer joining or leaving. Of course the special cases of peers that have all or no pieces are optimized to not require rebuilding the list.

picker strategy

The normal mode of the picker is of course rarest first, meaning pieces that few peers have are preferred to be downloaded over pieces that more peers have. This is a fundamental algorithm that is the basis of the performance of bittorrent. However, the user may set the piece picker into sequential download mode. This mode simply picks pieces sequentially, always preferring lower piece indices.

When a torrent starts out, picking the rarest pieces means increased risk that pieces won't be completed early (since there are only a few peers they can be downloaded from), leading to a delay of having any piece to offer to other peers. This lack of pieces to trade, delays the client from getting started into the normal tit-for-tat mode of bittorrent, and will result in a long ramp-up time. The heuristic to mitigate this problem is to, for the first few pieces, pick random pieces rather than rare pieces. The threshold for when to leave this initial picker mode is determined by session_settings::initial_picker_threshold.

reverse order

An orthogonal setting is reverse order, which is used for snubbed peers. Snubbed peers are peers that appear very slow, and might have timed out a piece request. The idea behind this is to make all snubbed peers more likely to be able to do download blocks from the same piece, concentrating slow peers on as few pieces as possible. The reverse order means that the most common pieces are picked, instead of the rarest pieces (or in the case of sequential download, the last pieces, intead of the first).

parole mode

Peers that have participated in a piece that failed the hash check, may be put in parole mode. This means we prefer downloading a full piece from this peer, in order to distinguish which peer is sending corrupt data. Whether to do this is or not is controlled by session_settings::use_parole_mode.

In parole mode, the piece picker prefers picking one whole piece at a time for a given peer, avoiding picking any blocks from a piece any other peer has contributed to (since that would defeat the purpose of parole mode).

prioritize partial pieces

This setting determines if partially downloaded or requested pieces should always be preferred over other pieces. The benefit of doing this is that the number of partial pieces is minimized (and hence the turn-around time for downloading a block until it can be uploaded to others is minimized). It also puts less stress on the disk cache, since fewer partial pieces need to be kept in the cache. Whether or not to enable this is controlled by session_settings::prioritize_partial_pieces.

The main benefit of not prioritizing partial pieces is that the rarest first algorithm gets to have more influence on which pieces are picked. The picker is more likely to truly pick the rarest piece, and hence improving the performance of the swarm.

This setting is turned on automatically whenever the number of partial pieces in the piece picker exceeds the number of peers we're connected to times 1.5. This is in order to keep the waste of partial pieces to a minimum, but still prefer rarest pieces.

prefer whole pieces

The prefer whole pieces setting makes the piece picker prefer picking entire pieces at a time. This is used by web connections (both http seeding standards), in order to be able to coalesce the small bittorrent requests to larger HTTP requests. This significantly improves performance when downloading over HTTP.

It is also used by peers that are downloading faster than a certain threshold. The main advantage is that these peers will better utilize the other peer's disk cache, by requesting all blocks in a single piece, from the same peer.

This threshold is controlled by session_settings::whole_pieces_threshold.

TODO: piece affinity by speed category TODO: piece priorities

+-----------------------+
| BitTorrent protocol   |
+-----------------------+
| SSL                   |
+-----------+-----------+
| TCP       | uTP       |
|           +-----------+
|           | UDP       |
+-----------+-----------+

testing

To test incoming SSL connections to an SSL torrent, one can use the following openssl command:

openssl s_client -cert <peer-certificate>.pem -key <peer-private-key>.pem -CAfile <torrent-cert>.pem -debug -connect 127.0.0.1:4433 -tls1 -servername <info-hash>

To create a root certificate, the Distinguished Name (DN) is not taken into account by bittorrent peers. You still need to specify something, but from libtorrent's point of view, it doesn't matter what it is. libtorrent only makes sure the peer certificates are signed by the correct root certificate.

One way to create the certificates is to use the CA.sh script that comes with openssl, like thisi (don't forget to enter a common Name for the certificate):

CA.sh -newca
CA.sh -newreq
CA.sh -sign

The torrent certificate is located in ./demoCA/private/demoCA/cacert.pem, this is the pem file to include in the .torrent file.

The peer's certificate is located in ./newcert.pem and the certificate's private key in ./newkey.pem.