libtorrent API Documentation

Author: Arvid Norberg,
Version: 1.1.1


The interface of libtorrent consists of a few classes. The main class is the session, it contains the main loop that serves all torrents.

The basic usage is as follows:

Each class and function is described in this manual, you may want to have a look at the tutorial as well.

For a description on how to create torrent files, see create_torrent.

things to keep in mind

A common problem developers are facing is torrents stopping without explanation. Here is a description on which conditions libtorrent will stop your torrents, how to find out about it and what to do about it.

Make sure to keep track of the paused state, the error state and the upload mode of your torrents. By default, torrents are auto-managed, which means libtorrent will pause them, unpause them, scrape them and take them out of upload-mode automatically.

Whenever a torrent encounters a fatal error, it will be stopped, and the torrent_status::error will describe the error that caused it. If a torrent is auto managed, it is scraped periodically and paused or resumed based on the number of downloaders per seed. This will effectively seed torrents that are in the greatest need of seeds.

If a torrent hits a disk write error, it will be put into upload mode. This means it will not download anything, but only upload. The assumption is that the write error is caused by a full disk or write permission errors. If the torrent is auto-managed, it will periodically be taken out of the upload mode, trying to write things to the disk again. This means torrent will recover from certain disk errors if the problem is resolved. If the torrent is not auto managed, you have to call set_upload_mode() to turn downloading back on again.

network primitives

There are a few typedefs in the libtorrent namespace which pulls in network types from the boost::asio namespace. These are:

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

These are declared in the <libtorrent/socket.hpp> header.

The using statements will give easy access to:


Which are the endpoint types used in libtorrent. An endpoint is an address with an associated port.

For documentation on these types, please refer to the asio documentation.


Many functions in libtorrent have two versions, one that throws exceptions on errors and one that takes an error_code reference which is filled with the error code on errors.

There is one exception class that is used for errors in libtorrent, it is based on boost.system's error_code class to carry the error code.

For more information, see libtorrent_exception and error_code_enum.

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",
                "torrentfilen 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];


libtorrent supports queuing. Queuing is a mechanism to automatically pause and resume torrents based on certain criteria. The criteria depends on the overall state the torrent is in (checking, downloading or seeding).

To opt-out of the queuing logic, make sure your torrents are added with the add_torrent_params::flag_auto_managed bit cleared. Or call torrent_handle::auto_managed(false) on the torrent handle.

The overall purpose of the queuing logic is to improve performance under arbitrary torrent downloading and seeding load. For example, if you want to download 100 torrents on a limited home connection, you improve performance by downloading them one at a time (or maybe two at a time), over downloading them all in parallel. The benefits are:

  • the average completion time of a torrent is half of what it would be if all downloaded in parallel.
  • The amount of upload capacity is more likely to reach the reciprocation rate of your peers, and is likely to improve your return on investment (download to upload ratio)
  • your disk I/O load is likely to be more local which may improve I/O performance and decrease fragmentation.

There are fundamentally 3 seaparate queues:

  • checking torrents
  • downloading torrents
  • seeding torrents

Every torrent that is not seeding has a queue number associated with it, this is its place in line to be started. See torrent_status::queue_position.

On top of the limits of each queue, there is an over arching limit, set in settings_pack::active_limit. The auto manager will never start more than this number of torrents (with one exception described below). Non-auto-managed torrents are exempt from this logic, and not counted.

At a regular interval, torrents are checked if there needs to be any re-ordering of which torrents are active and which are queued. This interval can be controlled via settings_pack::auto_manage_interval.

For queuing to work, resume data needs to be saved and restored for all torrents. See torrent_handle::save_resume_data().

queue position

The torrents in the front of the queue are started and the rest are ordered by 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 contiguous, 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: torrent_handle::queue_position(), torrent_handle::queue_position_up(), torrent_handle::queue_position_down(), torrent_handle::queue_position_top() and torrent_handle::queue_position_bottom().

checking queue

The checking queue affects torrents in the torrent_status::checking or torrent_status::allocating state that are auto-managed.

The checking queue will make sure that (of the torrents in its queue) no more than settings_pack::active_checking_limit torrents are started at any given time. Once a torrent completes checking and moves into a diffferent state, the next in line will be started for checking.

Any torrent added force-started or force-stopped (i.e. the auto managed flag is _not_ set), will not be subject to this limit and they will all check independently and in parallel.

downloading queue

Similarly to the checking queue, the downloading queue will make sure that no more than settings_pack::active_downloads torrents are in the downloading state at any given time.

The torrent_status::queue_position is used again here to determine who is next in line to be started once a downloading torrent completes or is stopped/removed.

seeding queue

The seeding queue does not use torrent_status::queue_position to determine which torrent to seed. Instead, it estimates the demand for the torrent to be seeded. A torrent with few other seeds and many downloaders is assumed to have a higher demand of more seeds than one with many seeds and few downloaders.

It limits the number of started seeds to settings_pack::active_seeds.

On top of this basic bias, seed priority can be controller by specifying a seed ratio (the upload to download ratio), a seed-time ratio (the download time to seeding time ratio) and a seed-time (the abosulte time to be seeding a torrent). Until all those targets are hit, the torrent will be prioritized for seeding.

Among torrents that have met their seed target, torrents where we don't know of any other seed take strict priority.

In order to avoid flapping, torrents that were started less than 30 minutes ago also have priority to keep seeding.

Finally, for torrents where none of the above apply, they are prioritized based on the download to seed ratio.

The relevant settings to control these limits are settings_pack::share_ratio_limit, settings_pack::seed_time_ratio_limit and settings_pack::seed_time_limit.

queuing options

In addition to simply starting and stopping torrents, the queuing mechanism can have more fine grained control of the resources used by torrents.

half-started torrents

In addition to the downloading and seeding limits, there are limits on actions torrents perform. The downloading and seeding limits control whether peers are allowed at all, and if peers are not allowed, torrents are stopped and don't do anything. If peers are allowed, torrents may:

  1. announce to trackers
  2. announce to the DHT
  3. announce to local peer discovery (local service discovery)

Each of those actions are associated with a cost and hence may need a seprarate limit. These limits are controlled by settings_pack::active_tracker_limit, settings_pack::active_dht_limit and settings_pack::active_lsd_limit respectively.

Specifically, announcing to a tracker is typically cheaper than announcing to the DHT. active_dht_limit will limit the number of torrents that are allowed to announce to the DHT. The highest priority ones will, and the lower priority ones won't. The will still be considered started though, and any incoming peers will still be accepted.

If you do not wish to impose such limits (basically, if you do not wish to have half-started torrents) make sure to set these limits to -1 (infinite).

prefer seeds

In the case where active_downloads + active_seeds > active_limit, there's an ambiguity whether the downloads should be satisfied first or the seeds. To disambiguate this case, the settings_pack::auto_manage_prefer_seeds determines whether seeds are preferred or not.

inactive torrents

Torrents that are not transferring any bytes (downloading or uploading) have a relatively low cost to be started. It's possible to exempt such torrents from the download and seed queues by setting settings_pack::dont_count_slow_torrents to true.

Since it sometimes may take a few minutes for a newly started torrent to find peers and be unchoked, or find peers that are interested in requesting data, torrents are not considered inactive immadiately. There must be an extended period of no transfers before it is considered inactive and exempt from the queuing limits.

fast resume

The fast resume mechanism is a way to remember which pieces are downloaded and where they are put between sessions. You can generate fast resume data by calling save_resume_data() on torrent_handle. You can then save this data to disk and use it when resuming the torrent. libtorrent will not check the piece hashes then, and rely on the information given in the fast-resume data. The fast-resume data also contains information about which blocks, in the unfinished pieces, were downloaded, so it will not have to start from scratch on the partially downloaded pieces.

To use the fast-resume data you simply give it to async_add_torrent() and add_torrent(), and it will skip the time consuming checks. It may have to do the checking anyway, if the fast-resume data is corrupt or doesn't fit the storage for that torrent, then it will not trust the fast-resume data and just do the checking.

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:
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.
save_path string. The save path where this torrent was saved. This is especially useful when moving torrents with move_storage() since this will be updated.
peers string. This string contains IPv4 and port pairs of peers we were connected to last session. The endpoints are in compact representation. 4 bytes IPv4 address followed by 2 bytes port. Hence, the length of this string should be divisible by 6.
banned_peers string. This string has the same format as peers but instead represent IPv4 peers that we have banned.
peers6 string. This string contains IPv6 and port pairs of peers we were connected to last session. The endpoints are in compact representation. 16 bytes IPv6 address followed by 2 bytes port. The length of this string should be divisible by 18.
banned_peers6 string. This string has the same format as peers6 but instead represent IPv6 peers that we have banned.
info If this field is present, it should be the info-dictionary of the torrent this resume data is for. Its SHA-1 hash must match the one in the info-hash field. When present, the torrent is loaded from here, meaning the torrent can be added purely from resume data (no need to load the .torrent file separately). This may have performance advantages.

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 sparse. 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.

storage allocation

There are two modes in which storage (files on disk) are allocated in libtorrent.

  1. The traditional full allocation mode, where the entire files are filled up with zeros before anything is downloaded. Files are allocated on demand, the first time anything is written to them. The main benefit of this mode is that it avoids creating heavily fragmented files.
  2. The sparse allocation, sparse files are used, and pieces are downloaded directly to where they belong. This is the recommended (and default) mode.

sparse allocation

On filesystems that supports sparse files, this allocation mode will only use as much space as has been downloaded.

The main drawback of this mode is that it may create heavily fragmented files.

  • It does not require an allocation pass on startup.

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. 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 zeroes. This delay is linear to the size of the download.
  • The download may occupy unnecessary disk space between download sessions.
  • Disk caches usually perform poorly with random access to large files and may slow down the download some.

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, once all files have been created.

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.

dynamic loading of torrent files

libtorrent has a feature that can unload idle torrents from memory. The purpose of this is to support being active on many more torrents than the RAM permits. This is useful for both embedded devices that have limited RAM and servers seeding tens of thousands of torrents.

The most significant parts of loaded torrents that use RAM are the piece hashes (20 bytes per piece) and the file list. The entire info-dictionary of the .torrent file is kept in RAM.

In order to activate the dynamic loading of torrent files, set the load function on the session. See set_load_function().

When a load function is set on the session, the dynamic load/unload feature is enabled. Torrents are kept in an LRU. Every time an operation is performed, on a torrent or from a peer, that requires the metadata of the torrent to be loaded, the torrent is bumped up in the LRU. When a torrent is paused or queued, it is demoted to the least recently used torrent in the LRU, since it's a good candidate for eviction.

To configure how many torrents are allowed to be loaded at the same time, set settings_pack::active_loaded_limit on the session.

Torrents can be exempt from being unloaded by being pinned. Pinned torrents still count against the limit, but are never considered for eviction. You can either pin a torrent when adding it, in add_torrent_params (see async_add_torrent() and add_torrent()), or after ading it with the set_pinned() function on torrent_handle.

Torrents that start out without metadata (e.g. magnet links or http downloads) are automatically pinned. This is important in order to give the client a chance to save the metadata to disk once it's received (see metadata_received_alert).

Once the metadata is saved to disk, it might make sense to unpin the torrent.

piece picker

The piece picker in libtorrent has the following features:

  • rarest first
  • sequential download
  • random pick
  • reverse order picking
  • parole mode
  • prioritize partial pieces
  • prefer whole pieces
  • piece affinity by speed category
  • piece priorities

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 settings_pack::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 settings_pack::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 setting_pack::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 the settings_pack::whole_pieces_threshold setting.

TODO: piece priorities

predictive piece announce

In order to improve performance, libtorrent supports a feature called predictive piece announce. When enabled, it will make libtorrent announce that we have pieces to peers, before we truly have them. The most important case is to announce a piece as soon as it has been downloaded and passed the hash check, but not yet been written to disk. In this case, there is a risk the piece will fail to be written to disk, in which case we won't have the piece anymore, even though we announced it to peers.

The other case is when we're very close to completing the download of a piece and assume it will pass the hash check, we can announce it to peers to make it available one round-trip sooner than otherwise. This lets libtorrent start uploading the piece to interested peers immediately when the piece complete, instead of waiting one round-trip for the peers to request it.

This makes for the implementation slightly more complicated, since piece will have more states and more complicated transitions. For instance, a piece could be:

  1. hashed but not fully written to disk
  2. fully written to disk but not hashed
  3. not fully downloaded
  4. downloaded and hash checked

Once a piece is fully downloaded, the hash check could complete before any of the write operations or it could complete after all write operations are complete.

peer classes

The peer classes feature in libtorrent allows a client to define custom groups of peers and rate limit them individually. Each such group is called a peer class. There are a few default peer classes that are always created:

  • global - all peers belong to this class, except peers on the local network
  • local peers - all peers on the local network belongs to this class TCP peers
  • tcp class - all peers connected over TCP belong to this class

The TCP peers class is used by the uTP/TCP balancing logic, if it's enabled, to throttle TCP peers. The global and local classes are used to adjust the global rate limits.

When the rate limits are adjusted for a specific torrent, a class is created implicitly for that torrent.

The default peer class IDs are defined as enums in the session class:

enum {

A peer class can be considered a more general form of lables that some clients have. Peer classes however are not just applied to torrents, but ultimately the peers.

Peer classes can be created with the create_peer_class() call (on the session object), and deleted with the delete_peer_class() call.

Peer classes are configured with the set_peer_class() get_peer_class() calls.

Custom peer classes can be assigned to torrents, with the ??? call, in which case all its peers will belong to the class. They can also be assigned based on the peer's IP address. See set_peer_class_filter() for more information.

SSL torrents

Torrents may have an SSL root (CA) certificate embedded in them. Such torrents are called SSL torrents. An SSL torrent talks to all bittorrent peers over SSL. The protocols are layered like this:


During the SSL handshake, both peers need to authenticate by providing a certificate that is signed by the CA certificate found in the .torrent file. These peer certificates are expected to be privided to peers through some other means than bittorrent. Typically by a peer generating a certificate request which is sent to the publisher of the torrent, and the publisher returning a signed certificate.

In libtorrent, set_ssl_certificate() in torrent_handle is used to tell libtorrent where to find the peer certificate and the private key for it. When an SSL torrent is loaded, the torrent_need_cert_alert is posted to remind the user to provide a certificate.

A peer connecting to an SSL torrent MUST provide the SNI TLS extension (server name indication). The server name is the hex encoded info-hash of the torrent to connect to. This is required for the client accepting the connection to know which certificate to present.

SSL connections are accepted on a separate socket from normal bittorrent connections. To pick which port the SSL socket should bind to, set settings_pack::ssl_listen to a different port. It defaults to port 4433. This setting is only taken into account when the normal listen socket is opened (i.e. just changing this setting won't necessarily close and re-open the SSL socket). To not listen on an SSL socket at all, set ssl_listen to 0.

This feature is only available if libtorrent is build with openssl support (TORRENT_USE_OPENSSL) and requires at least openSSL version 1.0, since it needs SNI support.

Peer certificates must have at least one SubjectAltName field of type dNSName. At least one of the fields must exactly match the name of the torrent. This is a byte-by-byte comparison, the UTF-8 encoding must be identical (i.e. there's no unicode normalization going on). This is the recommended way of verifying certificates for HTTPS servers according to RFC 2818. Note the difference that for torrents only dNSName fields are taken into account (not IP address fields). The most specific (i.e. last) Common Name field is also taken into account if no SubjectAltName did not match.

If any of these fields contain a single asterisk ("*"), the certificate is considered covering any torrent, allowing it to be reused for any torrent.

The purpose of matching the torrent name with the fields in the peer certificate is to allow a publisher to have a single root certificate for all torrents it distributes, and issue separate peer certificates for each torrent. A peer receiving a certificate will not necessarily be able to access all torrents published by this root certificate (only if it has a "star cert").


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 -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 script that comes with openssl, like thisi (don't forget to enter a common Name for the certificate): -newca -newreq -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.

session statistics

libtorrent provides a mechanism to query performance and statistics counters from its internals. This is primarily useful for troubleshooting of production systems and performance tuning.

The statistics consists of two fundamental types. counters and gauges. A counter is a monotonically increasing value, incremented every time some event occurs. For example, every time the network thread wakes up because a socket became readable will increment a counter. Another example is every time a socket receives n bytes, a counter is incremented by n.

Counters are the most flexible of metrics. It allows the program to sample the counter at any interval, and calculate average rates of increments to the counter. Some events may be rare and need to be sampled over a longer period in order to get userful rates, where other events may be more frequent and evenly distributed that sampling it frequently yields useful values. Counters also provides accurate overall counts. For example, converting samples of a download rate into a total transfer count is not accurate and takes more samples. Converting an increasing counter into a rate is easy and flexible.

Gauges measure the instantaneous state of some kind. This is used for metrics that are not counting events or flows, but states that can fluctuate. For example, the number of torrents that are currenly being downloaded.

It's important to know whether a value is a counter or a gauge in order to interpret it correctly. In order to query libtorrent for which counters and gauges are available, call session_stats_metrics(). This will return metadata about the values available for inspection in libtorrent. It will include whether a value is a counter or a gauge. The key information it includes is the index used to extract the actual measurements for a specific counter or gauge.

In order to take a sample, call post_session_stats() in the session object. This will result in a session_stats_alert being posted. In this alert object, there is an array of values, these values make up the sample. The value index in the stats metric indicates which index the metric's value is stored in.

The mapping between metric and value is not stable across versions of libtorrent. Always query the metrics first, to find out the index at which the value is stored, before interpreting the values array in the session_stats_alert. The mapping will not change during the runtime of your process though, it's tied to a specific libtorrent version. You only have to query the mapping once on startup (or every time is loaded, if it's done dynamically).

The available stats metrics are:

name type
peer.error_peers counter
peer.disconnected_peers counter

error_peers is the total number of peer disconnects caused by an error (not initiated by this client) and disconnected initiated by this client (disconnected_peers).

name type
peer.eof_peers counter
peer.connreset_peers counter
peer.connrefused_peers counter
peer.connaborted_peers counter
peer.notconnected_peers counter
peer.perm_peers counter
peer.buffer_peers counter
peer.unreachable_peers counter
peer.broken_pipe_peers counter
peer.addrinuse_peers counter
peer.no_access_peers counter
peer.invalid_arg_peers counter
peer.aborted_peers counter

these counters break down the peer errors into more specific categories. These errors are what the underlying transport reported (i.e. TCP or uTP)

name type
peer.piece_requests counter
peer.max_piece_requests counter
peer.invalid_piece_requests counter
peer.choked_piece_requests counter
peer.cancelled_piece_requests counter
peer.piece_rejects counter

the total number of incoming piece requests we've received followed by the number of rejected piece requests for various reasons. max_piece_requests mean we already had too many outstanding requests from this peer, so we rejected it. cancelled_piece_requests are ones where the other end explicitly asked for the piece to be rejected.

name type
peer.error_incoming_peers counter
peer.error_outgoing_peers counter

these counters break down the peer errors into whether they happen on incoming or outgoing peers.

name type
peer.error_rc4_peers counter
peer.error_encrypted_peers counter

these counters break down the peer errors into whether they happen on encrypted peers (just encrypted handshake) and rc4 peers (full stream encryption). These can indicate whether encrypted peers are more or less likely to fail

name type
peer.error_tcp_peers counter
peer.error_utp_peers counter

these counters break down the peer errors into whether they happen on uTP peers or TCP peers. these may indicate whether one protocol is more error prone

name type
peer.connect_timeouts counter
peer.uninteresting_peers counter
peer.timeout_peers counter
peer.no_memory_peers counter
peer.too_many_peers counter
peer.transport_timeout_peers counter
peer.num_banned_peers counter
peer.banned_for_hash_failure counter
peer.connection_attempts counter
peer.connection_attempt_loops counter
peer.incoming_connections counter

these counters break down the reasons to disconnect peers.

name type
peer.num_tcp_peers counter
peer.num_socks5_peers counter
peer.num_http_proxy_peers counter
peer.num_utp_peers counter
peer.num_i2p_peers counter
peer.num_ssl_peers counter
peer.num_ssl_socks5_peers counter
peer.num_ssl_http_proxy_peers counter
peer.num_ssl_utp_peers counter
peer.num_peers_half_open counter
peer.num_peers_connected counter
peer.num_peers_up_interested counter
peer.num_peers_down_interested counter
peer.num_peers_up_unchoked_all counter
peer.num_peers_up_unchoked_optimistic counter
peer.num_peers_up_unchoked counter
peer.num_peers_down_unchoked counter
peer.num_peers_up_requests counter
peer.num_peers_down_requests counter
peer.num_peers_end_game counter
peer.num_peers_up_disk counter
peer.num_peers_down_disk counter

the number of peer connections for each kind of socket. these counts include half-open (connecting) peers. num_peers_up_unchoked_all is the total number of unchoked peers, whereas num_peers_up_unchoked only are unchoked peers that count against the limit (i.e. excluding peers that are unchoked because the limit doesn't apply to them). num_peers_up_unchoked_optimistic is the number of optimistically unchoked peers.

name type
net.on_read_counter counter
net.on_write_counter counter
net.on_tick_counter counter
net.on_lsd_counter counter
net.on_lsd_peer_counter counter
net.on_udp_counter counter
net.on_accept_counter counter
net.on_disk_queue_counter counter
net.on_disk_counter counter

These counters count the number of times the network thread wakes up for each respective reason. If these counters are very large, it may indicate a performance issue, causing the network thread to wake up too ofte, wasting CPU. mitigate it by increasing buffers and limits for the specific trigger that wakes up the thread.

name type
net.sent_payload_bytes counter
net.sent_bytes counter
net.sent_ip_overhead_bytes counter
net.sent_tracker_bytes counter
net.recv_payload_bytes counter
net.recv_bytes counter
net.recv_ip_overhead_bytes counter
net.recv_tracker_bytes counter

total number of bytes sent and received by the session

name type
net.limiter_up_queue counter
net.limiter_down_queue counter

the number of sockets currently waiting for upload and download bandwidht from the rate limiter.

name type
net.limiter_up_bytes counter
net.limiter_down_bytes counter

the number of upload and download bytes waiting to be handed out from the rate limiter.

name type
net.recv_failed_bytes counter

the number of bytes downloaded that had to be discarded because they failed the hash check

name type
net.recv_redundant_bytes counter

the number of downloaded bytes that were discarded because they were downloaded multiple times (from different peers)

name type
net.has_incoming_connections counter

is false by default and set to true when the first incoming connection is established this is used to know if the client is behind NAT or not.

name type
ses.num_checking_torrents counter
ses.num_stopped_torrents counter
ses.num_upload_only_torrents counter
ses.num_downloading_torrents counter
ses.num_seeding_torrents counter
ses.num_queued_seeding_torrents counter
ses.num_queued_download_torrents counter
ses.num_error_torrents counter

these gauges count the number of torrents in different states. Each torrent only belongs to one of these states. For torrents that could belong to multiple of these, the most prominent in picked. For instance, a torrent with an error counts as an error-torrent, regardless of its other state.

name type
ses.non_filter_torrents counter

the number of torrents that don't have the IP filter applied to them.

name type
ses.num_loaded_torrents counter
ses.num_pinned_torrents counter

the number of torrents that are currently loaded

name type
ses.num_piece_passed counter
ses.num_piece_failed counter
ses.num_have_pieces counter
ses.num_total_pieces_added counter

these count the number of times a piece has passed the hash check, the number of times a piece was successfully written to disk and the number of total possible pieces added by adding torrents. e.g. when adding a torrent with 1000 piece, num_total_pieces_added is incremented by 1000.

name type
ses.torrent_evicted_counter counter

this counts the number of times a torrent has been evicted (only applies when dynamic loading of torrent files is enabled).

name type
ses.num_unchoke_slots counter

the number of allowed unchoked peers

name type
ses.num_incoming_choke counter
ses.num_incoming_unchoke counter
ses.num_incoming_interested counter
ses.num_incoming_not_interested counter
ses.num_incoming_have counter
ses.num_incoming_bitfield counter
ses.num_incoming_request counter
ses.num_incoming_piece counter
ses.num_incoming_cancel counter
ses.num_incoming_dht_port counter
ses.num_incoming_suggest counter
ses.num_incoming_have_all counter
ses.num_incoming_have_none counter
ses.num_incoming_reject counter
ses.num_incoming_allowed_fast counter
ses.num_incoming_ext_handshake counter
ses.num_incoming_pex counter
ses.num_incoming_metadata counter
ses.num_incoming_extended counter
ses.num_outgoing_choke counter
ses.num_outgoing_unchoke counter
ses.num_outgoing_interested counter
ses.num_outgoing_not_interested counter
ses.num_outgoing_have counter
ses.num_outgoing_bitfield counter
ses.num_outgoing_request counter
ses.num_outgoing_piece counter
ses.num_outgoing_cancel counter
ses.num_outgoing_dht_port counter
ses.num_outgoing_suggest counter
ses.num_outgoing_have_all counter
ses.num_outgoing_have_none counter
ses.num_outgoing_reject counter
ses.num_outgoing_allowed_fast counter
ses.num_outgoing_ext_handshake counter
ses.num_outgoing_pex counter
ses.num_outgoing_metadata counter
ses.num_outgoing_extended counter

bittorrent message counters. These counters are incremented every time a message of the corresponding type is received from or sent to a bittorrent peer.

name type
ses.waste_piece_timed_out counter
ses.waste_piece_cancelled counter
ses.waste_piece_unknown counter
ses.waste_piece_seed counter
ses.waste_piece_end_game counter
ses.waste_piece_closing counter

the number of wasted downloaded bytes by reason of the bytes being wasted.

name type
picker.piece_picker_partial_loops counter
picker.piece_picker_suggest_loops counter
picker.piece_picker_sequential_loops counter
picker.piece_picker_reverse_rare_loops counter
picker.piece_picker_rare_loops counter
picker.piece_picker_rand_start_loops counter
picker.piece_picker_rand_loops counter
picker.piece_picker_busy_loops counter

the number of pieces considered while picking pieces

name type
picker.reject_piece_picks counter
picker.unchoke_piece_picks counter
picker.incoming_redundant_piece_picks counter
picker.incoming_piece_picks counter
picker.end_game_piece_picks counter
picker.snubbed_piece_picks counter
picker.interesting_piece_picks counter
picker.hash_fail_piece_picks counter
disk.write_cache_blocks counter
disk.read_cache_blocks counter

This breaks down the piece picks into the event that triggered it

name type
disk.request_latency counter
disk.pinned_blocks counter
disk.disk_blocks_in_use counter
disk.queued_disk_jobs counter
disk.num_running_disk_jobs counter
disk.num_read_jobs counter
disk.num_write_jobs counter
disk.num_jobs counter
disk.num_writing_threads counter
disk.num_running_threads counter
disk.blocked_disk_jobs counter

the number of microseconds it takes from receiving a request from a peer until we're sending the response back on the socket.

name type
disk.queued_write_bytes counter
disk.arc_mru_size counter
disk.arc_mru_ghost_size counter
disk.arc_mfu_size counter
disk.arc_mfu_ghost_size counter
disk.arc_write_size counter
disk.arc_volatile_size counter

the number of bytes we have sent to the disk I/O thread for writing. Every time we hear back from the disk I/O thread with a completed write job, this is updated to the number of bytes the disk I/O thread is actually waiting for to be written (as opposed to bytes just hanging out in the cache)

name type
disk.num_blocks_written counter
disk.num_blocks_read counter

the number of blocks written and read from disk in total. A block is 16 kiB.

name type
disk.num_blocks_hashed counter

the total number of blocks run through SHA-1 hashing

name type
disk.num_blocks_cache_hits counter

the number of blocks read from the disk cache

name type
disk.num_write_ops counter
disk.num_read_ops counter

the number of disk I/O operation for reads and writes. One disk operation may transfer more then one block.

name type
disk.num_read_back counter

the number of blocks that had to be read back from disk in order to hash a piece (when verifying against the piece hash)

name type
disk.disk_read_time counter
disk.disk_write_time counter
disk.disk_hash_time counter
disk.disk_job_time counter

cumulative time spent in various disk jobs, as well as total for all disk jobs. Measured in microseconds

name type
disk.num_fenced_read counter
disk.num_fenced_write counter
disk.num_fenced_hash counter
disk.num_fenced_move_storage counter
disk.num_fenced_release_files counter
disk.num_fenced_delete_files counter
disk.num_fenced_check_fastresume counter
disk.num_fenced_save_resume_data counter
disk.num_fenced_rename_file counter
disk.num_fenced_stop_torrent counter
disk.num_fenced_cache_piece counter
disk.num_fenced_flush_piece counter
disk.num_fenced_flush_hashed counter
disk.num_fenced_flush_storage counter
disk.num_fenced_trim_cache counter
disk.num_fenced_file_priority counter
disk.num_fenced_load_torrent counter
disk.num_fenced_clear_piece counter
disk.num_fenced_tick_storage counter

for each kind of disk job, a counter of how many jobs of that kind are currently blocked by a disk fence

name type
dht.dht_nodes counter

The number of nodes in the DHT routing table

name type
dht.dht_node_cache counter

The number of replacement nodes in the DHT routing table

name type
dht.dht_torrents counter

the number of torrents currently tracked by our DHT node

name type
dht.dht_peers counter

the number of peers currently tracked by our DHT node

name type
dht.dht_immutable_data counter

the number of immutable data items tracked by our DHT node

name type
dht.dht_mutable_data counter

the number of mutable data items tracked by our DHT node

name type
dht.dht_allocated_observers counter

the number of RPC observers currently allocated

name type
dht.dht_messages_in counter
dht.dht_messages_out counter

the total number of DHT messages sent and received

name type
dht.dht_messages_out_dropped counter

the number of outgoing messages that failed to be sent

name type
dht.dht_bytes_in counter
dht.dht_bytes_out counter

the total number of bytes sent and received by the DHT

name type
dht.dht_ping_in counter
dht.dht_ping_out counter
dht.dht_find_node_in counter
dht.dht_find_node_out counter
dht.dht_get_peers_in counter
dht.dht_get_peers_out counter
dht.dht_announce_peer_in counter
dht.dht_announce_peer_out counter
dht.dht_get_in counter
dht.dht_get_out counter
dht.dht_put_in counter
dht.dht_put_out counter

the number of DHT messages we've sent and received by kind.

name type
dht.dht_invalid_announce counter
dht.dht_invalid_get_peers counter
dht.dht_invalid_put counter
dht.dht_invalid_get counter

the number of failed incoming DHT requests by kind of request

name type
utp.utp_packet_loss counter
utp.utp_timeout counter
utp.utp_packets_in counter
utp.utp_packets_out counter
utp.utp_fast_retransmit counter
utp.utp_packet_resend counter
utp.utp_samples_above_target counter
utp.utp_samples_below_target counter
utp.utp_payload_pkts_in counter
utp.utp_payload_pkts_out counter
utp.utp_invalid_pkts_in counter
utp.utp_redundant_pkts_in counter

uTP counters. Each counter represents the number of time each event has occurred.

name type
utp.num_utp_idle counter
utp.num_utp_syn_sent counter
utp.num_utp_connected counter
utp.num_utp_fin_sent counter
utp.num_utp_close_wait counter
utp.num_utp_deleted counter

the number of uTP sockets in each respective state

name type
sock_bufs.socket_send_size3 counter
sock_bufs.socket_send_size4 counter
sock_bufs.socket_send_size5 counter
sock_bufs.socket_send_size6 counter
sock_bufs.socket_send_size7 counter
sock_bufs.socket_send_size8 counter
sock_bufs.socket_send_size9 counter
sock_bufs.socket_send_size10 counter
sock_bufs.socket_send_size11 counter
sock_bufs.socket_send_size12 counter
sock_bufs.socket_send_size13 counter
sock_bufs.socket_send_size14 counter
sock_bufs.socket_send_size15 counter
sock_bufs.socket_send_size16 counter
sock_bufs.socket_send_size17 counter
sock_bufs.socket_send_size18 counter
sock_bufs.socket_send_size19 counter
sock_bufs.socket_send_size20 counter
sock_bufs.socket_recv_size3 counter
sock_bufs.socket_recv_size4 counter
sock_bufs.socket_recv_size5 counter
sock_bufs.socket_recv_size6 counter
sock_bufs.socket_recv_size7 counter
sock_bufs.socket_recv_size8 counter
sock_bufs.socket_recv_size9 counter
sock_bufs.socket_recv_size10 counter
sock_bufs.socket_recv_size11 counter
sock_bufs.socket_recv_size12 counter
sock_bufs.socket_recv_size13 counter
sock_bufs.socket_recv_size14 counter
sock_bufs.socket_recv_size15 counter
sock_bufs.socket_recv_size16 counter
sock_bufs.socket_recv_size17 counter
sock_bufs.socket_recv_size18 counter
sock_bufs.socket_recv_size19 counter
sock_bufs.socket_recv_size20 counter

the buffer sizes accepted by socket send and receive calls respectively. The larger the buffers are, the more efficient, because it reqire fewer system calls per byte. The size is 1 << n, where n is the number at the end of the counter name. i.e. 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576 bytes