258 lines
11 KiB
ReStructuredText
258 lines
11 KiB
ReStructuredText
=================
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libtorrent manual
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=================
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:Author: Arvid Norberg, arvid@rasterbar.com
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:Version: 0.15.0
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.. contents:: Table of contents
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:depth: 2
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:backlinks: none
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introduction
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============
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libtorrent is a feature complete C++ bittorrent implementation focusing
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on efficiency and scalability. It runs on embedded devices as well as
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desktops. It boasts a well documented library interface that is easy to
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use. It comes with a simple bittorrent client demonstrating the use of
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the library.
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features
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========
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libtorrent is under active development. It is an ongoing project. Its
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current state supports and includes the following features:
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extensions
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----------
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* plugin interface for implementing custom bittorrent extensions
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without having to modify libtorrent
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* supports trackerless torrents (using the Mainline kademlia DHT protocol) with
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some `DHT extensions`_. `BEP 5`_.
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* supports the bittorrent `extension protocol`_. See extensions_. `BEP 10`_.
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* supports the uTorrent metadata transfer protocol `BEP 9`_ (i.e. magnet links).
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* supports the uTorrent peer exchange protocol (PEX).
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* supports local peer discovery (multicasts for peers on the same local network)
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* multitracker extension support (supports both strict `BEP 12`_ and the
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uTorrent interpretation).
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* tracker scrapes
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* supports lt_trackers extension, to exchange trackers between peers
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* `HTTP seeding`_, as specified in `BEP 17`_ and `BEP 19`_.
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* supports the udp-tracker protocol. (`BEP 15`_).
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* supports the ``no_peer_id=1`` extension that will ease the load off trackers.
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* supports the ``compact=1`` tracker parameter.
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* super seeding/initial seeding (`BEP 16`_).
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* private torrents (`BEP 27`_).
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* support for IPv6, including `BEP 7`_ and `BEP 24`_.
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* support for merkle hash tree torrents. This makes the size of torrent files
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scale well with the size of the content.
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.. _extensions: manual.html#extensions
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.. _`http seeding`: manual.html#http-seeding
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disk management
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---------------
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* uses a separate disk I/O thread to not have the disk ever block on network or
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client interaction. (see threads_).
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* supports files > 2 gigabytes.
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* fast resume support, a way to get rid of the costly piece check at the
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start of a resumed torrent. Saves the storage state, piece_picker state
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as well as all local peers in a separate fast-resume file.
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* has an adjustable read and write disk cache for improved disk throughput.
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* queues torrents for file check, instead of checking all of them in parallel.
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* does not have any requirements on the piece order in a torrent that it
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resumes. This means it can resume a torrent downloaded by any client.
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* supports both sparse files and compact file allocation (where pieces
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are kept consolidated on disk)
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* seed mode, where the files on disk are assumed to be complete, and each
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piece's hash is verified the first time it is requested.
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.. _threads: manual.html#threads
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network
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-------
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* adjusts the length of the request queue depending on download rate.
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* serves multiple torrents on a single port and in a single thread
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* piece picking on block-level (as opposed to piece-level).
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This means it can download parts of the same piece from different peers.
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It will also prefer to download whole pieces from single peers if the
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download speed is high enough from that particular peer.
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* supports http proxies and basic proxy authentication
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* supports gzipped tracker-responses
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* can limit the upload and download bandwidth usage and the maximum number of
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unchoked peers
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* implements fair trade. User settable trade-ratio, must at least be 1:1,
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but one can choose to trade 1 for 2 or any other ratio that isn't unfair
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to the other party.
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* possibility to limit the number of connections.
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* delays have messages if there's no other outgoing traffic to the peer, and
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doesn't send have messages to peers that already has the piece. This saves
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bandwidth.
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* selective downloading. The ability to select which parts of a torrent you
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want to download.
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* ip filter to disallow ip addresses and ip ranges from connecting and
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being connected
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* NAT-PMP and UPnP support (automatic port mapping on routers that supports it)
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.. _`DHT extensions`: dht_extensions.html
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.. _`BEP 5`: http://bittorrent.org/beps/bep_0005.html
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.. _`BEP 7`: http://bittorrent.org/beps/bep_0007.html
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.. _`BEP 9`: http://bittorrent.org/beps/bep_0009.html
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.. _`BEP 10`: http://bittorrent.org/beps/bep_0010.html
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.. _`BEP 12`: http://bittorrent.org/beps/bep_0012.html
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.. _`BEP 15`: http://bittorrent.org/beps/bep_0015.html
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.. _`BEP 16`: http://bittorrent.org/beps/bep_0016.html
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.. _`BEP 17`: http://bittorrent.org/beps/bep_0017.html
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.. _`BEP 19`: http://bittorrent.org/beps/bep_0019.html
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.. _`BEP 24`: http://bittorrent.org/beps/bep_0024.html
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.. _`BEP 27`: http://bittorrent.org/beps/bep_0027.html
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.. _`extension protocol`: extension_protocol.html
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highlighted features
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====================
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disk caching
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------------
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All disk I/O in libtorrent is done asynchronously to the network thread, by the
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disk io thread. When a block is read, the disk io thread reads all subsequent
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blocks from that piece into the read cache, assuming that the peer requesting
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the block will also request more blocks from the same piece. This decreases the
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number of syscalls for reading data. It also decreases delay from seeking.
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Similarly, for write requests, blocks are cached and flushed to disk once one full
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piece is complete or the piece is the least recently updated one when more cache
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space is needed. The cache dynamically allocates space between the write and read
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cache. The write cache is strictly prioritized over the read cache.
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The cache blocks that are in used, are locked into physical memory to avoid it
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being paged out to disk. Allowing the disk cache to be paged out to disk means
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that it would become extremely inefficient to flush it, since it would have to be
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read back into physical memory only to be flushed back out to disk again.
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In order to conserve memory, and system calls, iovec file operations are
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used to flush multiple cache blocks in a single call.
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On low-memory systems, the disk cache can be disabled altogether or set to smaller
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limit, to save memory.
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network buffers
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---------------
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On CPUs with small L2 caches, copying memory can be expensive operations. It is important
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to keep copying to a minimum on such machines. This mostly applies to embedded systems.
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In order to minimize the number of times received data is copied, the receive buffer
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for payload data is received directly into a page aligned disk buffer. If the connection
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is encrypted, the buffer is decrypted in-place. The buffer is then moved into the disk
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cache without being copied. Once all the blocks for a piece have been received, or the
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cache needs to be flushed, all the blocks are passed directly to ``writev()`` to flush
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them in a single syscall. This means a single copy into user space memory, and a single
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copy back into kernel memory, as illustrated by this figure:
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.. image:: write_disk_buffers.png
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:width: 100%
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When seeding and uploading in general, unnecessary copying is avoided by caching blocks
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in aligned buffers, that are copied once into the peer's send buffer. The peer's send buffer
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is not guaranteed to be aligned, even though it is most of the time. The send buffer is
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then encrypted with the peer specific key and chained onto the ``iovec`` for sending.
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This means there is one user space copy in order to allow unaligned peer requests and
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peer-specific encryption. This is illustrated by the following figure:
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.. image:: read_disk_buffers.png
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:width: 100%
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piece picker
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------------
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The piece picker is a central component in a bittorrent implementation. The piece picker
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in libtorrent is optimized for quickly finding the rarest pieces. It keeps a list of all
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available pieces sorted by rarity, and pieces with the same rarity, shuffled. The rarest
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first mode is the dominant piece picker mode. Other modes are supported as well, and
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used by peers in specific situations.
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The piece picker allows to combine the availability of a piece with a priority. Together
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they determine the sort order of the piece list. Pieces with priority 0 will never be
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picked, which is used for the selective download feature.
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In order to have as few partially finished pieces as possible, peers have an affinity
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towards picking blocks from the same pieces as other peers in the same speed category.
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The speed category is a coarse categorization of peers based on their download rate. This
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makes slow peers pick blocks from the same piece, and fast peers pick from the same piece,
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and hence decreasing the likelihood of slow peers blocking the completion of pieces.
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The piece picker can also be set to download pieces in sequential order.
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merkle hash tree torrents
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-------------------------
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Merkle hash tree torrents is an extension that lets a torrent file only contain the
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root hash of the hash tree forming the piece hashes. The main benefit of this feature
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is that regardless of how many pieces there is in a torrent, the .torrent file will
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always be the same size. It will only grow with the number of files (since it still
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has to contain the file names).
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With regular torrents, clients have to request multiple blocks for pieces, typically
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from different peers, before the data can be verified against the piece hash. The
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larger the pieces are, the longer it will take to download a complete piece and verify
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it. Before the piece is verified, it cannot be shared with the swarm, which means the
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larger piece sizes, the slower turnaround data has when it is downloaded by peers.
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Since on average the data has to sit around, waiting, in client buffers before it has
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been verified and can be uploaded again.
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Another problem with large piece sizes is that it is harder for a client to pinpoint
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the malicious or buggy peer when a piece fails, and it will take longer to re-download
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it and take more tries before the piece succeeds the larger the pieces are.
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The piece size in regular torrents is a tradeoff between the size of the .torrent file
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itself and the piece size. Often, for files that are 4 GB, the piece size is 2 or 4 MB,
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just to avoid making the .torrent file too big.
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Merkle torrents solves these problems by removing the tradeoff between .torrent size and
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piece size. With merkle torrents, the piece size can be the minimum block size (16 kB),
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which lets peers verify every block of data received from peers, immediately. This
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gives a minimum turnaround time and completely removes the problem of identifying malicious
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peers.
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portability
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===========
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libtorrent runs on most major operating systems, including Windows,
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MacOS X, Linux, BSD and Solaris.
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It uses Boost.Thread, Boost.Filesystem, Boost.Date_time and various other
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boost libraries as well as zlib_ (shipped) and asio_ (shipped). At least version
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1.34.1 of boost is required.
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.. _zlib: http://www.zlib.org
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.. _asio: http://asio.sf.net
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libtorrent uses asio, hence it will take full advantage of high performance
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network APIs on the most popular platforms. I/O completion ports on windows,
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epoll on linux and kqueue on MacOS X and BSD.
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libtorrent has been successfully compiled and tested on:
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* Windows 2000, XP and Vista vc7.1, vc8
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* Linux x86 GCC 3.3, GCC 3.4.2, 4.x
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* Linux PPC GCC 4.1.1
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* MacOS X (darwin), (Apple's) GCC 3.3, (Apple's) GCC 4.0
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* SunOS 5.8 GCC 3.1 and Sunpro
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* Cygwin GCC 3.3.3
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Fails on:
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* GCC 2.95.4
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* msvc6
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