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moved session_settings documentation into header file

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Arvid Norberg 2013-08-10 22:15:02 +00:00
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docs/manual.rst
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@ -352,922 +352,6 @@ session_settings
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_.
.. _paper: http://bittyrant.cs.washington.edu/#papers
``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.
.. warning:: setting outgoing ports will limit the ability to keep multiple
connections to the same client, even for different torrents. It is not
recommended to change this setting. Its main purpose is to use as an
escape hatch for cheap routers with QoS capability but can only classify
flows based on port numbers.
``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_.
.. _`QBSS`: http://qbone.internet2.edu/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.
exceptions
==========

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include/libtorrent/session_settings.hpp
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