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tcp_abort_on_overflow (Boolean; default: disabled) |
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Enable resetting connections if the listening service is too
slow and unable to keep up and accept them.
It means that if overflow occurred due
to a burst, the connection will recover. Enable this option
only if you are really sure that the listening daemon
cannot be tuned to accept connections faster. Enabling this
option can harm the clients of your server.
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tcp_adv_win_scale (integer; default: 2) |
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Count buffering overhead as bytes/2^tcp_adv_win_scale
(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
if it is <= 0.
The socket receive buffer space is shared between the
application and kernel. TCP maintains part of the buffer as
the TCP window, this is the size of the receive window
advertised to the other end. The rest of the space is used
as the "application" buffer, used to isolate the network
from scheduling and application latencies. The
tcp_adv_win_scale default value of 2 implies that the space
used for the application buffer is one fourth that of the
total.
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tcp_app_win (integer; default: 31) |
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This variable defines how many
bytes of the TCP window are reserved for buffering
overhead.
A maximum of (window/2^tcp_app_win, mss) bytes in the window
are reserved for the application buffer. A value of 0
implies that no amount is reserved.
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tcp_bic (Boolean; default: disabled) |
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Enable BIC TCP congestion control algorithm.
BIC-TCP is a sender-side only change that ensures a linear RTT
fairness under large windows while offering both scalability and
bounded TCP-friendliness. The protocol combines two schemes
called additive increase and binary search increase. When the
congestion window is large, additive increase with a large
increment ensures linear RTT fairness as well as good
scalability. Under small congestion windows, binary search
increase provides TCP friendliness.
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tcp_bic_low_window (integer; default: 14) |
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Sets the threshold window (in packets) where BIC TCP starts to
adjust the congestion window. Below this threshold BIC TCP behaves
the same as the default TCP Reno.
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tcp_bic_fast_convergence (Boolean; default: enabled) |
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Forces BIC TCP to more quickly respond to changes in congestion
window. Allows two flows sharing the same connection to converge
more rapidly.
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tcp_dsack (Boolean; default: enabled) |
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Enable RFC 2883 TCP Duplicate SACK support.
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tcp_ecn (Boolean; default: disabled) |
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Enable RFC 2884 Explicit Congestion Notification.
When enabled, connectivity to some
destinations could be affected due to older, misbehaving
routers along the path causing connections to be dropped.
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tcp_fack (Boolean; default: enabled) |
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Enable TCP Forward Acknowledgement support.
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tcp_fin_timeout (integer; default: 60) |
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This specifies how many seconds to wait for a final FIN packet before the
socket is forcibly closed. This is strictly a violation of
the TCP specification, but required to prevent
denial-of-service attacks.
In Linux 2.2, the default value was 180.
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tcp_frto (Boolean; default: disabled) |
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Enables F-RTO, an enhanced recovery algorithm for TCP retransmission
timeouts. It is particularly beneficial in wireless environments
where packet loss is typically due to random radio interference
rather than intermediate router congestion.
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tcp_keepalive_intvl (integer; default: 75) |
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The number of seconds between TCP keep-alive probes.
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tcp_keepalive_probes (integer; default: 9) |
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The maximum number of TCP keep-alive probes to send
before giving up and killing the connection if
no response is obtained from the other end.
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tcp_keepalive_time (integer; default: 7200) |
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The number of seconds a connection needs to be idle
before TCP begins sending out keep-alive probes.
Keep-alives are only sent when the
SO_KEEPALIVE socket option is enabled. The default value is 7200 seconds
(2 hours). An idle connection is terminated after
approximately an additional 11 minutes (9 probes an interval
of 75 seconds apart) when keep-alive is enabled.
Note that underlying connection tracking mechanisms and
application timeouts may be much shorter.
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tcp_low_latency (Boolean; default: disabled) |
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If enabled, the TCP stack makes decisions that prefer lower
latency as opposed to higher throughput.
It this option is disabled, then higher throughput is preferred.
An example of an application where this default should be
changed would be a Beowulf compute cluster.
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tcp_max_orphans (integer; default: see below) |
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The maximum number of orphaned (not attached to any user file
handle) TCP sockets allowed in the system. When this number
is exceeded, the orphaned connection is reset and a warning
is printed. This limit exists only to prevent simple denial-of-service
attacks. Lowering this limit is not recommended. Network
conditions might require you to increase the number of
orphans allowed, but note that each orphan can eat up to ~64K
of unswappable memory. The default initial value is set
equal to the kernel parameter NR_FILE. This initial default
is adjusted depending on the memory in the system.
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tcp_max_syn_backlog (integer; default: see below) |
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The maximum number of queued connection requests which have
still not received an acknowledgement from the connecting
client. If this number is exceeded, the kernel will begin
dropping requests. The default value of 256 is increased to
1024 when the memory present in the system is adequate or
greater (>= 128Mb), and reduced to 128 for those systems with
very low memory (<= 32Mb). It is recommended that if this
needs to be increased above 1024, TCP_SYNQ_HSIZE in
include/net/tcp.h be modified to keep
TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel be
recompiled.
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tcp_max_tw_buckets (integer; default: see below) |
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The maximum number of sockets in TIME_WAIT state allowed in
the system. This limit exists only to prevent simple denial-of-service
attacks. The default value of NR_FILE*2 is adjusted
depending on the memory in the system. If this number is
exceeded, the socket is closed and a warning is printed.
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tcp_mem |
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This is a vector of 3 integers: [low, pressure, high]. These
bounds are used by TCP to track its memory usage. The
defaults are calculated at boot time from the amount of
available memory.
(TCP can only use
low memory for this, which is limited to around 900 megabytes on 32-bit systems.
64-bit systems do not suffer this limitation.)
low - TCP doesn’t regulate its memory allocation when the number
of pages it has allocated globally is below this number.
pressure - when the amount of memory allocated by TCP
exceeds this number of pages, TCP moderates its memory
consumption. This memory pressure state is exited
once the number of pages allocated falls below
the
low mark.
high - the maximum number of pages, globally, that TCP
will allocate. This value overrides any other limits
imposed by the kernel.
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tcp_orphan_retries (integer; default: 8) |
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The maximum number of attempts made to probe the other
end of a connection which has been closed by our end.
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tcp_reordering (integer; default: 3) |
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The maximum a packet can be reordered in a TCP packet stream
without TCP assuming packet loss and going into slow start.
It is not advisable to change this number.
This is a packet reordering detection metric designed to
minimize unnecessary back off and retransmits provoked by
reordering of packets on a connection.
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tcp_retrans_collapse (Boolean; default: enabled) |
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Try to send full-sized packets during retransmit.
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tcp_retries1 (integer; default: 3) |
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The number of times TCP will attempt to retransmit a
packet on an established connection normally,
without the extra effort of getting the network
layers involved. Once we exceed this number of
retransmits, we first have the network layer
update the route if possible before each new retransmit.
The default is the RFC specified minimum of 3.
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tcp_retries2 (integer; default: 15) |
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The maximum number of times a TCP packet is retransmitted
in established state before giving up. The default
value is 15, which corresponds to a duration of
approximately between 13 to 30 minutes, depending
on the retransmission timeout. The RFC 1122 specified
minimum limit of 100 seconds is typically deemed too
short.
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tcp_rfc1337 (Boolean; default: disabled) |
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Enable TCP behaviour conformant with RFC 1337.
When disabled,
if a RST is received in TIME_WAIT state, we close
the socket immediately without waiting for the end
of the TIME_WAIT period.
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tcp_rmem |
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This is a vector of 3 integers: [min, default,
max]. These parameters are used by TCP to regulate receive
buffer sizes. TCP dynamically adjusts the size of the
receive buffer from the defaults listed below, in the range
of these sysctl variables, depending on memory available
in the system.
min - minimum size of the receive buffer used by each TCP
socket. The default value is 4K, and is lowered to
PAGE_SIZE bytes in low-memory systems. This value
is used to ensure that in memory pressure mode,
allocations below this size will still succeed. This is not
used to bound the size of the receive buffer declared
using
SO_RCVBUF on a socket.
default - the default size of the receive buffer for a TCP socket.
This value overwrites the initial default buffer size from
the generic global
net.core.rmem_default defined for all protocols. The default value is 87380
bytes, and is lowered to 43689 in low-memory systems. If
larger receive buffer sizes are desired, this value should
be increased (to affect all sockets). To employ large TCP
windows, the
net.ipv4.tcp_window_scaling must be enabled (default).
max - the maximum size of the receive buffer used by
each TCP socket. This value does not override the global
net.core.rmem_max. This is not used to limit the size of the receive buffer
declared using
SO_RCVBUF on a socket.
The default value of 87380*2 bytes is lowered to 87380
in low-memory systems.
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tcp_sack (Boolean; default: enabled) |
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Enable RFC 2018 TCP Selective Acknowledgements.
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tcp_stdurg (Boolean; default: disabled) |
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If this option is enabled, then use the RFC 1122 interpretation
of the TCP urgent-pointer field.
According to this interpretation, the urgent pointer points
to the last byte of urgent data.
If this option is disabled, then use the BSD-compatible interpretation of
the urgent pointer:
the urgent pointer points to the first byte after the urgent data.
Enabling this option may lead to interoperability problems.
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tcp_synack_retries (integer; default: 5) |
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The maximum number of times a SYN/ACK segment
for a passive TCP connection will be retransmitted.
This number should not be higher than 255.
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tcp_syncookies (Boolean) |
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Enable TCP syncookies. The kernel must be compiled with
CONFIG_SYN_COOKIES. Send out syncookies when the syn backlog queue of a socket
overflows. The syncookies feature attempts to protect a
socket from a SYN flood attack. This should be used as a
last resort, if at all. This is a violation of the TCP
protocol, and conflicts with other areas of TCP such as TCP
extensions. It can cause problems for clients and relays.
It is not recommended as a tuning mechanism for heavily
loaded servers to help with overloaded or misconfigured
conditions. For recommended alternatives see
tcp_max_syn_backlog,
tcp_synack_retries, and
tcp_abort_on_overflow.
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tcp_syn_retries (integer; default: 5) |
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The maximum number of times initial SYNs for an active TCP
connection attempt will be retransmitted. This value should
not be higher than 255. The default value is 5, which
corresponds to approximately 180 seconds.
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tcp_timestamps (Boolean; default: enabled) |
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Enable RFC 1323 TCP timestamps.
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tcp_tw_recycle (Boolean; default: disabled) |
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Enable fast recycling of TIME-WAIT sockets.
Enabling this option is not
recommended since this causes problems when working
with NAT (Network Address Translation).
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tcp_tw_reuse (Boolean; default: disabled) |
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Allow to reuse TIME-WAIT sockets for new connections when it is
safe from protocol viewpoint.
It should not be changed without advice/request of technical
experts.
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tcp_window_scaling (Boolean; default: enabled) |
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Enable RFC 1323 TCP window scaling.
This feature allows the use of a large window
(> 64K) on a TCP connection, should the other end support it.
Normally, the 16 bit window length field in the TCP header
limits the window size to less than 64K bytes. If larger
windows are desired, applications can increase the size of
their socket buffers and the window scaling option will be
employed. If
tcp_window_scaling is disabled, TCP will not negotiate the use of window
scaling with the other end during connection setup.
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tcp_vegas_cong_avoid (Boolean; default: disabled) |
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Enable TCP Vegas congestion avoidance algorithm.
TCP Vegas is a sender-side only change to TCP that anticipates
the onset of congestion by estimating the bandwidth. TCP Vegas
adjusts the sending rate by modifying the congestion
window. TCP Vegas should provide less packet loss, but it is
not as aggressive as TCP Reno.
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tcp_westwood (Boolean; default: disabled) |
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Enable TCP Westwood+ congestion control algorithm.
TCP Westwood+ is a sender-side only modification of the TCP Reno
protocol stack that optimizes the performance of TCP congestion
control. It is based on end-to-end bandwidth estimation to set
congestion window and slow start threshold after a congestion
episode. Using this estimation, TCP Westwood+ adaptively sets a
slow start threshold and a congestion window which takes into
account the bandwidth used at the time congestion is experienced.
TCP Westwood+ significantly increases fairness with respect to
TCP Reno in wired networks and throughput over wireless links.
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tcp_wmem |
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This is a vector of 3 integers: [min, default, max]. These
parameters are used by TCP to regulate send buffer sizes.
TCP dynamically adjusts the size of the send buffer from the
default values listed below, in the range of these sysctl
variables, depending on memory available.
min - minimum size of the send buffer used by each TCP socket.
The default value is 4K bytes.
This value is used to ensure that in memory pressure mode,
allocations below this size will still succeed. This is not
used to bound the size of the send buffer declared
using
SO_SNDBUF on a socket.
default - the default size of the send buffer for a TCP socket.
This value overwrites the initial default buffer size from
the generic global
net.core.wmem_default defined for all protocols. The default value is 16K bytes.
If larger send buffer sizes are desired, this value
should be increased (to affect all sockets). To employ
large TCP windows, the sysctl variable
net.ipv4.tcp_window_scaling must be enabled (default).
max - the maximum size of the send buffer used by
each TCP socket. This value does not override the global
net.core.wmem_max. This is not used to limit the size of the send buffer
declared using
SO_SNDBUF on a socket.
The default value is 128K bytes. It is lowered to 64K
depending on the memory available in the system.
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