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TCP(7) Linux Programmer's Manual TCP(7)
tcp - TCP protocol
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
This is an implementation of the TCP protocol defined in RFC 793,
RFC 1122 and RFC 2001 with the NewReno and SACK extensions. It
provides a reliable, stream-oriented, full-duplex connection between
two sockets on top of ip(7), for both v4 and v6 versions. TCP
guarantees that the data arrives in order and retransmits lost
packets. It generates and checks a per-packet checksum to catch
transmission errors. TCP does not preserve record boundaries.
A newly created TCP socket has no remote or local address and is not
fully specified. To create an outgoing TCP connection use connect(2)
to establish a connection to another TCP socket. To receive new
incoming connections, first bind(2) the socket to a local address and
port and then call listen(2) to put the socket into the listening
state. After that a new socket for each incoming connection can be
accepted using accept(2). A socket which has had accept(2) or
connect(2) successfully called on it is fully specified and may
transmit data. Data cannot be transmitted on listening or not yet
connected sockets.
Linux supports RFC 1323 TCP high performance extensions. These
include Protection Against Wrapped Sequence Numbers (PAWS), Window
Scaling and Timestamps. Window scaling allows the use of large (>
64 kB) TCP windows in order to support links with high latency or
bandwidth. To make use of them, the send and receive buffer sizes
must be increased. They can be set globally with the
/proc/sys/net/ipv4/tcp_wmem and /proc/sys/net/ipv4/tcp_rmem files, or
on individual sockets by using the SO_SNDBUF and SO_RCVBUF socket
options with the setsockopt(2) call.
The maximum sizes for socket buffers declared via the SO_SNDBUF and
SO_RCVBUF mechanisms are limited by the values in the
/proc/sys/net/core/rmem_max and /proc/sys/net/core/wmem_max files.
Note that TCP actually allocates twice the size of the buffer
requested in the setsockopt(2) call, and so a succeeding
getsockopt(2) call will not return the same size of buffer as
requested in the setsockopt(2) call. TCP uses the extra space for
administrative purposes and internal kernel structures, and the /proc
file values reflect the larger sizes compared to the actual TCP
windows. On individual connections, the socket buffer size must be
set prior to the listen(2) or connect(2) calls in order to have it
take effect. See socket(7) for more information.
TCP supports urgent data. Urgent data is used to signal the receiver
that some important message is part of the data stream and that it
should be processed as soon as possible. To send urgent data specify
the MSG_OOB option to send(2). When urgent data is received, the
kernel sends a SIGURG signal to the process or process group that has
been set as the socket "owner" using the SIOCSPGRP or FIOSETOWN
ioctls (or the POSIX.1-specified fcntl(2) F_SETOWN operation). When
the SO_OOBINLINE socket option is enabled, urgent data is put into
the normal data stream (a program can test for its location using the
SIOCATMARK ioctl described below), otherwise it can be received only
when the MSG_OOB flag is set for recv(2) or recvmsg(2).
When out-of-band data is present, select(2) indicates the file
descriptor as having an exceptional condition and poll(2) indicates a
POLLPRI event.
Linux 2.4 introduced a number of changes for improved throughput and
scaling, as well as enhanced functionality. Some of these features
include support for zero-copy sendfile(2), Explicit Congestion
Notification, new management of TIME_WAIT sockets, keep-alive socket
options and support for Duplicate SACK extensions.
Address formats
TCP is built on top of IP (see ip(7)). The address formats defined
by ip(7) apply to TCP. TCP supports point-to-point communication
only; broadcasting and multicasting are not supported.
/proc interfaces
System-wide TCP parameter settings can be accessed by files in the
directory /proc/sys/net/ipv4/. In addition, most IP /proc interfaces
also apply to TCP; see ip(7). Variables described as Boolean take an
integer value, with a nonzero value ("true") meaning that the
corresponding option is enabled, and a zero value ("false") meaning
that the option is disabled.
tcp_abc (Integer; default: 0; Linux 2.6.15 to Linux 3.8)
Control the Appropriate Byte Count (ABC), defined in RFC 3465.
ABC is a way of increasing the congestion window (cwnd) more
slowly in response to partial acknowledgments. Possible
values are:
0 increase cwnd once per acknowledgment (no ABC)
1 increase cwnd once per acknowledgment of full sized segment
2 allow increase cwnd by two if acknowledgment is of two
segments to compensate for delayed acknowledgments.
tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
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.
tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
Count buffering overhead as bytes/2^tcp_adv_win_scale, if
tcp_adv_win_scale is greater than 0; or bytes-
bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less
than or equal to zero.
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.
tcp_allowed_congestion_control (String; default: see text; since
Linux 2.4.20)
Show/set the congestion control algorithm choices available to
unprivileged processes (see the description of the
TCP_CONGESTION socket option). The items in the list are
separated by white space and terminated by a newline
character. The list is a subset of those listed in
tcp_available_congestion_control. The default value for this
list is "reno" plus the default setting of
tcp_congestion_control.
tcp_autocorking (Boolean; default: enabled; since Linux 3.14)
If this option is enabled, the kernel tries to coalesce small
writes (from consecutive write(2) and sendmsg(2) calls) as
much as possible, in order to decrease the total number of
sent packets. Coalescing is done if at least one prior packet
for the flow is waiting in Qdisc queues or device transmit
queue. Applications can still use the TCP_CORK socket option
to obtain optimal behavior when they know how/when to uncork
their sockets.
tcp_available_congestion_control (String; read-only; since Linux
2.4.20)
Show a list of the congestion-control algorithms that are
registered. The items in the list are separated by white
space and terminated by a newline character. This list is a
limiting set for the list in tcp_allowed_congestion_control.
More congestion-control algorithms may be available as
modules, but not loaded.
tcp_app_win (integer; default: 31; since Linux 2.4)
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.
tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
The initial value of search_low to be used by the
packetization layer Path MTU discovery (MTU probing). If MTU
probing is enabled, this is the initial MSS used by the
connection.
tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
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.
tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to
2.6.13)
Set 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.
tcp_bic_fast_convergence (Boolean; default: enabled; Linux
2.4.27/2.6.6 to 2.6.13)
Force BIC TCP to more quickly respond to changes in congestion
window. Allows two flows sharing the same connection to
converge more rapidly.
tcp_congestion_control (String; default: see text; since Linux
2.4.13)
Set the default congestion-control algorithm to be used for
new connections. The algorithm "reno" is always available,
but additional choices may be available depending on kernel
configuration. The default value for this file is set as part
of kernel configuration.
tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
Lower limit, in bytes, of the size of socket reads that will
be offloaded to a DMA copy engine, if one is present in the
system and the kernel was configured with the CONFIG_NET_DMA
option.
tcp_dsack (Boolean; default: enabled; since Linux 2.4)
Enable RFC 2883 TCP Duplicate SACK support.
tcp_ecn (Integer; default: see below; since Linux 2.4)
Enable RFC 3168 Explicit Congestion Notification.
This file can have one of the following values:
0 Disable ECN. Neither initiate nor accept ECN. This
was the default up to and including Linux 2.6.30.
1 Enable ECN when requested by incoming connections and
also request ECN on outgoing connection attempts.
2 Enable ECN when requested by incoming connections, but
do not request ECN on outgoing connections. This value
is supported, and is the default, since Linux 2.6.31.
When enabled, connectivity to some destinations could be
affected due to older, misbehaving middle boxes along the
path, causing connections to be dropped. However, to
facilitate and encourage deployment with option 1, and to work
around such buggy equipment, the tcp_ecn_fallback option has
been introduced.
tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
Enable RFC 3168, Section 6.1.1.1. fallback. When enabled,
outgoing ECN-setup SYNs that time out within the normal SYN
retransmission timeout will be resent with CWR and ECE
cleared.
tcp_fack (Boolean; default: enabled; since Linux 2.2)
Enable TCP Forward Acknowledgement support.
tcp_fin_timeout (integer; default: 60; since Linux 2.2)
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.
tcp_frto (integer; default: see below; since Linux 2.4.21/2.6)
Enable F-RTO, an enhanced recovery algorithm for TCP
retransmission timeouts (RTOs). It is particularly beneficial
in wireless environments where packet loss is typically due to
random radio interference rather than intermediate router
congestion. See RFC 4138 for more details.
This file can have one of the following values:
0 Disabled. This was the default up to and including Linux
2.6.23.
1 The basic version F-RTO algorithm is enabled.
2 Enable SACK-enhanced F-RTO if flow uses SACK. The basic
version can be used also when SACK is in use though in that
case scenario(s) exists where F-RTO interacts badly with
the packet counting of the SACK-enabled TCP flow. This
value is the default since Linux 2.6.24.
Before Linux 2.6.22, this parameter was a Boolean value,
supporting just values 0 and 1 above.
tcp_frto_response (integer; default: 0; since Linux 2.6.22)
When F-RTO has detected that a TCP retransmission timeout was
spurious (i.e., the timeout would have been avoided had TCP
set a longer retransmission timeout), TCP has several options
concerning what to do next. Possible values are:
0 Rate halving based; a smooth and conservative response,
results in halved congestion window (cwnd) and slow-start
threshold (ssthresh) after one RTT.
1 Very conservative response; not recommended because even
though being valid, it interacts poorly with the rest of
Linux TCP; halves cwnd and ssthresh immediately.
2 Aggressive response; undoes congestion-control measures
that are now known to be unnecessary (ignoring the
possibility of a lost retransmission that would require TCP
to be more cautious); cwnd and ssthresh are restored to the
values prior to timeout.
tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
The number of seconds between TCP keep-alive probes.
tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
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.
tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
The number of seconds a connection needs to be idle before TCP
begins sending out keep-alive probes. Keep-alives are sent
only 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.
tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
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.
tcp_max_orphans (integer; default: see below; since Linux 2.4)
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
~64 kB 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.
tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
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 (>= 128 MB), and reduced to 128 for those systems with
very low memory (<= 32 MB).
Prior to Linux 2.6.20, it was recommended that if this needed
to be increased above 1024, the size of the SYNACK hash table
(TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to
keep
TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
and the kernel should be recompiled. In Linux 2.6.20, the
fixed sized TCP_SYNQ_HSIZE was removed in favor of dynamic
sizing.
tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
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.
tcp_moderate_rcvbuf (Boolean; default: enabled; since Linux
2.4.17/2.6.7)
If enabled, TCP performs receive buffer auto-tuning,
attempting to automatically size the buffer (no greater than
tcp_rmem[2]) to match the size required by the path for full
throughput.
tcp_mem (since Linux 2.4)
This is a vector of 3 integers: [low, pressure, high]. These
bounds, measured in units of the system page size, 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.
tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
This parameter controls TCP Packetization-Layer Path MTU
Discovery. The following values may be assigned to the file:
0 Disabled
1 Disabled by default, enabled when an ICMP black hole
detected
2 Always enabled, use initial MSS of tcp_base_mss.
tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
By default, TCP saves various connection metrics in the route
cache when the connection closes, so that connections
established in the near future can use these to set initial
conditions. Usually, this increases overall performance, but
it may sometimes cause performance degradation. If
tcp_no_metrics_save is enabled, TCP will not cache metrics on
closing connections.
tcp_orphan_retries (integer; default: 8; since Linux 2.4)
The maximum number of attempts made to probe the other end of
a connection which has been closed by our end.
tcp_reordering (integer; default: 3; since Linux 2.4)
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.
tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
Try to send full-sized packets during retransmit.
tcp_retries1 (integer; default: 3; since Linux 2.2)
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.
tcp_retries2 (integer; default: 15; since Linux 2.2)
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.
tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
Enable TCP behavior 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.
tcp_rmem (since Linux 2.4)
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 values,
depending on memory available in the system.
min minimum size of the receive buffer used by each TCP
socket. The default value is the system page size.
(On Linux 2.4, the default value is 4 kB, 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. (On Linux 2.4,
this will be 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 is calculated using
the formula
max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
(On Linux 2.4, the default is 87380*2 bytes, lowered
to 87380 in low-memory systems).
tcp_sack (Boolean; default: enabled; since Linux 2.2)
Enable RFC 2018 TCP Selective Acknowledgements.
tcp_slow_start_after_idle (Boolean; default: enabled; since Linux
2.6.18)
If enabled, provide RFC 2861 behavior and time out the
congestion window after an idle period. An idle period is
defined as the current RTO (retransmission timeout). If
disabled, the congestion window will not be timed out after an
idle period.
tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
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.
tcp_syn_retries (integer; default: 5; since Linux 2.2)
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.
tcp_synack_retries (integer; default: 5; since Linux 2.2)
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.
tcp_syncookies (Boolean; since Linux 2.2)
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.
tcp_timestamps (integer; default: 1; since Linux 2.2)
Set to one of the following values to enable or disable
RFC 1323 TCP timestamps:
0 Disable timestamps.
1 Enable timestamps as defined in RFC1323 and use random
offset for each connection rather than only using the
current time.
2 As for the value 1, but without random offsets. Setting
tcp_timestamps to this value is meaningful since Linux
4.10.
tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
This parameter controls what percentage of the congestion
window can be consumed by a single TCP Segmentation Offload
(TSO) frame. The setting of this parameter is a tradeoff
between burstiness and building larger TSO frames.
tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11)
Enable fast recycling of TIME_WAIT sockets. Enabling this
option is not recommended as the remote IP may not use
monotonically increasing timestamps (devices behind NAT,
devices with per-connection timestamp offsets). See RFC 1323
(PAWS) and RFC 6191.
tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
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.
tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to
2.6.13)
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.
tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to
2.6.13)
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.
tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
Enable RFC 1323 TCP window scaling. This feature allows the
use of a large window (> 64 kB) 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
64 kB. 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.
tcp_wmem (since Linux 2.4)
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 values,
depending on memory available.
min Minimum size of the send buffer used by each TCP
socket. The default value is the system page size.
(On Linux 2.4, the default value is 4 kB.) 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
/proc/sys/net/core/wmem_default defined for all
protocols. The default value is 16 kB. If larger
send buffer sizes are desired, this value should be
increased (to affect all sockets). To employ large
TCP windows, the
/proc/sys/net/ipv4/tcp_window_scaling must be set to
a nonzero value (default).
max The maximum size of the send buffer used by each TCP
socket. This value does not override the value in
/proc/sys/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
calculated using the formula
max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
(On Linux 2.4, the default value is 128 kB, lowered
64 kB depending on low-memory systems.)
tcp_workaround_signed_windows (Boolean; default: disabled; since
Linux 2.6.26)
If enabled, assume that no receipt of a window-scaling option
means that the remote TCP is broken and treats the window as a
signed quantity. If disabled, assume that the remote TCP is
not broken even if we do not receive a window scaling option
from it.
Socket options
To set or get a TCP socket option, call getsockopt(2) to read or
setsockopt(2) to write the option with the option level argument set
to IPPROTO_TCP. Unless otherwise noted, optval is a pointer to an
int. In addition, most IPPROTO_IP socket options are valid on TCP
sockets. For more information see ip(7).
TCP_CONGESTION (since Linux 2.6.13)
The argument for this option is a string. This option allows
the caller to set the TCP congestion control algorithm to be
used, on a per-socket basis. Unprivileged processes are
restricted to choosing one of the algorithms in
tcp_allowed_congestion_control (described above). Privileged
processes (CAP_NET_ADMIN) can choose from any of the available
congestion-control algorithms (see the description of
tcp_available_congestion_control above).
TCP_CORK (since Linux 2.2)
If set, don't send out partial frames. All queued partial
frames are sent when the option is cleared again. This is
useful for prepending headers before calling sendfile(2), or
for throughput optimization. As currently implemented, there
is a 200 millisecond ceiling on the time for which output is
corked by TCP_CORK. If this ceiling is reached, then queued
data is automatically transmitted. This option can be
combined with TCP_NODELAY only since Linux 2.5.71. This
option should not be used in code intended to be portable.
TCP_DEFER_ACCEPT (since Linux 2.4)
Allow a listener to be awakened only when data arrives on the
socket. Takes an integer value (seconds), this can bound the
maximum number of attempts TCP will make to complete the
connection. This option should not be used in code intended
to be portable.
TCP_INFO (since Linux 2.4)
Used to collect information about this socket. The kernel
returns a struct tcp_info as defined in the file
/usr/include/linux/tcp.h. This option should not be used in
code intended to be portable.
TCP_KEEPCNT (since Linux 2.4)
The maximum number of keepalive probes TCP should send before
dropping the connection. This option should not be used in
code intended to be portable.
TCP_KEEPIDLE (since Linux 2.4)
The time (in seconds) the connection needs to remain idle
before TCP starts sending keepalive probes, if the socket
option SO_KEEPALIVE has been set on this socket. This option
should not be used in code intended to be portable.
TCP_KEEPINTVL (since Linux 2.4)
The time (in seconds) between individual keepalive probes.
This option should not be used in code intended to be
portable.
TCP_LINGER2 (since Linux 2.4)
The lifetime of orphaned FIN_WAIT2 state sockets. This option
can be used to override the system-wide setting in the file
/proc/sys/net/ipv4/tcp_fin_timeout for this socket. This is
not to be confused with the socket(7) level option SO_LINGER.
This option should not be used in code intended to be
portable.
TCP_MAXSEG
The maximum segment size for outgoing TCP packets. In Linux
2.2 and earlier, and in Linux 2.6.28 and later, if this option
is set before connection establishment, it also changes the
MSS value announced to the other end in the initial packet.
Values greater than the (eventual) interface MTU have no
effect. TCP will also impose its minimum and maximum bounds
over the value provided.
TCP_NODELAY
If set, disable the Nagle algorithm. This means that segments
are always sent as soon as possible, even if there is only a
small amount of data. When not set, data is buffered until
there is a sufficient amount to send out, thereby avoiding the
frequent sending of small packets, which results in poor
utilization of the network. This option is overridden by
TCP_CORK; however, setting this option forces an explicit
flush of pending output, even if TCP_CORK is currently set.
TCP_QUICKACK (since Linux 2.4.4)
Enable quickack mode if set or disable quickack mode if
cleared. In quickack mode, acks are sent immediately, rather
than delayed if needed in accordance to normal TCP operation.
This flag is not permanent, it only enables a switch to or
from quickack mode. Subsequent operation of the TCP protocol
will once again enter/leave quickack mode depending on
internal protocol processing and factors such as delayed ack
timeouts occurring and data transfer. This option should not
be used in code intended to be portable.
TCP_SYNCNT (since Linux 2.4)
Set the number of SYN retransmits that TCP should send before
aborting the attempt to connect. It cannot exceed 255. This
option should not be used in code intended to be portable.
TCP_USER_TIMEOUT (since Linux 2.6.37)
This option takes an unsigned int as an argument. When the
value is greater than 0, it specifies the maximum amount of
time in milliseconds that transmitted data may remain
unacknowledged before TCP will forcibly close the
corresponding connection and return ETIMEDOUT to the
application. If the option value is specified as 0, TCP will
to use the system default.
Increasing user timeouts allows a TCP connection to survive
extended periods without end-to-end connectivity. Decreasing
user timeouts allows applications to "fail fast", if so
desired. Otherwise, failure may take up to 20 minutes with
the current system defaults in a normal WAN environment.
This option can be set during any state of a TCP connection,
but is effective only during the synchronized states of a
connection (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,
CLOSING, and LAST-ACK). Moreover, when used with the TCP
keepalive (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will
override keepalive to determine when to close a connection due
to keepalive failure.
The option has no effect on when TCP retransmits a packet, nor
when a keepalive probe is sent.
This option, like many others, will be inherited by the socket
returned by accept(2), if it was set on the listening socket.
Further details on the user timeout feature can be found in
RFC 793 and RFC 5482 ("TCP User Timeout Option").
TCP_WINDOW_CLAMP (since Linux 2.4)
Bound the size of the advertised window to this value. The
kernel imposes a minimum size of SOCK_MIN_RCVBUF/2. This
option should not be used in code intended to be portable.
Sockets API
TCP provides limited support for out-of-band data, in the form of (a
single byte of) urgent data. In Linux this means if the other end
sends newer out-of-band data the older urgent data is inserted as
normal data into the stream (even when SO_OOBINLINE is not set).
This differs from BSD-based stacks.
Linux uses the BSD compatible interpretation of the urgent pointer
field by default. This violates RFC 1122, but is required for
interoperability with other stacks. It can be changed via
/proc/sys/net/ipv4/tcp_stdurg.
It is possible to peek at out-of-band data using the recv(2) MSG_PEEK
flag.
Since version 2.4, Linux supports the use of MSG_TRUNC in the flags
argument of recv(2) (and recvmsg(2)). This flag causes the received
bytes of data to be discarded, rather than passed back in a caller-
supplied buffer. Since Linux 2.4.4, MSG_TRUNC also has this effect
when used in conjunction with MSG_OOB to receive out-of-band data.
Ioctls
The following ioctl(2) calls return information in value. The
correct syntax is:
int value;
error = ioctl(tcp_socket, ioctl_type, &value);
ioctl_type is one of the following:
SIOCINQ
Returns the amount of queued unread data in the receive
buffer. The socket must not be in LISTEN state, otherwise an
error (EINVAL) is returned. SIOCINQ is defined in
<linux/sockios.h>. Alternatively, you can use the synonymous
FIONREAD, defined in <sys/ioctl.h>.
SIOCATMARK
Returns true (i.e., value is nonzero) if the inbound data
stream is at the urgent mark.
If the SO_OOBINLINE socket option is set, and SIOCATMARK
returns true, then the next read from the socket will return
the urgent data. If the SO_OOBINLINE socket option is not
set, and SIOCATMARK returns true, then the next read from the
socket will return the bytes following the urgent data (to
actually read the urgent data requires the recv(MSG_OOB)
flag).
Note that a read never reads across the urgent mark. If an
application is informed of the presence of urgent data via
select(2) (using the exceptfds argument) or through delivery
of a SIGURG signal, then it can advance up to the mark using a
loop which repeatedly tests SIOCATMARK and performs a read
(requesting any number of bytes) as long as SIOCATMARK returns
false.
SIOCOUTQ
Returns the amount of unsent data in the socket send queue.
The socket must not be in LISTEN state, otherwise an error
(EINVAL) is returned. SIOCOUTQ is defined in
<linux/sockios.h>. Alternatively, you can use the synonymous
TIOCOUTQ, defined in <sys/ioctl.h>.
Error handling
When a network error occurs, TCP tries to resend the packet. If it
doesn't succeed after some time, either ETIMEDOUT or the last
received error on this connection is reported.
Some applications require a quicker error notification. This can be
enabled with the IPPROTO_IP level IP_RECVERR socket option. When
this option is enabled, all incoming errors are immediately passed to
the user program. Use this option with care — it makes TCP less
tolerant to routing changes and other normal network conditions.
EAFNOTSUPPORT
Passed socket address type in sin_family was not AF_INET.
EPIPE The other end closed the socket unexpectedly or a read is
executed on a shut down socket.
ETIMEDOUT
The other end didn't acknowledge retransmitted data after some
time.
Any errors defined for ip(7) or the generic socket layer may also be
returned for TCP.
Support for Explicit Congestion Notification, zero-copy sendfile(2),
reordering support and some SACK extensions (DSACK) were introduced
in 2.4. Support for forward acknowledgement (FACK), TIME_WAIT
recycling, and per-connection keepalive socket options were
introduced in 2.3.
Not all errors are documented.
IPv6 is not described.
accept(2), bind(2), connect(2), getsockopt(2), listen(2), recvmsg(2),
sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)
RFC 793 for the TCP specification.
RFC 1122 for the TCP requirements and a description of the Nagle
algorithm.
RFC 1323 for TCP timestamp and window scaling options.
RFC 1337 for a description of TIME_WAIT assassination hazards.
RFC 3168 for a description of Explicit Congestion Notification.
RFC 2581 for TCP congestion control algorithms.
RFC 2018 and RFC 2883 for SACK and extensions to SACK.
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 TCP(7)
Pages that refer to this page: getsockopt(2), listen(2), poll(2), recv(2), select_tut(2), send(2), sendfile(2), socket(2), splice(2), sockatmark(3), proc(5), services(5), slapd.conf(5), slapd-config(5), systemd.socket(5), ip(7), sock_diag(7), socket(7), udp(7), lttng-relayd(8)
Copyright and license for this manual page
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