|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | BUGS | SEE ALSO | COLOPHON |
|
|
|
tcp(7) Miscellaneous Information 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 acknowledgements.
Possible values are:
0 increase cwnd once per acknowledgement (no ABC)
1 increase cwnd once per acknowledgement of full sized
segment
2 allow increase cwnd by two if acknowledgement is of
two segments to compensate for delayed
acknowledgements.
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 Linux
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
Linux 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 Linux 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_fastopen (Bitmask; default: 0x1; since Linux 3.7)
Enables RFC 7413 Fast Open support. The flag is used as a
bitmap with the following values:
0x1 Enables client side Fast Open support
0x2 Enables server side Fast Open support
0x4 Allows client side to transmit data in SYN without
Fast Open option
0x200 Allows server side to accept SYN data without Fast
Open option
0x400 Enables Fast Open on all listeners without
TCP_FASTOPEN socket option
tcp_fastopen_key (since Linux 3.7)
Set server side RFC 7413 Fast Open key to generate Fast
Open cookie when server side Fast Open support is enabled.
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; obsolete since Linux 4.14)
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. Since Linux 4.14, this
file still exists, but its value is ignored.
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).
Before 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: 6; 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 6,
which corresponds to retrying for up to approximately 127
seconds. Before Linux 3.7, the default value was 5, which
(in conjunction with calculation based on other kernel
parameters) corresponded 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 (integer; default: 1; since Linux 2.2)
Enable TCP syncookies. The kernel must be compiled with
CONFIG_SYN_COOKIES. 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. Set to one of the following values:
0 Disable TCP syncookies.
1 Send out syncookies when the syn backlog queue of a
socket overflows.
2 (since Linux 3.12) Send out syncookies
unconditionally. This can be useful for network
testing.
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 Linux
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
Linux 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
Linux 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).
Following is a list of TCP-specific socket options. For details
of some other socket options that are also applicable for TCP
sockets, see socket(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, or buffered data may remain untransmitted
(due to zero window size) before TCP will forcibly close
the corresponding connection and return ETIMEDOUT to the
application. If the option value is specified as 0, TCP
will 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.
TCP_FASTOPEN (since Linux 3.6)
This option enables Fast Open (RFC 7413) on the listener
socket. The value specifies the maximum length of pending
SYNs (similar to the backlog argument in listen(2)). Once
enabled, the listener socket grants the TCP Fast Open
cookie on incoming SYN with TCP Fast Open option.
More importantly it accepts the data in SYN with a valid
Fast Open cookie and responds SYN-ACK acknowledging both
the data and the SYN sequence. accept(2) returns a socket
that is available for read and write when the handshake has
not completed yet. Thus the data exchange can commence
before the handshake completes. This option requires
enabling the server-side support on sysctl
net.ipv4.tcp_fastopen (see above). For TCP Fast Open
client-side support, see send(2) MSG_FASTOPEN or
TCP_FASTOPEN_CONNECT below.
TCP_FASTOPEN_CONNECT (since Linux 4.11)
This option enables an alternative way to perform Fast Open
on the active side (client). When this option is enabled,
connect(2) would behave differently depending on if a Fast
Open cookie is available for the destination.
If a cookie is not available (i.e. first contact to the
destination), connect(2) behaves as usual by sending a SYN
immediately, except the SYN would include an empty Fast
Open cookie option to solicit a cookie.
If a cookie is available, connect(2) would return 0
immediately but the SYN transmission is deferred. A
subsequent write(2) or sendmsg(2) would trigger a SYN with
data plus cookie in the Fast Open option. In other words,
the actual connect operation is deferred until data is
supplied.
Note: While this option is designed for convenience,
enabling it does change the behaviors and certain system
calls might set different errno values. With cookie
present, write(2) or sendmsg(2) must be called right after
connect(2) in order to send out SYN+data to complete 3WHS
and establish connection. Calling read(2) right after
connect(2) without write(2) will cause the blocking socket
to be blocked forever.
The application should either set TCP_FASTOPEN_CONNECT
socket option before write(2) or sendmsg(2), or call
write(2) or sendmsg(2) with MSG_FASTOPEN flag directly,
instead of both on the same connection.
Here is the typical call flow with this new option:
s = socket();
setsockopt(s, IPPROTO_TCP, TCP_FASTOPEN_CONNECT, 1, ...);
connect(s);
write(s); /* write() should always follow connect()
* in order to trigger SYN to go out. */
read(s)/write(s);
/* ... */
close(s);
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 Linux 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 Linux 2.4. Support for forward acknowledgement
(FACK), TIME_WAIT recycling, and per-connection keepalive socket
options were introduced in Linux 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)
The kernel source file Documentation/networking/ip-sysctl.txt.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 tcp(7)
Pages that refer to this page: getsockopt(2), listen(2), poll(2), recv(2), send(2), sendfile(2), socket(2), splice(2), sockatmark(3), lloadd.conf(5), proc_sys_net(5), services(5), slapd.conf(5), slapd-config(5), systemd.socket(5), ip(7), sock_diag(7), socket(7), udp(7), lttng-relayd(8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|