tcp(7) Miscellaneous Information Manual tcp(7)
NAME
tcp - TCP protocol
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
DESCRIPTION
This is an implementation of the TCP protocol defined in RFC 793,
RFC 1122 and RFC 2001 with the NewReno and SACK extensions. It pro-
vides 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 gener-
ates 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 incom-
ing 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) suc-
cessfully 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 win-
dows 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 inter-
nal 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 informa-
tion.
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 ker-
nel 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_OOBIN-
LINE 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 descrip-
tor 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 in-
clude support for zero-copy sendfile(2), Explicit Congestion Notifica-
tion, 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 di-
rectory /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 corre-
sponding 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 seg-
ment
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 listen-
ing daemon cannot be tuned to accept connections faster. En-
abling 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 applica-
tion 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 applica-
tion latencies. The tcp_adv_win_scale default value of 2 im-
plies 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_CONGES-
TION 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 set-
ting 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 pack-
ets. Coalescing is done if at least one prior packet for the
flow is waiting in Qdisc queues or device transmit queue. Ap-
plications can still use the TCP_CORK socket option to obtain
optimal behavior when they know how/when to uncork their sock-
ets.
tcp_available_congestion_control (String; read-only; since Linux
2.4.20)
Show a list of the congestion-control algorithms that are regis-
tered. 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 conges-
tion-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 re-
served 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 en-
abled, 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 lin-
ear RTT fairness as well as good scalability. Under small con-
gestion windows, binary search increase provides TCP friendli-
ness.
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 be-
haves 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 con-
verge 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 ad-
ditional choices may be available depending on kernel configura-
tion. 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 af-
fected 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 intro-
duced.
tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
Enable RFC 3168, Section 6.1.1.1. fallback. When enabled, out-
going ECN-setup SYNs that time out within the normal SYN re-
transmission 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 viola-
tion 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 retransmis-
sion 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, sup-
porting 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 con-
cerning 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 possi-
bility 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 giv-
ing 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 termi-
nated 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 applica-
tion 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 la-
tency as opposed to higher throughput. It this option is dis-
abled, then higher throughput is preferred. An example of an
application where this default should be changed would be a Be-
owulf compute cluster. Since Linux 4.14, this file still ex-
ists, 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. Net-
work conditions might require you to increase the number of or-
phans 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 drop-
ping 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 mem-
ory (<= 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-ser-
vice attacks. The default value of NR_FILE*2 is adjusted de-
pending on the memory in the system. If this number is ex-
ceeded, 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 num-
ber of pages it has allocated globally is below this num-
ber.
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 al-
locate. 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 Discov-
ery. The following values may be assigned to the file:
0 Disabled
1 Disabled by default, enabled when an ICMP black hole de-
tected
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 estab-
lished in the near future can use these to set initial condi-
tions. 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 re-
ordering detection metric designed to minimize unnecessary back
off and retransmits provoked by reordering of packets on a con-
nection.
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 es-
tablished 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 imme-
diately 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 pa-
rameters are used by TCP to regulate receive buffer sizes. TCP
dynamically adjusts the size of the receive buffer from the de-
faults 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_win-
dow_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 conges-
tion window after an idle period. An idle period is defined as
the current RTO (retransmission timeout). If disabled, the con-
gestion 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 interpreta-
tion, the urgent pointer points to the last byte of urgent data.
If this option is disabled, then use the BSD-compatible inter-
pretation 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 con-
nection 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 CON-
FIG_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 recom-
mended 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 op-
tion is not recommended as the remote IP may not use monotoni-
cally 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 West-
wood+ is a sender-side-only modification of the TCP Reno proto-
col stack that optimizes the performance of TCP congestion con-
trol. 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 experi-
enced. TCP Westwood+ significantly increases fairness with re-
spect 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 nego-
tiate the use of window scaling with the other end during con-
nection setup.
tcp_wmem (since Linux 2.4)
This is a vector of 3 integers: [min, default, max]. These pa-
rameters are used by TCP to regulate send buffer sizes. TCP dy-
namically 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_win-
dow_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 for-
mula
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 set-
sockopt(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 re-
stricted to choosing one of the algorithms in tcp_allowed_con-
gestion_control (described above). Privileged processes
(CAP_NET_ADMIN) can choose from any of the available congestion-
control algorithms (see the description of tcp_available_conges-
tion_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 use-
ful 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 connec-
tion. 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 re-
turns a struct tcp_info as defined in the file /usr/in-
clude/linux/tcp.h. This option should not be used in code in-
tended 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 uti-
lization 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 pro-
cessing 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 connec-
tion 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 ex-
tended 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 fail-
ure.
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 ker-
nel 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 lis-
tener 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 com-
pletes. 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, con-
nect(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 destina-
tion), 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 nor-
mal 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 interop-
erability 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 argu-
ment 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 (EIN-
VAL) is returned. SIOCINQ is defined in <linux/sockios.h>. Al-
ternatively, 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 SIOCAT-
MARK returns true, then the next read from the socket will re-
turn 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 ap-
plication is informed of the presence of urgent data via se-
lect(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 (request-
ing 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>. Alter-
natively, 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.
ERRORS
EAFNOTSUPPORT
Passed socket address type in sin_family was not AF_INET.
EPIPE The other end closed the socket unexpectedly or a read is exe-
cuted 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.
VERSIONS
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 recy-
cling, and per-connection keepalive socket options were introduced in
Linux 2.3.
BUGS
Not all errors are documented.
IPv6 is not described.
SEE ALSO
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 algo-
rithm.
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.
Linux man-pages 6.7 2023-10-31 tcp(7)
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