Ubuntu Manpage: tcp - TCP protocol

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 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.

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 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.

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
       recycling, and per-connection keepalive socket options were introduced in Linux 2.3.

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