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.

BUGS