TCP(7) Linux Programmer's Manual TCP(7)
tcp - TCP protocol
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
This is an implementation of the TCP protocol defined in RFC 793, RFC 1122 and RFC 2001
with the NewReno and SACK extensions. It provides a reliable, stream-oriented, full-
duplex connection between two sockets on top of ip(7), for both v4 and v6 versions. TCP
guarantees that the data arrives in order and retransmits lost packets. It generates and
checks a per-packet checksum to catch transmission errors. TCP does not preserve record
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 (> 64K) 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 setsock-
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 connec-
tions, 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.1-2001-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 SIOCAT-
MARK ioctl described below), otherwise it can be received only when the MSG_OOB flag is
set for recv(2) or recvmsg(2).
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.
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 sup-
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") mean-
ing that the corresponding option is enabled, and a zero value ("false") meaning that the
option is disabled.
tcp_abc (Integer; default: 0; since Linux 2.6.15)
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 acknowl-
edgments. Possible values are:
0 increase cwnd once per acknowledgment (no ABC)
1 increase cwnd once per acknowledgment of full sized segment
2 allow increase cwnd by two if acknowledgment is of two segments to compensate
for delayed acknowledgments.
tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
Enable resetting connections if the listening service is too slow and unable to
keep up and accept them. It means that if overflow occurred due to a burst, the
connection will recover. Enable this option only if you are really sure that the
listening daemon cannot be tuned to accept connections faster. Enabling this
option can harm the clients of your server.
tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
Count buffering overhead as bytes/2^tcp_adv_win_scale, if tcp_adv_win_scale is
greater than 0; or bytes-bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less
than or equal to zero.
The socket receive buffer space is shared between the application and kernel. TCP
maintains part of the buffer as the TCP window, this is the size of the receive
window advertised to the other end. The rest of the space is used as the "applica-
tion" buffer, used to isolate the network from scheduling and application laten-
cies. 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 pro-
cesses (see the description of the TCP_CONGESTION socket option). 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_available_congestion_control (String; read-only; since Linux 2.4.20)
Show a list of the congestion-control algorithms that are registered. 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
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 dis-
covery (MTU probing). If MTU probing is enabled, this is the initial MSS used by
tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
Enable BIC TCP congestion control algorithm. BIC-TCP is a sender-side only change
that ensures a linear RTT fairness under large windows while offering both scala-
bility 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_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
Set the threshold window (in packets) where BIC TCP starts to adjust the congestion
window. Below this threshold BIC TCP behaves the same as the default TCP Reno.
tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6 to 2.6.13)
Force BIC TCP to more quickly respond to changes in congestion window. Allows two
flows sharing the same connection to converge more rapidly.
tcp_congestion_control (String; default: see text; since Linux 2.4.13)
Set the default congestion-control algorithm to be used for new connections. The
algorithm "reno" is always available, but additional choices may be available
depending on kernel configuration. The default value for this file is set as part
of kernel configuration.
tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
Lower limit, in bytes, of the size of socket reads that will be offloaded to a DMA
copy engine, if one is present in the system and the kernel was configured with the
tcp_dsack (Boolean; default: enabled; since Linux 2.4)
Enable RFC 2883 TCP Duplicate SACK support.
tcp_ecn (Boolean; default: disabled; since Linux 2.4)
Enable RFC 2884 Explicit Congestion Notification. When enabled, connectivity to
some destinations could be affected due to older, misbehaving routers along the
path causing connections to be dropped.
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
tcp_frto (integer; default: 0; 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 con-
gestion. See RFC 4138 for more details.
This file can have one of the following values:
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.
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 con-
gestion 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 immedi-
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
tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
If enabled, the TCP stack makes decisions that prefer lower latency as opposed to
higher throughput. It this option is disabled, then higher throughput is pre-
ferred. An example of an application where this default should be changed would be
a Beowulf compute cluster.
tcp_max_orphans (integer; default: see below; since Linux 2.4)
The maximum number of orphaned (not attached to any user file handle) TCP sockets
allowed in the system. When this number is exceeded, the orphaned connection is
reset and a warning is printed. This limit exists only to prevent simple denial-
of-service attacks. Lowering this limit is not recommended. Network conditions
might require you to increase the number of orphans allowed, but note that each
orphan can eat up to ~64K 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 (>= 128Mb), and reduced to
128 for those systems with very low memory (<= 32Mb). It is recommended that if
this needs to be increased above 1024, TCP_SYNQ_HSIZE in include/net/tcp.h be modi-
fied to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel be recompiled.
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) 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:
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 con-
nection 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 con-
nection 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 4K,
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(4MB, tcp_mem*PAGE_SIZE/128))
(On Linux 2.4, the default is 87380*2 bytes, lowered to 87380 in low-mem-
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 time-
out). If disabled, the congestion window will not be timed out after an idle
tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
If this option is enabled, then use the RFC 1122 interpretation of the TCP urgent-
pointer field. According to this interpretation, the urgent pointer points to the
last byte of urgent data. If this option is disabled, then use the BSD-compatible
interpretation of the urgent pointer: the urgent pointer points to the first byte
after the urgent data. Enabling this option may lead to interoperability problems.
tcp_syn_retries (integer; default: 5; since Linux 2.2)
The maximum number of times initial SYNs for an active TCP connection attempt will
be retransmitted. This value should not be higher than 255. The default value is
5, which corresponds to approximately 180 seconds.
tcp_synack_retries (integer; default: 5; since Linux 2.2)
The maximum number of times a SYN/ACK segment for a passive TCP connection will be
retransmitted. This number should not be higher than 255.
tcp_syncookies (Boolean; since Linux 2.2)
Enable TCP syncookies. The kernel must be compiled with CONFIG_SYN_COOKIES. Send
out syncookies when the syn backlog queue of a socket overflows. The syncookies
feature attempts to protect a socket from a SYN flood attack. This should be used
as a last resort, if at all. This is a violation of the TCP protocol, and con-
flicts 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_over-
tcp_timestamps (Boolean; default: enabled; since Linux 2.2)
Enable RFC 1323 TCP timestamps.
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; since Linux 2.4)
Enable fast recycling of TIME_WAIT sockets. Enabling this option is not recom-
mended since this causes problems when working with NAT (Network Address Transla-
tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
Allow to reuse TIME_WAIT sockets for new connections when it is safe from protocol
viewpoint. It should not be changed without advice/request of technical experts.
tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
Enable TCP Vegas congestion avoidance algorithm. TCP Vegas is a sender-side only
change to TCP that anticipates the onset of congestion by estimating the bandwidth.
TCP Vegas adjusts the sending rate by modifying the congestion window. TCP Vegas
should provide less packet loss, but it is not as aggressive as TCP Reno.
tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
Enable TCP Westwood+ congestion control algorithm. TCP Westwood+ is a sender-side
only modification of the TCP Reno protocol stack that optimizes the performance of
TCP congestion control. It is based on end-to-end bandwidth estimation to set con-
gestion 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 expe-
rienced. 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
(> 64K) 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 64K
bytes. 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_scal-
ing 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, depend-
ing 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 4K
bytes.) This value is used to ensure that in memory pressure mode, allo-
cations 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 over-
writes the initial default buffer size from the generic global
/proc/sys/net/core/wmem_default defined for all protocols. The default
value is 16K bytes. If larger send buffer sizes are desired, this value
should be increased (to affect all sockets). To employ large TCP win-
dows, the /proc/sys/net/ipv4/tcp_window_scaling must be set to a nonzero
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(4MB, tcp_mem*PAGE_SIZE/128))
(On Linux 2.4, the default value is 128K bytes, lowered 64K depending on
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
To set or get a TCP socket option, call getsockopt(2) to read or setsockopt(2) to write
the option with the option level argument set to IPPROTO_TCP. Unless otherwise noted,
optval is a pointer to an int. In addition, most IPPROTO_IP socket options are valid on
TCP sockets. For more information see ip(7).
TCP_CONGESTION (since Linux 2.6.13)
Get or set the congestion-control algorithm for this socket. The optval argument
is a pointer to a character-string buffer.
For getsockopt() *optlen specifies the amount of space available in the buffer
pointed to by optval, which should be at least 16 bytes (defined by the kernel-
internal constant TCP_CA_NAME_MAX). On return, the buffer pointed to by optval is
set to a null-terminated string containing the name of the congestion-control algo-
rithm for this socket, and *optlen is set to the minimum of its original value and
TCP_CA_NAME_MAX. If the value passed in *optlen is too small, then the string
returned in *optval is silently truncated, and no terminating null byte is added.
If an empty string is returned, then the socket is using the default congestion-
control algorithm, determined as described under tcp_congestion_control above.
For setsockopt() optlen specifies the length of the congestion-control algorithm
name contained in the buffer pointed to by optval; this length need not include any
terminating null byte. The algorithm "reno" is always permitted; other algorithms
may be available, depending on kernel configuration. Possible errors from setsock-
opt() include: algorithm not found/available (ENOENT); setting this algorithm
requires the CAP_NET_ADMIN capability (EPERM); and failure getting kernel module
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 connec-
tion. 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 over-
ride 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.
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.
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 net-
work. 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_WINDOW_CLAMP (since Linux 2.4)
Bound the size of the advertised window to this value. The kernel imposes a mini-
mum size of SOCK_MIN_RCVBUF/2. This option should not be used in code intended to
TCP provides limited support for out-of-band data, in the form of (a single byte of)
urgent data. In Linux this means if the other end sends newer out-of-band data the older
urgent data is inserted as normal data into the stream (even when SO_OOBINLINE is not
set). This differs from BSD-based stacks.
Linux uses the BSD compatible interpretation of the urgent pointer field by default. This
violates RFC 1122, but is required for interoperability with other stacks. It can be
changed via /proc/sys/net/ipv4/tcp_stdurg.
It is possible to peek at out-of-band data using the recv(2) MSG_PEEK flag.
Since version 2.4, Linux supports the use of MSG_TRUNC in the flags argument of recv(2)
(and recvmsg(2)). This flag causes the received bytes of data to be discarded, rather
than passed back in a caller-supplied buffer. Since Linux 2.4.4, MSG_PEEK also has this
effect when used in conjunction with MSG_OOB to receive out-of-band data.
The following ioctl(2) calls return information in value. The correct syntax is:
error = ioctl(tcp_socket, ioctl_type, &value);
ioctl_type is one of the following:
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>.
Returns true (i.e., value is nonzero) if the inbound data stream is at the urgent
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.
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
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.
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
The other end didn't acknowledge retransmitted data after some time.
Any errors defined for ip(7) or the generic socket layer may also be returned for TCP.
Support for Explicit Congestion Notification, zero-copy sendfile(2), reordering support
and some SACK extensions (DSACK) were introduced in 2.4. Support for forward acknowledge-
ment (FACK), TIME_WAIT recycling, and per-connection keepalive socket options were intro-
duced in 2.3.
Not all errors are documented.
IPv6 is not described.
accept(2), bind(2), connect(2), getsockopt(2), listen(2), recvmsg(2), sendfile(2),
sendmsg(2), socket(2), ip(7), socket(7)
RFC 793 for the TCP specification.
RFC 1122 for the TCP requirements and a description of the Nagle algorithm.
RFC 1323 for TCP timestamp and window scaling options.
RFC 1337 for a description of TIME_WAIT assassination hazards.
RFC 3168 for a description of Explicit Congestion Notification.
RFC 2581 for TCP congestion control algorithms.
RFC 2018 and RFC 2883 for SACK and extensions to SACK.
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2013-06-21 TCP(7)