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 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)
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
. A socket which has had accept(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
files, or on individual sockets by using
socket options with the
The maximum sizes for socket buffers declared via the SO_SNDBUF
mechanisms are limited by the values in the
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
call. TCP uses the extra space for administrative
purposes and internal kernel structures, and the /proc
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)
calls in order to have it take effect. See
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
. 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
the POSIX.1-2001-specified fcntl(2) F_SETOWN
socket option is enabled, urgent data is put into the
normal data stream (a program can test for its location using the
ioctl described below), otherwise it can be received only
when the MSG_OOB
flag is set for recv(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
, Explicit Congestion Notification, new management
of TIME_WAIT sockets, keep-alive socket options and support for Duplicate SACK
TCP is built on top of IP (see ip(7)
). The address formats defined by
apply to TCP. TCP supports point-to-point communication only;
broadcasting and multicasting are not supported.
System-wide TCP parameter settings can be accessed by files in the directory
. In addition, most IP /proc
apply to TCP; see ip(7)
. Variables described as Boolean
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; 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 acknowledgments. Possible values are:
- increase cwnd once per acknowledgment (no ABC)
- increase cwnd once per acknowledgment of full sized segment
- 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
- 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
- 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
- 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_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
- 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
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
- 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
- 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
- tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to
- 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
- tcp_congestion_control (String; default: see text; since Linux
- 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_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 180.
- 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 congestion. See RFC 4138 for more details.
This file can have one of the following values:
- The basic version F-RTO algorithm is enabled.
- 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
- 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:
- Rate halving based; a smooth and conservative response, results in halved
congestion window (cwnd) and slow-start threshold (ssthresh)
after one RTT.
- Very conservative response; not recommended because even though being
valid, it interacts poorly with the rest of Linux TCP; halves cwnd
and ssthresh immediately.
- 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
- 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.
- 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
- 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
modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel
- tcp_max_tw_buckets (integer; default: see below; since Linux
- 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
- tcp_moderate_rcvbuf (Boolean; default: enabled; since Linux
- 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.)
- TCP doesn't regulate its memory allocation when the number of pages it has
allocated globally is below this number.
- 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.
- 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:
- Disabled by default, enabled when an ICMP black hole detected
- Always enabled, use initial MSS of tcp_base_mss.
- tcp_no_metrics_save (Boolean; default: disabled; since Linux
- 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
- 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
- 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.
- 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.
- 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).
- 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-memory
- 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
- 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: 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
- 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 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,
- 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
- tcp_tw_recycle (Boolean; default: disabled; since Linux 2.4)
- Enable fast recycling of TIME_WAIT sockets. Enabling this option is not
recommended since this causes problems when working with NAT (Network
- tcp_tw_reuse (Boolean; default: disabled; since Linux
- 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
- tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to
- 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
- 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
- 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_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.
- 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, 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.
- 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 16K bytes. 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).
- 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
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
- 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.
To set or get a TCP socket option, call getsockopt(2)
to read or
to write the option with the option level argument set to
. Unless otherwise noted, optval
is a pointer to an
. 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)
- 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_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
- 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.
- 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 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
- 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 before TCP will forcibly
close the corresponding connection and return ETIMEDOUT to the
application. If the option value is specified as 0, TCP will to use the
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
This option can be set during any state of a TCP connection, but is only
effective 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 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
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)
Since version 2.4, Linux supports the use of MSG_TRUNC
argument of recv(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
The following ioctl(2)
calls return information in value
correct syntax is:
error = ioctl(tcp_socket, ioctl_type, &value);
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
- 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.
- 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
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
- The other end closed the socket unexpectedly or a read is executed on a
shut down socket.
- 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 acknowledgement (FACK), TIME_WAIT recycling, and
per-connection keepalive socket options were introduced in 2.3.
Not all errors are documented.
IPv6 is not described.
RFC 793 for the TCP specification.
RFC 1122 for the TCP requirements and a description of the Nagle
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.
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