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 (> 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
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-specified fcntl(2) F_SETOWN
operation). When the
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)
When out-of-band data is present, select(2)
indicates the file descriptor
as having an exceptional condition and poll (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; Linux 2.6.15 to Linux
- 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
- 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
- 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_autocorking (Boolean; default: enabled; since
- 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
- 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
- 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 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
- 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
- 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
- Enable RFC 2883 TCP Duplicate SACK support.
- tcp_ecn (Integer; default: see below; since Linux
- Enable RFC 3168 Explicit Congestion
- This file can have one of the following values:
- Disable ECN. Neither initiate nor accept ECN. This was the
default up to and including Linux 2.6.30.
- Enable ECN when requested by incoming connections and also
request ECN on outgoing connection attempts.
- 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
- Enable RFC 3168, Section 188.8.131.52. 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
- Enable TCP Forward Acknowledgement support.
- tcp_fin_timeout (integer; default: 60; since Linux
- 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
- 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
- This file can have one of the following values:
- Disabled. This was the default up to and including Linux
- 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 flow. This value is the default since Linux
- 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
- 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
- The number of seconds between TCP keep-alive probes.
- tcp_keepalive_probes (integer; default: 9; since
- The maximum number of TCP keep-alive probes to send before
giving up and killing the connection if no response is obtained from the
- tcp_keepalive_time (integer; default: 7200; since
- 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
- 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
- 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).
- Prior to 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
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
- 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
- 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
- tcp_mtu_probing (integer; default: 0; since Linux
- 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
- Always enabled, use initial MSS of
- 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
- 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
- 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
- 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
- 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
- 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 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
- 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
- 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
max(87380, min(4 MB, tcp_mem*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
- 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
- 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
- tcp_syn_retries (integer; default: 5; since Linux
- 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
- The maximum number of times a SYN/ACK segment for a passive
TCP connection will be retransmitted. This number should not be higher
- 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 (integer; default: 1; since Linux
- Set to one of the following values to enable or disable
RFC 1323 TCP timestamps:
- Disable timestamps.
- Enable timestamps as defined in RFC1323 and use random
offset for each connection rather than only using the current time.
- 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
- 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 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
- 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 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
- 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.
- 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.
- 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).
- 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*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.
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 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.
- 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
- 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 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 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
- 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
- Further details on the user timeout feature can be found in
RFC 793 and RFC 5482 ("TCP User Timeout
- 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 <sys/ioctl.h>.
- 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
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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