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TCP(7) Linux Programmer's Manual TCP(7)
NAME
tcp - TCP protocol
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
DESCRIPTION
This is an implementation of the TCP protocol defined in RFC 793, RFC 1122 and RFC 2001 with the NewReno and SACK exten-
sions. 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 connec-
tion 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 setsockopt(2) call.
The maximum sizes for socket buffers declared via the SO_SNDBUF and SO_RCVBUF mechanisms are limited by the values in the
/proc/sys/net/core/rmem_max and /proc/sys/net/core/wmem_max files. Note that TCP actually allocates twice the size of
the buffer requested in the setsockopt(2) call, and so a succeeding getsockopt(2) call will not return the same size of
buffer as requested in the setsockopt(2) call. TCP uses the extra space for administrative purposes and internal kernel
structures, and the /proc file values reflect the larger sizes compared to the actual TCP windows. On individual 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 SIOCATMARK ioctl described below), otherwise it can be only received 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.
Address Formats
TCP is built on top of IP (see ip(7)). The address formats defined by ip(7) apply to TCP. TCP only supports point-to-
point communication; broadcasting and multicasting are not supported.
/proc interfaces
System-wide TCP parameter settings can be accessed by files in the directory /proc/sys/net/ipv4/. In addition, most IP
/proc interfaces also apply to TCP; see ip(7). Variables described as Boolean take an integer value, with a nonzero
value ("true") meaning that the corresponding option is enabled, and a zero value ("false") meaning that the option is
disabled.
tcp_abc (Integer; default: 0; 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:
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 "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 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 overhead.
A maximum of (window/2^tcp_app_win, mss) bytes in the window are reserved for the application buffer. A value of
0 implies that no amount is reserved.
tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
The initial value of search_low to be used by the packetization layer Path MTU discovery (MTU probing). If MTU
probing is enabled, this is the initial MSS used by the connection.
tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
Enable BIC TCP congestion control algorithm. BIC-TCP is a sender-side only change that ensures a linear RTT fair-
ness under large windows while offering both scalability and bounded TCP-friendliness. The protocol combines two
schemes called additive increase and binary search increase. When the congestion window is large, additive
increase with a large increment ensures linear RTT fairness as well as good scalability. Under small congestion
windows, binary search increase provides TCP friendliness.
tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 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 connec-
tion to converge more rapidly.
tcp_congestion_control (String; default: see text; since Linux 2.4.13)
Set the default congestion-control algorithm to be used for new connections. The algorithm "reno" is always
available, but additional choices may be available depending on kernel configuration. The default value for this
file is set as part of kernel configuration.
tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
Lower limit, in bytes, of the size of socket reads that will be offloaded to a DMA copy engine, if one is present
in the system and the kernel was configured with the CONFIG_NET_DMA option.
tcp_dsack (Boolean; default: enabled; since Linux 2.4)
Enable RFC 2883 TCP Duplicate SACK support.
tcp_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 benefi-
cial in wireless environments where packet loss is typically due to random radio interference rather than interme-
diate router congestion. See RFC 4138 for more details.
This file can have one of the following values:
0 Disabled.
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 congestion window (cwnd) and slow-
start threshold (ssthresh) after one RTT.
1 Very conservative response; not recommended because even though being valid, it interacts poorly with the rest
of Linux TCP; halves cwnd and ssthresh immediately.
2 Aggressive response; undoes congestion-control measures that are now known to be unnecessary (ignoring the pos-
sibility 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 only sent when the SO_KEEPALIVE socket option is enabled. The default value is 7200 seconds (2 hours). An
idle connection is terminated after approximately an additional 11 minutes (9 probes an interval of 75 seconds
apart) when keep-alive is enabled.
Note that underlying connection tracking mechanisms and application timeouts may be much shorter.
tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
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 2.2)
The maximum number of queued connection requests which have still not received an acknowledgement from the con-
necting 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 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[2]) to match the size required by the path for full throughput.
tcp_mem (since Linux 2.4)
This is a vector of 3 integers: [low, pressure, high]. These bounds, measured in units of the system page size,
are used by TCP to track its memory usage. The defaults are calculated at boot time from the amount of available
memory. (TCP can only use low memory for this, which is limited to around 900 megabytes on 32-bit systems.
64-bit systems do not suffer this limitation.)
low TCP doesn't regulate its memory allocation when the number of pages it has allocated globally is below
this number.
pressure When the amount of memory allocated by TCP exceeds this number of pages, TCP moderates its memory con-
sumption. This memory pressure state is exited once the number of pages allocated falls below the low
mark.
high The maximum number of pages, globally, that TCP will allocate. This value overrides any other limits
imposed by the kernel.
tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
This parameter controls TCP Packetization-Layer Path MTU Discovery. The following values may be assigned to the
file:
0 Disabled
1 Disabled by default, enabled when an ICMP black hole detected
2 Always enabled, use initial MSS of tcp_base_mss.
tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
By default, TCP saves various connection metrics in the route cache when the connection closes, so that connec-
tions 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 mini-
mize unnecessary back off and retransmits provoked by reordering of packets on a connection.
tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
Try to send full-sized packets during retransmit.
tcp_retries1 (integer; default: 3; since Linux 2.2)
The number of times TCP will attempt to retransmit a packet on an established connection normally, without the
extra effort of getting the network layers involved. Once we exceed this number of retransmits, we first have the
network layer update the route if possible before each new retransmit. The default is the RFC specified minimum
of 3.
tcp_retries2 (integer; default: 15; since Linux 2.2)
The maximum number of times a TCP packet is retransmitted in established state before giving up. The default
value is 15, which corresponds to a duration of approximately between 13 to 30 minutes, depending on the retrans-
mission 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 buf-
fer 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[1]*PAGE_SIZE/128))
(On Linux 2.4, the default is 87380*2 bytes, lowered to 87380 in low-memory systems).
tcp_sack (Boolean; default: enabled; since Linux 2.2)
Enable RFC 2018 TCP Selective Acknowledgements.
tcp_slow_start_after_idle (Boolean; default: enabled; since Linux 2.6.18)
If enabled, provide RFC 2861 behavior and time out the congestion window after an idle period. An idle period is
defined as the current RTO (retransmission timeout). If disabled, the congestion window will not be timed out
after an idle period.
tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
If this option is enabled, then use the RFC 1122 interpretation of the TCP urgent-pointer field. According to
this interpretation, the urgent pointer points to the last byte of urgent data. If this option is disabled, then
use the BSD-compatible interpretation of the urgent pointer: the urgent pointer points to the first byte after the
urgent data. Enabling this option may lead to interoperability problems.
tcp_syn_retries (integer; default: 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 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 recom-
mended alternatives see tcp_max_syn_backlog, tcp_synack_retries, and tcp_abort_on_overflow.
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 Off-
load (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 recommended since this causes problems
when working with NAT (Network Address Translation).
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 conges-
tion 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 band-
width used at the time congestion is experienced. TCP Westwood+ significantly increases fairness with respect to
TCP Reno in wired networks and throughput over wireless links.
tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
Enable RFC 1323 TCP window scaling. This feature allows the use of a large window (> 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.
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, allocations below this size will still succeed. This is not used to bound the size of the send
buffer declared using SO_SNDBUF on a socket.
default The default size of the send buffer for a TCP socket. This value overwrites the initial default buffer
size from the generic global /proc/sys/net/core/wmem_default defined for all protocols. The default
value is 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).
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[1]*PAGE_SIZE/128))
(On Linux 2.4, the default value is 128K bytes, lowered 64K depending on low-memory systems.)
tcp_workaround_signed_windows (Boolean; default: disabled; since Linux 2.6.26)
If enabled, assume that no receipt of a window-scaling option means that the remote TCP is broken and treats the
window as a signed quantity. If disabled, assume that the remote TCP is not broken even if we do not receive a
window scaling option from it.
Socket Options
To set or get a TCP socket option, call getsockopt(2) to read or setsockopt(2) to write the option with the option level
argument set to IPPROTO_TCP. In addition, most IPPROTO_IP socket options are valid on TCP sockets. For more information
see ip(7).
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 imple-
mented, there is a 200 millisecond ceiling on the time for which output is corked by TCP_CORK. If this ceiling is
reached, then queued data is automatically transmitted. This option can be combined with TCP_NODELAY only since
Linux 2.5.71. This option should not be used in code intended to be portable.
TCP_DEFER_ACCEPT (since Linux 2.4)
Allow a listener to be awakened only when data arrives on the socket. Takes an integer value (seconds), this can
bound the maximum number of attempts TCP will make to complete the connection. This option should not be used in
code intended to be portable.
TCP_INFO (since Linux 2.4)
Used to collect information about this socket. The kernel returns a struct tcp_info as defined in the file
/usr/include/linux/tcp.h. This option should not be used in code intended to be portable.
TCP_KEEPCNT (since Linux 2.4)
The maximum number of keepalive probes TCP should send before dropping the connection. This option should not be
used in code intended to be portable.
TCP_KEEPIDLE (since Linux 2.4)
The time (in seconds) the connection needs to remain idle before TCP starts sending keepalive probes, if the
socket option SO_KEEPALIVE has been set on this socket. This option should not be used in code intended to be
portable.
TCP_KEEPINTVL (since Linux 2.4)
The time (in seconds) between individual keepalive probes. This option should not be used in code intended to be
portable.
TCP_LINGER2 (since Linux 2.4)
The lifetime of orphaned FIN_WAIT2 state sockets. This option can be used to override the system-wide setting in
the file /proc/sys/net/ipv4/tcp_fin_timeout for this socket. This is not to be confused with the socket(7) level
option SO_LINGER. This option should not be used in code intended to be portable.
TCP_MAXSEG
The maximum segment size for outgoing TCP packets. 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) inter-
face MTU have no effect. TCP will also impose its minimum and maximum bounds over the value provided.
TCP_NODELAY
If set, disable the Nagle algorithm. This means that segments are always sent as soon as possible, even if there
is only a small amount of data. When not set, data is buffered until there is a sufficient amount to send out,
thereby avoiding the frequent sending of small packets, which results in poor utilization of the network. This
option is overridden by TCP_CORK; however, setting this option forces an explicit flush of pending output, even if
TCP_CORK is currently set.
TCP_QUICKACK (since Linux 2.4.4)
Enable quickack mode if set or disable quickack mode if cleared. In quickack mode, acks are sent immediately,
rather than delayed if needed in accordance to normal TCP operation. This flag is not permanent, it only enables
a switch to or from quickack mode. Subsequent operation of the TCP protocol will once again enter/leave quickack
mode depending on internal protocol processing and factors such as delayed ack timeouts occurring and data trans-
fer. 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 minimum size of SOCK_MIN_RCVBUF/2.
This option should not be used in code intended to be portable.
Sockets API
TCP provides limited support for out-of-band data, in the form of (a single byte of) urgent data. In Linux this means if
the other end sends newer out-of-band data the older urgent data is inserted as normal data into the stream (even when
SO_OOBINLINE is not set). This differs from BSD-based stacks.
Linux uses the BSD compatible interpretation of the urgent pointer field by default. This violates RFC 1122, but is
required for interoperability with other stacks. It can be changed via /proc/sys/net/ipv4/tcp_stdurg.
It is possible to peek at out-of-band data using the recv(2) MSG_PEEK flag.
Since 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.
Ioctls
These following ioctl(2) calls return information in value. The correct syntax is:
int value;
error = ioctl(tcp_socket, ioctl_type, &value);
ioctl_type is one of the following:
SIOCINQ
Returns the amount of queued unread data in the receive buffer. The socket must not be in LISTEN state, otherwise
an error (EINVAL) is returned.
SIOCATMARK
Returns true (i.e., value is nonzero) if the inbound data stream is at the urgent mark.
If the SO_OOBINLINE socket option is set, and SIOCATMARK returns true, then the next read from the socket will
return the urgent data. If the SO_OOBINLINE socket option is not set, and SIOCATMARK returns true, then the next
read from the socket will return the bytes following the urgent data (to actually read the urgent data requires
the recv(MSG_OOB) flag).
Note that a read never reads across the urgent mark. If an application is informed of the presence of urgent data
via select(2) (using the exceptfds argument) or through delivery of a SIGURG signal, then it can advance up to the
mark using a loop which repeatedly tests SIOCATMARK and performs a read (requesting any number of bytes) as long
as SIOCATMARK returns false.
SIOCOUTQ
Returns the amount of unsent data in the socket send queue. The socket must not be in LISTEN state, otherwise an
error (EINVAL) is returned.
Error Handling
When a network error occurs, TCP tries to resend the packet. If it doesn't succeed after some time, either ETIMEDOUT or
the last received error on this connection is reported.
Some applications require a quicker error notification. This can be enabled with the IPPROTO_IP level IP_RECVERR socket
option. When this option is enabled, all incoming errors are immediately passed to the user program. Use this option
with care -- it makes TCP less tolerant to routing changes and other normal network conditions.
ERRORS
EAFNOTSUPPORT
Passed socket address type in sin_family was not AF_INET.
EPIPE The other end closed the socket unexpectedly or a read is executed on a shut down socket.
ETIMEDOUT
The other end didn't acknowledge retransmitted data after some time.
Any errors defined for ip(7) or the generic socket layer may also be returned for TCP.
VERSIONS
Support for Explicit Congestion Notification, zero-copy sendfile(2), reordering support and some SACK extensions (DSACK)
were introduced in 2.4. Support for forward acknowledgement (FACK), TIME_WAIT recycling, and per-connection keepalive
socket options were introduced in 2.3.
BUGS
Not all errors are documented.
IPv6 is not described.
SEE ALSO
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 1644 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.
COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about
reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2009-09-30 TCP(7)
