.\" SPDX-License-Identifier: Linux-man-pages-1-para
.\"
.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\" Note also that many pieces are drawn from the kernel source file
.\" Documentation/networking/ip-sysctl.txt.
.\"
.\" 2.4 Updates by Nivedita Singhvi 4/20/02 <nivedita@us.ibm.com>.
.\" Modified, 2004-11-11, Michael Kerrisk and Andries Brouwer
.\"	Updated details of interaction of TCP_CORK and TCP_NODELAY.
.\"
.\" 2008-11-21, mtk, many, many updates.
.\"     The descriptions of /proc files and socket options should now
.\"     be more or less up to date and complete as at Linux 2.6.27
.\"     (other than the remaining FIXMEs in the page source below).
.\"
.\" FIXME The following need to be documented
.\"	TCP_MD5SIG (2.6.20)
.\"	    commit cfb6eeb4c860592edd123fdea908d23c6ad1c7dc
.\"	    Author was yoshfuji@linux-ipv6.org
.\"	    Needs CONFIG_TCP_MD5SIG
.\" From net/inet/Kconfig:
.\" bool "TCP: MD5 Signature Option support (RFC2385) (EXPERIMENTAL)"
.\" RFC2385 specifies a method of giving MD5 protection to TCP sessions.
.\" Its main (only?) use is to protect BGP sessions between core routers
.\" on the Internet.
.\"
.\" There is a TCP_MD5SIG option documented in FreeBSD's tcp(4),
.\" but probably many details are different on Linux
.\"	    http://thread.gmane.org/gmane.linux.network/47490
.\"	    http://www.daemon-systems.org/man/tcp.4.html
.\"	    http://article.gmane.org/gmane.os.netbsd.devel.network/3767/match=tcp_md5sig+freebsd
.\"
.\"	TCP_COOKIE_TRANSACTIONS (2.6.33)
.\"	    commit 519855c508b9a17878c0977a3cdefc09b59b30df
.\"	    Author: William Allen Simpson <william.allen.simpson@gmail.com>
.\"	    commit e56fb50f2b7958b931c8a2fc0966061b3f3c8f3a
.\"	    Author: William Allen Simpson <william.allen.simpson@gmail.com>
.\"
.\"	    REMOVED in Linux 3.10
.\" 		commit 1a2c6181c4a1922021b4d7df373bba612c3e5f04
.\"		Author: Christoph Paasch <christoph.paasch@uclouvain.be>
.\"
.\"	TCP_THIN_LINEAR_TIMEOUTS (2.6.34)
.\"	    commit 36e31b0af58728071e8023cf8e20c5166b700717
.\"	    Author: Andreas Petlund <apetlund@simula.no>
.\"
.\"	TCP_THIN_DUPACK (2.6.34)
.\"	    commit 7e38017557bc0b87434d184f8804cadb102bb903
.\"	    Author: Andreas Petlund <apetlund@simula.no>
.\"
.\"	TCP_REPAIR (3.5)
.\"	    commit ee9952831cfd0bbe834f4a26489d7dce74582e37
.\"	    Author: Pavel Emelyanov <xemul@parallels.com>
.\"	    See also
.\"		http://criu.org/TCP_connection
.\"		https://lwn.net/Articles/495304/
.\"
.\"	TCP_REPAIR_QUEUE (3.5)
.\"	    commit ee9952831cfd0bbe834f4a26489d7dce74582e37
.\"	    Author: Pavel Emelyanov <xemul@parallels.com>
.\"
.\"	TCP_QUEUE_SEQ (3.5)
.\"	    commit ee9952831cfd0bbe834f4a26489d7dce74582e37
.\"	    Author: Pavel Emelyanov <xemul@parallels.com>
.\"
.\"	TCP_REPAIR_OPTIONS (3.5)
.\"	    commit b139ba4e90dccbf4cd4efb112af96a5c9e0b098c
.\"	    Author: Pavel Emelyanov <xemul@parallels.com>
.\"
.\"     TCP_FASTOPEN (3.6)
.\"         (Fast Open server side implementation completed in Linux 3.7)
.\"	    http://lwn.net/Articles/508865/
.\"
.\"     TCP_TIMESTAMP (3.9)
.\"        commit 93be6ce0e91b6a94783e012b1857a347a5e6e9f2
.\"        Author: Andrey Vagin <avagin@openvz.org>
.\"
.\"     TCP_NOTSENT_LOWAT (3.12)
.\"        commit c9bee3b7fdecb0c1d070c7b54113b3bdfb9a3d36
.\"        Author: Eric Dumazet <edumazet@google.com>
.\"
.\"	TCP_CC_INFO (4.1)
.\"	   commit 6e9250f59ef9efb932c84850cd221f22c2a03c4a
.\"	   Author: Eric Dumazet <edumazet@google.com>
.\"
.\"	TCP_SAVE_SYN, TCP_SAVED_SYN (4.2)
.\"	    commit cd8ae85299d54155702a56811b2e035e63064d3d
.\"	    Author: Eric Dumazet <edumazet@google.com>
.\"
.TH tcp 7 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
tcp \- TCP protocol
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netinet/in.h>
.B #include <netinet/tcp.h>
.P
.IB tcp_socket " = socket(AF_INET, SOCK_STREAM, 0);"
.fi
.SH DESCRIPTION
This is an implementation of the TCP protocol defined in
RFC\ 793, RFC\ 1122 and RFC\ 2001 with the NewReno and SACK
extensions.
It provides a reliable, stream-oriented,
full-duplex connection between two sockets on top of
.BR 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.
.P
A newly created TCP socket has no remote or local address and is not
fully specified.
To create an outgoing TCP connection use
.BR connect (2)
to establish a connection to another TCP socket.
To receive new incoming connections, first
.BR bind (2)
the socket to a local address and port and then call
.BR listen (2)
to put the socket into the listening state.
After that a new socket for each incoming connection can be accepted using
.BR accept (2).
A socket which has had
.BR accept (2)
or
.BR connect (2)
successfully called on it is fully specified and may transmit data.
Data cannot be transmitted on listening or not yet connected sockets.
.P
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
.I /proc/sys/net/ipv4/tcp_wmem
and
.I /proc/sys/net/ipv4/tcp_rmem
files, or on individual sockets by using the
.B SO_SNDBUF
and
.B SO_RCVBUF
socket options with the
.BR setsockopt (2)
call.
.P
The maximum sizes for socket buffers declared via the
.B SO_SNDBUF
and
.B SO_RCVBUF
mechanisms are limited by the values in the
.I /proc/sys/net/core/rmem_max
and
.I /proc/sys/net/core/wmem_max
files.
Note that TCP actually allocates twice the size of
the buffer requested in the
.BR setsockopt (2)
call, and so a succeeding
.BR getsockopt (2)
call will not return the same size of buffer as requested in the
.BR setsockopt (2)
call.
TCP uses the extra space for administrative purposes and internal
kernel structures, and the
.I /proc
file values reflect the
larger sizes compared to the actual TCP windows.
On individual connections, the socket buffer size must be set prior to the
.BR listen (2)
or
.BR connect (2)
calls in order to have it take effect.
See
.BR socket (7)
for more information.
.P
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
.B MSG_OOB
option to
.BR send (2).
When urgent data is received, the kernel sends a
.B SIGURG
signal to the process or process group that has been set as the
socket "owner" using the
.B SIOCSPGRP
or
.B FIOSETOWN
ioctls (or the POSIX.1-specified
.BR fcntl (2)
.B F_SETOWN
operation).
When the
.B SO_OOBINLINE
socket option is enabled, urgent data is put into the normal
data stream (a program can test for its location using the
.B SIOCATMARK
ioctl described below),
otherwise it can be received only when the
.B MSG_OOB
flag is set for
.BR recv (2)
or
.BR recvmsg (2).
.P
When out-of-band data is present,
.BR select (2)
indicates the file descriptor as having an exceptional condition and
.I poll (2)
indicates a
.B POLLPRI
event.
.P
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
.BR sendfile (2),
Explicit Congestion Notification, new
management of TIME_WAIT sockets, keep-alive socket options
and support for Duplicate SACK extensions.
.SS Address formats
TCP is built on top of IP (see
.BR ip (7)).
The address formats defined by
.BR ip (7)
apply to TCP.
TCP supports point-to-point communication only;
broadcasting and multicasting are not
supported.
.SS /proc interfaces
System-wide TCP parameter settings can be accessed by files in the directory
.IR /proc/sys/net/ipv4/ .
In addition, most IP
.I /proc
interfaces also apply to TCP; see
.BR ip (7).
Variables described as
.I 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.
.TP
.IR tcp_abc " (Integer; default: 0; Linux 2.6.15 to Linux 3.8)"
.\" Since Linux 2.6.15; removed in Linux 3.9
.\" commit ca2eb5679f8ddffff60156af42595df44a315ef0
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
Control the Appropriate Byte Count (ABC), defined in RFC 3465.
ABC is a way of increasing the congestion window
.RI ( cwnd )
more slowly in response to partial acknowledgements.
Possible values are:
.RS
.TP
.B 0
increase
.I cwnd
once per acknowledgement (no ABC)
.TP
.B 1
increase
.I cwnd
once per acknowledgement of full sized segment
.TP
.B 2
allow increase
.I cwnd
by two if acknowledgement is
of two segments to compensate for delayed acknowledgements.
.RE
.TP
.IR tcp_abort_on_overflow " (Boolean; default: disabled; since Linux 2.4)"
.\" Since Linux 2.3.41
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
.I 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.
.TP
.IR tcp_adv_win_scale " (integer; default: 2; since Linux 2.4)"
.\" Since Linux 2.4.0-test7
Count buffering overhead as
.IR "bytes/2\[ha]tcp_adv_win_scale" ,
if
.I tcp_adv_win_scale
is greater than 0; or
.IR "bytes\-bytes/2\[ha](\-tcp_adv_win_scale)" ,
if
.I tcp_adv_win_scale
is less than or equal to zero.
.IP
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
.I tcp_adv_win_scale
default value of 2 implies that the space
used for the application buffer is one fourth that of the total.
.TP
.IR tcp_allowed_congestion_control " (String; default: see text; since Linux 2.4.20)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
Show/set the congestion control algorithm choices available to unprivileged
processes (see the description of the
.B 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
.IR tcp_available_congestion_control .
The default value for this list is "reno" plus the default setting of
.IR tcp_congestion_control .
.TP
.IR tcp_autocorking " (Boolean; default: enabled; since Linux 3.14)"
.\" commit f54b311142a92ea2e42598e347b84e1655caf8e3
.\" Text heavily based on Documentation/networking/ip-sysctl.txt
If this option is enabled, the kernel tries to coalesce small writes
(from consecutive
.BR write (2)
and
.BR 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
.B TCP_CORK
socket option to obtain optimal behavior
when they know how/when to uncork their sockets.
.TP
.IR tcp_available_congestion_control " (String; read-only; since Linux 2.4.20)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
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
.IR tcp_allowed_congestion_control .
More congestion-control algorithms may be available as modules,
but not loaded.
.TP
.IR tcp_app_win  " (integer; default: 31; since Linux 2.4)"
.\" Since Linux 2.4.0-test7
This variable defines how many
bytes of the TCP window are reserved for buffering overhead.
.IP
A maximum of (\fIwindow/2\[ha]tcp_app_win\fP, mss) bytes in the window
are reserved for the application buffer.
A value of 0 implies that no amount is reserved.
.\"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_base_mss " (Integer; default: 512; since Linux 2.6.17)"
The initial value of
.I 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.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_bic " (Boolean; default: disabled; Linux 2.4.27/2.6.6 to Linux 2.6.13)"
Enable BIC TCP congestion control algorithm.
BIC-TCP is a sender-side-only change that ensures a linear RTT
fairness under large windows while offering both scalability and
bounded TCP-friendliness.
The protocol combines two schemes
called additive increase and binary search increase.
When the congestion window is large, additive increase with a large
increment ensures linear RTT fairness as well as good scalability.
Under small congestion windows, binary search
increase provides TCP friendliness.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_bic_low_window " (integer; default: 14; Linux 2.4.27/2.6.6 to Linux 2.6.13)"
Set the threshold window (in packets) where BIC TCP starts to
adjust the congestion window.
Below this threshold BIC TCP behaves the same as the default TCP Reno.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_bic_fast_convergence " (Boolean; default: enabled; Linux 2.4.27/2.6.6 to Linux 2.6.13)"
Force BIC TCP to more quickly respond to changes in congestion window.
Allows two flows sharing the same connection to converge more rapidly.
.TP
.IR tcp_congestion_control " (String; default: see text; since Linux 2.4.13)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
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.
.TP
.IR 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
.B CONFIG_NET_DMA
option.
.TP
.IR tcp_dsack " (Boolean; default: enabled; since Linux 2.4)"
.\" Since Linux 2.4.0-test7
Enable RFC\ 2883 TCP Duplicate SACK support.
.TP
.IR tcp_fastopen " (Bitmask; default: 0x1; since Linux 3.7)"
Enables RFC\~7413 Fast Open support.
The flag is used as a bitmap with the following values:
.RS
.TP
.B 0x1
Enables client side Fast Open support
.TP
.B 0x2
Enables server side Fast Open support
.TP
.B 0x4
Allows client side to transmit data in SYN without Fast Open option
.TP
.B 0x200
Allows server side to accept SYN data without Fast Open option
.TP
.B 0x400
Enables Fast Open on all listeners without
.B TCP_FASTOPEN
socket option
.RE
.TP
.IR tcp_fastopen_key " (since Linux 3.7)"
Set server side RFC\~7413 Fast Open key to generate Fast Open cookie
when server side Fast Open support is enabled.
.TP
.IR tcp_ecn " (Integer; default: see below; since Linux 2.4)"
.\" Since Linux 2.4.0-test7
Enable RFC\ 3168 Explicit Congestion Notification.
.IP
This file can have one of the following values:
.RS
.TP
.B 0
Disable ECN.
Neither initiate nor accept ECN.
This was the default up to and including Linux 2.6.30.
.TP
.B 1
Enable ECN when requested by incoming connections and also
request ECN on outgoing connection attempts.
.TP
.B 2
.\" commit 255cac91c3c9ce7dca7713b93ab03c75b7902e0e
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.
.RE
.IP
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
.B tcp_ecn_fallback
option has been introduced.
.TP
.IR tcp_ecn_fallback " (Boolean; default: enabled; since Linux 4.1)"
.\" commit 492135557dc090a1abb2cfbe1a412757e3ed68ab
Enable RFC\ 3168, Section 6.1.1.1. fallback.
When enabled, outgoing ECN-setup SYNs that time out within the
normal SYN retransmission timeout will be resent with CWR and
ECE cleared.
.TP
.IR tcp_fack " (Boolean; default: enabled; since Linux 2.2)"
.\" Since Linux 2.1.92
Enable TCP Forward Acknowledgement support.
.TP
.IR tcp_fin_timeout " (integer; default: 60; since Linux 2.2)"
.\" Since Linux 2.1.53
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.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_frto " (integer; default: see below; since Linux 2.4.21/2.6)"
.\" Since Linux 2.4.21/2.5.43
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.
.IP
This file can have one of the following values:
.RS
.TP
.B 0
Disabled.
This was the default up to and including Linux 2.6.23.
.TP
.B 1
The basic version F-RTO algorithm is enabled.
.TP
.B 2
.\" commit c96fd3d461fa495400df24be3b3b66f0e0b152f9
Enable SACK-enhanced F-RTO if flow uses SACK.
The basic version can be used also when
SACK is in use though in that case scenario(s) exists where F-RTO
interacts badly with the packet counting of the SACK-enabled TCP flow.
This value is the default since Linux 2.6.24.
.RE
.IP
Before Linux 2.6.22, this parameter was a Boolean value,
supporting just values 0 and 1 above.
.TP
.IR 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:
.RS
.TP
.B 0
Rate halving based; a smooth and conservative response,
results in halved congestion window
.RI ( cwnd )
and slow-start threshold
.RI ( ssthresh )
after one RTT.
.TP
.B 1
Very conservative response; not recommended because even
though being valid, it interacts poorly with the rest of Linux TCP; halves
.I cwnd
and
.I ssthresh
immediately.
.TP
.B 2
Aggressive response; undoes congestion-control measures
that are now known to be unnecessary
(ignoring the possibility of a lost retransmission that would require
TCP to be more cautious);
.I cwnd
and
.I ssthresh
are restored to the values prior to timeout.
.RE
.TP
.IR tcp_keepalive_intvl " (integer; default: 75; since Linux 2.4)"
.\" Since Linux 2.3.18
The number of seconds between TCP keep-alive probes.
.TP
.IR tcp_keepalive_probes " (integer; default: 9; since Linux 2.2)"
.\" Since Linux 2.1.43
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.
.TP
.IR tcp_keepalive_time " (integer; default: 7200; since Linux 2.2)"
.\" Since Linux 2.1.43
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
.B 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.
.IP
Note that underlying connection tracking mechanisms and
application timeouts may be much shorter.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_low_latency  " (Boolean; default: disabled; since Linux 2.4.21/2.6; \
obsolete since Linux 4.14)"
.\" Since Linux 2.4.21/2.5.60
If enabled, the TCP stack makes decisions that prefer lower
latency as opposed to higher throughput.
It this option is disabled, then higher throughput is preferred.
An example of an application where this default should be
changed would be a Beowulf compute cluster.
Since Linux 4.14,
.\" commit b6690b14386698ce2c19309abad3f17656bdfaea
this file still exists, but its value is ignored.
.TP
.IR tcp_max_orphans  " (integer; default: see below; since Linux 2.4)"
.\" Since Linux 2.3.41
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 \[ti]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.
.TP
.IR tcp_max_syn_backlog " (integer; default: see below; since Linux 2.2)"
.\" Since Linux 2.1.53
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).
.IP
Before Linux 2.6.20,
.\" commit 72a3effaf633bcae9034b7e176bdbd78d64a71db
it was recommended that if this needed to be increased above 1024,
the size of the SYNACK hash table
.RB ( TCP_SYNQ_HSIZE )
in
.I include/net/tcp.h
should be modified to keep
.IP
.in +4n
.EX
TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
.EE
.in
.IP
and the kernel should be
recompiled.
In Linux 2.6.20, the fixed sized
.B TCP_SYNQ_HSIZE
was removed in favor of dynamic sizing.
.TP
.IR tcp_max_tw_buckets " (integer; default: see below; since Linux 2.4)"
.\" Since Linux 2.3.41
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.
.TP
.IR tcp_moderate_rcvbuf " (Boolean; default: enabled; since Linux 2.4.17/2.6.7)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
If enabled, TCP performs receive buffer auto-tuning,
attempting to automatically size the buffer (no greater than
.IR tcp_rmem[2] )
to match the size required by the path for full throughput.
.TP
.IR tcp_mem " (since Linux 2.4)"
.\" Since Linux 2.4.0-test7
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
.I "low memory"
for this, which is limited to around 900 megabytes on 32-bit systems.
64-bit systems do not suffer this limitation.)
.RS
.TP
.I low
TCP doesn't regulate its memory allocation when the number
of pages it has allocated globally is below this number.
.TP
.I pressure
When the amount of memory allocated by TCP
exceeds this number of pages, TCP moderates its memory consumption.
This memory pressure state is exited
once the number of pages allocated falls below
the
.I low
mark.
.TP
.I high
The maximum number of pages, globally, that TCP will allocate.
This value overrides any other limits imposed by the kernel.
.RE
.TP
.IR tcp_mtu_probing " (integer; default: 0; since Linux 2.6.17)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
This parameter controls TCP Packetization-Layer Path MTU Discovery.
The following values may be assigned to the file:
.RS
.TP
.B 0
Disabled
.TP
.B 1
Disabled by default, enabled when an ICMP black hole detected
.TP
.B 2
Always enabled, use initial MSS of
.IR tcp_base_mss .
.RE
.TP
.IR tcp_no_metrics_save " (Boolean; default: disabled; since Linux 2.6.6)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
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
.I tcp_no_metrics_save
is enabled, TCP will not cache metrics on closing connections.
.TP
.IR tcp_orphan_retries " (integer; default: 8; since Linux 2.4)"
.\" Since Linux 2.3.41
The maximum number of attempts made to probe the other
end of a connection which has been closed by our end.
.TP
.IR tcp_reordering " (integer; default: 3; since Linux 2.4)"
.\" Since Linux 2.4.0-test7
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.
.TP
.IR tcp_retrans_collapse " (Boolean; default: enabled; since Linux 2.2)"
.\" Since Linux 2.1.96
Try to send full-sized packets during retransmit.
.TP
.IR tcp_retries1 " (integer; default: 3; since Linux 2.2)"
.\" Since Linux 2.1.43
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.
.TP
.IR tcp_retries2 " (integer; default: 15; since Linux 2.2)"
.\" Since Linux 2.1.43
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.
.TP
.IR tcp_rfc1337 " (Boolean; default: disabled; since Linux 2.2)"
.\" Since Linux 2.1.90
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.
.TP
.IR tcp_rmem " (since Linux 2.4)"
.\" Since Linux 2.4.0-test7
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.
.RS
.TP
.I min
minimum size of the receive buffer used by each TCP socket.
The default value is the system page size.
(On Linux 2.4, the default value is 4\ kB, lowered to
.B 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
.B SO_RCVBUF
on a socket.
.TP
.I default
the default size of the receive buffer for a TCP socket.
This value overwrites the initial default buffer size from
the generic global
.I 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
.I net.ipv4.tcp_window_scaling
must be enabled (default).
.TP
.I max
the maximum size of the receive buffer used by each TCP socket.
This value does not override the global
.IR net.core.rmem_max .
This is not used to limit the size of the receive buffer declared using
.B SO_RCVBUF
on a socket.
The default value is calculated using the formula
.IP
.in +4n
.EX
max(87380, min(4\ MB, \fItcp_mem\fP[1]*PAGE_SIZE/128))
.EE
.in
.IP
(On Linux 2.4, the default is 87380*2 bytes,
lowered to 87380 in low-memory systems).
.RE
.TP
.IR tcp_sack " (Boolean; default: enabled; since Linux 2.2)"
.\" Since Linux 2.1.36
Enable RFC\ 2018 TCP Selective Acknowledgements.
.TP
.IR tcp_slow_start_after_idle " (Boolean; default: enabled; since Linux 2.6.18)"
.\" The following is from Linux 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
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.
.TP
.IR tcp_stdurg " (Boolean; default: disabled; since Linux 2.2)"
.\" Since Linux 2.1.44
If this option is enabled, then use the RFC\ 1122 interpretation
of the TCP urgent-pointer field.
.\" RFC 793 was ambiguous in its specification of the meaning of the
.\" urgent pointer.  RFC 1122 (and RFC 961) fixed on a particular
.\" resolution of this ambiguity (unfortunately the "wrong" one).
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.
.TP
.IR tcp_syn_retries  " (integer; default: 6; since Linux 2.2)"
.\" Since Linux 2.1.38
The maximum number of times initial SYNs for an active TCP
connection attempt will be retransmitted.
This value should not be higher than 255.
The default value is 6, which corresponds to retrying for up to
approximately 127 seconds.
Before Linux 3.7,
.\" commit 6c9ff979d1921e9fd05d89e1383121c2503759b9
the default value was 5, which
(in conjunction with calculation based on other kernel parameters)
corresponded to approximately 180 seconds.
.TP
.IR tcp_synack_retries " (integer; default: 5; since Linux 2.2)"
.\" Since Linux 2.1.38
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.
.TP
.IR tcp_syncookies " (integer; default: 1; since Linux 2.2)"
.\" Since Linux 2.1.43
Enable TCP syncookies.
The kernel must be compiled with
.BR CONFIG_SYN_COOKIES .
The syncookies feature attempts to protect a
socket from a SYN flood attack.
This should be used as a last resort, if at all.
This is a violation of the TCP protocol,
and conflicts with other areas of TCP such as TCP extensions.
It can cause problems for clients and relays.
It is not recommended as a tuning mechanism for heavily
loaded servers to help with overloaded or misconfigured conditions.
For recommended alternatives see
.IR tcp_max_syn_backlog ,
.IR tcp_synack_retries ,
and
.IR tcp_abort_on_overflow .
Set to one of the following values:
.RS
.TP
.B 0
Disable TCP syncookies.
.TP
.B 1
Send out syncookies when the syn backlog queue of a socket overflows.
.TP
.B 2
(since Linux 3.12)
.\" commit 5ad37d5deee1ff7150a2d0602370101de158ad86
Send out syncookies unconditionally.
This can be useful for network testing.
.RE
.TP
.IR tcp_timestamps " (integer; default: 1; since Linux 2.2)"
.\" Since Linux 2.1.36
Set to one of the following values to enable or disable RFC\ 1323
TCP timestamps:
.RS
.TP
.B 0
Disable timestamps.
.TP
.B 1
Enable timestamps as defined in RFC1323 and use random offset for
each connection rather than only using the current time.
.TP
.B 2
As for the value 1, but without random offsets.
.\" commit 25429d7b7dca01dc4f17205de023a30ca09390d0
Setting
.I tcp_timestamps
to this value is meaningful since Linux 4.10.
.RE
.TP
.IR tcp_tso_win_divisor " (integer; default: 3; since Linux 2.6.9)"
This parameter controls what percentage of the congestion window
can be consumed by a single TCP Segmentation Offload (TSO) frame.
The setting of this parameter is a tradeoff between burstiness and
building larger TSO frames.
.TP
.IR tcp_tw_recycle " (Boolean; default: disabled; Linux 2.4 to Linux 4.11)"
.\" Since Linux 2.3.15
.\" removed in Linux 4.12; commit 4396e46187ca5070219b81773c4e65088dac50cc
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.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_tw_reuse " (Boolean; default: disabled; since Linux 2.4.19/2.6)"
.\" Since Linux 2.4.19/2.5.43
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.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_vegas_cong_avoid  " (Boolean; default: disabled; Linux 2.2 to Linux 2.6.13)"
.\" Since Linux 2.1.8; removed in Linux 2.6.13
Enable TCP Vegas congestion avoidance algorithm.
TCP Vegas is a sender-side-only change to TCP that anticipates
the onset of congestion by estimating the bandwidth.
TCP Vegas adjusts the sending rate by modifying the congestion window.
TCP Vegas should provide less packet loss, but it is
not as aggressive as TCP Reno.
.\"
.\" The following is from Linux 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_westwood " (Boolean; default: disabled; Linux 2.4.26/2.6.3 to Linux 2.6.13)"
Enable TCP Westwood+ congestion control algorithm.
TCP Westwood+ is a sender-side-only modification of the TCP Reno
protocol stack that optimizes the performance of TCP congestion control.
It is based on end-to-end bandwidth estimation to set
congestion window and slow start threshold after a congestion episode.
Using this estimation, TCP Westwood+ adaptively sets a
slow start threshold and a congestion window which takes into
account the bandwidth used at the time congestion is experienced.
TCP Westwood+ significantly increases fairness with respect to
TCP Reno in wired networks and throughput over wireless links.
.TP
.IR tcp_window_scaling " (Boolean; default: enabled; since Linux 2.2)"
.\" Since Linux 2.1.36
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
.I tcp_window_scaling
is disabled, TCP will not negotiate the use of window
scaling with the other end during connection setup.
.TP
.IR tcp_wmem " (since Linux 2.4)"
.\" Since Linux 2.4.0-test7
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.
.RS
.TP
.I min
Minimum size of the send buffer used by each TCP socket.
The default value is the system page size.
(On Linux 2.4, the default value is 4\ kB.)
This value is used to ensure that in memory pressure mode,
allocations below this size will still succeed.
This is not used to bound the size of the send buffer declared using
.B SO_SNDBUF
on a socket.
.TP
.I default
The default size of the send buffer for a TCP socket.
This value overwrites the initial default buffer size from
the generic global
.I /proc/sys/net/core/wmem_default
defined for all protocols.
The default value is 16\ kB.
.\" True in Linux 2.4 and 2.6
If larger send buffer sizes are desired, this value
should be increased (to affect all sockets).
To employ large TCP windows, the
.I /proc/sys/net/ipv4/tcp_window_scaling
must be set to a nonzero value (default).
.TP
.I max
The maximum size of the send buffer used by each TCP socket.
This value does not override the value in
.IR /proc/sys/net/core/wmem_max .
This is not used to limit the size of the send buffer declared using
.B SO_SNDBUF
on a socket.
The default value is calculated using the formula
.IP
.in +4n
.EX
max(65536, min(4\ MB, \fItcp_mem\fP[1]*PAGE_SIZE/128))
.EE
.in
.IP
(On Linux 2.4, the default value is 128\ kB,
lowered 64\ kB depending on low-memory systems.)
.RE
.TP
.IR 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.
.SS Socket options
To set or get a TCP socket option, call
.BR getsockopt (2)
to read or
.BR setsockopt (2)
to write the option with the option level argument set to
.BR IPPROTO_TCP .
Unless otherwise noted,
.I optval
is a pointer to an
.IR int .
.\" or SOL_TCP on Linux
In addition,
most
.B IPPROTO_IP
socket options are valid on TCP sockets.
For more information see
.BR ip (7).
.P
Following is a list of TCP-specific socket options.
For details of some other socket options that are also applicable
for TCP sockets, see
.BR socket (7).
.TP
.BR TCP_CONGESTION " (since Linux 2.6.13)"
.\"	    commit 5f8ef48d240963093451bcf83df89f1a1364f51d
.\"	    Author: Stephen Hemminger <shemminger@osdl.org>
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
.I tcp_allowed_congestion_control
(described above).
Privileged processes
.RB ( CAP_NET_ADMIN )
can choose from any of the available congestion-control algorithms
(see the description of
.I tcp_available_congestion_control
above).
.TP
.BR TCP_CORK " (since Linux 2.2)"
.\" precisely: since Linux 2.1.127
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
.BR sendfile (2),
or for throughput optimization.
As currently implemented, there is a 200 millisecond ceiling on the time
for which output is corked by
.BR TCP_CORK .
If this ceiling is reached, then queued data is automatically transmitted.
This option can be combined with
.B TCP_NODELAY
only since Linux 2.5.71.
This option should not be used in code intended to be portable.
.TP
.BR TCP_DEFER_ACCEPT " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.38
.\" Useful references:
.\" http://www.techrepublic.com/article/take-advantage-of-tcp-ip-options-to-optimize-data-transmission/
.\" http://unix.stackexchange.com/questions/94104/real-world-use-of-tcp-defer-accept
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.
.TP
.BR TCP_INFO " (since Linux 2.4)"
Used to collect information about this socket.
The kernel returns a \fIstruct tcp_info\fP as defined in the file
.IR /usr/include/linux/tcp.h .
This option should not be used in code intended to be portable.
.TP
.BR TCP_KEEPCNT " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.18
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.
.TP
.BR TCP_KEEPIDLE " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.18
The time (in seconds) the connection needs to remain idle
before TCP starts sending keepalive probes, if the socket
option
.B SO_KEEPALIVE
has been set on this socket.
This option should not be used in code intended to be portable.
.TP
.BR TCP_KEEPINTVL " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.18
The time (in seconds) between individual keepalive probes.
This option should not be used in code intended to be portable.
.TP
.BR TCP_LINGER2 " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.41
The lifetime of orphaned FIN_WAIT2 state sockets.
This option can be used to override the system-wide setting in the file
.I /proc/sys/net/ipv4/tcp_fin_timeout
for this socket.
This is not to be confused with the
.BR socket (7)
level option
.BR SO_LINGER .
This option should not be used in code intended to be portable.
.TP
.B TCP_MAXSEG
.\" Present in Linux 1.0
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.
.TP
.B TCP_NODELAY
.\" Present in Linux 1.0
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
.BR TCP_CORK ;
however, setting this option forces an explicit flush of
pending output, even if
.B TCP_CORK
is currently set.
.TP
.BR 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.
.TP
.BR TCP_SYNCNT " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.18
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.
.TP
.BR TCP_USER_TIMEOUT " (since Linux 2.6.37)"
.\"	    commit dca43c75e7e545694a9dd6288553f55c53e2a3a3
.\"	    Author: Jerry Chu <hkchu@google.com>
.\" The following text taken nearly verbatim from Jerry Chu's (excellent)
.\" commit message.
.\"
This option takes an
.I unsigned int
as an argument.
When the value is greater than 0,
it specifies the maximum amount of time in milliseconds that transmitted
data may remain unacknowledged, or buffered data may remain untransmitted
(due to zero window size) before TCP will forcibly close the
corresponding connection and return
.B ETIMEDOUT
to the application.
If the option value is specified as 0,
TCP will use the system default.
.IP
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.
.IP
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
.RB ( SO_KEEPALIVE )
option,
.B TCP_USER_TIMEOUT
will override keepalive to determine when to close a
connection due to keepalive failure.
.IP
The option has no effect on when TCP retransmits a packet,
nor when a keepalive probe is sent.
.IP
This option, like many others, will be inherited by the socket returned by
.BR accept (2),
if it was set on the listening socket.
.IP
Further details on the user timeout feature can be found in
RFC\ 793 and RFC\ 5482 ("TCP User Timeout Option").
.TP
.BR TCP_WINDOW_CLAMP " (since Linux 2.4)"
.\" Precisely: since Linux 2.3.41
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.
.TP
.BR TCP_FASTOPEN " (since Linux 3.6)"
This option enables Fast Open (RFC\~7413) on the listener socket.
The value specifies the maximum length of pending SYNs
(similar to the backlog argument in
.BR listen (2)).
Once enabled,
the listener socket grants the TCP Fast Open cookie
on incoming SYN with TCP Fast Open option.
.IP
More importantly it accepts the data in SYN with a valid Fast Open cookie
and responds SYN-ACK acknowledging both the data and the SYN sequence.
.BR accept (2)
returns a socket that is available for read and write
when the handshake has not completed yet.
Thus the data exchange can commence before the handshake completes.
This option requires enabling the server-side support on sysctl
.I net.ipv4.tcp_fastopen
(see above).
For TCP Fast Open client-side support,
see
.BR send (2)
.B MSG_FASTOPEN
or
.B TCP_FASTOPEN_CONNECT
below.
.TP
.BR TCP_FASTOPEN_CONNECT " (since Linux 4.11)"
This option enables an alternative way to perform Fast Open
on the active side (client).
When this option is enabled,
.BR connect (2)
would behave differently depending on
if a Fast Open cookie is available for the destination.
.IP
If a cookie is not available (i.e. first contact to the destination),
.BR connect (2)
behaves as usual by sending a SYN immediately,
except the SYN would include an empty Fast Open cookie option
to solicit a cookie.
.IP
If a cookie is available,
.BR connect (2)
would return 0 immediately but the SYN transmission is deferred.
A subsequent
.BR write (2)
or
.BR sendmsg (2)
would trigger a SYN with data plus cookie in the Fast Open option.
In other words,
the actual connect operation is deferred until data is supplied.
.IP
.B Note:
While this option is designed for convenience,
enabling it does change the behaviors and certain system calls might set
different
.I errno
values.
With cookie present,
.BR write (2)
or
.BR sendmsg (2)
must be called right after
.BR connect (2)
in order to send out SYN+data to complete 3WHS and establish connection.
Calling
.BR read (2)
right after
.BR connect (2)
without
.BR write (2)
will cause the blocking socket to be blocked forever.
.IP
The application should either set
.B TCP_FASTOPEN_CONNECT
socket option before
.BR write (2)
or
.BR sendmsg (2),
or call
.BR write (2)
or
.BR sendmsg (2)
with
.B MSG_FASTOPEN
flag directly,
instead of both on the same connection.
.IP
Here is the typical call flow with this new option:
.IP
.in +4n
.EX
s = socket();
setsockopt(s, IPPROTO_TCP, TCP_FASTOPEN_CONNECT, 1, ...);
connect(s);
write(s); /* write() should always follow connect()
           * in order to trigger SYN to go out. */
read(s)/write(s);
/* ... */
close(s);
.EE
.in
.SS 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
.B SO_OOBINLINE
is not set).
This differs from BSD-based stacks.
.P
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
.IR /proc/sys/net/ipv4/tcp_stdurg .
.P
It is possible to peek at out-of-band data using the
.BR recv (2)
.B MSG_PEEK
flag.
.P
Since Linux 2.4, Linux supports the use of
.B MSG_TRUNC
in the
.I flags
argument of
.BR recv (2)
(and
.BR 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,
.B MSG_TRUNC
also has this effect when used in conjunction with
.B MSG_OOB
to receive out-of-band data.
.SS Ioctls
The following
.BR ioctl (2)
calls return information in
.IR value .
The correct syntax is:
.P
.RS
.nf
.BI int " value";
.IB error " = ioctl(" tcp_socket ", " ioctl_type ", &" value ");"
.fi
.RE
.P
.I ioctl_type
is one of the following:
.TP
.B SIOCINQ
Returns the amount of queued unread data in the receive buffer.
The socket must not be in LISTEN state, otherwise an error
.RB ( EINVAL )
is returned.
.B SIOCINQ
is defined in
.IR <linux/sockios.h> .
.\" FIXME https://www.sourceware.org/bugzilla/show_bug.cgi?id=12002,
.\" filed 2010-09-10, may cause SIOCINQ to be defined in glibc headers
Alternatively,
you can use the synonymous
.BR FIONREAD ,
defined in
.IR <sys/ioctl.h> .
.TP
.B SIOCATMARK
Returns true (i.e.,
.I value
is nonzero) if the inbound data stream is at the urgent mark.
.IP
If the
.B SO_OOBINLINE
socket option is set, and
.B SIOCATMARK
returns true, then the
next read from the socket will return the urgent data.
If the
.B SO_OOBINLINE
socket option is not set, and
.B 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
.B recv(MSG_OOB)
flag).
.IP
Note that a read never reads across the urgent mark.
If an application is informed of the presence of urgent data via
.BR select (2)
(using the
.I exceptfds
argument) or through delivery of a
.B SIGURG
signal,
then it can advance up to the mark using a loop which repeatedly tests
.B SIOCATMARK
and performs a read (requesting any number of bytes) as long as
.B SIOCATMARK
returns false.
.TP
.B SIOCOUTQ
Returns the amount of unsent data in the socket send queue.
The socket must not be in LISTEN state, otherwise an error
.RB ( EINVAL )
is returned.
.B SIOCOUTQ
is defined in
.IR <linux/sockios.h> .
.\" FIXME . https://www.sourceware.org/bugzilla/show_bug.cgi?id=12002,
.\" filed 2010-09-10, may cause SIOCOUTQ to be defined in glibc headers
Alternatively,
you can use the synonymous
.BR TIOCOUTQ ,
defined in
.IR <sys/ioctl.h> .
.SS Error handling
When a network error occurs, TCP tries to resend the packet.
If it doesn't succeed after some time, either
.B ETIMEDOUT
or the last received error on this connection is reported.
.P
Some applications require a quicker error notification.
This can be enabled with the
.B IPPROTO_IP
level
.B IP_RECVERR
socket option.
When this option is enabled, all incoming
errors are immediately passed to the user program.
Use this option with care \[em] it makes TCP less tolerant to routing
changes and other normal network conditions.
.SH ERRORS
.TP
.B EAFNOTSUPPORT
Passed socket address type in
.I sin_family
was not
.BR AF_INET .
.TP
.B EPIPE
The other end closed the socket unexpectedly or a read is
executed on a shut down socket.
.TP
.B ETIMEDOUT
The other end didn't acknowledge retransmitted data after some time.
.P
Any errors defined for
.BR ip (7)
or the generic socket layer may also be returned for TCP.
.SH VERSIONS
Support for Explicit Congestion Notification, zero-copy
.BR sendfile (2),
reordering support and some SACK extensions
(DSACK) were introduced in Linux 2.4.
Support for forward acknowledgement (FACK), TIME_WAIT recycling,
and per-connection keepalive socket options were introduced in Linux 2.3.
.SH BUGS
Not all errors are documented.
.P
IPv6 is not described.
.\" Only a single Linux kernel version is described
.\" Info for 2.2 was lost. Should be added again,
.\" or put into a separate page.
.\" .SH AUTHORS
.\" This man page was originally written by Andi Kleen.
.\" It was updated for 2.4 by Nivedita Singhvi with input from
.\" Alexey Kuznetsov's Documentation/networking/ip-sysctl.txt
.\" document.
.SH SEE ALSO
.BR accept (2),
.BR bind (2),
.BR connect (2),
.BR getsockopt (2),
.BR listen (2),
.BR recvmsg (2),
.BR sendfile (2),
.BR sendmsg (2),
.BR socket (2),
.BR ip (7),
.BR socket (7)
.P
The kernel source file
.IR Documentation/networking/ip\-sysctl.txt .
.P
RFC\ 793 for the TCP specification.
.br
RFC\ 1122 for the TCP requirements and a description of the Nagle algorithm.
.br
RFC\ 1323 for TCP timestamp and window scaling options.
.br
RFC\ 1337 for a description of TIME_WAIT assassination hazards.
.br
RFC\ 3168 for a description of Explicit Congestion Notification.
.br
RFC\ 2581 for TCP congestion control algorithms.
.br
RFC\ 2018 and RFC\ 2883 for SACK and extensions to SACK.