path: root/man7/tcp.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\"
.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
.\" %%%LICENSE_END
.\"
.\" 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_CONGESTION (new in 2.6.13)
.\"	    commit 5f8ef48d240963093451bcf83df89f1a1364f51d
.\"	    Author: Stephen Hemminger <shemminger@osdl.org>
.\"	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>
.\"	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_USER_TIMEOUT (new in 2.6.37)
.\"	    Author: Jerry Chu <hkchu@google.com>
.\"	    commit dca43c75e7e545694a9dd6288553f55c53e2a3a3
.\"	TCP_REPAIR (3.5)
.\"	    commit ee9952831cfd0bbe834f4a26489d7dce74582e37
.\"	    Author: Pavel Emelyanov <xemul@parallels.com>
.\"	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>
.\"
.TH TCP  7 2012-04-23 "Linux" "Linux Programmer's Manual"
.SH NAME
tcp \- TCP protocol
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.B #include <netinet/in.h>
.br
.B #include <netinet/tcp.h>
.sp
.B tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
.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.

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.

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
.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.

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.
.PP
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.1-2001-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 only received when the
.B MSG_OOB
flag is set for
.BR recv (2)
or
.BR 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
.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 only supports point-to-point
communication; 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; since Linux 2.6.15)"
.\" The following is from 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 acknowledgments.
Possible values are:
.RS
.IP 0 3
increase
.I cwnd
once per acknowledgment (no ABC)
.IP 1
increase
.I cwnd
once per acknowledgment of full sized segment
.IP 2
allow increase
.I cwnd
by two if acknowledgment is
of two segments to compensate for delayed acknowledgments.
.RE
.TP
.IR tcp_abort_on_overflow " (Boolean; default: disabled; since Linux 2.4)"
.\" Since 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 2.4.0-test7
Count buffering overhead as
.IR "bytes/2^tcp_adv_win_scale" ,
if
.I tcp_adv_win_scale
is greater than 0; or
.IR "bytes-bytes/2^(\-tcp_adv_win_scale)" ,
if
.I 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
.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 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 list is a subset of those listed in
.IR tcp_available_congestion_control .
.\" FIXME How are the items in this delimited? Null bytes, spaces, commas?
The default value for this list is "reno" plus the default setting of
.IR tcp_congestion_control .
.TP
.IR tcp_available_congestion_control " (String; read-only; since Linux 2.4.20)"
.\" The following is from 2.6.28-rc4: Documentation/networking/ip-sysctl.txt
Show a list of the congestion-control algorithms
that are registered.
.\" FIXME How are the items in this delimited? Null bytes, spaces, commas?
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 2.4.0-test7
This variable defines how many
bytes of the TCP window are reserved for buffering overhead.

A maximum of (\fIwindow/2^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 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 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR 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 friendliness.
.\"
.\" The following is from 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 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 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 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 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 2.4.0-test7
Enable RFC\ 2883 TCP Duplicate SACK support.
.TP
.IR tcp_ecn " (Boolean; default: disabled; since Linux 2.4)"
.\" Since 2.4.0-test7
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.
.TP
.IR tcp_fack " (Boolean; default: enabled; since Linux 2.2)"
.\" Since 2.1.92
Enable TCP Forward Acknowledgement support.
.TP
.IR tcp_fin_timeout " (integer; default: 60; since Linux 2.2)"
.\" Since 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 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_frto " (integer; default: 0; since Linux 2.4.21/2.6)"
.\" Since 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.

This file can have one of the following values:
.RS
.IP 0 3
Disabled.
.IP 1
The basic version F-RTO algorithm is enabled.
.IP 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.
.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
.IP 0 3
Rate halving based; a smooth and conservative response,
results in halved congestion window
.RI ( cwnd )
and slow-start threshold
.RI ( ssthresh )
after one RTT.
.IP 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.
.IP 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 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 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 2.1.43
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
.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.

Note that underlying connection tracking mechanisms and
application timeouts may be much shorter.
.\"
.\" The following is from 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_low_latency  " (Boolean; default: disabled; since Linux 2.4.21/2.6)"
.\" Since 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.
.TP
.IR tcp_max_orphans  " (integer; default: see below; since Linux 2.4)"
.\" Since 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 ~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.
.TP
.IR tcp_max_syn_backlog " (integer; default: see below; since Linux 2.2)"
.\" Since 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 (>= 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
.I include/net/tcp.h
be modified to keep
TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel be
recompiled.
.TP
.IR tcp_max_tw_buckets " (integer; default: see below; since Linux 2.4)"
.\" Since 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 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 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 10
.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 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
.IP 0 3
Disabled
.IP 1
Disabled by default, enabled when an ICMP black hole detected
.IP 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 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 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 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 2.1.96
Try to send full-sized packets during retransmit.
.TP
.IR tcp_retries1 " (integer; default: 3; since Linux 2.2)"
.\" Since 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 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 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 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 10
.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 4K, 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

    max(87380, min(4MB, \fItcp_mem\fP[1]*PAGE_SIZE/128))

(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 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 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 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: 5; since Linux 2.2)"
.\" Since 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 5, which corresponds to approximately 180 seconds.
.TP
.IR tcp_synack_retries " (integer; default: 5; since Linux 2.2)"
.\" Since 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 " (Boolean; since Linux 2.2)"
.\" Since 2.1.43
Enable TCP syncookies.
The kernel must be compiled with
.BR 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
.IR tcp_max_syn_backlog ,
.IR tcp_synack_retries ,
and
.IR tcp_abort_on_overflow .
.TP
.IR tcp_timestamps " (Boolean; default: enabled; since Linux 2.2)"
.\" Since 2.1.36
Enable RFC\ 1323 TCP timestamps.
.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; since Linux 2.4)"
.\" Since 2.3.15
Enable fast recycling of TIME_WAIT sockets.
Enabling this option is not
recommended since this causes problems when working
with NAT (Network Address Translation).
.\"
.\" The following is from 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_tw_reuse " (Boolean; default: disabled; since Linux 2.4.19/2.6)"
.\" Since 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 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR tcp_vegas_cong_avoid  " (Boolean; default: disabled; Linux 2.2 to 2.6.13)"
.\" Since 2.1.8; removed in 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 2.6.12: Documentation/networking/ip-sysctl.txt
.TP
.IR 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.
.TP
.IR tcp_window_scaling " (Boolean; default: enabled; since Linux 2.2)"
.\" Since 2.1.36
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
.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 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 10
.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 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
.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 16K bytes.
.\" 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

    max(65536, min(4MB, \fItcp_mem\fP[1]*PAGE_SIZE/128))

(On Linux 2.4, the default value is 128K bytes,
lowered 64K 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 .
.\" 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).
.TP
.BR TCP_CORK " (since Linux 2.2)"
.\" precisely: since 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 2.3.38
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 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 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 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 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 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_WINDOW_CLAMP " (since Linux 2.4)"
.\" Precisely: since 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.
.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.
.PP
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 .

It is possible to peek at out-of-band data using the
.IR recv (2)
.B MSG_PEEK
flag.

Since version 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,
.BR MSG_PEEK
also has this effect when used in conjunction with
.BR 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:
.PP
.RS
.nf
.BI int " value";
.IB error " = ioctl(" tcp_socket ", " ioctl_type ", &" value ");"
.fi
.RE
.PP
.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 http://sources.redhat.com/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.

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).

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 http://sources.redhat.com/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.
.PP
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.
.PP
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 2.4.
Support for forward acknowledgement (FACK), TIME_WAIT recycling,
and per-connection keepalive socket options were introduced in 2.3.
.SH BUGS
Not all errors are documented.
.br
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)
.sp
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.