.\" 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.

.\"

.\" %%%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_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 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 2017-09-15 "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>

.PP

.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.

.PP

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.

.PP

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.

.PP

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-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).

.PP

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.

.PP

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 2.6.15; removed in 3.9

.\" commit ca2eb5679f8ddffff60156af42595df44a315ef0

.\" 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.

.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 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 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 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^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 " (Integer; default: see below; since Linux 2.4)"

.\" Since 2.4.0-test7

Enable RFC\ 3168 Explicit Congestion Notification.

.IP

This file can have one of the following values:

.RS

.IP 0

Disable ECN.

Neither initiate nor accept ECN.

This was the default up to and including Linux 2.6.30.

.IP 1

Enable ECN when requested by incoming connections and also

request ECN on outgoing connection attempts.

.IP 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 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: see below; 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.

.IP

This file can have one of the following values:

.RS

.IP 0 3

Disabled.

This was the default up to and including Linux 2.6.23.

.IP 1

The basic version F-RTO algorithm is enabled.

.IP 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

.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 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 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 ~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 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

Prior to 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

    TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

.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 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 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

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

.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 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 " (integer; default: 1; since Linux 2.2)"

.\" Since 2.1.36

Set to one of the following values to enable or disable RFC\ 1323

TCP timestamps:

.RS

.IP 0 3

Disable timestamps.

.IP 1

Enable timestamps as defined in RFC1323 and use random offset for

each connection rather than only using the current time.

.IP 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 4.11)"

.\" Since 2.3.15

.\" removed in 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 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