sched.7: Note error that occurs when writing invalid value to /proc/PID/autogroup

[thirdparty/man-pages.git] / man7 / tcp.7

.\" 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 2015-12-05 "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-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).

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; since Linux 2.6.15)"
.\" The following is from 2.6.28-rc4: Documentation/networking/ip-sysctl.txt