man-pages/man7/tcp.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\" 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.
.\"
.\" 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.
.\"
.TH TCP  7 2003-08-21 "Linux Man Page" "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>
.br
.B tcp_socket = socket(PF_INET, SOCK_STREAM, 0); 
.SH DESCRIPTION
This is an implementation of the TCP protocol defined in
RFC793, RFC1122 and RFC2001 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 fresh 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
.BR bind (2) 
the socket first to a local address and port and then call 
.BR listen (2)
to put the socket into listening state.  After that a new
socket for each incoming connection can be accepted 
using
.BR accept (2).
A socket which has had
.B accept
or 
.B connect
successfully called on it is fully specified and may
transmit data.  Data cannot be transmitted on listening or
not yet connected sockets.

Linux supports RFC1323 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
.B net.ipv4.tcp_wmem
and
.B net.ipv4.tcp_rmem
sysctl variables, 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 global
.B net.core.rmem_max
and
.B net.core.wmem_max
sysctls.  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 this for administrative purposes and internal
kernel structures, and the sysctl variables reflect the
larger sizes compared to the actual TCP windows. 
On individual connections, the socket buffer size must be
set prior to the
.B listen()
or 
.B connect()
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 reading process or the process or process
group that has been set for the socket using the
.B SIOCSPGRP
or 
.B FIOSETOWN 
ioctls. When the 
.B SO_OOBINLINE
socket option is enabled, urgent data is put into the normal
data stream (and can be tested for by the
.B SIOCATMARK
ioctl), 
otherwise it can be only received when the
.B MSG_OOB
flag is set for 
.BR sendmsg (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 zerocopy
.BR sendfile (2), 
Explicit Congestion Notification, new
management of TIME_WAIT sockets, keep-alive socket options
and support for Duplicate SACK extensions.
.SH "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.
.SH SYSCTLS
These variables can be accessed by the 
.B /proc/sys/net/ipv4/* 
files or with the 
.BR sysctl (2)
interface.  In addition, most IP sysctls also apply to TCP; see
.BR ip (7). 
.TP
.B tcp_abort_on_overflow
Enable resetting connections if the listening service is too
slow and unable to keep up and accept them.  It is not
enabled by default.  It means that if overflow occurred due
to a burst, the connection will recover.  Enable this option
_only_ if you are really sure that the listening daemon
cannot be tuned to accept connections faster.  Enabling this
option can harm the clients of your server. 
.TP
.B tcp_adv_win_scale
Count buffering overhead as bytes/2^tcp_adv_win_scale
(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
if it is <= 0. The default is 2.  

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 
.B 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 
.B tcp_app_win 
This variable defines how many
bytes of the TCP window are reserved for buffering
overhead.

A maximum of (window/2^tcp_app_win, mss) bytes in the window
are reserved for the application buffer.  A value of 0
implies that no amount is reserved.  The default value is 31.
.TP
.B tcp_dsack
Enable RFC2883 TCP Duplicate SACK support. 
It is enabled by default.
.TP
.B tcp_ecn
Enable RFC2884 Explicit Congestion Notification.  It is not
enabled by default.  When enabled, connectivity to some
destinations could be affected due to older, misbehaving
routers along the path causing connections to be dropped.
.TP
.B tcp_fack
Enable TCP Forward Acknowledgement support.  It is enabled by
default.
.TP
.B tcp_fin_timeout
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 (DoS) attacks.  The default value in 2.4
kernels is 60, down from 180 in 2.2.
.TP
.B tcp_keepalive_intvl
The number of seconds between TCP keep-alive probes.
The default value is 75 seconds.
.TP
.B tcp_keepalive_probes
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.
The default value is 9. 
.TP
.B tcp_keepalive_time
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.
.TP
.B tcp_max_orphans
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 DoS
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
.B tcp_max_syn_backlog
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
include/net/tcp.h be modified to keep
TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the kernel be
recompiled.
.TP
.B tcp_max_tw_buckets
The maximum number of sockets in TIME_WAIT state allowed in
the system.  This limit exists only to prevent simple DoS
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 
.B tcp_mem
This is a vector of 3 integers: [low, pressure, high].  These
bounds are used by TCP to track its memory usage.  The
defaults are calculated at boot time from the amount of
available memory.

.I low 
- TCP doesn't regulate its memory allocation when the number
of pages it has allocated globally is below this number.

.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 
.B low 
mark.

.I high
- the maximum number of pages, globally, that TCP
will allocate.  This value overrides any other limits
imposed by the kernel. 
.TP
.B tcp_orphan_retries
The maximum number of attempts made to probe the other
end of a connection which has been closed by our end.
The default value is 8.
.TP
.B tcp_reordering
The maximum a packet can be reordered in a TCP packet stream
without TCP assuming packet loss and going into slow start.
The default is 3.  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
.B tcp_retrans_collapse
Try to send full-sized packets during retransmit. 
This is enabled by default. 
.TP
.B tcp_retries1
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
.B tcp_retries2
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 RFC1122 specified
minimum limit of 100 seconds is typically deemed too
short.
.TP
.B tcp_rfc1337
Enable TCP behaviour conformant with RFC 1337.
This is not enabled by default.  When not enabled,
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
.B tcp_rmem 
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 sysctl variables, depending on memory available
in the system.

.I min
- minimum size of the receive buffer used by each TCP
socket.  The default value is 4K, and is lowered to
PAGE_SIZE bytes in low memory systems.  This value
is used to ensure that in memory pressure mode,
allocations below this size will still succeed.  This is not
used to bound the size of the receive buffer declared
using
.B SO_RCVBUF
on a socket.  

.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 
.B net.core.rmem_default
defined for all protocols.  The default value is 87380
bytes, and is 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
.B net.ipv4.tcp_window_scaling 
must be enabled (default).

.I max
- the maximum size of the receive buffer used by
each TCP socket.  This value does not override the global 
.BR 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 of 87380*2 bytes is lowered to 87380
in low memory systems.
.TP
.B tcp_sack
Enable RFC2018 TCP Selective Acknowledgements. 
It is enabled by default.
.TP
.B tcp_stdurg
Enable the strict RFC793 interpretation of the TCP
urgent-pointer field.  The default is to use the
BSD-compatible interpretation of the urgent-pointer, pointing
to the first byte after the urgent data.  The RFC793
interpretation is to have it point to the last byte of urgent
data.  Enabling this option may lead to interoperatibility
problems.
.TP
.B tcp_synack_retries
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. The default
value is 5.
.TP
.B tcp_syncookies
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
.BR tcp_max_syn_backlog ,
.BR tcp_synack_retries ,
.BR tcp_abort_on_overflow .
.TP
.B tcp_syn_retries
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
.B tcp_timestamps
Enable RFC1323 TCP timestamps.  This is enabled
by default.
.TP
.B tcp_tw_recycle
Enable fast recycling of TIME-WAIT sockets.  It is 
not enabled by default.  Enabling this option is not
recommended since this causes problems when working
with NAT (Network Address Translation). 
.TP
.B tcp_window_scaling
Enable RFC1323 TCP window scaling.  It is enabled by
default.  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
.B tcp_window_scaling 
is disabled, TCP will not negotiate the use of window
scaling with the other end during connection setup.
.TP
.B tcp_wmem
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 sysctl
variables, depending on memory available.

.I min
- minimum size of the send buffer used by each TCP socket.
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.

.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
.B net.core.wmem_default
defined for all protocols.  The default value is 16K bytes.
If larger send buffer sizes are desired, this value
should be increased (to affect all sockets).  To employ
large TCP windows, the sysctl variable
.B net.ipv4.tcp_window_scaling
must be enabled (default).

.I max
- the maximum size of the send buffer used by
each TCP socket.  This value does not override the global 
.BR 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 128K bytes.  It is lowered to 64K
depending on the memory available in the system.
.SH "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 SOL_TCP.
In addition,
most 
.B SOL_IP 
socket options are valid on TCP sockets. For more
information see
.BR ip (7).
.TP
.B TCP_CORK
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.
This option can be
combined with
.BR TCP_NODELAY
only since Linux 2.5.71.
This option should not be used in code intended to be
portable.
.TP
.B TCP_DEFER_ACCEPT
Allows 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
.B TCP_INFO
Used to collect information about this socket.  The kernel
returns a struct tcp_info as defined in the file
/usr/include/linux/tcp.h.  This option should not be used in
code intended to be portable.
.TP
.B TCP_KEEPCNT
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
.B TCP_KEEPIDLE
The time (in seconds) the connection needs to remain idle
before TCP starts sending keepalive probes, if the socket
option SO_KEEPALIVE has been set on this socket.  This
option should not be used in code intended to be portable.
.TP
.B TCP_KEEPINTVL
The time (in seconds) between individual keepalive probes.
This option should not be used in code intended to be
portable.
.TP
.B TCP_LINGER2
The lifetime of orphaned FIN_WAIT2 state sockets.  This
option can be used to override the system wide sysctl
.B tcp_fin_timeout
on 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
The maximum segment size for outgoing TCP packets.  If this
option is set before connection establishment, it also
changes the MSS value announced to the other end in the
initial packet.  Values greater than the (eventual)
interface MTU have no effect.  TCP will also impose 
its minimum and maximum bounds over the value provided.
.TP
.B TCP_NODELAY
If set, disable the Nagle algorithm.  This means that segments
are always sent as soon as possible, even if there is only a
small amount of data.  When not set, data is buffered until there
is a sufficient amount to send out, thereby avoiding the
frequent sending of small packets, which results in poor
utilization of the network.
This option is overriden by
.BR TCP_CORK ; 
however, setting this option forces an explicit flush of
pending output, even if
.B TCP_CORK
is currently set.
.TP
.B TCP_QUICKACK
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
.B TCP_SYNCNT
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
.B TCP_WINDOW_CLAMP
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.
.SH IOCTLS
These ioctls can be accessed using 
.BR ioctl (2).
The correct syntax is:
.PP
.RS
.nf
.BI int " value";
.IB error " = ioctl(" tcp_socket ", " ioctl_type ", &" value ");"
.fi
.RE
.TP
.BR SIOCINQ
Returns the amount of queued unread data in the receive
buffer.  Argument is a pointer to an integer.  The socket
must not be in LISTEN state, otherwise an error (EINVAL)
is returned.
.TP
.B SIOCATMARK
Returns true when the all urgent data has been already
received by the user program.  This is used together with
.BR SO_OOBINLINE .
Argument is an pointer to an integer for the test result.
.TP
.B SIOCOUTQ
Returns the amount of unsent data in the socket send queue
in the passed integer value pointer.  The socket must not 
be in LISTEN state, otherwise an error (EINVAL)
is returned.
.SH "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 SOL_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 \- it makes TCP less tolerant to routing
changes and other normal network conditions.
.SH NOTES
When an error occurs doing a connection setup occurring in a
socket write
.B SIGPIPE
is only raised when the
.B SO_KEEPALIVE
socket option is set.
.PP
TCP has no real out-of-band data; it has 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 RFC1122, but is
required for interoperability with other stacks.  It can be
changed by the
.B tcp_stdurg
sysctl.
.SH ERRORS
.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.
.TP
.B EAFNOTSUPPORT
Passed socket address type in
.I sin_family 
was not 
.BR AF_INET .
.PP
Any errors defined for
.BR ip (7)
or the generic socket layer may also be returned for TCP.
.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 VERSIONS
Support for Explicit Congestion Notification, zerocopy 
sendfile, reordering support and some SACK extensions
(DSACK) were introduced in 2.4.
Support for forward acknowledgement (FACK), TIME_WAIT recycling, 
per connection keepalive socket options and sysctls
were introduced in 2.3.

The default values and descriptions for the sysctl variables 
given above are applicable for the 2.4 kernel.
.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-sysctls.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 sysctl (2),
.BR ip (7),
.BR socket (7)
.sp
RFC793 for the TCP specification.
.br
RFC1122 for the TCP requirements and a description 
of the Nagle algorithm.
.br
RFC1323 for TCP timestamp and window scaling options.
.br
RFC1644 for a description of TIME_WAIT assassination
hazards.
.br
RFC2481 for a description of Explicit Congestion
Notification.
.br
RFC2581 for TCP congestion control algorithms.
.br
RFC2018 and RFC2883 for SACK and extensions to SACK.