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TCP(7)			   Linux Programmer's Manual			TCP(7)

NAME
       tcp - TCP protocol

SYNOPSIS
       #include	<sys/socket.h>
       #include	<netinet/in.h>
       #include	<netinet/tcp.h>

       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

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  pro-
       vides  a	 reliable, stream-oriented, full-duplex	connection between two
       sockets on top of ip(7),	for both v4 and	v6 versions.   TCP  guarantees
       that the	data arrives in	order and retransmits lost packets.  It	gener-
       ates 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  connect(2)
       to establish a connection to another TCP	socket.	 To receive new	incom-
       ing connections,	first bind(2) the socket to a local address  and  port
       and  then  call	listen(2)  to put the socket into the listening	state.
       After that a new	socket for each	incoming connection  can  be  accepted
       using  accept(2).   A socket which has had accept(2) or connect(2) suc-
       cessfully 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    /proc/sys/net/ipv4/tcp_wmem	   and
       /proc/sys/net/ipv4/tcp_rmem  files,  or	on individual sockets by using
       the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call.

       The maximum sizes for socket buffers declared  via  the	SO_SNDBUF  and
       SO_RCVBUF    mechanisms	  are	limited	  by   the   values   in   the
       /proc/sys/net/core/rmem_max  and	  /proc/sys/net/core/wmem_max	files.
       Note that TCP actually allocates	twice the size of the buffer requested
       in the setsockopt(2) call, and so a succeeding getsockopt(2) call  will
       not  return  the	 same size of buffer as	requested in the setsockopt(2)
       call.  TCP uses the extra space for administrative purposes and	inter-
       nal  kernel  structures,	 and  the /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 listen(2) or connect(2)
       calls in	order to have it take effect.  See socket(7) for more informa-
       tion.

       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 MSG_OOB option to send(2).  When urgent data	is received, the  ker-
       nel sends a SIGURG signal to the	process	or process group that has been
       set as the socket "owner" using the SIOCSPGRP or	FIOSETOWN  ioctls  (or
       the  POSIX.1-2001-specified  fcntl(2)  F_SETOWN	operation).   When the
       SO_OOBINLINE socket option is enabled, urgent data is put into the nor-
       mal  data stream	(a program can test for	its location using the SIOCAT-
       MARK ioctl described below), otherwise it can be	received only when the
       MSG_OOB flag is set for recv(2) or recvmsg(2).

       Linux  2.4  introduced  a number	of changes for improved	throughput and
       scaling,	as well	as enhanced functionality.  Some of these features in-
       clude  support for zero-copy sendfile(2), Explicit Congestion Notifica-
       tion, new management of TIME_WAIT sockets,  keep-alive  socket  options
       and support for Duplicate SACK extensions.

   Address formats
       TCP  is built on	top of IP (see ip(7)).	The address formats defined by
       ip(7) apply to TCP.  TCP	supports  point-to-point  communication	 only;
       broadcasting and	multicasting are not supported.

   /proc interfaces
       System-wide  TCP	parameter settings can be accessed by files in the di-
       rectory /proc/sys/net/ipv4/.  In	addition,  most	 IP  /proc  interfaces
       also  apply  to TCP; see	ip(7).	Variables described as Boolean take an
       integer value, with a nonzero value ("true") meaning  that  the	corre-
       sponding	option is enabled, and a zero value ("false") meaning that the
       option is disabled.

       tcp_abc (Integer; default: 0; since Linux 2.6.15)
	      Control the Appropriate Byte Count (ABC),	defined	in  RFC	 3465.
	      ABC  is  a  way  of increasing the congestion window (cwnd) more
	      slowly in	response to partial acknowledgments.  Possible	values
	      are:

	      0	 increase cwnd once per	acknowledgment (no ABC)

	      1	 increase cwnd once per	acknowledgment of full sized segment

	      2	 allow	increase  cwnd by two if acknowledgment	is of two seg-
		 ments to compensate for delayed acknowledgments.

       tcp_abort_on_overflow (Boolean; default:	disabled; since	Linux 2.4)
	      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 only if you are really sure that the listen-
	      ing daemon cannot	be tuned to accept  connections	 faster.   En-
	      abling this option can harm the clients of your server.

       tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
	      Count   buffering	  overhead  as	bytes/2^tcp_adv_win_scale,  if
	      tcp_adv_win_scale	   is	 greater    than    0;	  or	bytes-
	      bytes/2^(-tcp_adv_win_scale),  if	tcp_adv_win_scale is less than
	      or equal to zero.

	      The socket receive buffer	space is shared	between	 the  applica-
	      tion  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 applica-
	      tion  latencies.	 The  tcp_adv_win_scale	default	value of 2 im-
	      plies that the space used	for  the  application  buffer  is  one
	      fourth that of the total.

       tcp_allowed_congestion_control  (String;	default: see text; since Linux
       2.4.20)
	      Show/set the congestion control algorithm	choices	 available  to
	      unprivileged  processes  (see the	description of the TCP_CONGES-
	      TION 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  tcp_available_congestion_control.
	      The  default value for this list is "reno" plus the default set-
	      ting of tcp_congestion_control.

       tcp_autocorking (Boolean; default: enabled; since Linux 3.14)
	      If this option is	enabled, the kernel tries  to  coalesce	 small
	      writes  (from consecutive	write(2) and sendmsg(2)	calls) as much
	      as possible, in order to decrease	the total number of sent pack-
	      ets.   Coalescing	 is  done if at	least one prior	packet for the
	      flow is waiting in Qdisc queues or device	transmit  queue.   Ap-
	      plications  can  still  use the TCP_CORK socket option to	obtain
	      optimal behavior when they know how/when to uncork  their	 sock-
	      ets.

       tcp_available_congestion_control	  (String;   read-only;	  since	 Linux
       2.4.20)
	      Show a list of the congestion-control algorithms that are	regis-
	      tered.   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 tcp_allowed_congestion_control.  More conges-
	      tion-control algorithms may be available	as  modules,  but  not
	      loaded.

       tcp_app_win (integer; default: 31; since	Linux 2.4)
	      This  variable  defines how many bytes of	the TCP	window are re-
	      served 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.

       tcp_base_mss (Integer; default: 512; since Linux	2.6.17)
	      The initial value	of search_low to be used by the	 packetization
	      layer  Path  MTU discovery (MTU probing).	 If MTU	probing	is en-
	      abled, this is the initial MSS used by the connection.

       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 lin-
	      ear RTT fairness as well as good scalability.  Under small  con-
	      gestion  windows,	 binary	search increase	provides TCP friendli-
	      ness.

       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 be-
	      haves the	same as	the default TCP	Reno.

       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  con-
	      verge more rapidly.

       tcp_congestion_control (String; default:	see text; since	Linux 2.4.13)
	      Set  the default congestion-control algorithm to be used for new
	      connections.  The	algorithm "reno" is always available, but  ad-
	      ditional choices may be available	depending on kernel configura-
	      tion.  The default value for this	file is	set as part of	kernel
	      configuration.

       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 CONFIG_NET_DMA option.

       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
	      Enable RFC 2883 TCP Duplicate SACK support.

       tcp_ecn (Boolean; default: disabled; since Linux	2.4)
	      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.

       tcp_fack	(Boolean; default: enabled; since Linux	2.2)
	      Enable TCP Forward Acknowledgement support.

       tcp_fin_timeout (integer; default: 60; since Linux 2.2)
	      This specifies how many seconds to wait for a final  FIN	packet
	      before the socket	is forcibly closed.  This is strictly a	viola-
	      tion of the TCP specification, but required to  prevent  denial-
	      of-service attacks.  In Linux 2.2, the default value was 180.

       tcp_frto	(integer; default: 0; since Linux 2.4.21/2.6)
	      Enable F-RTO, an enhanced	recovery algorithm for TCP retransmis-
	      sion 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:

	      0	 Disabled.

	      1	 The basic version F-RTO algorithm is enabled.

	      2	 Enable	SACK-enhanced F-RTO if flow uses SACK.	The basic ver-
		 sion 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.

	      Before Linux 2.6.22, this	parameter was a	 Boolean  value,  sup-
	      porting just values 0 and	1 above.

       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 concern-
	      ing what to do next.  Possible values are:

	      0	 Rate halving based; a smooth and conservative	response,  re-
		 sults	in  halved  congestion	window	(cwnd)	and slow-start
		 threshold (ssthresh) after one	RTT.

	      1	 Very conservative  response;  not  recommended	 because  even
		 though	 being	valid,	it  interacts  poorly with the rest of
		 Linux TCP; halves cwnd	and ssthresh immediately.

	      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  cau-
		 tious); cwnd and ssthresh are restored	to the values prior to
		 timeout.

       tcp_keepalive_intvl (integer; default: 75; since	Linux 2.4)
	      The number of seconds between TCP	keep-alive probes.

       tcp_keepalive_probes (integer; default: 9; since	Linux 2.2)
	      The maximum number of TCP	keep-alive probes to send before  giv-
	      ing  up  and  killing  the connection if no response is obtained
	      from the other end.

       tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
	      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 SO_KEEPALIVE socket option  is  enabled.	  The  default
	      value  is	 7200 seconds (2 hours).  An idle connection is	termi-
	      nated 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 applica-
	      tion timeouts may	be much	shorter.

       tcp_low_latency (Boolean; default: disabled; since Linux	2.4.21/2.6)
	      If enabled, the TCP stack	makes decisions	that prefer lower  la-
	      tency  as	 opposed to higher throughput.	It this	option is dis-
	      abled, then higher throughput is preferred.  An  example	of  an
	      application  where this default should be	changed	would be a Be-
	      owulf compute cluster.

       tcp_max_orphans (integer; default: see below; since Linux 2.4)
	      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.	  Net-
	      work  conditions might require you to increase the number	of or-
	      phans 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.

       tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
	      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 drop-
	      ping 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  mem-
	      ory  (<=	32Mb).	It is recommended that if this needs to	be in-
	      creased above 1024, TCP_SYNQ_HSIZE in include/net/tcp.h be modi-
	      fied  to	keep  TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog,  and the
	      kernel be	recompiled.

       tcp_max_tw_buckets (integer; default: see below;	since Linux 2.4)
	      The maximum number of sockets in TIME_WAIT state allowed in  the
	      system.  This limit exists only to prevent simple	denial-of-ser-
	      vice attacks.  The default value of NR_FILE*2  is	 adjusted  de-
	      pending  on  the	memory	in  the	system.	 If this number	is ex-
	      ceeded, the socket is closed and a warning is printed.

       tcp_moderate_rcvbuf   (Boolean;	 default:   enabled;	since	 Linux
       2.4.17/2.6.7)
	      If  enabled, TCP performs	receive	buffer auto-tuning, attempting
	      to automatically size the	buffer (no greater  than  tcp_rmem[2])
	      to match the size	required by the	path for full throughput.

       tcp_mem (since Linux 2.4)
	      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  low	memory	for  this,  which  is  limited	to  around 900
	      megabytes	on 32-bit systems.  64-bit systems do not suffer  this
	      limitation.)

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

	      pressure	When  the  amount  of  memory allocated	by TCP exceeds
			this number of pages, TCP moderates  its  memory  con-
			sumption.   This  memory pressure state	is exited once
			the number of pages  allocated	falls  below  the  low
			mark.

	      high	The  maximum  number of	pages, globally, that TCP will
			allocate.  This	value overrides	any other  limits  im-
			posed by the kernel.

       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
	      This parameter controls TCP Packetization-Layer Path MTU Discov-
	      ery.  The	following values may be	assigned to the	file:

	      0	 Disabled

	      1	 Disabled by default, enabled when an ICMP black hole detected

	      2	 Always	enabled, use initial MSS of tcp_base_mss.

       tcp_no_metrics_save (Boolean; default: disabled;	since Linux 2.6.6)
	      By default, TCP saves various connection metrics	in  the	 route
	      cache  when  the	connection  closes, so that connections	estab-
	      lished in	the near future	can use	these to  set  initial	condi-
	      tions.   Usually,	this increases overall performance, but	it may
	      sometimes	cause performance degradation.	If tcp_no_metrics_save
	      is enabled, TCP will not cache metrics on	closing	connections.

       tcp_orphan_retries (integer; default: 8;	since Linux 2.4)
	      The  maximum number of attempts made to probe the	other end of a
	      connection which has been	closed by our end.

       tcp_reordering (integer;	default: 3; since Linux	2.4)
	      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  re-
	      ordering	detection metric designed to minimize unnecessary back
	      off and retransmits provoked by reordering of packets on a  con-
	      nection.

       tcp_retrans_collapse (Boolean; default: enabled;	since Linux 2.2)
	      Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3; since	Linux 2.2)
	      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.

       tcp_retries2 (integer; default: 15; since Linux 2.2)
	      The maximum number of times a TCP	packet is retransmitted	in es-
	      tablished	 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.

       tcp_rfc1337 (Boolean; default: disabled;	since Linux 2.2)
	      Enable TCP behavior conformant with RFC 1337.  When disabled, if
	      a	RST is received	in TIME_WAIT state, we close the socket	 imme-
	      diately without waiting for the end of the TIME_WAIT period.

       tcp_rmem	(since Linux 2.4)
	      This  is a vector	of 3 integers: [min, default, max].  These pa-
	      rameters are used	by TCP to regulate receive buffer sizes.   TCP
	      dynamically  adjusts the size of the receive buffer from the de-
	      faults listed below, in the range	of these values, depending  on
	      memory available in the system.

	      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
			PAGE_SIZE bytes	in low-memory systems.)	 This value is
			used  to  ensure that in memory	pressure mode, alloca-
			tions below this size will still succeed.  This	is not
			used  to bound the size	of the receive buffer declared
			using SO_RCVBUF	on a socket.

	      default	the default size of  the  receive  buffer  for	a  TCP
			socket.	  This	value  overwrites  the initial default
			buffer size from the generic global  net.core.rmem_de-
			fault 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 buf-
			fer sizes are desired, this value should be  increased
			(to affect all sockets).  To employ large TCP windows,
			the net.ipv4.tcp_window_scaling	must be	 enabled  (de-
			fault).

	      max	the  maximum  size  of the receive buffer used by each
			TCP socket.  This value	does not override  the	global
			net.core.rmem_max.  This is not	used to	limit the size
			of the receive buffer declared using  SO_RCVBUF	 on  a
			socket.	  The  default	value  is calculated using the
			formula

			    max(87380, min(4MB,	tcp_mem[1]*PAGE_SIZE/128))

			(On Linux 2.4, the default is 87380*2  bytes,  lowered
			to 87380 in low-memory systems).

       tcp_sack	(Boolean; default: enabled; since Linux	2.2)
	      Enable RFC 2018 TCP Selective Acknowledgements.

       tcp_slow_start_after_idle   (Boolean;  default:	enabled;  since	 Linux
       2.6.18)
	      If enabled, provide RFC 2861 behavior and	time out  the  conges-
	      tion  window after an idle period.  An idle period is defined as
	      the current RTO (retransmission timeout).	 If disabled, the con-
	      gestion window will not be timed out after an idle period.

       tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
	      If  this option is enabled, then use the RFC 1122	interpretation
	      of the TCP urgent-pointer	field.	According to this  interpreta-
	      tion, the	urgent pointer points to the last byte of urgent data.
	      If this option is	disabled, then use the	BSD-compatible	inter-
	      pretation	 of  the  urgent pointer: the urgent pointer points to
	      the first	byte after the urgent data.  Enabling this option  may
	      lead to interoperability problems.

       tcp_syn_retries (integer; default: 5; since Linux 2.2)
	      The  maximum number of times initial SYNs	for an active TCP con-
	      nection attempt will be retransmitted.  This value should	not be
	      higher  than  255.  The default value is 5, which	corresponds to
	      approximately 180	seconds.

       tcp_synack_retries (integer; default: 5;	since Linux 2.2)
	      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.

       tcp_syncookies (Boolean;	since Linux 2.2)
	      Enable TCP syncookies.  The kernel must be  compiled  with  CON-
	      FIG_SYN_COOKIES.	Send out syncookies when the syn backlog queue
	      of a socket overflows.  The syncookies feature attempts to  pro-
	      tect 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	proto-
	      col,  and	 conflicts  with other areas of	TCP such as TCP	exten-
	      sions.  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	recom-
	      mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
	      and tcp_abort_on_overflow.

       tcp_timestamps (Boolean;	default: enabled; since	Linux 2.2)
	      Enable RFC 1323 TCP timestamps.

       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.

       tcp_tw_recycle (Boolean;	default: disabled; since Linux 2.4)
	      Enable  fast  recycling of TIME_WAIT sockets.  Enabling this op-
	      tion is not recommended since this causes	problems when  working
	      with NAT (Network	Address	Translation).

       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
	      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.

       tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 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.

       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
	      Enable TCP Westwood+ congestion control  algorithm.   TCP	 West-
	      wood+  is	a sender-side only modification	of the TCP Reno	proto-
	      col stack	that optimizes the performance of TCP congestion  con-
	      trol.   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 experi-
	      enced.  TCP Westwood+ significantly increases fairness with  re-
	      spect to TCP Reno	in wired networks and throughput over wireless
	      links.

       tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
	      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 tcp_window_scaling is disabled, TCP	will not nego-
	      tiate  the  use of window	scaling	with the other end during con-
	      nection setup.

       tcp_wmem	(since Linux 2.4)
	      This is a	vector of 3 integers: [min, default, max].  These  pa-
	      rameters are used	by TCP to regulate send	buffer sizes.  TCP dy-
	      namically	adjusts	the size of the	send buffer from  the  default
	      values  listed below, in the range of these values, depending on
	      memory available.

	      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 de-
			clared using SO_SNDBUF on a socket.

	      default	The default size of the	send buffer for	a TCP  socket.
			This  value overwrites the initial default buffer size
			from the  generic  global  /proc/sys/net/core/wmem_de-
			fault 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 sock-
			ets).	 To   employ   large	TCP    windows,	   the
			/proc/sys/net/ipv4/tcp_window_scaling must be set to a
			nonzero	value (default).

	      max	The maximum size of the	send buffer used by  each  TCP
			socket.	  This	value  does  not override the value in
			/proc/sys/net/core/wmem_max.   This  is	 not  used  to
			limit  the  size  of  the  send	 buffer	declared using
			SO_SNDBUF on a socket.	The default  value  is	calcu-
			lated using the	formula

			    max(65536, min(4MB,	tcp_mem[1]*PAGE_SIZE/128))

			(On  Linux  2.4, the default value is 128K bytes, low-
			ered 64K depending on low-memory systems.)

       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.

   Socket options
       To  set	or get a TCP socket option, call getsockopt(2) to read or set-
       sockopt(2) to write the option with the option level  argument  set  to
       IPPROTO_TCP.   Unless  otherwise	 noted,	optval is a pointer to an int.
       In addition, most IPPROTO_IP socket options are valid on	 TCP  sockets.
       For more	information see	ip(7).

       TCP_CONGESTION (since Linux 2.6.13)
	      The  argument  for  this option is a string.  This option	allows
	      the caller to set	the TCP	congestion  control  algorithm	to  be
	      used,  on	 a  per-socket	basis.	Unprivileged processes are re-
	      stricted to choosing one of the algorithms  in  tcp_allowed_con-
	      gestion_control	(described   above).	Privileged   processes
	      (CAP_NET_ADMIN) can choose from any of the available congestion-
	      control algorithms (see the description of tcp_available_conges-
	      tion_control above).

       TCP_CORK	(since Linux 2.2)
	      If set, don't send  out  partial	frames.	  All  queued  partial
	      frames  are sent when the	option is cleared again.  This is use-
	      ful for prepending headers before	calling	 sendfile(2),  or  for
	      throughput  optimization.	  As currently implemented, there is a
	      200 millisecond ceiling on the time for which output  is	corked
	      by  TCP_CORK.   If  this ceiling is reached, then	queued data is
	      automatically transmitted.  This option  can  be	combined  with
	      TCP_NODELAY  only	since Linux 2.5.71.  This option should	not be
	      used in code intended to be portable.

       TCP_DEFER_ACCEPT	(since Linux 2.4)
	      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 connec-
	      tion.   This  option  should  not	be used	in code	intended to be
	      portable.

       TCP_INFO	(since Linux 2.4)
	      Used to collect information about	this socket.  The  kernel  re-
	      turns  a	struct	tcp_info  as  defined  in  the	file  /usr/in-
	      clude/linux/tcp.h.  This option should not be used in  code  in-
	      tended to	be portable.

       TCP_KEEPCNT (since Linux	2.4)
	      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.

       TCP_KEEPIDLE (since Linux 2.4)
	      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.

       TCP_KEEPINTVL (since Linux 2.4)
	      The time (in seconds) between individual keepalive probes.  This
	      option should not	be used	in code	intended to be portable.

       TCP_LINGER2 (since Linux	2.4)
	      The  lifetime  of	orphaned FIN_WAIT2 state sockets.  This	option
	      can be used to override the  system-wide	setting	 in  the  file
	      /proc/sys/net/ipv4/tcp_fin_timeout for this socket.  This	is not
	      to be confused with the socket(7)	level option SO_LINGER.	  This
	      option should not	be used	in code	intended to be portable.

       TCP_MAXSEG
	      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.

       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 uti-
	      lization of the network.	This option is overridden by TCP_CORK;
	      however, setting this option forces an explicit flush of pending
	      output, even if TCP_CORK is currently set.

       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 pro-
	      cessing and factors such as delayed ack timeouts	occurring  and
	      data  transfer.  This option should not be used in code intended
	      to be portable.

       TCP_SYNCNT (since Linux 2.4)
	      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.

       TCP_USER_TIMEOUT	(since Linux 2.6.37)
	      This option takes	an unsigned int	 as  an	 argument.   When  the
	      value is greater than 0, it specifies the	maximum	amount of time
	      in milliseconds that transmitted data may	remain	unacknowledged
	      before  TCP will forcibly	close the corresponding	connection and
	      return ETIMEDOUT to the application.  If	the  option  value  is
	      specified	as 0, TCP will to use the system default.

	      Increasing  user timeouts	allows a TCP connection	to survive ex-
	      tended periods without end-to-end	connectivity.  Decreasing user
	      timeouts	allows	applications  to  "fail	 fast",	if so desired.
	      Otherwise, failure may take up to	20 minutes  with  the  current
	      system defaults in a normal WAN environment.

	      This option can be set during any	state of a TCP connection, but
	      is only effective	during the synchronized	states of a connection
	      (ESTABLISHED,  FIN-WAIT-1,  FIN-WAIT-2, CLOSE-WAIT, CLOSING, and
	      LAST-ACK).   Moreover,  when  used  with	 the   TCP   keepalive
	      (SO_KEEPALIVE)  option, TCP_USER_TIMEOUT will override keepalive
	      to determine when	to close a connection due to  keepalive	 fail-
	      ure.

	      The  option  has no effect on when TCP retransmits a packet, nor
	      when a keepalive probe is	sent.

	      This option, like	many others, will be inherited by  the	socket
	      returned by accept(2), if	it was set on the listening socket.

	      Further  details	on  the	 user  timeout feature can be found in
	      RFC 793 and RFC 5482 ("TCP User Timeout Option").

       TCP_WINDOW_CLAMP	(since Linux 2.4)
	      Bound the	size of	the advertised window to this value.  The ker-
	      nel  imposes  a  minimum size of SOCK_MIN_RCVBUF/2.  This	option
	      should not be used in code intended to be	portable.

   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  nor-
       mal  data  into	the  stream (even when SO_OOBINLINE is not set).  This
       differs from BSD-based stacks.

       Linux uses the BSD compatible  interpretation  of  the  urgent  pointer
       field by	default.  This violates	RFC 1122, but is required for interop-
       erability   with	  other	  stacks.    It	   can	  be	changed	   via
       /proc/sys/net/ipv4/tcp_stdurg.

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

       Since version 2.4, Linux	supports the use of MSG_TRUNC in the flags ar-
       gument  of  recv(2)  (and  recvmsg(2)).	 This flag causes the received
       bytes of	data to	be discarded, rather than passed back in a caller-sup-
       plied  buffer.	Since Linux 2.4.4, MSG_TRUNC also has this effect when
       used in conjunction with	MSG_OOB	to receive out-of-band data.

   Ioctls
       The following ioctl(2) calls return information in value.  The  correct
       syntax is:

	      int value;
	      error = ioctl(tcp_socket,	ioctl_type, &value);

       ioctl_type is one of the	following:

       SIOCINQ
	      Returns  the amount of queued unread data	in the receive buffer.
	      The socket must not be in	LISTEN state, otherwise	an error (EIN-
	      VAL) is returned.	 SIOCINQ is defined in _linux/sockios.h_.  Al-
	      ternatively, you can use the  synonymous	FIONREAD,  defined  in
	      _sys/ioctl.h_.

       SIOCATMARK
	      Returns true (i.e., value	is nonzero) if the inbound data	stream
	      is at the	urgent mark.

	      If the SO_OOBINLINE socket option	is set,	and SIOCATMARK returns
	      true,  then the next read	from the socket	will return the	urgent
	      data.  If	the SO_OOBINLINE socket	option is not set, and SIOCAT-
	      MARK  returns  true, then	the next read from the socket will re-
	      turn the bytes following the urgent data (to actually  read  the
	      urgent data requires the recv(MSG_OOB) flag).

	      Note  that a read	never reads across the urgent mark.  If	an ap-
	      plication	is informed of the presence of	urgent	data  via  se-
	      lect(2)  (using the exceptfds argument) or through delivery of a
	      SIGURG signal, then it can advance up to the mark	using  a  loop
	      which  repeatedly	tests SIOCATMARK and performs a	read (request-
	      ing any number of	bytes) as long as SIOCATMARK returns false.

       SIOCOUTQ
	      Returns the amount of unsent data	in the socket send queue.  The
	      socket  must not be in LISTEN state, otherwise an	error (EINVAL)
	      is returned.  SIOCOUTQ is	defined	in _linux/sockios.h_.	Alter-
	      natively,	 you  can  use	the  synonymous	 TIOCOUTQ,  defined in
	      _sys/ioctl.h_.

   Error handling
       When a network error occurs, TCP	tries to resend	 the  packet.	If  it
       doesn't	succeed	after some time, either	ETIMEDOUT or the last received
       error on	this connection	is reported.

       Some applications require a quicker error notification.	 This  can  be
       enabled	with the IPPROTO_IP level 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.

ERRORS
       EAFNOTSUPPORT
	      Passed socket address type in sin_family was not AF_INET.

       EPIPE  The other	end closed the socket unexpectedly or a	read  is  exe-
	      cuted on a shut down socket.

       ETIMEDOUT
	      The  other  end didn't acknowledge retransmitted data after some
	      time.

       Any errors defined for ip(7) or the generic socket layer	 may  also  be
       returned	for TCP.

VERSIONS
       Support	for  Explicit  Congestion Notification,	zero-copy 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.

BUGS
       Not all errors are documented.
       IPv6 is not described.

SEE ALSO
       accept(2), bind(2), connect(2), getsockopt(2),  listen(2),  recvmsg(2),
       sendfile(2), sendmsg(2),	socket(2), ip(7), socket(7)

       RFC 793 for the TCP specification.
       RFC 1122	 for the TCP requirements and a	description of the Nagle algo-
       rithm.
       RFC 1323	for TCP	timestamp and window scaling options.
       RFC 1337	for a description of TIME_WAIT assassination hazards.
       RFC 3168	for a description of Explicit Congestion Notification.
       RFC 2581	for TCP	congestion control algorithms.
       RFC 2018	and RFC	2883 for SACK and extensions to	SACK.

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-03-31				TCP(7)

NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | BUGS | SEE ALSO | COLOPHON

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