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

       tcp - TCP protocol

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

       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

       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-

       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-specified fcntl(2) F_SETOWN operation).  When the SO_OOBIN-
       LINE socket option is enabled, urgent data is put into the normal  data
       stream  (a program can test for its location using the SIOCATMARK 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
       include	support for zero-copy sendfile(2), Explicit Congestion Notifi-
       cation, 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
       directory /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

	      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.
	      Enabling 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
	      implies 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
	      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.
	      Applications  can still use the TCP_CORK socket option to obtain
	      optimal behavior when they know how/when to uncork  their	 sock-

       tcp_available_congestion_control	  (String;   read-only;	  since	 Linux
	      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

       tcp_app_win (integer; default: 31; since Linux 2.4)
	      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.

       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
	      enabled, 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-

       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.

       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
	      additional choices may be available depending on kernel configu-
	      ration.  The default value for this file is set as part of  ker-
	      nel 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 (Integer; default: se below; since Linux 2.4)
	      Enable RFC 3168 Explicit Congestion Notification.

	      This file can have one of the following values:

	      0	     Disable  ECN.  Neither initiate nor accept ECN.  This was
		     the default up to and including Linux 2.6.30.

	      1	     Enable ECN when requested	by  incoming  connections  and
		     also request ECN on outgoing connection attempts.

	      2	     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.

	      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  tcp_ecn_fallback	option has been intro-

       tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
	      Enable RFC 3168, Section fallback.  When enabled,  out-
	      going  ECN-setup	SYNs  that  time  out  within  the  normal SYN
	      retransmission timeout will be resent with CWR and ECE cleared.

       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,
		 results  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

       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
	      latency 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
	      Beowulf 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
	      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.

       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).

	      Prior to Linux 2.6.20, it was recommended that if this needed to
	      be increased above 1024, the  size  of  the  SYNACK  hash	 table
	      (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep

		  TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

	      and the kernel should be recompiled.  In Linux 2.6.20, the fixed
	      sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.

       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
	      depending	 on  the  memory  in  the  system.   If this number is
	      exceeded, the socket is closed and a warning is printed.

       tcp_moderate_rcvbuf   (Boolean;	 default:   enabled;	since	 Linux
	      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

	      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

	      high	The  maximum  number of pages, globally, that TCP will
			allocate.   This  value	 overrides  any	 other	limits
			imposed 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
	      reordering  detection  metric  designed  to minimize unnecessary
	      back off and retransmits provoked by reordering of packets on  a

       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
	      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.

       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
	      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.

	      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_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   net.ipv4.tcp_win-
			dow_scaling must be enabled (default).

	      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

			    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
	      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
	      option is not recommended for  devices  communicating  with  the
	      general  Internet	 or  using  NAT (Network Address Translation).
	      Since some NAT gateways pass through IP timestamp values, one IP
	      can  appear  to  have  non-increasing  timestamps.  See RFC 1323
	      (PAWS), RFC 6191.

       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
	      respect to TCP Reno in wired networks and throughput over	 wire-
	      less 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
	      parameters are used by TCP to regulate send buffer  sizes.   TCP
	      dynamically adjusts the size of the send buffer from the default
	      values listed below, in the range of these values, depending  on
	      memory available.

	      min	Minimum	 size  of  the	send  buffer  used by each TCP
			socket.	 The default value is the  system  page	 size.
			(On  Linux  2.4, the default value is 4K bytes.)  This
			value is used to ensure that in memory pressure	 mode,
			allocations  below this size will still succeed.  This
			is not used to bound  the  size	 of  the  send	buffer
			declared using 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_default defined for all proto-
			cols.  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 /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
	      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

   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
	      restricted 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

       TCP_INFO (since Linux 2.4)
	      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.

       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.

	      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

	      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
	      extended	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 effective only during the synchronized states of a connection
	      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-

	      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

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

       Since  version  2.4,  Linux  supports the use of MSG_TRUNC in the flags
       argument 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.

       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:

	      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>.
	      Alternatively,  you  can use the synonymous FIONREAD, defined in

	      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
	      return 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
	      application  is  informed	 of  the  presence  of urgent data via
	      select(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.

	      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

   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.

	      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.

	      The other end didn't acknowledge retransmitted data  after  some

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

       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.

       Not all errors are documented.
       IPv6 is not described.

       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-
       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.

       This  page  is  part of release 4.04 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

Linux				  2015-12-05				TCP(7)