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CentOS 7.0 - man page for tcp (centos section 7)

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  provides  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 generates  and
       checks  a  per-packet checksum to catch transmission errors.  TCP does not preserve record
       boundaries.

       A newly created TCP socket has no remote or local address and is not fully specified.   To
       create  an outgoing TCP connection use connect(2) to establish a connection to another TCP
       socket.	To receive new incoming 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)  successfully  called on it is fully specified and may
       transmit data.  Data cannot be transmitted on listening or not yet connected sockets.

       Linux supports RFC 1323 TCP high performance extensions.  These include Protection Against
       Wrapped Sequence Numbers (PAWS), Window Scaling and Timestamps.	Window scaling allows the
       use of large (> 64K) TCP windows in order to support links with high latency or bandwidth.
       To make use of them, the send and receive buffer sizes must be increased.  They can be set
       globally with the /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 setsock-
       opt(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 internal kernel structures, and the /proc file
       values reflect the larger sizes compared to the actual TCP windows.  On individual connec-
       tions,  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 information.

       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  kernel 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 normal 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 include support for zero-copy sendfile(2),
       Explicit Congestion Notification, 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 sup-
       ported.

   /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") mean-
       ing that the corresponding 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 acknowl-
	      edgments.  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 segments 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
	      listening  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 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 "applica-
	      tion"  buffer,  used  to isolate the network from scheduling and application laten-
	      cies.  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 2.4.20)
	      Show/set	the  congestion  control algorithm choices available to unprivileged pro-
	      cesses (see the description of the TCP_CONGESTION socket option).  The  list  is	a
	      subset  of those listed in tcp_available_congestion_control.  The default value for
	      this list is "reno" plus the default setting of tcp_congestion_control.

       tcp_available_congestion_control (String; read-only; since Linux 2.4.20)
	      Show a list of the congestion-control algorithms that are registered.  This list is
	      a  limiting  set	for the list in tcp_allowed_congestion_control.  More congestion-
	      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 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 dis-
	      covery  (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 scala-
	      bility and bounded TCP-friendliness.  The  protocol  combines  two  schemes  called
	      additive increase and binary search increase.  When the congestion window is large,
	      additive increase with a large increment ensures linear RTT  fairness  as  well  as
	      good  scalability.  Under small congestion windows, binary search increase provides
	      TCP friendliness.

       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 converge 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 configuration.  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  violation	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  retransmission  timeouts
	      (RTOs).	It  is particularly beneficial in wireless environments where packet loss
	      is typically due to random radio interference rather than intermediate router  con-
	      gestion.	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 version can be used
		 also when SACK is in use though in that  case	scenario(s)  exists  where  F-RTO
		 interacts badly with the packet counting of the SACK-enabled TCP flow.

	      Before  Linux  2.6.22, this parameter was a Boolean value, supporting 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 concerning what to do next.  Possible values are:

	      0  Rate halving based; a smooth and conservative response, results in  halved  con-
		 gestion 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  immedi-
		 ately.

	      2  Aggressive response; undoes congestion-control measures that are now known to be
		 unnecessary (ignoring the  possibility  of  a	lost  retransmission  that  would
		 require  TCP  to be more cautious); 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 giving  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
	      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.

       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 disabled, then higher throughput is pre-
	      ferred.  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.   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.

       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 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 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-service attacks.  The default value of
	      NR_FILE*2 is adjusted depending on the memory in the system.   If  this  number  is
	      exceeded, the socket is closed and a warning is printed.

       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  consumption.   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 imposed by the kernel.

       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
	      This  parameter controls TCP Packetization-Layer Path MTU Discovery.  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  con-
	      nection closes, so that connections established in the near future can use these to
	      set initial conditions.  Usually, this increases overall performance,  but  it  may
	      sometimes  cause	performance  degradation.  If 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 connection.

       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  con-
	      nection  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 immediately 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, allocations 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_window_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 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-mem-
			ory 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 congestion window after an
	      idle period.  An idle period is defined as the current  RTO  (retransmission  time-
	      out).   If  disabled,  the  congestion  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 interpretation, the urgent pointer points to the
	      last byte of urgent data.  If this option is disabled, then use the  BSD-compatible
	      interpretation  of  the urgent pointer: the urgent pointer points to the first byte
	      after the urgent data.  Enabling this option may lead to interoperability problems.

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

       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 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  con-
	      flicts  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 tcp_max_syn_backlog,  tcp_synack_retries,  and  tcp_abort_on_over-
	      flow.

       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 recom-
	      mended since this causes problems when working with NAT (Network	Address  Transla-
	      tion).

       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 Westwood+ is a sender-side
	      only modification of the TCP Reno protocol stack that optimizes the performance  of
	      TCP congestion control.  It is based on end-to-end bandwidth estimation to set con-
	      gestion 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  expe-
	      rienced.	 TCP  Westwood+ significantly increases fairness with respect 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_scal-
	      ing  is  disabled,  TCP will not negotiate the use of window scaling with the other
	      end during connection 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,  depend-
	      ing 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, allo-
			cations 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 over-
			writes	the  initial  default  buffer  size  from  the	 generic   global
			/proc/sys/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 win-
			dows, 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 calculated using the formula

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

			(On Linux 2.4, the default value is 128K bytes, lowered 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  setsockopt(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)
	      Get  or  set the congestion-control algorithm for this socket.  The optval argument
	      is a pointer to a character-string buffer.

	      For getsockopt() *optlen specifies the amount of	space  available  in  the  buffer
	      pointed  to  by  optval,	which should be at least 16 bytes (defined by the kernel-
	      internal constant TCP_CA_NAME_MAX).  On return, the buffer pointed to by optval  is
	      set to a null-terminated string containing the name of the congestion-control algo-
	      rithm for this socket, and *optlen is set to the minimum of its original value  and
	      TCP_CA_NAME_MAX.	 If  the  value  passed  in *optlen is too small, then the string
	      returned in *optval is silently truncated, and no terminating null byte  is  added.
	      If  an  empty  string is returned, then the socket is using the default congestion-
	      control algorithm, determined as described under tcp_congestion_control above.

	      For setsockopt() optlen specifies the length of  the  congestion-control	algorithm
	      name contained in the buffer pointed to by optval; this length need not include any
	      terminating null byte.  The algorithm "reno" is always permitted; other  algorithms
	      may be available, depending on kernel configuration.  Possible errors from setsock-
	      opt() include: algorithm	not  found/available  (ENOENT);  setting  this	algorithm
	      requires	the  CAP_NET_ADMIN  capability (EPERM); and failure getting kernel module
	      (EBUSY).

       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 useful 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 connection.	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 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 connec-
	      tion.  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 over-
	      ride 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 utilization of the net-
	      work.  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 processing 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_WINDOW_CLAMP (since Linux 2.4)
	      Bound  the size of the advertised window to this value.  The kernel imposes a mini-
	      mum 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  normal 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 interoperability 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  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-supplied buffer.  Since Linux 2.4.4, MSG_PEEK 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  (EINVAL)  is  returned.   SIOCINQ  is
	      defined  in <linux/sockios.h>.  Alternatively, 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 SIOCATMARK 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 (requesting  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>.   Alternatively, 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 executed 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 acknowledge-
       ment  (FACK), TIME_WAIT recycling, and per-connection keepalive socket options were intro-
       duced 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 algorithm.
       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 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-06-21					   TCP(7)


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