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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(PF_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() or connect() success-
       fully called on it is fully specified and may transmit data.  Data can-
       not 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 net.ipv4.tcp_wmem and net.ipv4.tcp_rmem sysctl
       variables,  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 global net.core.rmem_max and
       net.core.wmem_max sysctls.  Note that TCP actually allocates twice  the
       size  of	 the buffer requested in the setsockopt(2) call, and so a suc-
       ceeding 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	sysctl
       variables  reflect the larger sizes compared to the actual TCP windows.
       On individual connections, the socket buffer size must be set prior  to
       the  listen()  or  connect() 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 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 only received 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 only supports point-to-point communication;
       broadcasting and multicasting are not supported.

SYSCTLS
       These variables can be accessed by the  /proc/sys/net/ipv4/*  files  or
       with  the sysctl(2) interface.  In addition, most IP sysctls also apply
       to TCP; see ip(7).  Variables described	as  Boolean  take  an  integer
       value,  with  a	non-zero value ("true") meaning that the corresponding
       option is enabled, and a zero value ("false") meaning that  the	option
       is disabled.

       tcp_abort_on_overflow (Boolean; default: disabled)
	      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)
	      Count  buffering	overhead  as   bytes/2^tcp_adv_win_scale   (if
	      tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale), if
	      it is <= 0.

	      The 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_app_win (integer; default: 31)
	      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_bic (Boolean; default: disabled)
	      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)
	      Sets 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)
	      Forces BIC TCP to more quickly respond to changes in  congestion
	      window. Allows two flows sharing the same connection to converge
	      more rapidly.

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

       tcp_ecn (Boolean; default: disabled)
	      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)
	      Enable TCP Forward Acknowledgement support.

       tcp_fin_timeout (integer; default: 60)
	      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 (Boolean; default: disabled)
	      Enables F-RTO, an enhanced recovery algorithm for	 TCP  retrans-
	      mission  timeouts.   It  is  particularly beneficial in wireless
	      environments where packet loss is typically due to random	 radio
	      interference rather than intermediate router congestion.

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

       tcp_keepalive_probes (integer; default: 9)
	      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)
	      The  number  of seconds a connection needs to be idle before TCP
	      begins sending out keep-alive probes.  Keep-alives are only sent
	      when  the	 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)
	      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)
	      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)
	      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
	      increased	 above	1024,  TCP_SYNQ_HSIZE  in include/net/tcp.h be
	      modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the
	      kernel be recompiled.

       tcp_max_tw_buckets (integer; default: see below)
	      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_mem
	      This  is	a  vector of 3 integers: [low, pressure, high].	 These
	      bounds are used by TCP to track its memory usage.	 The  defaults
	      are calculated at boot time from the amount of available memory.
	      (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_orphan_retries (integer; default: 8)
	      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)
	      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)
	      Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3)
	      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)
	      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)
	      Enable TCP behaviour 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
	      This  is	a  vector  of  3 integers: [min, default, max].	 These
	      parameters are used by TCP to  regulate  receive	buffer	sizes.
	      TCP  dynamically adjusts the size of the receive buffer from the
	      defaults listed below, in the range of these  sysctl  variables,
	      depending on memory available in the system.

	      min  -  minimum  size  of	 the  receive  buffer used by each TCP
	      socket.  The default value is 4K, and is	lowered	 to  PAGE_SIZE
	      bytes  in low-memory systems.  This value is used to ensure that
	      in memory pressure mode, allocations below this size will	 still
	      succeed.	 This  is  not	used  to bound the size of the receive
	      buffer declared using 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, and is lowered to
	      43689 in low-memory systems.  If larger receive buffer sizes are
	      desired, this value should be increased (to affect all sockets).
	      To employ large  TCP  windows,  the  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 of 87380*2 bytes is lowered to 87380 in low-memory
	      systems.

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

       tcp_stdurg (Boolean; default: disabled)
	      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_synack_retries (integer; default: 5)
	      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)
	      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_syn_retries (integer; default: 5)
	      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_timestamps (Boolean; default: enabled)
	      Enable RFC 1323 TCP timestamps.

       tcp_tw_recycle (Boolean; default: disabled)
	      Enable  fast  recycling  of  TIME-WAIT  sockets.	 Enabling this
	      option is not recommended since this causes problems when	 work-
	      ing with NAT (Network Address Translation).

       tcp_tw_reuse (Boolean; default: disabled)
	      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_window_scaling (Boolean; default: enabled)
	      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_vegas_cong_avoid (Boolean; default: disabled)
	      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)
	      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 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 experi-
	      enced.  TCP  Westwood+  significantly  increases	fairness  with
	      respect  to TCP Reno in wired networks and throughput over wire-
	      less links.

       tcp_wmem
	      This is a vector of 3  integers:	[min,  default,	 max].	 These
	      parameters  are  used by TCP to regulate send buffer sizes.  TCP
	      dynamically adjusts the size of the send buffer from the default
	      values  listed  below,  in  the range of these sysctl variables,
	      depending on memory available.

	      min - minimum size of the send buffer used by each  TCP  socket.
	      The  default  value  is  4K bytes.  This value is used to ensure
	      that in memory pressure mode, allocations below this  size  will
	      still  succeed.	This is not used to bound the size of the send
	      buffer declared using 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 net.core.wmem_default defined for all  protocols.
	      The default value is 16K bytes.  If larger send buffer sizes are
	      desired, this value should be increased (to affect all sockets).
	      To    employ    large   TCP   windows,   the   sysctl   variable
	      net.ipv4.tcp_window_scaling must be enabled (default).

	      max - the maximum size of the  send  buffer  used	 by  each  TCP
	      socket.	  This	  value	  does	 not   override	  the	global
	      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 128K bytes.  It is lowered to 64K depending on the mem-
	      ory available in the system.

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.   In addition, most IPPROTO_IP socket options are valid on
       TCP sockets. For more information see ip(7).

       TCP_CORK
	      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
	      Allows a listener to be awakened only when data arrives  on  the
	      socket.	Takes  an  integer value (seconds), this can bound the
	      maximum number of attempts TCP will make to complete the connec-
	      tion.   This  option  should  not be used in code intended to be
	      portable.

       TCP_INFO
	      Used to collect  information  about  this	 socket.   The	kernel
	      returns	a   struct   tcp_info	as   defined   in   the	  file
	      /usr/include/linux/tcp.h.	 This option should  not  be  used  in
	      code intended to be portable.

       TCP_KEEPCNT
	      The  maximum  number  of keepalive probes TCP should send before
	      dropping the connection.	This option should not be used in code
	      intended to be portable.

       TCP_KEEPIDLE
	      The time (in seconds) the connection needs to remain idle before
	      TCP starts  sending  keepalive  probes,  if  the	socket	option
	      SO_KEEPALIVE  has	 been  set on this socket.  This option should
	      not be used in code intended to be portable.

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

       TCP_LINGER2
	      The  lifetime  of orphaned FIN_WAIT2 state sockets.  This option
	      can be used to override the system wide  sysctl  tcp_fin_timeout
	      on  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.  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
	      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
	      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
	      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.

IOCTLS
       These  following	 ioctl(2) calls return information in value.  The cor-
       rect 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.

       SIOCATMARK
	      Returns true (i.e., value	 is  non-zero)	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.

       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.

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.

NOTES
       TCP  has	 no  real  out-of-band data; it has urgent data. In Linux this
       means if the other end sends newer out-of-band data  the	 older	urgent
       data is inserted as normal data into the stream (even when 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  by  the  tcp_stdurg
       sysctl.

ERRORS
       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.

       EAFNOTSUPPORT
	      Passed socket address type in sin_family was not AF_INET.

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

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

VERSIONS
       Support for Explicit  Congestion	 Notification,	zero-copy  sendfile(),
       reordering  support and some SACK extensions (DSACK) were introduced in
       2.4.  Support for forward acknowledgement (FACK), TIME_WAIT  recycling,
       per  connection keepalive socket options and sysctls were introduced in
       2.3.

       The default values and descriptions  for	 the  sysctl  variables	 given
       above are applicable for the 2.4 kernel.

AUTHORS
       This man page was originally written by Andi Kleen.  It was updated for
       2.4 by Nivedita Singhvi with input from Alexey  Kuznetsov's  Documenta-
       tion/networking/ip-sysctls.txt document.

SEE ALSO
       accept(2),  bind(2),  connect(2), getsockopt(2), listen(2), recvmsg(2),
       sendfile(2), sendmsg(2), socket(2), sysctl(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 1644 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.



Linux Man Page			  2005-06-15				TCP(7)
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