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TCP(7)			   Linux Programmers Manual		       TCP(7)



NAME
       tcp - TCP protocol

SYNOPSIS
       #include 
       #include 
       #include 

       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(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 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(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 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    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_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  con
	      verge 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 doesnt 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 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
	      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,  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, 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
	      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_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
	      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_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,	dont  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
       doesnt  succeed after some time, either ETIMEDOUT or the last received
       error on this connection is reported.

       Some applications require a quicker error notification.	 This  can  be
       enabled	with the IPPROTO_IP level IP_RECVERR socket option.  When this
       option is enabled, all incoming errors are immediately  passed  to  the
       user  program.	Use this option with care  it makes TCP less tolerant
       to routing changes and other normal network conditions.

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

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

       ETIMEDOUT
	      The  other  end didnt acknowledge retransmitted data after some
	      time.

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

VERSIONS
       Support	for  Explicit  Congestion Notification, zero-copy sendfile(2),
       reordering support and some SACK extensions (DSACK) were introduced  in
       2.4.   Support for forward acknowledgement (FACK), TIME_WAIT recycling,
       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.

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.

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), 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.

COLOPHON
       This page is part of release 3.05 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				  2007-11-25				TCP(7)




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