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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
       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 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
       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 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
       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
       SIOCATMARK 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
       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 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 nonzero 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
              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)
              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
              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
              "application"   buffer,   used   to  isolate  the  network  from
              scheduling and  application  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
              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)
              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
              violation 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
              retransmission  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
              giving  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
              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)
              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 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.
              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)
              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 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-
              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_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 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
              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
              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_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
              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  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
              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_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
              working 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
              negotiate the use of window scaling with the  other  end  during
              connection 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
              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
              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
              experienced.  TCP  Westwood+  significantly  increases  fairness
              with  respect  to TCP Reno in wired networks and throughput over
              wireless 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
              memory available in the system.

   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.   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
              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
              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
              connection.   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
              utilization  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
              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
              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
              kernel 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
       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.

       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.

   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 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
       interoperability   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
       algorithm.
       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 2.77 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/.