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NAME

       tcp - TCP protocol

SYNOPSIS

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

       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION

       This  is  an  implementation  of  the  TCP protocol defined in RFC 793,
       RFC 1122 and  RFC 2001  with  the  NewReno  and  SACK  extensions.   It
       provides  a  reliable,  stream-oriented, full-duplex connection between
       two sockets on top  of  ip(7),  for  both  v4  and  v6  versions.   TCP
       guarantees that the data arrives in order and retransmits lost packets.
       It generates and checks a per-packet  checksum  to  catch  transmission
       errors.  TCP does not preserve record boundaries.

       A  newly  created  TCP socket has no remote or local address and is not
       fully specified.  To create an outgoing TCP connection  use  connect(2)
       to  establish  a  connection  to  another  TCP  socket.  To receive new
       incoming connections, first bind(2) the socket to a local  address  and
       port  and  then  call  listen(2)  to  put the socket into the listening
       state.  After that a new socket for each  incoming  connection  can  be
       accepted  using  accept(2).   A  socket  which  has  had  accept(2)  or
       connect(2) successfully  called  on  it  is  fully  specified  and  may
       transmit  data.   Data  cannot  be  transmitted on listening or not yet
       connected sockets.

       Linux supports RFC 1323 TCP high performance extensions.  These include
       Protection  Against Wrapped Sequence Numbers (PAWS), Window Scaling and
       Timestamps.  Window scaling allows the use of large (> 64K) TCP windows
       in  order to support links with high latency or bandwidth.  To make use
       of them, the send and receive buffer sizes must be increased.  They can
       be    set    globally    with   the   /proc/sys/net/ipv4/tcp_wmem   and
       /proc/sys/net/ipv4/tcp_rmem files, or on individual  sockets  by  using
       the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call.

       The  maximum  sizes  for  socket buffers declared via the SO_SNDBUF and
       SO_RCVBUF   mechanisms   are   limited   by   the   values    in    the
       /proc/sys/net/core/rmem_max   and   /proc/sys/net/core/wmem_max  files.
       Note that TCP actually allocates twice the size of the buffer requested
       in  the setsockopt(2) call, and so a succeeding getsockopt(2) call will
       not return the same size of buffer as requested  in  the  setsockopt(2)
       call.   TCP  uses  the  extra  space  for  administrative  purposes and
       internal kernel structures, and  the  /proc  file  values  reflect  the
       larger  sizes  compared  to  the  actual  TCP  windows.   On individual
       connections, the socket buffer size must be set prior to the  listen(2)
       or connect(2) calls in order to have it take effect.  See socket(7) for
       more 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.

   /proc interfaces
       System-wide TCP parameter settings can be  accessed  by  files  in  the
       directory  /proc/sys/net/ipv4/.   In addition, most IP /proc interfaces
       also apply to TCP; see ip(7).  Variables described as Boolean  take  an
       integer   value,  with  a  nonzero  value  ("true")  meaning  that  the
       corresponding option is enabled, and a  zero  value  ("false")  meaning
       that the option is disabled.

       tcp_abc (Integer; default: 0; since Linux 2.6.15)
              Control  the  Appropriate Byte Count (ABC), defined in RFC 3465.
              ABC is a way of increasing the  congestion  window  (cwnd)  more
              slowly  in response to partial acknowledgments.  Possible values
              are:

              0  increase cwnd once per acknowledgment (no ABC)

              1  increase cwnd once per acknowledgment of full sized segment

              2  allow increase cwnd  by  two  if  acknowledgment  is  of  two
                 segments to compensate for delayed acknowledgments.

       tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
              Enable  resetting  connections  if  the listening service is too
              slow and unable to keep up and accept them.  It  means  that  if
              overflow  occurred  due to a burst, the connection will recover.
              Enable this  option  only  if  you  are  really  sure  that  the
              listening  daemon  cannot be tuned to accept connections faster.
              Enabling this option can harm the clients of your server.

       tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
              Count  buffering  overhead  as   bytes/2^tcp_adv_win_scale,   if
              tcp_adv_win_scale    is    greater    than    0;    or    bytes-
              bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less  than
              or equal to zero.

              The   socket   receive   buffer  space  is  shared  between  the
              application and kernel.  TCP maintains part of the buffer as the
              TCP window, this is the size of the receive window advertised to
              the  other  end.   The  rest  of  the  space  is  used  as   the
              "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_allowed_congestion_control (String; default: see text; since  Linux
       2.4.20)
              Show/set  the  congestion control algorithm choices available to
              unprivileged   processes   (see   the   description    of    the
              TCP_CONGESTION  socket  option).   The list is a subset of those
              listed in tcp_available_congestion_control.  The  default  value
              for   this   list   is   "reno"  plus  the  default  setting  of
              tcp_congestion_control.

       tcp_available_congestion_control  (String;   read-only;   since   Linux
       2.4.20)
              Show  a  list  of  the  congestion-control  algorithms  that are
              registered.  This list  is  a  limiting  set  for  the  list  in
              tcp_allowed_congestion_control.      More     congestion-control
              algorithms may be available as modules, but not loaded.

       tcp_app_win (integer; default: 31; since Linux 2.4)
              This variable defines how many  bytes  of  the  TCP  window  are
              reserved for buffering overhead.

              A maximum of (window/2^tcp_app_win, mss) bytes in the window are
              reserved for the application buffer.  A value of 0 implies  that
              no amount is reserved.

       tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
              The  initial value of search_low to be used by the packetization
              layer Path MTU discovery  (MTU  probing).   If  MTU  probing  is
              enabled, this is the initial MSS used by the connection.

       tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
              Enable  BIC  TCP  congestion  control  algorithm.   BIC-TCP is a
              sender-side only change that ensures a linear RTT fairness under
              large  windows  while offering both scalability and bounded TCP-
              friendliness.  The protocol combines two schemes called additive
              increase and binary search increase.  When the congestion window
              is large, additive  increase  with  a  large  increment  ensures
              linear  RTT  fairness  as well as good scalability.  Under small
              congestion  windows,  binary  search   increase   provides   TCP
              friendliness.

       tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
              Set  the  threshold  window (in packets) where BIC TCP starts to
              adjust the congestion window.   Below  this  threshold  BIC  TCP
              behaves the same as the default TCP Reno.

       tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6
       to 2.6.13)
              Force BIC TCP to more quickly respond to changes  in  congestion
              window.   Allows  two  flows  sharing  the  same  connection  to
              converge more rapidly.

       tcp_congestion_control (String; default: see text; since Linux 2.4.13)
              Set the default congestion-control algorithm to be used for  new
              connections.   The  algorithm  "reno"  is  always available, but
              additional  choices  may  be  available  depending   on   kernel
              configuration.   The  default value for this file is set as part
              of kernel configuration.

       tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
              Lower limit, in bytes, of the size of socket reads that will  be
              offloaded  to a DMA copy engine, if one is present in the system
              and the kernel was configured with the CONFIG_NET_DMA option.

       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
              Enable RFC 2883 TCP Duplicate SACK support.

       tcp_ecn (Boolean; default: disabled; since Linux 2.4)
              Enable RFC 2884 Explicit Congestion Notification.  When enabled,
              connectivity  to  some  destinations  could  be  affected due to
              older, misbehaving routers along the path causing connections to
              be dropped.

       tcp_fack (Boolean; default: enabled; since Linux 2.2)
              Enable TCP Forward Acknowledgement support.

       tcp_fin_timeout (integer; default: 60; since Linux 2.2)
              This  specifies  how many seconds to wait for a final FIN packet
              before the socket  is  forcibly  closed.   This  is  strictly  a
              violation  of  the  TCP  specification,  but required to prevent
              denial-of-service attacks.  In Linux 2.2, the default value  was
              180.

       tcp_frto (integer; default: 0; since Linux 2.4.21/2.6)
              Enable   F-RTO,   an   enhanced   recovery   algorithm  for  TCP
              retransmission timeouts (RTOs).  It is  particularly  beneficial
              in  wireless  environments where packet loss is typically due to
              random  radio  interference  rather  than  intermediate   router
              congestion.  See RFC 4138 for more details.

              This file can have one of the following values:

              0  Disabled.

              1  The basic version F-RTO algorithm is enabled.

              2  Enable  SACK-enhanced  F-RTO  if  flow  uses SACK.  The basic
                 version can be used also when SACK is in use though  in  that
                 case  scenario(s) exists where F-RTO interacts badly with the
                 packet counting of the SACK-enabled TCP flow.

              Before  Linux  2.6.22,  this  parameter  was  a  Boolean  value,
              supporting just values 0 and 1 above.

       tcp_frto_response (integer; default: 0; since Linux 2.6.22)
              When  F-RTO  has  detected that a TCP retransmission timeout was
              spurious (i.e, the timeout would have been avoided had TCP set a
              longer   retransmission   timeout),   TCP  has  several  options
              concerning what to do next.  Possible values are:

              0  Rate halving  based;  a  smooth  and  conservative  response,
                 results  in  halved  congestion  window (cwnd) and slow-start
                 threshold (ssthresh) after one RTT.

              1  Very conservative  response;  not  recommended  because  even
                 though  being  valid,  it  interacts  poorly with the rest of
                 Linux TCP; halves cwnd and ssthresh immediately.

              2  Aggressive response; undoes congestion-control measures  that
                 are  now known to be unnecessary (ignoring the possibility of
                 a lost retransmission that  would  require  TCP  to  be  more
                 cautious); cwnd and ssthresh are restored to the values prior
                 to timeout.

       tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
              The number of seconds between TCP keep-alive probes.

       tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
              The maximum number of  TCP  keep-alive  probes  to  send  before
              giving  up and killing the connection if no response is obtained
              from the other end.

       tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
              The number of seconds a connection needs to be idle  before  TCP
              begins sending out keep-alive probes.  Keep-alives are 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; since Linux 2.4.21/2.6)
              If  enabled,  the  TCP  stack  makes decisions that prefer lower
              latency as opposed to higher  throughput.   It  this  option  is
              disabled, then higher throughput is preferred.  An example of an
              application where this default should  be  changed  would  be  a
              Beowulf compute cluster.

       tcp_max_orphans (integer; default: see below; since Linux 2.4)
              The  maximum  number  of orphaned (not attached to any user file
              handle) TCP sockets allowed in the system.  When this number  is
              exceeded,  the  orphaned  connection  is  reset and a warning is
              printed.  This limit exists only to  prevent  simple  denial-of-
              service  attacks.   Lowering  this  limit  is  not  recommended.
              Network conditions might require you to increase the  number  of
              orphans allowed, but note that each orphan can eat up to ~64K of
              unswappable memory.  The default initial value is set  equal  to
              the  kernel parameter NR_FILE.  This initial default is adjusted
              depending on the memory in the system.

       tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
              The maximum number of  queued  connection  requests  which  have
              still  not  received  an  acknowledgement  from  the  connecting
              client.  If this number  is  exceeded,  the  kernel  will  begin
              dropping  requests.   The  default  value of 256 is increased to
              1024 when the memory  present  in  the  system  is  adequate  or
              greater  (>=  128Mb),  and reduced to 128 for those systems with
              very low memory (<= 32Mb).  It is recommended that if this needs
              to  be increased above 1024, TCP_SYNQ_HSIZE in include/net/tcp.h
              be modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog,  and
              the kernel be recompiled.

       tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
              The  maximum number of sockets in TIME_WAIT state allowed in the
              system.  This limit exists only  to  prevent  simple  denial-of-
              service  attacks.   The  default  value of NR_FILE*2 is adjusted
              depending on the memory  in  the  system.   If  this  number  is
              exceeded, the socket is closed and a warning is printed.

       tcp_moderate_rcvbuf    (Boolean;    default:   enabled;   since   Linux
       2.4.17/2.6.7)
              If enabled, TCP performs receive buffer auto-tuning,  attempting
              to  automatically  size the buffer (no greater than tcp_rmem[2])
              to match the size required by the path for full throughput.

       tcp_mem (since Linux 2.4)
              This is a vector of 3 integers: [low,  pressure,  high].   These
              bounds,  measured  in units of the system page size, are used by
              TCP to track its memory usage.  The defaults are  calculated  at
              boot  time  from  the amount of available memory.  (TCP can only
              use low  memory  for  this,  which  is  limited  to  around  900
              megabytes  on 32-bit systems.  64-bit systems do not suffer this
              limitation.)

              low       TCP doesn't regulate its memory  allocation  when  the
                        number  of  pages  it  has allocated globally is below
                        this number.

              pressure  When the amount of memory  allocated  by  TCP  exceeds
                        this   number  of  pages,  TCP  moderates  its  memory
                        consumption.  This memory  pressure  state  is  exited
                        once the number of pages allocated falls below the low
                        mark.

              high      The maximum number of pages, globally, that  TCP  will
                        allocate.   This  value  overrides  any  other  limits
                        imposed by the kernel.

       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
              This  parameter  controls  TCP  Packetization-Layer   Path   MTU
              Discovery.  The following values may be assigned to the file:

              0  Disabled

              1  Disabled by default, enabled when an ICMP black hole detected

              2  Always enabled, use initial MSS of tcp_base_mss.

       tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
              By  default,  TCP  saves various connection metrics in the route
              cache  when  the  connection   closes,   so   that   connections
              established  in  the  near  future  can use these to set initial
              conditions.  Usually, this increases overall performance, but it
              may     sometimes    cause    performance    degradation.     If
              tcp_no_metrics_save is enabled, TCP will not  cache  metrics  on
              closing connections.

       tcp_orphan_retries (integer; default: 8; since Linux 2.4)
              The  maximum number of attempts made to probe the other end of a
              connection which has been closed by our end.

       tcp_reordering (integer; default: 3; since Linux 2.4)
              The maximum a packet can be reordered in  a  TCP  packet  stream
              without  TCP assuming packet loss and going into slow start.  It
              is not advisable to  change  this  number.   This  is  a  packet
              reordering  detection  metric  designed  to minimize unnecessary
              back off and retransmits provoked by reordering of packets on  a
              connection.

       tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
              Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3; since Linux 2.2)
              The  number  of times TCP will attempt to retransmit a packet on
              an established connection normally, without the extra effort  of
              getting the network layers involved.  Once we exceed this number
              of retransmits, we first have the network layer update the route
              if  possible before each new retransmit.  The default is the RFC
              specified minimum of 3.

       tcp_retries2 (integer; default: 15; since Linux 2.2)
              The maximum number of times a TCP  packet  is  retransmitted  in
              established  state  before  giving up.  The default value is 15,
              which corresponds to a duration of approximately between  13  to
              30  minutes,  depending  on  the  retransmission  timeout.   The
              RFC 1122 specified minimum limit of  100  seconds  is  typically
              deemed too short.

       tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
              Enable TCP behavior conformant with RFC 1337.  When disabled, if
              a RST is received  in  TIME_WAIT  state,  we  close  the  socket
              immediately without waiting for the end of the TIME_WAIT period.

       tcp_rmem (since Linux 2.4)
              This  is  a  vector  of  3 integers: [min, default, max].  These
              parameters are used by TCP to  regulate  receive  buffer  sizes.
              TCP  dynamically adjusts the size of the receive buffer from the
              defaults listed below, in the range of these  values,  depending
              on memory available in the system.

              min       minimum  size  of  the receive buffer used by each TCP
                        socket.  The default value is the  system  page  size.
                        (On  Linux  2.4,  the  default value is 4K, lowered to
                        PAGE_SIZE bytes in low-memory systems.)  This value is
                        used   to   ensure   that  in  memory  pressure  mode,
                        allocations below this size will still succeed.   This
                        is  not  used  to bound the size of the receive buffer
                        declared using SO_RCVBUF on a socket.

              default   the default size of  the  receive  buffer  for  a  TCP
                        socket.   This  value  overwrites  the initial default
                        buffer    size     from     the     generic     global
                        net.core.rmem_default  defined for all protocols.  The
                        default value is 87380 bytes.   (On  Linux  2.4,  this
                        will  be  lowered to 43689 in low-memory systems.)  If
                        larger receive buffer sizes are  desired,  this  value
                        should  be  increased  (to  affect  all  sockets).  To
                        employ       large       TCP       windows,        the
                        net.ipv4.tcp_window_scaling must be enabled (default).

              max       the  maximum  size  of the receive buffer used by each
                        TCP socket.  This value does not override  the  global
                        net.core.rmem_max.  This is not used to limit the size
                        of the receive buffer declared using  SO_RCVBUF  on  a
                        socket.   The  default  value  is calculated using the
                        formula

                            max(87380, min(4MB, tcp_mem[1]*PAGE_SIZE/128))

                        (On Linux 2.4, the default is 87380*2  bytes,  lowered
                        to 87380 in low-memory systems).

       tcp_sack (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 2018 TCP Selective Acknowledgements.

       tcp_slow_start_after_idle   (Boolean;  default:  enabled;  since  Linux
       2.6.18)
              If  enabled,  provide  RFC  2861  behavior  and  time  out   the
              congestion  window  after  an  idle  period.   An idle period is
              defined  as  the  current  RTO  (retransmission  timeout).    If
              disabled,  the  congestion window will not be timed out after an
              idle period.

       tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
              If this option is enabled, then use the RFC 1122  interpretation
              of   the   TCP   urgent-pointer   field.    According   to  this
              interpretation, the urgent pointer points to the  last  byte  of
              urgent  data.   If  this  option  is disabled, then use the BSD-
              compatible interpretation of  the  urgent  pointer:  the  urgent
              pointer  points  to  the  first  byte  after  the  urgent  data.
              Enabling this option may lead to interoperability problems.

       tcp_syn_retries (integer; default: 5; since Linux 2.2)
              The maximum number of times  initial  SYNs  for  an  active  TCP
              connection attempt will be retransmitted.  This value should not
              be higher than 255.  The default value is 5,  which  corresponds
              to approximately 180 seconds.

       tcp_synack_retries (integer; default: 5; since Linux 2.2)
              The  maximum number of times a SYN/ACK segment for a passive TCP
              connection will be retransmitted.  This  number  should  not  be
              higher than 255.

       tcp_syncookies (Boolean; since Linux 2.2)
              Enable  TCP  syncookies.   The  kernel  must  be  compiled  with
              CONFIG_SYN_COOKIES.  Send out syncookies when  the  syn  backlog
              queue of a socket overflows.  The syncookies feature attempts to
              protect a socket from a SYN flood attack.  This should  be  used
              as  a  last  resort,  if at all.  This is a violation of the TCP
              protocol, and 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_timestamps (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 1323 TCP timestamps.

       tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
              This parameter controls what percentage of the congestion window
              can be consumed by  a  single  TCP  Segmentation  Offload  (TSO)
              frame.   The  setting  of  this  parameter is a tradeoff between
              burstiness and building larger TSO frames.

       tcp_tw_recycle (Boolean; default: disabled; since Linux 2.4)
              Enable fast  recycling  of  TIME_WAIT  sockets.   Enabling  this
              option  is  not  recommended  since  this  causes  problems when
              working with NAT (Network Address Translation).

       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
              Allow to reuse TIME_WAIT sockets for new connections when it  is
              safe  from protocol viewpoint.  It should not be changed without
              advice/request of technical experts.

       tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
              Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is a
              sender-side  only  change  to  TCP that anticipates the onset of
              congestion by estimating the bandwidth.  TCP Vegas  adjusts  the
              sending  rate  by  modifying  the  congestion window.  TCP Vegas
              should provide less packet loss, but it is not as aggressive  as
              TCP Reno.

       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
              Enable   TCP   Westwood+   congestion  control  algorithm.   TCP
              Westwood+ is a sender-side only modification  of  the  TCP  Reno
              protocol  stack that optimizes the performance of TCP congestion
              control.  It is based on end-to-end bandwidth estimation to  set
              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_window_scaling (Boolean; default: enabled; since Linux 2.2)
              Enable RFC 1323 TCP window scaling.  This feature allows the use
              of a large window (> 64K) on a TCP connection, should the  other
              end support it.  Normally, the 16 bit window length field in the
              TCP header limits the window size to less than  64K  bytes.   If
              larger  windows  are desired, applications can increase the size
              of their socket buffers and the window scaling  option  will  be
              employed.   If  tcp_window_scaling  is  disabled,  TCP  will not
              negotiate the use of window scaling with the  other  end  during
              connection setup.

       tcp_wmem (since Linux 2.4)
              This  is  a  vector  of  3 integers: [min, default, max].  These
              parameters are used by TCP to regulate send buffer  sizes.   TCP
              dynamically adjusts the size of the send buffer from the default
              values listed below, in the range of these values, depending  on
              memory available.

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

              default   The  default size of the send buffer for a TCP socket.
                        This value overwrites the initial default buffer  size
                        from            the           generic           global
                        /proc/sys/net/core/wmem_default   defined   for    all
                        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 /proc/sys/net/ipv4/tcp_window_scaling
                        must be set to a nonzero value (default).

              max       The  maximum  size of the send buffer used by each TCP
                        socket.  This value does not  override  the  value  in
                        /proc/sys/net/core/wmem_max.   This  is  not  used  to
                        limit the size  of  the  send  buffer  declared  using
                        SO_SNDBUF   on   a   socket.   The  default  value  is
                        calculated using the formula

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

                        (On Linux  2.4,  the  default  value  is  128K  bytes,
                        lowered 64K depending on low-memory systems.)

       tcp_workaround_signed_windows  (Boolean; default: disabled; since Linux
       2.6.26)
              If enabled, assume that no receipt of  a  window-scaling  option
              means  that  the remote TCP is broken and treats the window as a
              signed quantity.  If disabled, assume that the remote TCP is not
              broken  even  if  we do not receive a window scaling option from
              it.

   Socket Options
       To set or get a TCP  socket  option,  call  getsockopt(2)  to  read  or
       setsockopt(2) to write the option with the option level argument set to
       IPPROTO_TCP.  In addition, most IPPROTO_IP socket options are valid  on
       TCP sockets.  For more information see ip(7).

       TCP_CORK (since Linux 2.2)
              If  set,  don't  send  out  partial  frames.  All queued partial
              frames are sent when the  option  is  cleared  again.   This  is
              useful for prepending headers before calling sendfile(2), or for
              throughput optimization.  As currently implemented, there  is  a
              200  millisecond  ceiling on the time for which output is corked
              by TCP_CORK.  If this ceiling is reached, then  queued  data  is
              automatically  transmitted.   This  option  can be combined with
              TCP_NODELAY only since Linux 2.5.71.  This option should not  be
              used in code intended to be portable.

       TCP_DEFER_ACCEPT (since Linux 2.4)
              Allow  a  listener  to be awakened only when data arrives on the
              socket.  Takes an integer value (seconds), this  can  bound  the
              maximum  number  of  attempts  TCP  will  make  to  complete the
              connection.  This option should not be used in code intended  to
              be portable.

       TCP_INFO (since Linux 2.4)
              Used  to  collect  information  about  this  socket.  The kernel
              returns   a   struct   tcp_info   as   defined   in   the   file
              /usr/include/linux/tcp.h.   This  option  should  not be used in
              code intended to be portable.

       TCP_KEEPCNT (since Linux 2.4)
              The maximum number of keepalive probes TCP  should  send  before
              dropping the connection.  This option should not be used in code
              intended to be portable.

       TCP_KEEPIDLE (since Linux 2.4)
              The time (in seconds) the connection needs to remain idle before
              TCP  starts  sending  keepalive  probes,  if  the  socket option
              SO_KEEPALIVE has been set on this socket.   This  option  should
              not be used in code intended to be portable.

       TCP_KEEPINTVL (since Linux 2.4)
              The time (in seconds) between individual keepalive probes.  This
              option should not be used in code intended to be portable.

       TCP_LINGER2 (since Linux 2.4)
              The lifetime of orphaned FIN_WAIT2 state sockets.   This  option
              can  be  used  to  override  the system-wide setting in the file
              /proc/sys/net/ipv4/tcp_fin_timeout for this socket.  This is not
              to  be confused with the socket(7) level option SO_LINGER.  This
              option should not be used in code intended to be portable.

       TCP_MAXSEG
              The maximum segment size for outgoing TCP packets.  In Linux 2.2
              and  earlier,  and  in Linux 2.6.28 and later, if this option is
              set before connection establishment, it  also  changes  the  MSS
              value  announced to the other end in the initial packet.  Values
              greater than the (eventual) interface MTU have no  effect.   TCP
              will  also  impose its minimum and maximum bounds over the value
              provided.

       TCP_NODELAY
              If set, disable the Nagle algorithm.  This means  that  segments
              are  always  sent  as  soon as possible, even if there is only a
              small amount of data.  When not  set,  data  is  buffered  until
              there  is  a sufficient amount to send out, thereby avoiding the
              frequent  sending  of  small  packets,  which  results  in  poor
              utilization  of  the  network.   This  option  is  overridden by
              TCP_CORK; however, setting this option forces an explicit  flush
              of pending output, even if TCP_CORK is currently set.

       TCP_QUICKACK (since Linux 2.4.4)
              Enable quickack mode if set or disable quickack mode if cleared.
              In quickack mode, acks are sent immediately, rather than delayed
              if  needed  in accordance to normal TCP operation.  This flag is
              not permanent, it only enables a  switch  to  or  from  quickack
              mode.   Subsequent operation of the TCP protocol will once again
              enter/leave  quickack  mode  depending  on   internal   protocol
              processing  and  factors  such as delayed ack timeouts occurring
              and data transfer.  This option  should  not  be  used  in  code
              intended to be portable.

       TCP_SYNCNT (since Linux 2.4)
              Set  the  number  of SYN retransmits that TCP should send before
              aborting the attempt to connect.  It cannot  exceed  255.   This
              option should not be used in code intended to be portable.

       TCP_WINDOW_CLAMP (since Linux 2.4)
              Bound  the  size  of  the  advertised window to this value.  The
              kernel imposes a minimum size of SOCK_MIN_RCVBUF/2.  This option
              should not be used in code intended to be portable.

   Sockets API
       TCP  provides  limited  support for out-of-band data, in the form of (a
       single byte of) urgent data.  In Linux this  means  if  the  other  end
       sends  newer  out-of-band  data  the  older  urgent data is inserted as
       normal data into the stream (even when SO_OOBINLINE is not set).   This
       differs from BSD-based stacks.

       Linux  uses  the  BSD  compatible  interpretation of the urgent pointer
       field  by  default.   This  violates  RFC 1122,  but  is  required  for
       interoperability   with   other   stacks.    It   can  be  changed  via
       /proc/sys/net/ipv4/tcp_stdurg.

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

       Since  version  2.4,  Linux  supports the use of MSG_TRUNC in the flags
       argument of recv(2) (and recvmsg(2)).  This flag  causes  the  received
       bytes  of  data  to  be discarded, rather than passed back in a caller-
       supplied buffer.  Since Linux 2.4.4, MSG_PEEK also has this effect when
       used in conjunction with MSG_OOB to receive out-of-band data.

   Ioctls
       The  following ioctl(2) calls return information in value.  The correct
       syntax is:

              int value;
              error = ioctl(tcp_socket, ioctl_type, &value);

       ioctl_type is one of the following:

       SIOCINQ
              Returns the amount of queued unread data in the receive  buffer.
              The  socket  must  not  be  in  LISTEN state, otherwise an error
              (EINVAL) is returned.  SIOCINQ is defined in  <linux/sockios.h>.
              Alternatively,  you  can use the synonymous FIONREAD, defined in
              <sys/ioctl.h>.

       SIOCATMARK
              Returns true (i.e., value is nonzero) if the inbound data stream
              is at the urgent mark.

              If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
              true, then the next read from the socket will return the  urgent
              data.   If  the  SO_OOBINLINE  socket  option  is  not  set, and
              SIOCATMARK returns true, then the next read from the socket will
              return the bytes following the urgent data (to actually read the
              urgent data requires the recv(MSG_OOB) flag).

              Note that a read never reads across  the  urgent  mark.   If  an
              application  is  informed  of  the  presence  of urgent data via
              select(2) (using the exceptfds argument) or through delivery  of
              a SIGURG signal, then it can advance up to the mark using a loop
              which  repeatedly  tests  SIOCATMARK   and   performs   a   read
              (requesting  any  number of bytes) as long as SIOCATMARK returns
              false.

       SIOCOUTQ
              Returns the amount of unsent data in the socket send queue.  The
              socket  must not be in LISTEN state, otherwise an error (EINVAL)
              is  returned.   SIOCOUTQ  is   defined   in   <linux/sockios.h>.
              Alternatively,  you  can use the synonymous TIOCOUTQ, defined in
              <sys/ioctl.h>.

   Error Handling
       When a network error occurs, TCP tries to resend  the  packet.   If  it
       doesn't  succeed after some time, either ETIMEDOUT or the last received
       error on this connection is reported.

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

ERRORS

       EAFNOTSUPPORT
              Passed socket address type in sin_family was not AF_INET.

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

       ETIMEDOUT
              The  other  end didn't acknowledge retransmitted data after some
              time.

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

VERSIONS

       Support  for  Explicit  Congestion Notification, zero-copy sendfile(2),
       reordering support and some SACK extensions (DSACK) were introduced  in
       2.4.   Support for forward acknowledgement (FACK), TIME_WAIT recycling,
       and per-connection keepalive socket options were introduced in 2.3.

BUGS

       Not all errors are documented.
       IPv6 is not described.

SEE ALSO

       accept(2), bind(2), connect(2), getsockopt(2),  listen(2),  recvmsg(2),
       sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)

       RFC 793 for the TCP specification.
       RFC 1122  for  the  TCP  requirements  and  a  description of the Nagle
       algorithm.
       RFC 1323 for TCP timestamp and window scaling options.
       RFC 1337 for a description of TIME_WAIT assassination hazards.
       RFC 3168 for a description of Explicit Congestion Notification.
       RFC 2581 for TCP congestion control algorithms.
       RFC 2018 and RFC 2883 for SACK and extensions to SACK.

COLOPHON

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