Provided by: manpages_3.21-1_all bug

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  non-zero  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)
              Controls 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  choices  available  to  non-
              privileged 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)
              Shows 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)
              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; Linux 2.4.27/2.6.6
       to 2.6.13)
              Forces 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: enabled; 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)
              Enables   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 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
                        non-zero 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)
              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 (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.   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
       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  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
              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,
       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 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 3.21 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/.