tcp


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  pro-
       vides  a  reliable, stream-oriented, full-duplex connection between two
       sockets on top of ip(7), for both v4 and v6 versions.   TCP  guarantees
       that the data arrives in order and retransmits lost packets.  It gener-
       ates and checks a per-packet checksum  to  catch  transmission  errors.
       TCP does not preserve record boundaries.

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

       Linux supports RFC 1323 TCP high performance extensions.  These include
       Protection Against Wrapped Sequence Numbers (PAWS), Window Scaling  and
       Timestamps.  Window scaling allows the use of large (> 64K) TCP windows
       in order to support links with high latency or bandwidth.  To make  use
       of them, the send and receive buffer sizes must be increased.  They can
       be   set   globally   with    the    /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  inter-
       nal  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 informa-
       tion.

       TCP  supports  urgent data.  Urgent data is used to signal the receiver
       that some important message is part of the  data  stream  and  that  it
       should  be  processed as soon as possible.  To send urgent data specify
       the MSG_OOB option to send(2).  When urgent data is received, the  ker-
       nel sends a SIGURG signal to the process or process group that has been

   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  corre-
       sponding 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 seg-
                 ments 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 listen-
              ing  daemon  cannot  be  tuned  to  accept  connections  faster.
              Enabling this option can harm the clients of your server.

       tcp_adv_win_scale (integer; default: 2; 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  applica-
              tion  and  kernel.   TCP maintains part of the buffer as the TCP
              window, this is the size of the receive window advertised to the
              other  end.   The rest of the space is used as the "application"
              buffer, used to isolate the network from scheduling and applica-
              tion  latencies.   The  tcp_adv_win_scale  default  value  of  2
              implies that the space used for the application  buffer  is  one
              fourth that of the total.

       tcp_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_CONGES-

       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 lin-
              ear RTT fairness as well as good scalability.  Under small  con-
              gestion  windows,  binary search increase provides TCP friendli-
              ness.

       tcp_bic_low_window (integer; default: 14; 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  con-
              verge 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 configu-
              ration.  The default value for this file is set as part of  ker-
              nel 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
              Enable F-RTO, an enhanced recovery algorithm for TCP retransmis-
              sion 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 ver-
                 sion 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,  sup-
              porting 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 concern-
              ing 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  cau-
                 tious); 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  giv-
              ing  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 termi-
              nated after approximately an additional 11 minutes (9 probes  an

       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.   Net-
              work  conditions  might  require  you  to increase the number of
              orphans allowed, but note that each orphan can eat up to ~64K of
              unswappable  memory.   The default initial value is set equal to
              the kernel parameter NR_FILE.  This initial default is  adjusted
              depending on the memory in the system.

       tcp_max_syn_backlog (integer; default: see below; 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 drop-
              ping requests.  The default value of 256 is  increased  to  1024
              when the memory present in the system is adequate or greater (>=
              128Mb), and reduced to 128 for those systems with very low  mem-
              ory  (<=  32Mb).   It  is  recommended  that if this needs to be
              increased above 1024,  TCP_SYNQ_HSIZE  in  include/net/tcp.h  be
              modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the
              kernel be recompiled.

       tcp_max_tw_buckets (integer; default: see below; 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-ser-
              vice attacks.   The  default  value  of  NR_FILE*2  is  adjusted
              depending  on  the  memory  in  the  system.   If this number is
              exceeded, the socket is closed and a warning is printed.

       tcp_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  con-
                        sumption.   This  memory pressure state is exited once
                        the number of pages  allocated  falls  below  the  low

              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 estab-
              lished in the near future can use these to  set  initial  condi-
              tions.   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  imme-
              diately 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.
                        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_win-
                        dow_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  conges-
              tion  window after an idle period.  An idle period is defined as
              the current RTO (retransmission timeout).  If disabled, the con-
              gestion 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  interpreta-
              tion, the urgent pointer points to the last byte of urgent data.
              If this option is disabled, then use the  BSD-compatible  inter-
              pretation  of  the  urgent pointer: the urgent pointer points to
              the first byte after the urgent data.  Enabling this option  may
              lead to interoperability problems.

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

       tcp_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  CON-
              FIG_SYN_COOKIES.  Send out syncookies when the syn backlog queue

       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  work-
              ing 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  West-
              wood+  is a sender-side only modification of the TCP Reno proto-
              col stack that optimizes the performance of TCP congestion  con-
              trol.   It  is  based  on end-to-end bandwidth estimation to set
              congestion window and slow start threshold  after  a  congestion
              episode.  Using this estimation, TCP Westwood+ adaptively sets a
              slow start threshold and a congestion window  which  takes  into
              account  the  bandwidth  used  at the time congestion is experi-
              enced.  TCP  Westwood+  significantly  increases  fairness  with
              respect  to TCP Reno in wired networks and throughput over wire-
              less links.

       tcp_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 nego-
              tiate  the  use of window scaling with the other end during con-
              nection 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
                        from           the           generic            global
                        /proc/sys/net/core/wmem_default defined for all proto-
                        cols.  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  calcu-
                        lated using the formula

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

                        (On  Linux  2.4, the default value is 128K bytes, low-
                        ered 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 set-
       sockopt(2) to write the option with the option level  argument  set  to
       IPPROTO_TCP.   In addition, most IPPROTO_IP socket options are valid on
       TCP sockets.  For more information see ip(7).

       TCP_CORK (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 use-
              ful for prepending headers before calling  sendfile(2),  or  for
              throughput  optimization.   As currently implemented, there is a
              200 millisecond ceiling on the time for which output  is  corked
              by  TCP_CORK.   If  this ceiling is reached, then queued data is
              automatically transmitted.  This option  can  be  combined  with
              TCP_NODELAY  only since Linux 2.5.71.  This option should not be
              used in code intended to be portable.

       TCP_DEFER_ACCEPT (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 connec-
              tion.   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
              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  uti-
              lization of the network.  This option is overridden by TCP_CORK;
              however, setting this option forces an explicit flush of pending
              output, even if TCP_CORK is currently set.

       TCP_QUICKACK (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  pro-
              cessing  and  factors such as delayed ack timeouts occurring and
              data transfer.  This option should not be used in code  intended
              to be portable.

       TCP_SYNCNT (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 ker-
              nel imposes a minimum size of  SOCK_MIN_RCVBUF/2.   This  option
              should not be used in code intended to be portable.

   Sockets API
       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-sup-
       plied 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 (EIN-
              VAL) 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 SIOCAT-
              MARK returns true, then the  next  read  from  the  socket  will
              return the bytes following the urgent data (to actually read the
              urgent data requires the recv(MSG_OOB) flag).

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

       SIOCOUTQ
              Returns the amount of unsent data in the socket send queue.  The
              socket must not be in LISTEN state, otherwise an error  (EINVAL)
              is  returned.  SIOCOUTQ is defined in <linux/sockios.h>.  Alter-
              natively, 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.


       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  algo-
       rithm.
       RFC 1323 for TCP timestamp and window scaling options.
       RFC 1644 for a description of TIME_WAIT assassination hazards.
       RFC 3168 for a description of Explicit Congestion Notification.
       RFC 2581 for TCP congestion control algorithms.
       RFC 2018 and RFC 2883 for SACK and extensions to SACK.

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-09-10                            TCP(7)
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