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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.  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 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

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       pages/.