tcp(7)


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-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 received only
   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 supports point-to-point communication only;
   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  items  in  the  list are
          separated by white space and terminated by a newline  character.
          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_autocorking (Boolean; default: enabled; since Linux 3.14)
          If this option is enabled, the kernel tries  to  coalesce  small
          writes  (from consecutive write(2) and sendmsg(2) calls) as much
          as possible, in order to  decrease  the  total  number  of  sent
          packets.   Coalescing  is  done if at least one prior packet for
          the flow is waiting in Qdisc queues or  device  transmit  queue.
          Applications  can still use the TCP_CORK socket option to obtain
          optimal  behavior  when  they  know  how/when  to  uncork  their
          sockets.

   tcp_available_congestion_control   (String;   read-only;   since  Linux
   2.4.20)
          Show a  list  of  the  congestion-control  algorithms  that  are
          registered.   The items in the list are separated by white space
          and terminated by a newline character.  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 (Integer; default: se below; since Linux 2.4)
          Enable RFC 3168 Explicit Congestion Notification.

          This file can have one of the following values:

          0      Disable  ECN.  Neither initiate nor accept ECN.  This was
                 the default up to and including Linux 2.6.30.

          1      Enable ECN when requested  by  incoming  connections  and
                 also request ECN on outgoing connection attempts.

          2      Enable ECN when requested by incoming connections, but do
                 not request ECN on outgoing connections.  This  value  is
                 supported, and is the default, since Linux 2.6.31.

          When   enabled,  connectivity  to  some  destinations  could  be
          affected due to older, misbehaving middle boxes along the  path,
          causing  connections  to be dropped.  However, to facilitate and
          encourage deployment with option 1,  and  to  work  around  such
          buggy   equipment,   the   tcp_ecn_fallback   option   has  been
          introduced.

   tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
          Enable  RFC 3168,  Section  6.1.1.1.  fallback.   When  enabled,
          outgoing  ECN-setup  SYNs  that  time  out within the normal SYN
          retransmission timeout will be resent with CWR and ECE cleared.

   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 sent only
          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).

          Prior to Linux 2.6.20, it was recommended that if this needed to
          be increased above 1024, the  size  of  the  SYNACK  hash  table
          (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep

              TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

          and the kernel should be recompiled.  In Linux 2.6.20, the fixed
          sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.

   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 for  devices  communicating  with  the
          general  Internet  or  using  NAT (Network Address Translation).
          Since some NAT gateways pass through IP timestamp values, one IP
          can  appear  to  have  non-increasing  timestamps.  See RFC 1323
          (PAWS), RFC 6191.

   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.  Unless otherwise noted, optval is a pointer  to  an  int.
   In  addition,  most IPPROTO_IP socket options are valid on TCP sockets.
   For more information see ip(7).

   TCP_CONGESTION (since Linux 2.6.13)
          The argument for this option is a string.   This  option  allows
          the  caller  to  set  the TCP congestion control algorithm to be
          used,  on  a  per-socket  basis.   Unprivileged  processes   are
          restricted    to    choosing    one   of   the   algorithms   in
          tcp_allowed_congestion_control  (described  above).   Privileged
          processes  (CAP_NET_ADMIN)  can choose from any of the available
          congestion-control   algorithms   (see   the   description    of
          tcp_available_congestion_control above).

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

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

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

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

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

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

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

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

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

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

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

   TCP_USER_TIMEOUT (since Linux 2.6.37)
          This  option  takes  an  unsigned  int as an argument.  When the
          value is greater than 0, it specifies the maximum amount of time
          in  milliseconds that transmitted data may remain unacknowledged
          before TCP will forcibly close the corresponding connection  and
          return  ETIMEDOUT  to  the  application.  If the option value is
          specified as 0, TCP will to use the system default.

          Increasing user timeouts allows  a  TCP  connection  to  survive
          extended  periods  without  end-to-end connectivity.  Decreasing
          user timeouts allows applications to "fail fast", if so desired.
          Otherwise,  failure  may  take up to 20 minutes with the current
          system defaults in a normal WAN environment.

          This option can be set during any state of a TCP connection, but
          is effective only during the synchronized states of a connection
          (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT,  CLOSING,  and
          LAST-ACK).    Moreover,   when   used  with  the  TCP  keepalive
          (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will override  keepalive
          to  determine  when  to  close  a  connection  due  to keepalive
          failure.

          The option has no effect on when TCP retransmits a  packet,  nor
          when a keepalive probe is sent.

          This  option,  like many others, will be inherited by the socket
          returned by accept(2), if it was set on the listening socket.

          Further details on the user timeout  feature  can  be  found  in
          RFC 793 and RFC 5482 ("TCP User Timeout Option").

   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_TRUNC  also  has  this  effect
   when used in conjunction with MSG_OOB to receive out-of-band data.

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

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

   ioctl_type is one of the following:

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

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

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

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

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

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

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

ERRORS

   EAFNOTSUPPORT
          Passed socket address type in sin_family was not AF_INET.

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

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

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

VERSIONS

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

BUGS

   Not all errors are documented.
   IPv6 is not described.

SEE ALSO

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

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

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.





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