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TCP(7)                     Linux Programmer's Manual                    TCP(7)



NAME
       tcp - TCP protocol

SYNOPSIS
       #include <&lt;sys/socket.h>&gt;
       #include <&lt;netinet/in.h>&gt;
       #include <&lt;netinet/tcp.h>&gt;

       tcp_socket = socket(PF_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 net.ipv4.tcp_wmem and net.ipv4.tcp_rmem sysctl
       variables,  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 global net.core.rmem_max and
       net.core.wmem_max sysctls.  Note that TCP actually allocates twice  the
       size  of  the buffer requested in the setsockopt(2) call, and so a suc-
       ceeding 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  sysctl
       variables  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 ker-
       nel 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 nor-
       mal data stream (a program can test for its location using the  SIOCAT-
       MARK 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  Notifi-
       cation,  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.

   Sysctls
       These variables can be accessed by the  /proc/sys/net/ipv4/*  files  or
       with  the sysctl(2) interface.  In addition, most IP sysctls also apply
       to TCP; see ip(7).  Variables described  as  Boolean  take  an  integer
       value,  with  a  non-zero value ("true") meaning that the corresponding
       option is enabled, and a zero value ("false") meaning that  the  option
       is disabled.

       tcp_abort_on_overflow (Boolean; default: disabled)
              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)
              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_app_win (integer; default: 31)
              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_bic (Boolean; default: disabled)
              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)
              Sets  the  threshold window (in packets) where BIC TCP starts to
              adjust the congestion window.   Below  this  threshold  BIC  TCP
              behaves the same as the default TCP Reno.

       tcp_bic_fast_convergence (Boolean; default: enabled)
              Forces  BIC TCP to more quickly respond to changes in congestion
              window.  Allows two flows sharing the same  connection  to  con-
              verge more rapidly.

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

       tcp_ecn (Boolean; default: disabled)
              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)
              Enable TCP Forward Acknowledgement support.

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

       tcp_frto (Boolean; default: disabled)
              Enables  F-RTO,  an enhanced recovery algorithm for TCP retrans-
              mission timeouts.  It is  particularly  beneficial  in  wireless
              environments  where packet loss is typically due to random radio
              interference rather than intermediate router congestion.

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

       tcp_keepalive_probes (integer; default: 9)
              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)
              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
              interval of 75 seconds apart) when keep-alive is enabled.

              Note that underlying connection tracking mechanisms and applica-
              tion timeouts may be much shorter.

       tcp_low_latency (Boolean; default: disabled)
              If enabled, the TCP stack  makes  decisions  that  prefer  lower
              latency as opposed to higher throughput.  It this option is dis-
              abled, 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)
              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)
              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)
              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_mem
              This is a vector of 3 integers: [low,  pressure,  high].   These
              bounds  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_orphan_retries (integer; default: 8)
              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)
              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)
              Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3)
              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)
              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)
              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
              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  sysctl  variables,
              depending on memory available in the system.

              min      minimum  size  of  the  receive buffer used by each TCP
                       socket.  The default value is 4K,  and  is  lowered  to
                       PAGE_SIZE  bytes  in low-memory systems.  This value is
                       used to ensure that in memory  pressure  mode,  alloca-
                       tions  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 buf-
                       fer  size from the generic global net.core.rmem_default
                       defined for all protocols.  The default value is  87380
                       bytes,  and  is 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 of 87380*2 bytes is lowered
                       to 87380 in low-memory systems.

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

       tcp_stdurg (Boolean; default: disabled)
              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_synack_retries (integer; default: 5)
              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)
              Enable TCP syncookies.  The kernel must be  compiled  with  CON-
              FIG_SYN_COOKIES.  Send out syncookies when the syn backlog queue
              of a socket overflows.  The syncookies feature attempts to  pro-
              tect 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  proto-
              col,  and  conflicts  with other areas of TCP such as TCP exten-
              sions.  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  recom-
              mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
              and tcp_abort_on_overflow.

       tcp_syn_retries (integer; default: 5)
              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_timestamps (Boolean; default: enabled)
              Enable RFC 1323 TCP timestamps.

       tcp_tw_recycle (Boolean; default: disabled)
              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)
              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_window_scaling (Boolean; default: enabled)
              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_vegas_cong_avoid (Boolean; default: disabled)
              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)
              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_wmem
              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 sysctl variables,
              depending on memory available.

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

              default - the default size of the send buffer for a TCP  socket.
              This  value  overwrites the initial default buffer size from the
              generic global net.core.wmem_default defined for all  protocols.
              The default value is 16K bytes.  If larger send buffer sizes are
              desired, this value should be increased (to affect all sockets).
              To    employ    large   TCP   windows,   the   sysctl   variable
              net.ipv4.tcp_window_scaling must be enabled (default).

              max - the maximum size of the  send  buffer  used  by  each  TCP
              socket.     This    value   does   not   override   the   global
              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 128K bytes.  It is lowered to 64K depending on the mem-
              ory available in the system.

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

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

       TCP_LINGER2
              The  lifetime  of orphaned FIN_WAIT2 state sockets.  This option
              can be used to override the system wide  sysctl  tcp_fin_timeout
              on  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
              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
              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
              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.

   Ioctls
       These  following  ioctl(2) calls return information in value.  The cor-
       rect syntax is:

              int value;
              error = ioctl(tcp_socket, ioctl_type, &&amp;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.

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

              If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
              true, then the next read from the socket will return the  urgent
              data.  If the SO_OOBINLINE socket option is not set, and 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.

   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 exe-
              cuted 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,
       per  connection keepalive socket options and sysctls were introduced in
       2.3.

       The default values and descriptions  for  the  sysctl  variables  given
       above are applicable for the 2.4 kernel.

NOTES
       TCP  has  no  real out-of-band data; it has 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 interop-
       erability with other stacks.  It  can  be  changed  by  the  tcp_stdurg
       sysctl.

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), sysctl(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 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-11-25                            TCP(7)