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

NAME
       tcp - TCP protocol

SYNOPSIS
       #include 
       #include 
       #include 

       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 setsock‐
       opt(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 connec‐
       tions,  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 sup‐
       ported.

   /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") mean‐
       ing 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 acknowl‐
              edgments.  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 "applica‐
              tion"  buffer,  used  to isolate the network from scheduling and application laten‐
              cies.  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 pro‐
              cesses (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 consecu‐
              tive 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  con‐
              gestion-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 dis‐
              covery (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  scala‐
              bility  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.  How‐
              ever, 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  con‐
              gestion.  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  con‐
                 gestion 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  immedi‐
                 ately.

              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 pre‐
              ferred.  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 con‐
              nection 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 con‐
              nection 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-mem‐
                        ory 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 time‐
              out).  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 con‐
              flicts 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_over‐
              flow.

       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  recom‐
              mended  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 con‐
              gestion  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 expe‐
              rienced.  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_scal‐
              ing 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, depend‐
              ing 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, allo‐
                        cations  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  over‐
                        writes   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  win‐
                        dows,  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_conges‐
              tion_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  connec‐
              tion.  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 over‐
              ride  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  net‐
              work.   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  connec‐
              tion  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  with‐
              out 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  cur‐
              rent 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 deter‐
              mine 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  mini‐
              mum  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 .  Alternatively, you can use the synonymous  FIONREAD,
              defined in .

       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
              .  Alternatively, you can use the synonymous TIOCOUTQ, defined  in
              .

   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 acknowledge‐
       ment (FACK), TIME_WAIT recycling, and per-connection keepalive socket options were  intro‐
       duced 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 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-12-05                                     TCP(7)

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