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TCP(7)                                                                  Linux Programmer's Manual                                                                 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 connecā€
       tion 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.   Winā€
       dow  scaling  allows  the use of large (> 64 kB) 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  getā€
       sockopt(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 interā€
       nal kernel structures, and the /proc file values reflect the larger sizes compared to the actual TCP windows.  On individual connections, the socket buffer  size
       must be set prior to the listen(2) or connect(2) calls in order to have it take effect.  See socket(7) for more 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).

       When out-of-band data is present, select(2) indicates the file descriptor as having an exceptional condition and poll (2) indicates a POLLPRI event.

       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  extenā€
       sions.

   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 mulā€
       ticasting 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; Linux 2.6.15 to Linux 3.8)
              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
              acknowledgements.  Possible values are:

              0  increase cwnd once per acknowledgement (no ABC)

              1  increase cwnd once per acknowledgement of full sized segment

              2  allow increase cwnd by two if acknowledgement is of two segments to compensate for delayed acknowledgements.

       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_congesā€
              tion_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 deā€
              crease  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  congesā€
              tion  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 configā€
              ured 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: see 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 inā€
              troduced.

       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  reā€
              sent 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 specificaā€
              tion, but required to prevent denial-of-service attacks.  In Linux 2.2, the default value was 180.

       tcp_frto (integer; default: see below; 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.  This was the default up to and including Linux 2.6.23.

              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.  This value is the default since Linux 2.6.24.

              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; obsolete since Linux 4.14)
              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.  Since Linux 4.14,  this  file  still
              exists, but its value is ignored.

       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 conā€
              nection 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 ~64 kB 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 (>=
              128 MB), and reduced to 128 for those systems with very low memory (<= 32 MB).

              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  inā€
              clude/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 usā€
              age.  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 exā€
                     ited 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 deā€
              fault 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  4 kB,
                     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(4 MB, 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  (retransmisā€
              sion 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: 6; 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 deā€
              fault value is 6, which corresponds to retrying for up to approximately 127 seconds.  Before Linux 3.7, the default value was  5,  which  (in  conjunction
              with calculation based on other kernel parameters) corresponded 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 (integer; default: 1; since Linux 2.2)
              Enable TCP syncookies.  The kernel must be compiled with CONFIG_SYN_COOKIES.  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 misconā€
              figured conditions.  For recommended alternatives see tcp_max_syn_backlog, tcp_synack_retries, and tcp_abort_on_overflow.  Set to  one  of  the  following
              values:

              0  Disable TCP syncookies.

              1  Send out syncookies when the syn backlog queue of a socket overflows.

              2  (since Linux 3.12) Send out syncookies unconditionally.  This can be useful for network testing.

       tcp_timestamps (integer; default: 1; since Linux 2.2)
              Set to one of the following values to enable or disable RFC 1323 TCP timestamps:

              0  Disable timestamps.

              1  Enable timestamps as defined in RFC1323 and use random offset for each connection rather than only using the current time.

              2  As for the value 1, but without random offsets.  Setting tcp_timestamps to this value is meaningful since Linux 4.10.

       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; Linux 2.4 to 4.11)
              Enable fast recycling of TIME_WAIT sockets.  Enabling this option is not recommended as the remote IP may not use monotonically increasing timestamps (deā€
              vices behind NAT, devices with per-connection timestamp offsets).  See RFC 1323 (PAWS) and 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 techniā€
              cal 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 congesā€
              tion 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 (> 64 kB) on a TCP connection, should the other end support  it.   Norā€
              mally,  the  16  bit window length field in the TCP header limits the window size to less than 64 kB.  If larger windows are desired, applications can inā€
              crease 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  4 kB.)
                     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 16 kB.  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  (deā€
                     fault).

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

                     (On Linux 2.4, the default value is 128 kB, lowered 64 kB 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  disā€
              abled, 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).

       Following is a list of TCP-specific socket options.  For details of some other socket options that are also applicable for TCP sockets, see socket(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  esā€
              tablishment, it also changes the MSS value announced to the other end in the initial packet.  Values greater than the (eventual) interface MTU have no efā€
              fect.  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 transā€
              mitted data may remain unacknowledged, or bufferred data may remain untransmitted (due to zero window size) before TCP will forcibly close the correspondā€
              ing connection and return ETIMEDOUT to the application.  If the option value is specified as 0, TCP will use the system default.

              Increasing  user  timeouts  allows a TCP connection to survive extended periods without end-to-end connectivity.  Decreasing user timeouts allows applicaā€
              tions 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 disā€
       carded, 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_OOBINā€
              LINE  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 actuā€
              ally 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  arā€
              gument) 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 (reā€
              questing 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 connecā€
       tion 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 ā€” it makes TCP less tolerant to routing changes and other normal netā€
       work 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)

       The kernel source file Documentation/networking/ip-sysctl.txt.

       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.

Linux                                                                          2021-03-22                                                                         TCP(7)