# tcp

## NAME



           tcp - TCP protocol
    


## SYNOPSIS



           **#include <sys/socket.h>**
           **#include <netinet/in.h>**
           **#include <netinet/tcp.h>**
    
           _tcp_socket_ **= socket(AF_INET, SOCK_STREAM, 0);**
    


## DESCRIPTION



           This is an implementation of the TCP protocol defined in RFC 793,
           RFC 1122 and RFC 2001 with the NewReno and SACK extensions.  It
           provides a reliable, stream-oriented, full-duplex connection
           between two sockets on top of ip(7), for both v4 and v6 versions.
           TCP guarantees that the data arrives in order and retransmits lost
           packets.  It generates and checks a per-packet checksum to catch
           transmission errors.  TCP does not preserve record boundaries.
    
           A newly created TCP socket has no remote or local address and is
           not fully specified.  To create an outgoing TCP connection use
           connect(2) to establish a connection to another TCP socket.  To
           receive new incoming connections, first bind(2) the socket to a
           local address and port and then call listen(2) to put the socket
           into the listening state.  After that a new socket for each
           incoming connection can be accepted using accept(2).  A socket
           which has had accept(2) or connect(2) successfully called on it is
           fully specified and may transmit data.  Data cannot be transmitted
           on listening or not yet connected sockets.
    
           Linux supports RFC 1323 TCP high performance extensions.  These
           include Protection Against Wrapped Sequence Numbers (PAWS), Window
           Scaling and Timestamps.  Window scaling allows the use of large (>
           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
           getsockopt(2) call will not return the same size of buffer as
           requested in the setsockopt(2) call.  TCP uses the extra space for
           administrative purposes and internal kernel structures, and the
           _/proc_ file values reflect the larger sizes compared to the actual
           TCP windows.  On individual connections, the socket buffer size
           must be set prior to the listen(2) or connect(2) calls in order to
           have it take effect.  See socket(7) for more information.
    
           TCP supports urgent data.  Urgent data is used to signal the
           receiver that some important message is part of the data stream
           and that it should be processed as soon as possible.  To send
           urgent data specify the **MSG_OOB** option to send(2).  When urgent
           data is received, the kernel sends a **SIGURG** signal to the process
           or process group that has been set as the socket "owner" using the
           **SIOCSPGRP** or **FIOSETOWN** ioctls (or the POSIX.1-specified fcntl(2)
           **F_SETOWN** operation).  When the **SO_OOBINLINE** socket option is
           enabled, urgent data is put into the normal data stream (a program
           can test for its location using the **SIOCATMARK** ioctl described
           below), otherwise it can be received only when the **MSG_OOB** flag is
           set for recv(2) or recvmsg(2).
    
           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
           extensions.
    
       **Address formats**
           TCP is built on top of IP (see ip(7)).  The address formats
           defined by ip(7) apply to TCP.  TCP supports point-to-point
           communication only; broadcasting and multicasting are not
           supported.
    
       **/proc interfaces**
           System-wide TCP parameter settings can be accessed by files in the
           directory _/proc/sys/net/ipv4/_.  In addition, most IP _/proc_
           interfaces also apply to TCP; see ip(7).  Variables described as
           _Boolean_ take an integer value, with a nonzero value ("true")
           meaning that the corresponding option is enabled, and a zero value
           ("false") meaning that the option is disabled.
    
           _tcp_abc_ (Integer; default: 0; 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_congestion_control_.  The default value for
                  this list is "reno" plus the default setting of
                  _tcp_congestion_control_.
    
           _tcp_autocorking_ (Boolean; default: enabled; since Linux 3.14)
                  If this option is enabled, the kernel tries to coalesce
                  small writes (from consecutive write(2) and sendmsg(2)
                  calls) as much as possible, in order to decrease the total
                  number of sent packets.  Coalescing is done if at least one
                  prior packet for the flow is waiting in Qdisc queues or
                  device transmit queue.  Applications can still use the
                  **TCP_CORK** socket option to obtain optimal behavior when they
                  know how/when to uncork their sockets.
    
           _tcp_available_congestion_control_ (String; read-only; since Linux
           2.4.20)
                  Show a list of the congestion-control algorithms that are
                  registered.  The items in the list are separated by white
                  space and terminated by a newline character.  This list is
                  a limiting set for the list in
                  _tcp_allowed_congestion_control_.  More congestion-control
                  algorithms may be available as modules, but not loaded.
    
           _tcp_app_win_ (integer; default: 31; since Linux 2.4)
                  This variable defines how many bytes of the TCP window are
                  reserved for buffering overhead.
    
                  A maximum of (_window/2^tcp_app_win_ , mss) bytes in the
                  window are reserved for the application buffer.  A value of
                  0 implies that no amount is reserved.
    
           _tcp_base_mss_ (Integer; default: 512; since Linux 2.6.17)
                  The initial value of _search_low_ to be used by the
                  packetization layer Path MTU discovery (MTU probing).  If
                  MTU probing is enabled, this is the initial MSS used by the
                  connection.
    
           _tcp_bic_ (Boolean; default: disabled; Linux 2.4.27/2.6.6 to Linux
           2.6.13)
                  Enable BIC TCP congestion control algorithm.  BIC-TCP is a
                  sender-side-only change that ensures a linear RTT fairness
                  under large windows while offering both scalability and
                  bounded TCP-friendliness.  The protocol combines two
                  schemes called additive increase and binary search
                  increase.  When the congestion window is large, additive
                  increase with a large increment ensures linear RTT fairness
                  as well as good scalability.  Under small congestion
                  windows, binary search increase provides TCP friendliness.
    
           _tcp_bic_low_window_ (integer; default: 14; Linux 2.4.27/2.6.6 to
           Linux 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 Linux 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_fastopen_ (Bitmask; default: 0x1; since Linux 3.7)
                  Enables RFC 7413 Fast Open support.  The flag is used as a
                  bitmap with the following values:
    
                  **0x1** Enables client side Fast Open support
    
                  **0x2** Enables server side Fast Open support
    
                  **0x4** Allows client side to transmit data in SYN without
                         Fast Open option
    
                  **0x200** Allows server side to accept SYN data without Fast
                         Open option
    
                  **0x400** Enables Fast Open on all listeners without
                         **TCP_FASTOPEN** socket option
    
           _tcp_fastopen_key_ (since Linux 3.7)
                  Set server side RFC 7413 Fast Open key to generate Fast
                  Open cookie when server side Fast Open support is enabled.
    
           _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 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: 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 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 ~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).
    
                  Before Linux 2.6.20, it was recommended that if this needed
                  to be increased above 1024, the size of the SYNACK hash
                  table (**TCP_SYNQ_HSIZE**) in _include/net/tcp.h_ should be
                  modified to keep
    
                      TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
    
                  and the kernel should be recompiled.  In Linux 2.6.20, the
                  fixed sized **TCP_SYNQ_HSIZE** was removed in favor of dynamic
                  sizing.
    
           _tcp_max_tw_buckets_ (integer; default: see below; since Linux 2.4)
                  The maximum number of sockets in TIME_WAIT state allowed in
                  the system.  This limit exists only to prevent simple
                  denial-of-service attacks.  The default value of NR_FILE*2
                  is adjusted depending on the memory in the system.  If this
                  number is exceeded, the socket is closed and a warning is
                  printed.
    
           _tcp_moderate_rcvbuf_ (Boolean; default: enabled; since Linux
           2.4.17/2.6.7)
                  If enabled, TCP performs receive buffer auto-tuning,
                  attempting to automatically size the buffer (no greater
                  than _tcp_rmem[2]_) to match the size required by the path
                  for full throughput.
    
           _tcp_mem_ (since Linux 2.4)
                  This is a vector of 3 integers: [low, pressure, high].
                  These bounds, measured in units of the system page size,
                  are used by TCP to track its memory usage.  The defaults
                  are calculated at boot time from the amount of available
                  memory.  (TCP can only use _low memory_ for this, which is
                  limited to around 900 megabytes on 32-bit systems.  64-bit
                  systems do not suffer this limitation.)
    
                  _low_    TCP doesn't regulate its memory allocation when the
                         number of pages it has allocated globally is below
                         this number.
    
                  _pressure_
                         When the amount of memory allocated by TCP exceeds
                         this number of pages, TCP moderates its memory
                         consumption.  This memory pressure state is exited
                         once the number of pages allocated falls below the
                         _low_ mark.
    
                  _high_   The maximum number of pages, globally, that TCP will
                         allocate.  This value overrides any other limits
                         imposed by the kernel.
    
           _tcp_mtu_probing_ (integer; default: 0; since Linux 2.6.17)
                  This parameter controls TCP Packetization-Layer Path MTU
                  Discovery.  The following values may be assigned to the
                  file:
    
                  **0** Disabled
    
                  **1** Disabled by default, enabled when an ICMP black hole
                         detected
    
                  **2** Always enabled, use initial MSS of _tcp_base_mss_.
    
           _tcp_no_metrics_save_ (Boolean; default: disabled; since Linux
           2.6.6)
                  By default, TCP saves various connection metrics in the
                  route cache when the connection closes, so that connections
                  established in the near future can use these to set initial
                  conditions.  Usually, this increases overall performance,
                  but it may sometimes cause performance degradation.  If
                  _tcp_no_metrics_save_ is enabled, TCP will not cache metrics
                  on closing connections.
    
           _tcp_orphan_retries_ (integer; default: 8; since Linux 2.4)
                  The maximum number of attempts made to probe the other end
                  of a connection which has been closed by our end.
    
           _tcp_reordering_ (integer; default: 3; since Linux 2.4)
                  The maximum a packet can be reordered in a TCP packet
                  stream without TCP assuming packet loss and going into slow
                  start.  It is not advisable to change this number.  This is
                  a packet reordering detection metric designed to minimize
                  unnecessary back off and retransmits provoked by reordering
                  of packets on a connection.
    
           _tcp_retrans_collapse_ (Boolean; default: enabled; since Linux 2.2)
                  Try to send full-sized packets during retransmit.
    
           _tcp_retries1_ (integer; default: 3; since Linux 2.2)
                  The number of times TCP will attempt to retransmit a packet
                  on an established connection normally, without the extra
                  effort of getting the network layers involved.  Once we
                  exceed this number of retransmits, we first have the
                  network layer update the route if possible before each new
                  retransmit.  The default is the RFC specified minimum of 3.
    
           _tcp_retries2_ (integer; default: 15; since Linux 2.2)
                  The maximum number of times a TCP packet is retransmitted
                  in established state before giving up.  The default value
                  is 15, which corresponds to a duration of approximately
                  between 13 to 30 minutes, depending on the retransmission
                  timeout.  The RFC 1122 specified minimum limit of 100
                  seconds is typically deemed too short.
    
           _tcp_rfc1337_ (Boolean; default: disabled; since Linux 2.2)
                  Enable TCP behavior conformant with RFC 1337.  When
                  disabled, if a RST is received in TIME_WAIT state, we close
                  the socket immediately without waiting for the end of the
                  TIME_WAIT period.
    
           _tcp_rmem_ (since Linux 2.4)
                  This is a vector of 3 integers: [min, default, max].  These
                  parameters are used by TCP to regulate receive buffer
                  sizes.  TCP dynamically adjusts the size of the receive
                  buffer from the defaults listed below, in the range of
                  these values, depending on memory available in the system.
    
                  _min_    minimum size of the receive buffer used by each TCP
                         socket.  The default value is the system page size.
                         (On Linux 2.4, the default value is 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 (retransmission timeout).  If
                  disabled, the congestion window will not be timed out after
                  an idle period.
    
           _tcp_stdurg_ (Boolean; default: disabled; since Linux 2.2)
                  If this option is enabled, then use the RFC 1122
                  interpretation of the TCP urgent-pointer field.  According
                  to this interpretation, the urgent pointer points to the
                  last byte of urgent data.  If this option is disabled, then
                  use the BSD-compatible interpretation of the urgent
                  pointer: the urgent pointer points to the first byte after
                  the urgent data.  Enabling this option may lead to
                  interoperability problems.
    
           _tcp_syn_retries_ (integer; default: 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 default 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
                  misconfigured 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 Linux
           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 (devices 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 technical experts.
    
           _tcp_vegas_cong_avoid_ (Boolean; default: disabled; Linux 2.2 to
           Linux 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
           Linux 2.6.13)
                  Enable TCP Westwood+ congestion control algorithm.  TCP
                  Westwood+ is a sender-side-only modification of the TCP
                  Reno protocol stack that optimizes the performance of TCP
                  congestion control.  It is based on end-to-end bandwidth
                  estimation to set congestion window and slow start
                  threshold after a congestion episode.  Using this
                  estimation, TCP Westwood+ adaptively sets a slow start
                  threshold and a congestion window which takes into account
                  the bandwidth used at the time congestion is experienced.
                  TCP Westwood+ significantly increases fairness with respect
                  to TCP Reno in wired networks and throughput over wireless
                  links.
    
           _tcp_window_scaling_ (Boolean; default: enabled; since Linux 2.2)
                  Enable RFC 1323 TCP window scaling.  This feature allows
                  the use of a large window (> 64 kB) 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 64 kB.  If larger windows are desired,
                  applications can increase the size of their socket buffers
                  and the window scaling option will be employed.  If
                  _tcp_window_scaling_ is disabled, TCP will not negotiate the
                  use of window scaling with the other end during connection
                  setup.
    
           _tcp_wmem_ (since Linux 2.4)
                  This is a vector of 3 integers: [min, default, max].  These
                  parameters are used by TCP to regulate send buffer sizes.
                  TCP dynamically adjusts the size of the send buffer from
                  the default values listed below, in the range of these
                  values, depending on memory available.
    
                  _min_    Minimum size of the send buffer used by each TCP
                         socket.  The default value is the system page size.
                         (On Linux 2.4, the default value is 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 (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(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 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).
    
           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 establishment, it also
                  changes the MSS value announced to the other end in the
                  initial packet.  Values greater than the (eventual)
                  interface MTU have no effect.  TCP will also impose its
                  minimum and maximum bounds over the value provided.
    
           **TCP_NODELAY**
                  If set, disable the Nagle algorithm.  This means that
                  segments are always sent as soon as possible, even if there
                  is only a small amount of data.  When not set, data is
                  buffered until there is a sufficient amount to send out,
                  thereby avoiding the frequent sending of small packets,
                  which results in poor utilization of the network.  This
                  option is overridden by **TCP_CORK** ; however, setting this
                  option forces an explicit flush of pending output, even if
                  **TCP_CORK** is currently set.
    
           **TCP_QUICKACK**(since Linux 2.4.4)
                  Enable quickack mode if set or disable quickack mode if
                  cleared.  In quickack mode, acks are sent immediately,
                  rather than delayed if needed in accordance to normal TCP
                  operation.  This flag is not permanent, it only enables a
                  switch to or from quickack mode.  Subsequent operation of
                  the TCP protocol will once again enter/leave quickack mode
                  depending on internal protocol processing and factors such
                  as delayed ack timeouts occurring and data transfer.  This
                  option should not be used in code intended to be portable.
    
           **TCP_SYNCNT**(since Linux 2.4)
                  Set the number of SYN retransmits that TCP should send
                  before aborting the attempt to connect.  It cannot exceed
                  255.  This option should not be used in code intended to be
                  portable.
    
           **TCP_USER_TIMEOUT**(since Linux 2.6.37)
                  This option takes an _unsigned int_ as an argument.  When the
                  value is greater than 0, it specifies the maximum amount of
                  time in milliseconds that transmitted data may remain
                  unacknowledged, or buffered data may remain untransmitted
                  (due to zero window size) before TCP will forcibly close
                  the corresponding 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 applications to "fail
                  fast", if so desired.  Otherwise, failure may take up to 20
                  minutes with the current system defaults in a normal WAN
                  environment.
    
                  This option can be set during any state of a TCP
                  connection, but is effective only during the synchronized
                  states of a connection (ESTABLISHED, FIN-WAIT-1, FIN-
                  WAIT-2, CLOSE-WAIT, CLOSING, and LAST-ACK).  Moreover, when
                  used with the TCP keepalive (**SO_KEEPALIVE**) option,
                  **TCP_USER_TIMEOUT** will override keepalive to determine when
                  to close a connection due to keepalive failure.
    
                  The option has no effect on when TCP retransmits a packet,
                  nor when a keepalive probe is sent.
    
                  This option, like many others, will be inherited by the
                  socket returned by accept(2), if it was set on the
                  listening socket.
    
                  Further details on the user timeout feature can be found in
                  RFC 793 and RFC 5482 ("TCP User Timeout Option").
    
           **TCP_WINDOW_CLAMP**(since Linux 2.4)
                  Bound the size of the advertised window to this value.  The
                  kernel imposes a minimum size of SOCK_MIN_RCVBUF/2.  This
                  option should not be used in code intended to be portable.
    
           **TCP_FASTOPEN**(since Linux 3.6)
                  This option enables Fast Open (RFC 7413) on the listener
                  socket.  The value specifies the maximum length of pending
                  SYNs (similar to the backlog argument in listen(2)).  Once
                  enabled, the listener socket grants the TCP Fast Open
                  cookie on incoming SYN with TCP Fast Open option.
    
                  More importantly it accepts the data in SYN with a valid
                  Fast Open cookie and responds SYN-ACK acknowledging both
                  the data and the SYN sequence.  accept(2) returns a socket
                  that is available for read and write when the handshake has
                  not completed yet.  Thus the data exchange can commence
                  before the handshake completes.  This option requires
                  enabling the server-side support on sysctl
                  _net.ipv4.tcp_fastopen_ (see above).  For TCP Fast Open
                  client-side support, see send(2) **MSG_FASTOPEN** or
                  **TCP_FASTOPEN_CONNECT** below.
    
           **TCP_FASTOPEN_CONNECT**(since Linux 4.11)
                  This option enables an alternative way to perform Fast Open
                  on the active side (client).  When this option is enabled,
                  connect(2) would behave differently depending on if a Fast
                  Open cookie is available for the destination.
    
                  If a cookie is not available (i.e. first contact to the
                  destination), connect(2) behaves as usual by sending a SYN
                  immediately, except the SYN would include an empty Fast
                  Open cookie option to solicit a cookie.
    
                  If a cookie is available, connect(2) would return 0
                  immediately but the SYN transmission is deferred.  A
                  subsequent write(2) or sendmsg(2) would trigger a SYN with
                  data plus cookie in the Fast Open option.  In other words,
                  the actual connect operation is deferred until data is
                  supplied.
    
                  **Note:** While this option is designed for convenience,
                  enabling it does change the behaviors and certain system
                  calls might set different _errno_ values.  With cookie
                  present, write(2) or sendmsg(2) must be called right after
                  connect(2) in order to send out SYN+data to complete 3WHS
                  and establish connection.  Calling read(2) right after
                  connect(2) without write(2) will cause the blocking socket
                  to be blocked forever.
    
                  The application should either set **TCP_FASTOPEN_CONNECT**
                  socket option before write(2) or sendmsg(2), or call
                  write(2) or sendmsg(2) with **MSG_FASTOPEN** flag directly,
                  instead of both on the same connection.
    
                  Here is the typical call flow with this new option:
    
                      s = socket();
                      setsockopt(s, IPPROTO_TCP, TCP_FASTOPEN_CONNECT, 1, ...);
                      connect(s);
                      write(s); /* write() should always follow connect()
                                 * in order to trigger SYN to go out. */
                      read(s)/write(s);
                      /* ... */
                      close(s);
    
       **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 Linux 2.4, Linux supports the use of **MSG_TRUNC** in the _flags_
           argument of recv(2) (and recvmsg(2)).  This flag causes the
           received bytes of data to be discarded, rather than passed back in
           a caller-supplied buffer.  Since Linux 2.4.4, **MSG_TRUNC** also has
           this effect when used in conjunction with **MSG_OOB** to receive out-
           of-band data.
    
       **Ioctls**
           The following ioctl(2) calls return information in _value_.  The
           correct syntax is:
    
                  **int** _value_**;**
                  _error_ **= ioctl(**_tcp_socket_**,**_ioctl_type_**, &**_value_**);**
    
           _ioctl_type_ is one of the following:
    
           **SIOCINQ**
                  Returns the amount of queued unread data in the receive
                  buffer.  The socket must not be in LISTEN state, otherwise
                  an error (**EINVAL**) is returned.  **SIOCINQ** is defined in
                  _< linux/sockios.h>_.  Alternatively, you can use the
                  synonymous **FIONREAD** , defined in _< sys/ioctl.h>_.
    
           **SIOCATMARK**
                  Returns true (i.e., _value_ is nonzero) if the inbound data
                  stream is at the urgent mark.
    
                  If the **SO_OOBINLINE** socket option is set, and **SIOCATMARK**
                  returns true, then the next read from the socket will
                  return the urgent data.  If the **SO_OOBINLINE** socket option
                  is not set, and **SIOCATMARK** returns true, then the next read
                  from the socket will return the bytes following the urgent
                  data (to actually read the urgent data requires the
                  **recv(MSG_OOB)** flag).
    
                  Note that a read never reads across the urgent mark.  If an
                  application is informed of the presence of urgent data via
                  select(2) (using the _exceptfds_ argument) or through
                  delivery of a **SIGURG** signal, then it can advance up to the
                  mark using a loop which repeatedly tests **SIOCATMARK** and
                  performs a read (requesting any number of bytes) as long as
                  **SIOCATMARK** returns false.
    
           **SIOCOUTQ**
                  Returns the amount of unsent data in the socket send queue.
                  The socket must not be in LISTEN state, otherwise an error
                  (**EINVAL**) is returned.  **SIOCOUTQ** is defined in
                  _< linux/sockios.h>_.  Alternatively, you can use the
                  synonymous **TIOCOUTQ** , defined in _< sys/ioctl.h>_.
    
       **Error handling**
           When a network error occurs, TCP tries to resend the packet.  If
           it doesn't succeed after some time, either **ETIMEDOUT** or the last
           received error on this connection is reported.
    
           Some applications require a quicker error notification.  This can
           be enabled with the **IPPROTO_IP** level **IP_RECVERR** socket option.
           When this option is enabled, all incoming errors are immediately
           passed to the user program.  Use this option with care — it makes
           TCP less tolerant to routing changes and other normal network
           conditions.
    


## ERRORS



           **EAFNOTSUPPORT**
                  Passed socket address type in _sin_family_ was not **AF_INET**.
    
           **EPIPE** The other end closed the socket unexpectedly or a read is
                  executed on a shut down socket.
    
           **ETIMEDOUT**
                  The other end didn't acknowledge retransmitted data after
                  some time.
    
           Any errors defined for ip(7) or the generic socket layer may also
           be returned for TCP.
    


## VERSIONS



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


## BUGS



           Not all errors are documented.
    
           IPv6 is not described.
    


* * *