TCP(7)                    (2020-12-21)                     TCP(7)

          tcp - TCP protocol

          #include <sys/socket.h>
          #include <netinet/in.h>
          #include <netinet/tcp.h>

          tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

          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 con-
          nection 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 lis-
          tening 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

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          setsockopt(2) call.  TCP uses the extra space for adminis-
          trative 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 sig-
          nal 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), other-
          wise 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 manage-
          ment 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

        /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.


               .}f Enable resetting connections if the listening ser-
               vice 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.


               .}f Show/set the congestion control algorithm choices
               available to unprivileged processes (see the descrip-
               tion 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.


          htmlmanrefstarttcp_available_congestion_control(String;read-only;sinceLinux2.4.20)    .}f Show a list of the congestion-control algorithms
               that are registered.  The items in the list are sepa-
               rated by white space and terminated by a newline char-
               acter.  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.


               .}f 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.



               .}f 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

     2.4.27/2.6.6 to 2.6.13)

               .}f Set the default congestion-control algorithm to be
               used for new connections.  The algorithm "reno" is
               always available, but additional choices may be avail-
               able depending on kernel configuration.  The default
               value for this file is set as part of kernel configura-


               .}f Enable RFC 2883 TCP Duplicate SACK support.


               .}f Enable RFC 3168, Section fallback.  When
               enabled, outgoing ECN-setup SYNs that time out within
               the normal SYN retransmission timeout will be resent
               with CWR and ECE cleared.



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               .}f 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.


               .}f 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

               2  Aggressive response; undoes congestion-control mea-
                  sures 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.

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               .}f 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.


               If enabled, the TCP stack makes decisions that prefer
               lower latency as opposed to higher throughput.  It this
               option is disabled, then higher throughput is pre-
               ferred.  An example of an application where this
               default should be changed would be a Beowulf compute
               cluster.  Since Linux 4.14, this file still exists, but
               its value is ignored.


               .}f 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

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               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 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.


               .}f 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.

               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 limita-

               low  TCP doesn't regulate its memory allocation when
                    the number of pages it has allocated globally is
                    below this number.

                    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.

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               .}f 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 con-


               .}f 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 con-


               .}f 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.


               .}f 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.

               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 pres-
                    sure 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

                    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 sys-
                    tems.)  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

               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,

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                    (On Linux 2.4, the default is 87380*2 bytes, low-
                    ered to 87380 in low-memory systems).


               .}f 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.


               .}f 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 calcu-
               lation based on other kernel parameters) corresponded
               to approximately 180 seconds.


               .}f 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 rec-
               ommended as a tuning mechanism for heavily loaded
               servers to help with overloaded or misconfigured condi-
               tions.  For recommended alternatives see
               tcp_max_syn_backlog, tcp_synack_retries, and
               tcp_abort_on_overflow. Set to one of the following val-

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               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 uncondition-
                  ally.  This can be useful for network testing.


               .}f This parameter controls what percentage of the con-
               gestion window can be consumed by a single TCP Segmen-
               tation Offload (TSO) frame.  The setting of this param-
               eter is a tradeoff between burstiness and building
               larger TSO frames.


               .}f Allow to reuse TIME_WAIT sockets for new connec-
               tions when it is safe from protocol viewpoint.  It
               should not be changed without advice/request of techni-
               cal experts.



               .}f 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.




          htmlmanrefstarttcp_workaround_signed_windows(Boolean;default:disabled;sinceLinux2.6.26)    .}f 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 informa-
          tion see ip(7).

          Following is a list of TCP-specific socket options.  For
          details of some other socket options that are also applica-
          ble 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.  Unprivi-
               leged 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 par-
               tial frames are sent when the option is cleared again.
               This is useful for prepending headers before calling
               sendfile(2), or for throughput optimization.  As cur-
               rently 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 automati-
               cally transmitted.  This option can be combined with

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               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

          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.

               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


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               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; how-
               ever, 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 proto-
               col processing and factors such as delayed ack timeouts
               occurring and data transfer.  This option should not be
               used in code intended to be portable.

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

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

               Increasing user timeouts allows a TCP connection to
               survive extended periods without end-to-end connectiv-
               ity.  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 con-
               nection, 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.

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               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 lis-
               tening socket.

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

          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

        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

          Linux uses the BSD compatible interpretation of the urgent
          pointer field by default.  This violates RFC 1122, but is
          required for interoperability with other stacks.  It can be
          changed via /proc/sys/net/ipv4/tcp_stdurg.

          It is possible to peek at out-of-band data using the recv(2)
          MSG_PEEK flag.

          Since version 2.4, Linux supports the use of MSG_TRUNC in
          the flags argument of recv(2) (and recvmsg(2)).  This flag
          causes the received bytes of data to be discarded, rather
          than passed back in a caller-supplied buffer.  Since Linux
          2.4.4, MSG_TRUNC also has this effect when used in conjunc-
          tion with MSG_OOB to receive out-of-band data.

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

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

          ioctl_type is one of the following:

               Returns the amount of queued unread data in the receive
               buffer.  The socket must not be in LISTEN state,

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               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>.

               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.

               Returns the amount of unsent data in the socket send
               queue.  The socket must not be in LISTEN state, other-
               wise 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.

               Passed socket address type in sin_family was not

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

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               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.

          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.

          Not all errors are documented.

          IPv6 is not described.

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

          The kernel source file Documentation/networking/ip-

          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 haz-
          RFC 3168 for a description of Explicit Congestion Notifica-
          RFC 2581 for TCP congestion control algorithms.
          RFC 2018 and RFC 2883 for SACK and extensions to SACK.

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
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