tcp

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 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 acknowledgments.  Possible values
              are:

              0  increase cwnd once per acknowledgment (no ABC)

              1  increase cwnd once per acknowledgment of full sized segment

              2  allow increase cwnd by two if acknowledgment is of two
                 segments to compensate for delayed acknowledgments.

       tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
              Enable resetting connections if the listening service is too
              slow and unable to keep up and accept them.  It means that if
              overflow occurred due to a burst, the connection will recover.
              Enable this option only if you are really sure that the
              listening daemon cannot be tuned to accept connections faster.
              Enabling this option can harm the clients of your server.

       tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
              Count buffering overhead as bytes/2^tcp_adv_win_scale, if
              tcp_adv_win_scale is greater than 0; or bytes-
              bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less than
              or equal to zero.

              The socket receive buffer space is shared between the
              application and kernel.  TCP maintains part of the buffer as the
              TCP window, this is the size of the receive window advertised to
              the other end.  The rest of the space is used as the
              "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 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 2.6.13)
              Set the threshold window (in packets) where BIC TCP starts to
              adjust the congestion window.  Below this threshold BIC TCP
              behaves the same as the default TCP Reno.

       tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6
       to 2.6.13)
              Force BIC TCP to more quickly respond to changes in congestion
              window.  Allows two flows sharing the same connection to
              converge more rapidly.

       tcp_congestion_control (String; default: see text; since Linux 2.4.13)
              Set the default congestion-control algorithm to be used for new
              connections.  The algorithm "reno" is always available, but
              additional choices may be available depending on kernel
              configuration.  The default value for this file is set as part
              of kernel configuration.

       tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
              Lower limit, in bytes, of the size of socket reads that will be
              offloaded to a DMA copy engine, if one is present in the system
              and the kernel was configured with the CONFIG_NET_DMA option.

       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
              Enable RFC 2883 TCP Duplicate SACK support.

       tcp_ecn (Integer; default: 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)
              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.

       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).

              Prior to Linux 2.6.20, it was recommended that if this needed to
              be increased above 1024, the size of the SYNACK hash table
              (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep

                  TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog

              and the kernel should be recompiled.  In Linux 2.6.20, the fixed
              sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.

       tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
              The maximum number of sockets in TIME_WAIT state allowed in the
              system.  This limit exists only to prevent simple denial-of-
              service attacks.  The default value of NR_FILE*2 is adjusted
              depending on the memory in the system.  If this number is
              exceeded, the socket is closed and a warning is printed.

       tcp_moderate_rcvbuf (Boolean; default: enabled; since Linux
       2.4.17/2.6.7)
              If enabled, TCP performs receive buffer auto-tuning, attempting
              to automatically size the buffer (no greater than tcp_rmem[2])
              to match the size required by the path for full throughput.

       tcp_mem (since Linux 2.4)
              This is a vector of 3 integers: [low, pressure, high].  These
              bounds, measured in units of the system page size, are used by
              TCP to track its memory usage.  The defaults are calculated at
              boot time from the amount of available memory.  (TCP can only
              use low memory for this, which is limited to around 900
              megabytes on 32-bit systems.  64-bit systems do not suffer this
              limitation.)

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

              pressure  When the amount of memory allocated by TCP exceeds
                        this number of pages, TCP moderates its memory
                        consumption.  This memory pressure state is exited
                        once the number of pages allocated falls below the low
                        mark.

              high      The maximum number of pages, globally, that TCP will
                        allocate.  This value overrides any other limits
                        imposed by the kernel.

       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
              This parameter controls TCP Packetization-Layer Path MTU
              Discovery.  The following values may be assigned to the file:

              0  Disabled

              1  Disabled by default, enabled when an ICMP black hole detected

              2  Always enabled, use initial MSS of tcp_base_mss.

       tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
              By default, TCP saves various connection metrics in the route
              cache when the 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: 5; since Linux 2.2)
              The maximum number of times initial SYNs for an active TCP
              connection attempt will be retransmitted.  This value should not
              be higher than 255.  The default value is 5, which corresponds
              to approximately 180 seconds.

       tcp_synack_retries (integer; default: 5; since Linux 2.2)
              The maximum number of times a SYN/ACK segment for a passive TCP
              connection will be retransmitted.  This number should not be
              higher than 255.

       tcp_syncookies (Boolean; since Linux 2.2)
              Enable TCP syncookies.  The kernel must be compiled with
              CONFIG_SYN_COOKIES.  Send out syncookies when the syn backlog
              queue of a socket overflows.  The syncookies feature attempts to
              protect a socket from a SYN flood attack.  This should be used
              as a last resort, if at all.  This is a violation of the TCP
              protocol, and 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.

       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 (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 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 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
              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.

   Sockets API
       TCP provides limited support for out-of-band data, in the form of (a
       single byte of) urgent data.  In Linux this means if the other end
       sends newer out-of-band data the older urgent data is inserted as
       normal data into the stream (even when SO_OOBINLINE is not set).  This
       differs from BSD-based stacks.

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

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

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

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

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

       ioctl_type is one of the following:

       SIOCINQ
              Returns the amount of queued unread data in the receive buffer.
              The socket must not be in LISTEN state, otherwise an error
              (EINVAL) is returned.  SIOCINQ is defined in <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
       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)

       RFC 793 for the TCP specification.
       RFC 1122 for the TCP requirements and a description of the Nagle
       algorithm.
       RFC 1323 for TCP timestamp and window scaling options.
       RFC 1337 for a description of TIME_WAIT assassination hazards.
       RFC 3168 for a description of Explicit Congestion Notification.
       RFC 2581 for TCP congestion control algorithms.
       RFC 2018 and RFC 2883 for SACK and extensions to SACK.

COLOPHON
       This page is part of release 5.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06                            TCP(7)