TCP(4)                    BSD Kernel Interfaces Manual                    TCP(4)

     tcp — Internet Transmission Control Protocol

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

     socket(AF_INET, SOCK_STREAM, 0);

     The TCP protocol provides reliable, flow-controlled, two-way transmission
     of data.  It is a byte-stream protocol used to support the SOCK_STREAM
     abstraction.  TCP uses the standard Internet address format and, in
     addition, provides a per-host collection of “port addresses”.  Thus, each
     address is composed of an Internet address specifying the host and network,
     with a specific TCP port on the host identifying the peer entity.

     Sockets utilizing the TCP protocol are either “active” or “passive”.
     Active sockets initiate connections to passive sockets.  By default, TCP
     sockets are created active; to create a passive socket, the listen(2)
     system call must be used after binding the socket with the bind(2) system
     call.  Only passive sockets may use the accept(2) call to accept incoming
     connections.  Only active sockets may use the connect(2) call to initiate

     Passive sockets may “underspecify” their location to match incoming
     connection requests from multiple networks.  This technique, termed
     “wildcard addressing”, allows a single server to provide service to clients
     on multiple networks.  To create a socket which listens on all networks,
     the Internet address INADDR_ANY must be bound.  The TCP port may still be
     specified at this time; if the port is not specified, the system will
     assign one.  Once a connection has been established, the socket's address
     is fixed by the peer entity's location.  The address assigned to the socket
     is the address associated with the network interface through which packets
     are being transmitted and received.  Normally, this address corresponds to
     the peer entity's network.

     TCP supports a number of socket options which can be set with setsockopt(2)
     and tested with getsockopt(2):

     TCP_INFO          Information about a socket's underlying TCP session may
                       be retrieved by passing the read-only option TCP_INFO to
                       getsockopt(2).  It accepts a single argument: a pointer
                       to an instance of struct tcp_info.

                       This API is subject to change; consult the source to
                       determine which fields are currently filled out by this
                       option.  FreeBSD specific additions include send window
                       size, receive window size, and bandwidth-controlled
                       window space.

     TCP_CCALGOOPT     Set or query congestion control algorithm specific
                       parameters.  See mod_cc(4) for details.

     TCP_CONGESTION    Select or query the congestion control algorithm that TCP
                       will use for the connection.  See mod_cc(4) for details.

     TCP_FUNCTION_BLK  Select or query the set of functions that TCP will use
                       for this connection.  This allows a user to select an
                       alternate TCP stack.  The alternate TCP stack must
                       already be loaded in the kernel.  To list the available
                       TCP stacks, see functions_available in the MIB Variables
                       section further down.  To list the default TCP stack, see
                       functions_default in the MIB Variables section.

     TCP_KEEPINIT      This setsockopt(2) option accepts a per-socket timeout
                       argument of u_int in seconds, for new, non-established
                       TCP connections.  For the global default in milliseconds
                       see keepinit in the MIB Variables section further down.

     TCP_KEEPIDLE      This setsockopt(2) option accepts an argument of u_int
                       for the amount of time, in seconds, that the connection
                       must be idle before keepalive probes (if enabled) are
                       sent for the connection of this socket.  If set on a
                       listening socket, the value is inherited by the newly
                       created socket upon accept(2).  For the global default in
                       milliseconds see keepidle in the MIB Variables section
                       further down.

     TCP_KEEPINTVL     This setsockopt(2) option accepts an argument of u_int to
                       set the per-socket interval, in seconds, between
                       keepalive probes sent to a peer.  If set on a listening
                       socket, the value is inherited by the newly created
                       socket upon accept(2).  For the global default in
                       milliseconds see keepintvl in the MIB Variables section
                       further down.

     TCP_KEEPCNT       This setsockopt(2) option accepts an argument of u_int
                       and allows a per-socket tuning of the number of probes
                       sent, with no response, before the connection will be
                       dropped.  If set on a listening socket, the value is
                       inherited by the newly created socket upon accept(2).
                       For the global default see the keepcnt in the MIB
                       Variables section further down.

     TCP_NODELAY       Under most circumstances, TCP sends data when it is
                       presented; when outstanding data has not yet been
                       acknowledged, it gathers small amounts of output to be
                       sent in a single packet once an acknowledgement is
                       received.  For a small number of clients, such as window
                       systems that send a stream of mouse events which receive
                       no replies, this packetization may cause significant
                       delays.  The boolean option TCP_NODELAY defeats this

     TCP_MAXSEG        By default, a sender- and receiver-TCP will negotiate
                       among themselves to determine the maximum segment size to
                       be used for each connection.  The TCP_MAXSEG option
                       allows the user to determine the result of this
                       negotiation, and to reduce it if desired.

     TCP_NOOPT         TCP usually sends a number of options in each packet,
                       corresponding to various TCP extensions which are
                       provided in this implementation.  The boolean option
                       TCP_NOOPT is provided to disable TCP option use on a per-
                       connection basis.

     TCP_NOPUSH        By convention, the sender-TCP will set the “push” bit,
                       and begin transmission immediately (if permitted) at the
                       end of every user call to write(2) or writev(2).  When
                       this option is set to a non-zero value, TCP will delay
                       sending any data at all until either the socket is
                       closed, or the internal send buffer is filled.

     TCP_MD5SIG        This option enables the use of MD5 digests (also known as
                       TCP-MD5) on writes to the specified socket.  Outgoing
                       traffic is digested; digests on incoming traffic are
                       verified.  When this option is enabled on a socket, all
                       inbound and outgoing TCP segments must be signed with MD5

                       One common use for this in a FreeBSD router deployment is
                       to enable based routers to interwork with Cisco equipment
                       at peering points.  Support for this feature conforms to
                       RFC 2385.

                       In order for this option to function correctly, it is
                       necessary for the administrator to add a tcp-md5 key
                       entry to the system's security associations database
                       (SADB) using the setkey(8) utility.  This entry can only
                       be specified on a per-host basis at this time.

                       If an SADB entry cannot be found for the destination, the
                       system does not send any outgoing segments and drops any
                       inbound segments.

                       Each dropped segment is taken into account in the TCP
                       protocol statistics.

     The option level for the setsockopt(2) call is the protocol number for TCP,
     available from getprotobyname(3), or IPPROTO_TCP.  All options are declared
     in <netinet/tcp.h>.

     Options at the IP transport level may be used with TCP; see ip(4).
     Incoming connection requests that are source-routed are noted, and the
     reverse source route is used in responding.

     The default congestion control algorithm for TCP is cc_newreno(4).  Other
     congestion control algorithms can be made available using the mod_cc(4)

   MIB Variables
     The TCP protocol implements a number of variables in the net.inet.tcp
     branch of the sysctl(3) MIB.

     TCPCTL_DO_RFC1323  (rfc1323) Implement the window scaling and timestamp
                        options of RFC 1323 (default is true).

     TCPCTL_MSSDFLT     (mssdflt) The default value used for the maximum segment
                        size (“MSS”) when no advice to the contrary is received
                        from MSS negotiation.

     TCPCTL_SENDSPACE   (sendspace) Maximum TCP send window.

     TCPCTL_RECVSPACE   (recvspace) Maximum TCP receive window.

     log_in_vain        Log any connection attempts to ports where there is not
                        a socket accepting connections.  The value of 1 limits
                        the logging to SYN (connection establishment) packets
                        only.  That of 2 results in any TCP packets to closed
                        ports being logged.  Any value unlisted above disables
                        the logging (default is 0, i.e., the logging is

     msl                The Maximum Segment Lifetime, in milliseconds, for a

     keepinit           Timeout, in milliseconds, for new, non-established TCP
                        connections.  The default is 75000 msec.

     keepidle           Amount of time, in milliseconds, that the connection
                        must be idle before keepalive probes (if enabled) are
                        sent.  The default is 7200000 msec (2 hours).

     keepintvl          The interval, in milliseconds, between keepalive probes
                        sent to remote machines, when no response is received on
                        a keepidle probe.  The default is 75000 msec.

     keepcnt            Number of probes sent, with no response, before a
                        connection is dropped.  The default is 8 packets.

     always_keepalive   Assume that SO_KEEPALIVE is set on all TCP connections,
                        the kernel will periodically send a packet to the remote
                        host to verify the connection is still up.

     icmp_may_rst       Certain ICMP unreachable messages may abort connections
                        in SYN-SENT state.

     do_tcpdrain        Flush packets in the TCP reassembly queue if the system
                        is low on mbufs.

     blackhole          If enabled, disable sending of RST when a connection is
                        attempted to a port where there is not a socket
                        accepting connections.  See blackhole(4).

     delayed_ack        Delay ACK to try and piggyback it onto a data packet.

     delacktime         Maximum amount of time, in milliseconds, before a
                        delayed ACK is sent.

                        Enable Path MTU Discovery.

     tcbhashsize        Size of the TCP control-block hash table (read-only).
                        This may be tuned using the kernel option TCBHASHSIZE or
                        by setting net.inet.tcp.tcbhashsize in the loader(8).

     pcbcount           Number of active process control blocks (read-only).

     syncookies         Determines whether or not SYN cookies should be
                        generated for outbound SYN-ACK packets.  SYN cookies are
                        a great help during SYN flood attacks, and are enabled
                        by default.  (See syncookies(4).)

                        The interval (in seconds) specifying how often the
                        secret data used in RFC 1948 initial sequence number
                        calculations should be reseeded.  By default, this
                        variable is set to zero, indicating that no reseeding
                        will occur.  Reseeding should not be necessary, and will
                        break TIME_WAIT recycling for a few minutes.

     reass.cursegments  The current total number of segments present in all
                        reassembly queues.

     reass.maxsegments  The maximum limit on the total number of segments across
                        all reassembly queues.  The limit can be adjusted as a

     reass.maxqueuelen  The maximum number of segments allowed in each
                        reassembly queue.  By default, the system chooses a
                        limit based on each TCP connection's receive buffer size
                        and maximum segment size (MSS).  The actual limit
                        applied to a session's reassembly queue will be the
                        lower of the system-calculated automatic limit and the
                        user-specified reass.maxqueuelen limit.

     rexmit_initial, rexmit_min, rexmit_slop
                        Adjust the retransmit timer calculation for TCP.  The
                        slop is typically added to the raw calculation to take
                        into account occasional variances that the SRTT
                        (smoothed round-trip time) is unable to accommodate,
                        while the minimum specifies an absolute minimum.  While
                        a number of TCP RFCs suggest a 1 second minimum, these
                        RFCs tend to focus on streaming behavior, and fail to
                        deal with the fact that a 1 second minimum has severe
                        detrimental effects over lossy interactive connections,
                        such as a 802.11b wireless link, and over very fast but
                        lossy connections for those cases not covered by the
                        fast retransmit code.  For this reason, we use 200ms of
                        slop and a near-0 minimum, which gives us an effective
                        minimum of 200ms (similar to Linux).  The initial value
                        is used before an RTT measurement has been performed.

     initcwnd_segments  Enable the ability to specify initial congestion window
                        in number of segments.  The default value is 10 as
                        suggested by RFC 6928.  Changing the value on fly would
                        not affect connections using congestion window from the
                        hostcache.  Caution: This regulates the burst of packets
                        allowed to be sent in the first RTT.  The value should
                        be relative to the link capacity.  Start with small
                        values for lower-capacity links.  Large bursts can cause
                        buffer overruns and packet drops if routers have small
                        buffers or the link is experiencing congestion.

     rfc3042            Enable the Limited Transmit algorithm as described in
                        RFC 3042.  It helps avoid timeouts on lossy links and
                        also when the congestion window is small, as happens on
                        short transfers.

     rfc3390            Enable support for RFC 3390, which allows for a
                        variable-sized starting congestion window on new
                        connections, depending on the maximum segment size.
                        This helps throughput in general, but particularly
                        affects short transfers and high-bandwidth large
                        propagation-delay connections.

     sack.enable        Enable support for RFC 2018, TCP Selective
                        Acknowledgment option, which allows the receiver to
                        inform the sender about all successfully arrived
                        segments, allowing the sender to retransmit the missing
                        segments only.

     sack.maxholes      Maximum number of SACK holes per connection.  Defaults
                        to 128.

                        Maximum number of SACK holes per system, across all
                        connections.  Defaults to 65536.

     maxtcptw           When a TCP connection enters the TIME_WAIT state, its
                        associated socket structure is freed, since it is of
                        negligible size and use, and a new structure is
                        allocated to contain a minimal amount of information
                        necessary for sustaining a connection in this state,
                        called the compressed TCP TIME_WAIT state.  Since this
                        structure is smaller than a socket structure, it can
                        save a significant amount of system memory.  The
                        net.inet.tcp.maxtcptw MIB variable controls the maximum
                        number of these structures allocated.  By default, it is
                        initialized to kern.ipc.maxsockets / 5.

     nolocaltimewait    Suppress creating of compressed TCP TIME_WAIT states for
                        connections in which both endpoints are local.

                        Recycle TCP FIN_WAIT_2 connections faster when the
                        socket is marked as SBS_CANTRCVMORE (no user process has
                        the socket open, data received on the socket cannot be
                        read).  The timeout used here is finwait2_timeout.

     finwait2_timeout   Timeout to use for fast recycling of TCP FIN_WAIT_2
                        connections.  Defaults to 60 seconds.

     ecn.enable         Enable support for TCP Explicit Congestion Notification
                        (ECN).  ECN allows a TCP sender to reduce the
                        transmission rate in order to avoid packet drops.
                        0       Disable ECN.
                        1       Allow incoming connections to request ECN.
                                Outgoing connections will request ECN.
                        2       Allow incoming connections to request ECN.
                                Outgoing connections will not request ECN.

     ecn.maxretries     Number of retries (SYN or SYN/ACK retransmits) before
                        disabling ECN on a specific connection.  This is needed
                        to help with connection establishment when a broken
                        firewall is in the network path.

                        Turn on automatic path MTU blackhole detection.  In case
                        of retransmits OS will lower the MSS to check if it's
                        MTU problem.  If current MSS is greater than configured
                        value to try, it will be set to configured value,
                        otherwise, MSS will be set to default values
                        (net.inet.tcp.mssdflt and net.inet.tcp.v6mssdflt).

                        MSS to try for IPv4 if PMTU blackhole detection is
                        turned on.

                        MSS to try for IPv6 if PMTU blackhole detection is
                        turned on.

                        Number of times configured values were used in an
                        attempt to downshift.

                        Number of times default MSS was used in an attempt to

                        Number of connections for which retransmits continued
                        even after MSS downshift.

                        List of available TCP function blocks (TCP stacks).

     functions_default  The default TCP function block (TCP stack).

                        Determines whether to inherit listen socket's tcp stack
                        or use the current system default tcp stack, as defined
                        by functions_default
                        ).  Default is true.

     insecure_rst       Use criteria defined in RFC793 instead of RFC5961 for
                        accepting RST segments.  Default is false.

     insecure_syn       Use criteria defined in RFC793 instead of RFC5961 for
                        accepting SYN segments.  Default is false.

                        When initializing the TCP timestamps, use a per
                        connection offset instead of a per host pair offset.
                        Default is to use per connection offsets as recommended
                        in RFC 7323.

     A socket operation may fail with one of the following errors returned:

     [EISCONN]          when trying to establish a connection on a socket which
                        already has one;

     [ENOBUFS] or [ENOMEM]
                        when the system runs out of memory for an internal data

     [ETIMEDOUT]        when a connection was dropped due to excessive

     [ECONNRESET]       when the remote peer forces the connection to be closed;

     [ECONNREFUSED]     when the remote peer actively refuses connection
                        establishment (usually because no process is listening
                        to the port);

     [EADDRINUSE]       when an attempt is made to create a socket with a port
                        which has already been allocated;

     [EADDRNOTAVAIL]    when an attempt is made to create a socket with a
                        network address for which no network interface exists;

     [EAFNOSUPPORT]     when an attempt is made to bind or connect a socket to a
                        multicast address.

     [EINVAL]           when trying to change TCP function blocks at an invalid
                        point in the session;

     [ENOENT]           when trying to use a TCP function block that is not

     getsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4),
     ip(4), mod_cc(4), siftr(4), syncache(4), setkey(8), tcp_functions(9)

     V. Jacobson, R. Braden, and D. Borman, TCP Extensions for High Performance,
     RFC 1323.

     A. Heffernan, Protection of BGP Sessions via the TCP MD5 Signature Option,
     RFC 2385.

     K. Ramakrishnan, S. Floyd, and D. Black, The Addition of Explicit
     Congestion Notification (ECN) to IP, RFC 3168.

     The TCP protocol appeared in 4.2BSD.  The RFC 1323 extensions for window
     scaling and timestamps were added in 4.4BSD.  The TCP_INFO option was
     introduced in Linux 2.6 and is subject to change.

BSD                               July 23, 2019                              BSD