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

     tcp -- Internet Transmission Control Protocol

     #include <&lt;sys/types.h>&gt;
     #include <&lt;sys/socket.h>&gt;
     #include <&lt;netinet/in.h>&gt;

     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 addi-
     tion, provides a per-host collection of ``port addresses''.  Thus, each
     address is composed of an Internet address specifying the host and net-
     work, 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
     connections.  TCP also supports a more datagram-like mode, called Trans-
     action TCP, which is described in ttcp(4).

     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 inter-
     face 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_NODELAY  Under most circumstances, TCP sends data when it is pre-
                  sented; 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 algorithm.

     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, corre-
                  sponding to various TCP extensions which are provided in
                  this implementation.  The boolean option TCP_NOOPT is pro-
                  vided 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).  The TCP_NOPUSH
                  option is provided to allow servers to easily make use of
                  Transaction TCP (see ttcp(4)).  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

     TCP_MD5SIG   This option enables the use of MD5 digests (also known as
                  TCP-MD5) on writes to the specified socket.  In the current
                  release, only outgoing traffic is digested; digests on
                  incoming traffic are not verified.  The current default
                  behavior for the system is to respond to a system advertis-
                  ing this option with TCP-MD5; this may change.

                  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.  Only IPv4 (AF_INET) sessions are supported.

                  In order for this option to function correctly, it is neces-
                  sary 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 must have an SPI of 0x1000
                  and can therefore only be specified on a per-host basis at
                  this time.

                  If an SADB entry cannot be found for the destination, the
                  outgoing traffic will have an invalid digest option
                  prepended, and the following error message will be visible
                  on the system console: tcp_signature_compute: SADB lookup
                  failed for %d.%d.%d.%d.

     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.

   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_DO_RFC1644  (rfc1644) Implement Transaction TCP, as described in
                        RFC 1644.

     TCPCTL_MSSDFLT     (mssdflt) The default value used for the maximum seg-
                        ment 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 disabled).

                        The number of packets allowed to be in-flight during
                        the TCP slow-start phase on a non-local network.

                        The number of packets allowed to be in-flight during
                        the TCP slow-start phase to local machines in the same

     msl                The Maximum Segment Lifetime, in milliseconds, for a

     keepinit           Timeout, in milliseconds, for new, non-established TCP

     keepidle           Amount of time, in milliseconds, that the connection
                        must be idle before keepalive probes (if enabled) are

     keepintvl          The interval, in milliseconds, between keepalive
                        probes sent to remote machines.  After TCPTV_KEEPCNT
                        (default 8) probes are sent, with no response, the
                        connection is dropped.

     always_keepalive   Assume that SO_KEEPALIVE is set on all TCP connec-
                        tions, 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 connec-
                        tions in SYN-SENT state.

     do_tcpdrain        Flush packets in the TCP reassembly queue if the sys-
                        tem 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.

     newreno            Enable TCP NewReno Fast Recovery algorithm, as
                        described in RFC 2582.

                        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

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

     syncookies         Determines whether or not SYN cookies should be gener-
                        ated 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.

     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 con-
                        nections, 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).

     inflight.enable    Enable TCP bandwidth-delay product limiting.  An
                        attempt will be made to calculate the bandwidth-delay
                        product for each individual TCP connection, and limit
                        the amount of inflight data being transmitted, to
                        avoid building up unnecessary packets in the network.
                        This option is recommended if you are serving a lot of
                        data over connections with high bandwidth-delay prod-
                        ucts, such as modems, GigE links, and fast long-haul
                        WANs, and/or you have configured your machine to
                        accommodate large TCP windows.  In such situations,
                        without this option, you may experience high interac-
                        tive latencies or packet loss due to the overloading
                        of intermediate routers and switches.  Note that band-
                        width-delay product limiting only effects the transmit
                        side of a TCP connection.

     inflight.debug     Enable debugging for the bandwidth-delay product algo-
                        rithm.  This may default to on (1), so if you enable
                        the algorithm, you should probably also disable debug-
                        ging by setting this variable to 0.

     inflight.min       This puts a lower bound on the bandwidth-delay product
                        window, in bytes.  A value of 1024 is typically used
                        for debugging.  6000-16000 is more typical in a pro-
                        duction installation.  Setting this value too low may
                        result in slow ramp-up times for bursty connections.
                        Setting this value too high effectively disables the

     inflight.max       This puts an upper bound on the bandwidth-delay prod-
                        uct window, in bytes.  This value should not generally
                        be modified, but may be used to set a global per-con-
                        nection limit on queued data, potentially allowing you
                        to intentionally set a less than optimum limit, to
                        smooth data flow over a network while still being able
                        to specify huge internal TCP buffers.

     inflight.stab      The bandwidth-delay product algorithm requires a
                        slightly larger window than it otherwise calculates
                        for stability.  This parameter determines the extra
                        window in maximal packets / 10.  The default value of
                        20 represents 2 maximal packets.  Reducing this value
                        is not recommended, but you may come across a situa-
                        tion with very slow links where the ping(8) time
                        reduction of the default inflight code is not suffi-
                        cient.  If this case occurs, you should first try
                        reducing inflight.min and, if that does not work,
                        reduce both inflight.min and inflight.stab, trying
                        values of 15, 10, or 5 for the latter.  Never use a
                        value less than 5.  Reducing inflight.stab can lead to
                        upwards of a 20% underutilization of the link as well
                        as reducing the algorithm's ability to adapt to chang-
                        ing situations and should only be done as a last

     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.  This is a standards track RFC and
                        is off by default.

     rfc3390            Enable support for RFC 3390, which allows for a vari-
                        able-sized starting congestion window on new connec-
                        tions, depending on the maximum segment size.  This
                        helps throughput in general, but particularly affects
                        short transfers and high-bandwidth large propagation-
                        delay connections.  This is a standards track RFC and
                        support for it is off by default.

                        When this feature is enabled, the slowstart_flightsize
                        and local_slowstart_flightsize settings are not
                        observed for new connection slow starts, but they are
                        still used for slow starts that occur when the connec-
                        tion has been idle and starts sending again.

     sack.enable        Enable support for RFC 2018, TCP Selective Acknowledg-
                        ment option, which allows the receiver to inform the
                        sender about all successfully arrived segments, allow-
                        ing the sender to retransmit the missing segments

     sack.initburst     Control the number of SACK retransmissions done upon
                        initiation of SACK recovery.

     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]          when the system runs out of memory for an internal
                        data structure;

     [ETIMEDOUT]        when a connection was dropped due to excessive

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

     [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 net-
                        work address for which no network interface exists;

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

     getsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4),
     ip(4), syncache(4), ttcp(4), setkey(8)

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

     R. Braden, T/TCP - TCP Extensions for Transactions, RFC 1644.

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

     The TCP protocol appeared in 4.2BSD.  The RFC 1323 extensions for window
     scaling and timestamps were added in 4.4BSD.

BSD                            October 12, 2004                            BSD