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NAME

     tcp - Internet Transmission Control Protocol

SYNOPSIS

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

     int
     socket(AF_INET, SOCK_STREAM, 0);

DESCRIPTION

     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
     connections.

     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_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 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,
                  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.  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
                  advertising 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
                  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 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

     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_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 disabled).

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

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

     msl                The Maximum Segment Lifetime, in milliseconds, for a
                        packet.

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

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

     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
                        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.

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

     path_mtu_discovery
                        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).)

     isn_reseed_interval
                        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
                        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).

     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
                        products, 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
                        interactive latencies or packet loss due to the
                        overloading of intermediate routers and switches.
                        Note that bandwidth-delay product limiting only
                        effects the transmit side of a TCP connection.

     inflight.debug     Enable debugging for the bandwidth-delay product
                        algorithm.

     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
                        production installation.  Setting this value too low
                        may result in slow ramp-up times for bursty
                        connections.  Setting this value too high effectively
                        disables the algorithm.

     inflight.max       This puts an upper bound on the bandwidth-delay
                        product window, in bytes.  This value should not
                        generally be modified, but may be used to set a global
                        per-connection 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
                        situation with very slow links where the ping(8) time
                        reduction of the default inflight code is not
                        sufficient.  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
                        changing situations and should only be done as a last
                        resort.

     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.

                        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
                        connection has been idle and starts sending again.

     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.

     sack.globalmaxholes
                        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.

     fast_finwait2_recycle
                        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.

ERRORS

     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
                        retransmissions;

     [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.

SEE ALSO

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

     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.

HISTORY

     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.