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         #include <sys/socket.h>
         #include <netinet/in.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 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
         Passive sockets may "underspecify" their location to match incoming con-
         nection 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_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 deter-
                      mine 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 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
         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 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
         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 seg-
                            ment size ("MSS") when no advice to the contrary is
                            received from MSS negotiation.
                            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
         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-
         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.
                            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.
         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.
                            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.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 allo-
                            cated to contain a minimal amount of information nec-
                            essary 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 maxi-
                            mum 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
                            to help with connection establishment when a broken
                            firewall is in the network path.


         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), 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.
         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 August 16, 2008 BSD


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