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     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

     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

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

     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 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.  Outgoing traffic is digested; digests on incoming
                     traffic are verified if the net.inet.tcp.signature_verify_input sysctl is
                     nonzero.  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

                     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 <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

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

   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).

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

     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

     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

     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

     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.

     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).

     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.

     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.


     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


     getsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4), ip(4), mod_cc(4),
     siftr(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.