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TCP(4)                 FreeBSD Kernel Interfaces Manual                 TCP(4)

NAME
     tcp - Internet Transmission Control Protocol

SYNOPSIS
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netinet/tcp.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_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 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.  When this option is
                           enabled on a socket, all inbound and outgoing TCP
                           segments must be signed with MD5 digests.

                           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)
     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 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.

     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).

     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.

     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.

     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.  Settings:
                            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.

     pmtud_blackhole_detection
                            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).

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

     v6pmtud_blackhole_mss  MSS to try for IPv6 if PMTU blackhole detection is
                            turned on.

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

     pmtud_blackhole_activated_min_mss
                            Number of times default MSS was used in an attempt
                            to downshift.

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

     functions_available    List of available TCP function blocks (TCP
                            stacks).

     functions_default      The default TCP function block (TCP stack).

     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.

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.

     [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
                        available;

SEE ALSO
     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.

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.

FreeBSD 11.1-RELEASE-p4        February 6, 2017        FreeBSD 11.1-RELEASE-p4
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