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tcp - Internet Transmission Control Protocol



      tcp - Internet Transmission Control Protocol


      #include <sys/types.h>
      #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
                   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).  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.
      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‐
      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.
      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
                         socket is marked as SBS_CANTRCVMORE (no user process
                         has the socket open, data received on the socket can‐
                         not be read).  The timeout used here is
      finwait2_timeout   Timeout to use for fast recycling of TCP FIN_WAIT_2
                         connections.  Defaults to 60 seconds.


      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.


      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.


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