bsd/man/man4/tcp.4

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  34 .\"     @(#)tcp.4       8.1 (Berkeley) 6/5/93
  35 .\"
  36 .Dd June 5, 1993
  37 .Dt TCP 4
  38 .Os BSD 4.2
  39 .Sh NAME
  40 .Nm tcp
  41 .Nd Internet Transmission Control Protocol
  42 .Sh SYNOPSIS
  43 .Fd #include <sys/socket.h>
  44 .Fd #include <netinet/in.h>
  45 .Ft int
  46 .Fn socket AF_INET SOCK_STREAM 0
  47 .Sh DESCRIPTION
  48 The
  49 .Tn TCP
  50 protocol provides reliable, flow-controlled, two-way
  51 transmission of data.  It is a byte-stream protocol used to
  52 support the
  53 .Dv SOCK_STREAM
  54 abstraction.  TCP uses the standard
  55 Internet address format and, in addition, provides a per-host
  56 collection of
  57 .Dq port addresses .
  58 Thus, each address is composed
  59 of an Internet address specifying the host and network, with
  60 a specific
  61 .Tn TCP
  62 port on the host identifying the peer entity.
  63 .Pp
  64 Sockets utilizing the tcp protocol are either
  65 .Dq active
  66 or
  67 .Dq passive .
  68 Active sockets initiate connections to passive
  69 sockets.  By default
  70 .Tn TCP
  71 sockets are created active; to create a
  72 passive socket the
  73 .Xr listen 2
  74 system call must be used
  75 after binding the socket with the
  76 .Xr bind 2
  77 system call.  Only
  78 passive sockets may use the
  79 .Xr accept 2
  80 call to accept incoming connections.  Only active sockets may
  81 use the
  82 .Xr connect 2
  83 call to initiate connections.
  84 .Pp
  85 Passive sockets may
  86 .Dq underspecify
  87 their location to match
  88 incoming connection requests from multiple networks.  This
  89 technique, termed
  90 .Dq wildcard addressing ,
  91 allows a single
  92 server to provide service to clients on multiple networks.
  93 To create a socket which listens on all networks, the Internet
  94 address
  95 .Dv INADDR_ANY
  96 must be bound.  The
  97 .Tn TCP
  98 port may still be specified
  99 at this time; if the port is not specified the system will assign one.
 100 Once a connection has been established the socket's address is
 101 fixed by the peer entity's location.   The address assigned the
 102 socket is the address associated with the network interface
 103 through which packets are being transmitted and received.  Normally
 104 this address corresponds to the peer entity's network.
 105 .Pp
 106 .Tn TCP
 107 supports one socket option which is set with
 108 .Xr setsockopt 2
 109 and tested with
 110 .Xr getsockopt 2 .
 111 Under most circumstances,
 112 .Tn TCP
 113 sends data when it is presented;
 114 when outstanding data has not yet been acknowledged, it gathers
 115 small amounts of output to be sent in a single packet once
 116 an acknowledgement is received.
 117 For a small number of clients, such as window systems
 118 that send a stream of mouse events which receive no replies,
 119 this packetization may cause significant delays.
 120 Therefore,
 121 .Tn TCP
 122 provides a boolean option,
 123 .Dv TCP_NODELAY
 124 (from
 125 .Aq Pa netinet/tcp.h ,
 126 to defeat this algorithm.
 127 The option level for the
 128 .Xr setsockopt
 129 call is the protocol number for
 130 .Tn TCP ,
 131 available from
 132 .Xr getprotobyname 3 .
 133 .Pp
 134 Options at the
 135 .Tn IP
 136 transport level may be used with
 137 .Tn TCP ;
 138 see
 139 .Xr ip 4 .
 140 Incoming connection requests that are source-routed are noted,
 141 and the reverse source route is used in responding.
 142 .Sh DIAGNOSTICS
 143 A socket operation may fail with one of the following errors returned:
 144 .Bl -tag -width [EADDRNOTAVAIL]
 145 .It Bq Er EISCONN
 146 when trying to establish a connection on a socket which
 147 already has one;
 148 .It Bq Er ENOBUFS
 149 when the system runs out of memory for
 150 an internal data structure;
 151 .It Bq Er ETIMEDOUT
 152 when a connection was dropped
 153 due to excessive retransmissions;
 154 .It Bq Er ECONNRESET
 155 when the remote peer
 156 forces the connection to be closed;
 157 .It Bq Er ECONNREFUSED
 158 when the remote
 159 peer actively refuses connection establishment (usually because
 160 no process is listening to the port);
 161 .It Bq Er EADDRINUSE
 162 when an attempt
 163 is made to create a socket with a port which has already been
 164 allocated;
 165 .It Bq Er EADDRNOTAVAIL
 166 when an attempt is made to create a
 167 socket with a network address for which no network interface
 168 exists.
 169 .El
 170 .Sh SEE ALSO
 171 .Xr getsockopt 2 ,
 172 .Xr socket 2 ,
 173 .Xr intro 4 ,
 174 .Xr inet 4 ,
 175 .Xr ip 4
 176 .Sh HISTORY
 177 The
 178 .Nm
 179 protocol stack appeared in
 180 .Bx 4.2 .