xref: /netbsd/share/man/man4/tcp.4 (revision bf9ec67e)
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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
39.Sh NAME
40.Nm tcp
41.Nd Internet Transmission Control Protocol
42.Sh SYNOPSIS
43.Fd #include \*[Lt]sys/socket.h\*[Gt]
44.Fd #include \*[Lt]netinet/in.h\*[Gt]
45.Ft int
46.Fn socket AF_INET SOCK_STREAM 0
47.Ft int
48.Fn socket AF_INET6 SOCK_STREAM 0
49.Sh DESCRIPTION
50The
51.Tn TCP
52provides reliable, flow-controlled, two-way transmission of data.
53It is a byte-stream protocol used to support the
54.Dv SOCK_STREAM
55abstraction.
56.Tn TCP
57uses the standard Internet address format and, in addition, provides
58a per-host collection of
59.Dq port addresses .
60Thus, each address is composed of an Internet address specifying
61the host and network, with a specific
62.Tn TCP
63port on the host identifying the peer entity.
64.Pp
65Sockets utilizing
66.Tn TCP
67are either
68.Dq active
69or
70.Dq passive .
71Active sockets initiate connections to passive
72sockets.
73By default
74.Tn TCP
75sockets are created active; to create a passive socket the
76.Xr listen 2
77system call must be used
78after binding the socket with the
79.Xr bind 2
80system call.
81Only passive sockets may use the
82.Xr accept 2
83call to accept incoming connections.
84Only active sockets may use the
85.Xr connect 2
86call to initiate connections.
87.Pp
88Passive sockets may
89.Dq underspecify
90their location to match incoming connection requests from multiple networks.
91This technique, termed
92.Dq wildcard addressing ,
93allows a single
94server to provide service to clients on multiple networks.
95To create a socket which listens on all networks, the Internet
96address
97.Dv INADDR_ANY
98must be bound.
99The
100.Tn TCP
101port may still be specified at this time; if the port is not
102specified the system will assign one.
103Once a connection has been established the socket's address is
104fixed by the peer entity's location.
105The address assigned the socket is the address associated with the
106network interface through which packets are being transmitted and received.
107Normally this address corresponds to the peer entity's network.
108.Pp
109.Tn TCP
110supports one socket option which is set with
111.Xr setsockopt 2
112and tested with
113.Xr getsockopt 2 .
114Under most circumstances,
115.Tn TCP
116sends data when it is presented;
117when outstanding data has not yet been acknowledged, it gathers
118small amounts of output to be sent in a single packet once
119an acknowledgement is received.
120For a small number of clients, such as window systems
121that send a stream of mouse events which receive no replies,
122this packetization may cause significant delays.
123Therefore,
124.Tn TCP
125provides a boolean option,
126.Dv TCP_NODELAY
127(from
128.Aq Pa netinet/tcp.h ,
129to defeat this algorithm.
130The option level for the
131.Xr setsockopt 2
132call is the protocol number for
133.Tn TCP ,
134available from
135.Xr getprotobyname 3 .
136.Pp
137Options at the
138.Tn IP
139network level may be used with
140.Tn TCP ;
141see
142.Xr ip 4
143or
144.Xr ip6 4 .
145Incoming connection requests that are source-routed are noted,
146and the reverse source route is used in responding.
147.Sh DIAGNOSTICS
148A socket operation may fail with one of the following errors returned:
149.Bl -tag -width [EADDRNOTAVAIL]
150.It Bq Er EISCONN
151when trying to establish a connection on a socket which
152already has one;
153.It Bq Er ENOBUFS
154when the system runs out of memory for
155an internal data structure;
156.It Bq Er ETIMEDOUT
157when a connection was dropped
158due to excessive retransmissions;
159.It Bq Er ECONNRESET
160when the remote peer
161forces the connection to be closed;
162.It Bq Er ECONNREFUSED
163when the remote
164peer actively refuses connection establishment (usually because
165no process is listening to the port);
166.It Bq Er EADDRINUSE
167when an attempt
168is made to create a socket with a port which has already been
169allocated;
170.It Bq Er EADDRNOTAVAIL
171when an attempt is made to create a
172socket with a network address for which no network interface
173exists.
174.El
175.Sh SEE ALSO
176.Xr getsockopt 2 ,
177.Xr socket 2 ,
178.Xr inet 4 ,
179.Xr inet6 4 ,
180.Xr intro 4 ,
181.Xr ip 4 ,
182.Xr ip6 4
183.Rs
184.%R RFC
185.%N 793
186.%D September 1981
187.%T "Transmission Control Protocol"
188.Re
189.Rs
190.%R RFC
191.%N 1122
192.%D October 1989
193.%T "Requirements for Internet Hosts -- Communication Layers"
194.Re
195.Sh HISTORY
196The
197.Nm
198protocol stack appeared in
199.Bx 4.2 .
200