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@(#)tcp.4 8.1 (Berkeley) 6/5/93
.Dd April 25, 2004 .Dt TCP 4 .Os .Sh NAME .Nm tcp .Nd Internet Transmission Control Protocol .Sh SYNOPSIS n sys/socket.h n netinet/in.h .Ft int .Fn socket AF_INET SOCK_STREAM 0 .Ft int .Fn socket AF_INET6 SOCK_STREAM 0 .Sh DESCRIPTION The .Tn TCP provides reliable, flow-controlled, two-way transmission of data. It is a byte-stream protocol used to support the .Dv SOCK_STREAM abstraction. .Tn TCP uses the standard Internet address format and, in addition, provides a per-host collection of .Dq port addresses . Thus, each address is composed of an Internet address specifying the host and network, with a specific .Tn TCP port on the host identifying the peer entity.
p Sockets using .Tn TCP are either .Dq active or .Dq passive . Active sockets initiate connections to passive sockets. By default .Tn TCP sockets are created active; to create a passive socket the .Xr listen 2 system call must be used after binding the socket with the .Xr bind 2 system call. Only passive sockets may use the .Xr accept 2 call to accept incoming connections. Only active sockets may use the .Xr connect 2 call to initiate connections.
p Passive sockets may .Dq underspecify their location to match incoming connection requests from multiple networks. This technique, termed .Dq 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 .Dv INADDR_ANY must be bound. The .Tn 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 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.
p .Tn TCP supports a number of socket options which can be set with .Xr setsockopt 2 and tested with .Xr getsockopt 2 : l -tag -width TCP_MD5SIGx t Dv TCP_NODELAY Under most circumstances, .Tn 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. Therefore, .Tn TCP provides a boolean option, .Dv TCP_NODELAY (from .Aq Pa netinet/tcp.h , to defeat this algorithm. t Dv 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 .Dv TCP_MAXSEG option allows the user to determine the result of this negotiation, and to reduce it if desired. .El
p The option level for the .Xr setsockopt 2 call is the protocol number for .Tn TCP , available from .Xr getprotobyname 3 .
p In the historical x .Tn TCP implementation, if the .Dv TCP_NODELAY option was set on a passive socket, the sockets returned by .Xr accept 2 erroneously did not have the .Dv TCP_NODELAY option set; the behavior was corrected to inherit .Dv TCP_NODELAY in .Nx 1.6 .
p Options at the .Tn IP network level may be used with .Tn TCP ; see .Xr ip 4 or .Xr ip6 4 . Incoming connection requests that are source-routed are noted, and the reverse source route is used in responding.
p There are many adjustable parameters that control various aspects of the .Nx TCP behavior; these parameters are documented in .Xr sysctl 3 , and they include: l -bullet -compact t RFC 1323 extensions for high performance t Send/receive buffer sizes t Default maximum segment size (MSS) t SYN cache parameters t Initial window size t Hughes/Touch/Heidemann Congestion Window Monitoring algorithm t Keepalive parameters t newReno algorithm for congestion control t Logging of connection refusals t RST packet rate limits .El .Sh DIAGNOSTICS A socket operation may fail with one of the following errors returned: l -tag -width [EADDRNOTAVAIL] t Bq Er EISCONN when trying to establish a connection on a socket which already has one; t Bq Er ENOBUFS when the system runs out of memory for an internal data structure; t Bq Er ETIMEDOUT when a connection was dropped due to excessive retransmissions; t Bq Er ECONNRESET when the remote peer forces the connection to be closed; t Bq Er ECONNREFUSED when the remote peer actively refuses connection establishment (usually because no process is listening to the port); t Bq Er EADDRINUSE when an attempt is made to create a socket with a port which has already been allocated; t Bq Er EADDRNOTAVAIL when an attempt is made to create a socket with a network address for which no network interface exists. .El .Sh SEE ALSO .Xr getsockopt 2 , .Xr socket 2 , .Xr sysctl 3 , .Xr inet 4 , .Xr inet6 4 , .Xr intro 4 , .Xr ip 4 , .Xr ip6 4 .Rs .%R RFC .%N 793 .%D September 1981 .%T "Transmission Control Protocol" .Re .Rs .%R RFC .%N 1122 .%D October 1989 .%T "Requirements for Internet Hosts -- Communication Layers" .Re .Sh HISTORY The .Nm protocol stack appeared in x 4.2 .
Indexes created Tue Sep 30 11:09:46 GMT 2025