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.\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd February 14, 1995
.Dt TCP 4
.Os BSD 4.2
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Fd #include <netinet/in.h>
.Ft int
.Fn socket AF_INET SOCK_STREAM 0
.Sh DESCRIPTION
The
.Tn TCP
protocol provides reliable, flow-controlled, two-way
transmission of data.  It is a byte-stream protocol used to
support the
.Dv SOCK_STREAM
abstraction.  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.
.Pp
Sockets utilizing the tcp protocol 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.
.Tn TCP
also supports a more datagram-like mode, called Transaction
.Tn TCP ,
which is described in
.Xr ttcp 4 .
.Pp
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.
.Pp
.Tn TCP
supports a number of socket options which can be set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 :
.Bl -tag -width TCP_NODELAYx
.It 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.
The boolean option
.Dv TCP_NODELAY
defeats this algorithm.
.It 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.
.It Dv TCP_NOOPT
.Tn TCP
usually sends a number of options in each packet, corresponding to
various
.Tn TCP
extensions which are provided in this implementation.  The boolean
option
.Dv TCP_NOOPT
is provided to disable 
.Tn TCP
option use on a per-connection basis.
.It Dv TCP_NOPUSH
By convention, the sender-TCP
will set the
.Dq push
bit and begin transmission immediately (if permitted) at the end of
every user call to
.Xr write 2
or
.Xr writev 2 .
The
.Dv TCP_NOPUSH
option is provided to allow servers to easily make use of Transaction
TCP (see
.Xr ttcp 4 ) .
When the option is set to a non-zero value,
.Tn TCP
will delay sending any data at all until either the socket is closed,
or the internal send buffer is filled.
.El
.Pp
The option level for the
.Xr setsockopt 2
call is the protocol number for
.Tn TCP ,
available from
.Xr getprotobyname 3 ,
or
.Dv IPPROTO_TCP .
All options are declared in
.Aq Pa netinet/tcp.h .
.Pp
Options at the
.Tn IP
transport level may be used with
.Tn TCP ;
see
.Xr ip 4 .
Incoming connection requests that are source-routed are noted,
and the reverse source route is used in responding.
.Sh MIB VARIABLES
The
.Nm
protocol implements a number of variables in the
.Li net.inet
branch of the
.Xr sysctl 3
MIB.
.Bl -tag -width TCPCTL_DO_RFC1644
.It Dv TCPCTL_DO_RFC1323
.Pq tcp.rfc1323
Implement the window scaling and timestamp options of RFC 1323
(default true).
.It Dv TCPCTL_DO_RFC1644
.Pq tcp.rfc1644
Implement Transaction
.Tn TCP ,
as described in RFC 1644.
.It Dv TCPCTL_MSSDFLT
.Pq tcp.mssdflt
The default value used for the maximum segment size
.Pq Dq MSS
when no advice to the contrary is received from MSS negotiation.
.It TCPCTL_RTTDFLT
.Pq tcp.rttdflt
The value of the default maximum TCP Round Trip Time.
.It TCPCTL_SENDSPACE
.Pq tcp.sendspace
Maximum TCP send window.
.It TCPCTL_RECVSPACE
.Pq tcp.recvspace
Maximum TCP receive window. 
.It tcp.log_in_vain
Log any connection attempts to ports where there is not a socket
accepting connections.
.It tcp.blackhole
If enabled, disable sending of RST when a connection is attempted
to a port where there is not a socket accepting connections.
See
.Xr blackhole 4 .
.It tcp.delayed_ack
Delay ACK to try and piggyback it onto a data packet.
.It tcp.newreno
Enable TCP NewReno Fast Recovery algorithm,
as described in RFC 2582.
.It tcp.path_mtu_discovery
Enable Path MTU Discovery
.El
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one;
.It Bq Er ENOBUFS
when the system runs out of memory for
an internal data structure;
.It Bq Er ETIMEDOUT
when a connection was dropped
due to excessive retransmissions;
.It Bq Er ECONNRESET
when the remote peer
forces the connection to be closed;
.It Bq Er ECONNREFUSED
when the remote
peer actively refuses connection establishment (usually because
no process is listening to the port);
.It Bq Er EADDRINUSE
when an attempt
is made to create a socket with a port which has already been
allocated;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a 
socket with a network address for which no network interface
exists.
.It Bq Er EAFNOSUPPORT
when an attempt is made to bind or connect a socket to a multicast
address.
.El
.Sh SEE ALSO
.Xr getsockopt 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ttcp 4
.Xr blackhole 4
.Rs
.%A V. Jacobson
.%A R. Braden
.%A D. Borman
.%T "TCP Extensions for High Performance"
.%O RFC 1323
.Re
.Rs
.%A R. Braden
.%T "T/TCP \- TCP Extensions for Transactions"
.%O RFC 1644
.Re
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.2 .
The RFC 1323 extensions for window scaling and timestamps were added
in
.Bx 4.4 .