freebsd-nq/share/man/man4/tcp.4

.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd February 14, 1995
.Dt TCP 4
.Os
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In 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 Dv TCPCTL_SENDSPACE
.Pq tcp.sendspace
Maximum TCP send window.
.It Dv 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.
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).
.It tcp.slowstart_flightsize
The number of packets allowed to be in-flight during the
.Tn TCP
slow-start phase on a non-local network.
.It tcp.local_slowstart_flightsize
The number of packets allowed to be in-flight during the
.Tn TCP
slow-start phase to local machines in the same subnet.
.It tcp.msl
The Maximum Segment Lifetime, in milliseconds, for a packet.
.It tcp.keepinit
Timeout, in milliseconds, for new, non-established TCP connections.
.It tcp.keepidle
Amount of time, in milliseconds, that the connection must be idle
before keepalive probes (if enabled) are sent.
.It tcp.keepintvl
The interval, in milliseconds, between keepalive probes sent to remote
machines.
After
.Dv TCPTV_KEEPCNT
(default 8) probes are sent, with no response, the connection is dropped.
.It tcp.always_keepalive
Assume that
.Dv SO_KEEPALIVE
is set on all
.Tn TCP
connections, the kernel will
periodically send a packet to the remote host to verify the connection
is still up.
.It tcp.icmp_may_rst
Certain
.Tn ICMP
unreachable messages may abort connections in
.Tn SYN-SENT
state.
.It tcp.do_tcpdrain
Flush packets in the
.Tn TCP
reassembly queue if the system is low on mbufs.
.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.delacktime
Maximum amount of time, in milliseconds, before a delayed ACK is sent.
.It tcp.newreno
Enable TCP NewReno Fast Recovery algorithm,
as described in RFC 2582.
.It tcp.path_mtu_discovery
Enable Path MTU Discovery
.It tcp.tcbhashsize
Size of the
.Tn TCP
control-block hashtable
(read-only).
This may be tuned using the kernel option
.Dv TCBHASHSIZE
or by setting
.Va net.inet.tcp.tcbhashsize
in the
.Xr loader 8 .
.It tcp.pcbcount
Number of active process control blocks
(read-only).
.It tcp.syncookies
Determines whether or not syn cookies should be generated for
outbound syn-ack packets.  Syn cookies are a great help during
syn flood attacks, and are enabled by default.
.It tcp.isn_reseed_interval
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
.Dv TIME_WAIT
recycling for a few minutes.
.El
.Sh ERRORS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width Er
.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 blackhole 4 ,
.Xr inet 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ttcp 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 .