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.\"     From: @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\" $FreeBSD: src/share/man/man4/tcp.4,v 1.11.2.14 2002/12/29 16:35:38 schweikh Exp $
.\"
.Dd April 21, 2018
.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.
.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 .
When the
.Dv TCP_NOPUSH
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.
.\".It Dv TCP_SIGNATURE_ENABLE
.\"This option enables the use of MD5 digests (also known as TCP-MD5)
.\"on writes to the specified socket.
.\"In the current release, only outgoing traffic is digested;
.\"digests on incoming traffic are not verified.
.\"The current default behavior for the system is to respond to a system
.\"advertising this option with TCP-MD5; this may change.
.\".Pp
.\"One common use for this in a DragonFlyBSD router deployment is to enable
.\"based routers to interwork with Cisco equipment at peering points.
.\"Support for this feature conforms to RFC 2385.
.\"Only IPv4 (AF_INET) sessions are supported.
.\".Pp
.\"In order for this option to function correctly, it is necessary for the
.\"administrator to add a tcp-md5 key entry to the system's security
.\"associations database (SADB) using the
.\".Xr setkey 8
.\"utility.
.\"This entry must have an SPI of 0x1000 and can therefore only be specified
.\"on a per-host basis at this time.
.\".Pp
.\"If an SADB entry cannot be found for the destination, the outgoing traffic
.\"will have an invalid digest option prepended, and the following error message
.\"will be visible on the system console:
.\".Em "tcpsignature_compute: SADB lookup failed for %d.%d.%d.%d" .
.It Dv TCP_KEEPINIT
If a
.Tn TCP
connection cannot be established within a period of time,
.Tn TCP
will time out the connection attempt.
The
.Dv TCP_KEEPINIT
option specifies the number of seconds to wait
before the connection attempt times out.
The default value for
.Dv TCP_KEEPINIT
is tcp.keepinit seconds.
For the accepted sockets, the
.Dv TCP_KEEPINIT
option value is inherited from the listening socket.
.It Dv TCP_KEEPIDLE
When the
.Dv SO_KEEPALIVE
option is enabled,
.Tn TCP
sends a keepalive probe to the remote system of a connection
that has been idle for a period of time.
The
.Dv TCP_KEEPIDLE
specifies the number of seconds before
.Tn TCP
will send the initial keepalive probe.
The default value for
.Dv TCP_KEEPIDLE
is tcp.keepidle seconds.
For the accepted sockets,
the
.Dv TCP_KEEPIDLE
option value is inherited from the listening socket.
.It Dv TCP_KEEPINTVL
When the
.Dv SO_KEEPALIVE
option is enabled,
.Tn TCP
sends a keepalive probe to the remote system of a connection
that has been idle for a period of time.
The
.Dv TCP_KEEPINTVL
option specifies the number of seconds to wait
before retransmitting a keepalive probe.
The default value for
.Dv TCP_KEEPINTVL
is tcp.keepintvl seconds.
For the accepted sockets,
the
.Dv TCP_KEEPINTVL
option value is inherited from the listening socket.
.It Dv TCP_KEEPCNT
When the
.Dv SO_KEEPALIVE
option is enabled,
.Tn TCP
sends a keepalive probe to the remote system of a connection
that has been idle for a period of time.
The
.Dv TCP_KEEPCNT
option specifies the maximum number of keepalive
probes to be sent before dropping the connection.
The default value for
.Dv TCP_KEEPCNT
is tcp.keepcnt seconds.
For the accepted sockets,
the
.Dv TCP_KEEPCNT
option value is inherited from the listening socket.
.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
.In 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_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.msl
The Maximum Segment Lifetime for a packet.
.It tcp.keepinit
Timeout for new, non-established TCP connections.
.It tcp.keepidle
Amount of time the connection should be idle before keepalive
probes (if enabled) are sent.
.It tcp.keepintvl
The interval between keepalive probes sent to remote machines.
After
tcp.keepcnt
(default 8) probes are sent, with no response, the connection is dropped.
.It tcp.keepcnt
The maximum number of keepalive probes to be sent
before dropping the connection.
.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 to piggyback it onto a data packet.
.It tcp.delacktime
Maximum amount of time 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
Enables Path MTU Discovery.  PMTU Discovery is helpful for avoiding
IP fragmentation when tranferring lots of data to the same client.
For web servers, where most of the connections are short and to
different clients, PMTU Discovery actually hurts performance due
to unnecessary retransmissions.  Turn this on only if most of your
TCP connections are long transfers or are repeatedly to the same
set of clients.
.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.
.It tcp.inet.tcp.rexmit_{min,slop}
Adjust the retransmit timer calculation for TCP.  The slop is
typically added to the raw calculation to take into account
occasional variances that the SRTT (smoothed round trip time)
is unable to accommodate, while the minimum specifies an
absolute minimum.  While a number of TCP RFCs suggest a 1
second minimum these RFCs tend to focus on streaming behavior
and fail to deal with the fact that a 1 second minimum has severe
detrimental effects over lossy interactive connections, such
as a 802.11b wireless link, and over very fast but lossy
connections for those cases not covered by the fast retransmit
code.  For this reason we suggest changing the slop to 200ms and
setting the minimum to something out of the way, like 20ms,
which gives you an effective minimum of 200ms (similar to Linux).
.It tcp.inflight_enable
Enable
.Tn TCP
bandwidth delay product limiting.  An attempt will be made to calculate
the bandwidth delay product for each individual TCP connection and limit
the amount of inflight data being transmitted to avoid building up
unnecessary packets in the network.  This option is recommended if you
are serving a lot of data over connections with high bandwidth-delay
products, such as modems, GigE links, and fast long-haul WANs, and/or
you have configured your machine to accommodate large TCP windows.  In such
situations, without this option, you may experience high interactive
latencies or packet loss due to the overloading of intermediate routers
and switches.  Note that bandwidth delay product limiting only affects
the transmit side of a TCP connection.
.It tcp.inflight_debug
Enable debugging for the bandwidth delay product algorithm.  This may
default to on (1) so if you enable the algorithm you should probably also
disable debugging by setting this variable to 0.
.It tcp.inflight_min
This puts an lower bound on the bandwidth delay product window, in bytes.
A value of 1024 is typically used for debugging.  6000-16000 is more typical
in a production installation.  Setting this value too low may result in
slow ramp-up times for bursty connections.  Setting this value too high
effectively disables the algorithm.
.It tcp.inflight_max
This puts an upper bound on the bandwidth delay product window, in bytes.
This value should not generally be modified but may be used to set a
global per-connection limit on queued data, potentially allowing you to
intentionally set a less than optimum limit to smooth data flow over a
network while still being able to specify huge internal TCP buffers.
.It tcp.inflight_stab
This value stabilizes the bwnd (write window) calculation at high speeds
by increasing the bandwidth calculation in 1/10% increments.  The default
value of 50 represents a +5% increase.  In addition, bwnd is further increased
by a fixed 2*maxseg bytes to stabilize the algorithm at low speeds.
Changing the stab value is not recommended, but you may come across
situations where tuning is beneficial.
However, our recommendation for tuning is to stick with only adjusting
tcp.inflight_min.
Reducing tcp.inflight_stab too much can lead to upwards of a 20%
underutilization of the link and prevent the algorithm from properly adapting
to changing situations.  Increasing tcp.inflight_stab too much can lead to
an excessive packet buffering situation.
.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
.Rs
.%A V. Jacobson
.%A R. Braden
.%A D. Borman
.%T "TCP Extensions for High Performance"
.%O RFC 1323
.Re
.Rs
.%A "A. Heffernan"
.%T "Protection of BGP Sessions via the TCP MD5 Signature Option"
.%O "RFC 2385"
.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 .