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.\"     From: @(#)tcp.4 8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
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.Dd November 19, 2012
.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.
.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.
.Pp
Sockets utilizing the
.Tn 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 to 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 ".Dv TCP_CONGESTION"
.It Dv TCP_INFO
Information about a socket's underlying TCP session may be retrieved
by passing the read-only option
.Dv TCP_INFO
to
.Xr getsockopt 2 .
It accepts a single argument: a pointer to an instance of
.Vt "struct tcp_info" .
.Pp
This API is subject to change; consult the source to determine
which fields are currently filled out by this option.
.Fx
specific additions include
send window size,
receive window size,
and
bandwidth-controlled window space.
.It Dv TCP_CONGESTION
Select or query the congestion control algorithm that TCP will use for the
connection.
See
.Xr mod_cc 4
for details.
.It Dv TCP_KEEPINIT
This write-only 
.Xr setsockopt 2
option accepts a per-socket timeout argument of
.Vt "u_int"
in seconds, for new, non-established
.Tn TCP
connections.
For the global default in milliseconds see
.Va keepinit
in the
.Sx MIB Variables
section further down.
.It Dv TCP_KEEPIDLE
This write-only 
.Xr setsockopt 2
option accepts an argument of
.Vt "u_int"
for the amount of time, in seconds, that the connection must be idle
before keepalive probes (if enabled) are sent for the connection of this
socket.
If set on a listening socket, the value is inherited by the newly created
socket upon
.Xr accept 2 .
For the global default in milliseconds see
.Va keepidle
in the
.Sx MIB Variables
section further down.
.It Dv TCP_KEEPINTVL
This write-only 
.Xr setsockopt 2
option accepts an argument of
.Vt "u_int"
to set the per-socket interval, in seconds, between keepalive probes sent
to a peer.
If set on a listening socket, the value is inherited by the newly created
socket upon
.Xr accept 2 .
For the global default in milliseconds see
.Va keepintvl
in the
.Sx MIB Variables
section further down.
.It Dv TCP_KEEPCNT
This write-only 
.Xr setsockopt 2
option accepts an argument of
.Vt "u_int"
and allows a per-socket tuning of the number of probes sent, with no response,
before the connection will be dropped.
If set on a listening socket, the value is inherited by the newly created
socket upon
.Xr accept 2 .
For the global default see the
.Va keepcnt
in the
.Sx MIB Variables
section further down.
.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
.No receiver- Ns Tn 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
.No sender- Ns Tn 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 this 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_MD5SIG
This option enables the use of MD5 digests (also known as TCP-MD5)
on writes to the specified socket.
Outgoing traffic is digested;
digests on incoming traffic are verified if the
.Va net.inet.tcp.signature_verify_input
sysctl is nonzero.
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
.Fx
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
.Pq Dv 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 "tcp_signature_compute: SADB lookup failed for %d.%d.%d.%d" .
.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.
.Pp
The default congestion control algorithm for
.Tn TCP
is
.Xr cc_newreno 4 .
Other congestion control algorithms can be made available using the
.Xr mod_cc 4
framework.
.Ss MIB Variables
The
.Tn TCP
protocol implements a number of variables in the
.Va net.inet.tcp
branch of the
.Xr sysctl 3
MIB.
.Bl -tag -width ".Va TCPCTL_DO_RFC1323"
.It Dv TCPCTL_DO_RFC1323
.Pq Va rfc1323
Implement the window scaling and timestamp options of RFC 1323
(default is true).
.It Dv TCPCTL_MSSDFLT
.Pq Va 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 Va sendspace
Maximum
.Tn TCP
send window.
.It Dv TCPCTL_RECVSPACE
.Pq Va recvspace
Maximum
.Tn TCP
receive window.
.It Va 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
.Tn SYN
(connection establishment) packets only.
That of 2 results in any
.Tn TCP
packets to closed ports being logged.
Any value unlisted above disables the logging
(default is 0, i.e., the logging is disabled).
.It Va slowstart_flightsize
The number of packets allowed to be in-flight during the
.Tn TCP
slow-start phase on a non-local network.
.It Va 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 Va msl
The Maximum Segment Lifetime, in milliseconds, for a packet.
.It Va keepinit
Timeout, in milliseconds, for new, non-established
.Tn TCP
connections.
The default is 75000 msec.
.It Va keepidle
Amount of time, in milliseconds, that the connection must be idle
before keepalive probes (if enabled) are sent.
The default is 7200000 msec (2 hours).
.It Va keepintvl
The interval, in milliseconds, between keepalive probes sent to remote
machines, when no response is received on a
.Va keepidle
probe.
The default is 75000 msec.
.It Va keepcnt
Number of probes sent, with no response, before a connection
is dropped.
The default is 8 packets.
.It Va 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 Va icmp_may_rst
Certain
.Tn ICMP
unreachable messages may abort connections in
.Tn SYN-SENT
state.
.It Va do_tcpdrain
Flush packets in the
.Tn TCP
reassembly queue if the system is low on mbufs.
.It Va 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 Va delayed_ack
Delay ACK to try and piggyback it onto a data packet.
.It Va delacktime
Maximum amount of time, in milliseconds, before a delayed ACK is sent.
.It Va path_mtu_discovery
Enable Path MTU Discovery.
.It Va tcbhashsize
Size of the
.Tn TCP
control-block hash table
(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 Va pcbcount
Number of active process control blocks
(read-only).
.It Va syncookies
Determines whether or not
.Tn SYN
cookies should be generated for outbound
.Tn SYN-ACK
packets.
.Tn SYN
cookies are a great help during
.Tn SYN
flood attacks, and are enabled by default.
(See
.Xr syncookies 4 . )
.It Va 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 Va rexmit_min , rexmit_slop
Adjust the retransmit timer calculation for
.Tn TCP .
The slop is
typically added to the raw calculation to take into account
occasional variances that the
.Tn SRTT
(smoothed round-trip time)
is unable to accommodate, while the minimum specifies an
absolute minimum.
While a number of
.Tn 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 use 200ms of slop and a near-0
minimum, which gives us an effective minimum of 200ms (similar to
.Tn Linux ) .
.It Va rfc3042
Enable the Limited Transmit algorithm as described in RFC 3042.
It helps avoid timeouts on lossy links and also when the congestion window
is small, as happens on short transfers.
.It Va rfc3390
Enable support for RFC 3390, which allows for a variable-sized
starting congestion window on new connections, depending on the
maximum segment size.
This helps throughput in general, but
particularly affects short transfers and high-bandwidth large
propagation-delay connections.
.Pp
When this feature is enabled, the
.Va slowstart_flightsize
and
.Va local_slowstart_flightsize
settings are not observed for new
connection slow starts, but they are still used for slow starts
that occur when the connection has been idle and starts sending
again.
.It Va sack.enable
Enable support for RFC 2018, TCP Selective Acknowledgment option,
which allows the receiver to inform the sender about all successfully
arrived segments, allowing the sender to retransmit the missing segments
only.
.It Va sack.maxholes
Maximum number of SACK holes per connection.
Defaults to 128.
.It Va sack.globalmaxholes
Maximum number of SACK holes per system, across all connections.
Defaults to 65536.
.It Va maxtcptw
When a TCP connection enters the
.Dv TIME_WAIT
state, its associated socket structure is freed, since it is of
negligible size and use, and a new structure is allocated to contain a
minimal amount of information necessary for sustaining a connection in
this state, called the compressed TCP TIME_WAIT state.
Since this structure is smaller than a socket structure, it can save
a significant amount of system memory.
The
.Va net.inet.tcp.maxtcptw
MIB variable controls the maximum number of these structures allocated.
By default, it is initialized to
.Va kern.ipc.maxsockets
/ 5.
.It Va nolocaltimewait
Suppress creating of compressed TCP TIME_WAIT states for connections in
which both endpoints are local.
.It Va fast_finwait2_recycle
Recycle
.Tn TCP
.Dv FIN_WAIT_2
connections faster when the socket is marked as
.Dv SBS_CANTRCVMORE
(no user process has the socket open, data received on
the socket cannot be read).
The timeout used here is
.Va finwait2_timeout .
.It Va finwait2_timeout
Timeout to use for fast recycling of
.Tn TCP
.Dv FIN_WAIT_2
connections.
Defaults to 60 seconds.
.It Va ecn.enable
Enable support for TCP Explicit Congestion Notification (ECN).
ECN allows a TCP sender to reduce the transmission rate in order to
avoid packet drops.
.It Va ecn.maxretries
Number of retries (SYN or SYN/ACK retransmits) before disabling ECN on a
specific connection. This is needed to help with connection establishment
when a broken firewall is in the network path.
.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 mod_cc 4 ,
.Xr syncache 4 ,
.Xr setkey 8
.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
.Rs
.%A "K. Ramakrishnan"
.%A "S. Floyd"
.%A "D. Black"
.%T "The Addition of Explicit Congestion Notification (ECN) to IP"
.%O "RFC 3168"
.Re
.Sh HISTORY
The
.Tn TCP
protocol appeared in
.Bx 4.2 .
The RFC 1323 extensions for window scaling and timestamps were added
in
.Bx 4.4 .
The
.Dv TCP_INFO
option was introduced in
.Tn Linux 2.6
and is
.Em subject to change .