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.\"     @(#)tcp.4	8.1 (Berkeley) 6/5/93
.\"
.Dd March 18, 2015
.Dt TCP 4
.Os BSD 4.2
.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
or
.Xr connectx 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_CONNECTIONTIMEOUT"
.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_KEEPALIVE
.Tn The
.Dv TCP_KEEPALIVE
options enable to specify the amount of time, in seconds, that the 
connection must be idle before keepalive probes (if enabled) are sent. 
The default value is specified by the 
.Tn MIB
variable
.Va net.inet.tcp.keepidle .  
.It Dv TCP_CONNECTIONTIMEOUT
.Tn The
.Dv TCP_CONNECTIONTIMEOUT
option allows to specify the timeout, in seconds, for new, non established
.Tn TCP
connections. This option can be useful for both active and passive
.Tn TCP
connections. The default value is specified by the 
.Tn MIB
variable
.Va net.inet.tcp.keepinit .
.It Dv TCP_KEEPINTVL
When keepalive probes are enabled, this option will set the amount of time in seconds between successive keepalives sent to probe an unresponsive peer. 
.It Dv TCP_KEEPCNT
.Tn When keepalive probes are enabled, this option will set the number of times a keepalive probe should be repeated if the peer is not responding. After this many probes, the connection will be closed. 
.It Dv TCP_SENDMOREACKS
When a stream of 
.Tn TCP
data packets are received, OS X uses an algorithm to reduce the number of acknowlegements by generating a 
.Tn TCP
acknowlegement for 8 data packets instead of acknowledging every other data packet. When this socket option is enabled, the connection will always send a
.Tn TCP
acknowledgement for every other data packet.
.It Dv TCP_ENABLE_ECN
Using Explicit Congestion Notification (ECN) on
.Tn TCP
allows bi-directional end-to-end notification of congestion without dropping packets. Conventionally TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The
.Tn TCP
receiver of the packet echoes congestion indication to the 
.Tn TCP
sender, which reduces it's transmission rate as if it detected a dropped packet. This will avoid unnecessary retransmissions and will improve latency by saving the time required for recovering a lost packet.
.It Dv TCP_NOTSENT_LOWAT
The send socket buffer of a
.Tn TCP sender has unsent and unacknowledged data. This option allows a 
.Tn TCP sender to control the amount of unsent data kept in the send socket buffer. The value of the option should be the maximum amount of unsent data in bytes. Kevent, poll and select will generate a write notification when the unsent data falls below the amount given by this option. This will allow an application to generate just-in-time fresh updates for real-time communication.
.It Dv TCP_FASTOPEN
The TCP listener can set this option to use TCP Fast Open feature. After
setting this option, an
.Xr accept 2
may return a socket that is in SYN_RECEIVED state but is readable and writable.
.It Dv TCP_CONNECTION_INFO
This socket option can be used to obtain TCP connection level statistics. The
"struct tcp_connection_info" defined in <netinet/tcp_var.h> is copied to the
user buffer.
.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.
.Ss "Non-blocking connect"
.Pp
When a
.Tn TCP
socket is set non-blocking, and the connection cannot be established immediately, 
.Xr connect 2
or
.Xr connectx 2
returns with the error
.Dv EINPROGRESS ,
and the connection is established asynchronously.
.Pp
When the asynchronous connection completes successfully,
.Xr select 2
or
.Xr poll 2
or
.Xr kqueue 2
will indicate the file descriptor is ready for writing. 
If the connection encounters an error, the file descriptor 
is marked ready for both reading and writing, and the pending error 
can be retrieved via the socket option 
.Dv SO_ERROR .
.Pp
Note that even if the socket is non-blocking, it is possible for the connection 
to be established immediately. In that case 
.Xr connect 2
or
.Xr connectx 2
does not return with 
.Dv EINPROGRESS .
.Sh DIAGNOSTICS
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;
.It Bq Er EINPROGRESS
returned by
.Xr connect 2
or
.Xr connectx 2
when the socket is set nonblocking, and the connection cannot be 
immediately established;
.It Bq Er EALREADY
returned by
.Xr connect 2
or
.Xr connectx 2
when connection request is already in progress for the specified socket.
.It Bq Er ENODATA
returned by
.Xr recv 2
or
.Xr send 2
in case a connection is experiencing a data-stall (probably due to a middlebox issue).
It is advised that the current connection gets closed by the application and a
new attempt is being made.
.
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr connectx 2 ,
.Xr getsockopt 2 ,
.Xr kqueue 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr sysctl 3 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr netintro 4 ,
.Xr setkey 8
.Sh HISTORY
The
.Tn TCP
protocol appeared in
.Bx 4.2 .
.Pp
The socket option
.Dv TCP_CONNECTIONTIMEOUT
first appeared in Mac OS X 10.6.