.\"
.\" @(#)tcp.4 8.1 (Berkeley) 6/5/93
.\"
-.Dd June 5, 1993
+.Dd March 18, 2015
.Dt TCP 4
.Os BSD 4.2
.Sh NAME
.Nm tcp
.Nd Internet Transmission Control Protocol
.Sh SYNOPSIS
-.Fd #include <sys/socket.h>
-.Fd #include <netinet/in.h>
+.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
+transmission of data.
+It is a byte-stream protocol used to
support the
.Dv SOCK_STREAM
-abstraction. TCP uses the standard
+abstraction.
+.Tn TCP
+uses the standard
Internet address format and, in addition, provides a per-host
collection of
-.Dq port addresses .
+.Dq "port addresses" .
Thus, each address is composed
-of an Internet address specifying the host and network, with
-a specific
+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
+Sockets utilizing the
+.Tn TCP
+protocol are either
.Dq active
or
.Dq passive .
Active sockets initiate connections to passive
-sockets. By default
+sockets.
+By default,
.Tn TCP
sockets are created active; to create a
-passive socket the
+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
+system call.
+Only passive sockets may use the
.Xr accept 2
-call to accept incoming connections. Only active sockets may
-use the
+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 ,
+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
+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
+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.
+through which packets are being transmitted and received.
+Normally, this address corresponds to the peer entity's network.
.Pp
.Tn TCP
-supports one socket option which is set with
+supports a number of socket options which can be set with
.Xr setsockopt 2
and tested with
-.Xr getsockopt 2 .
+.Xr getsockopt 2 :
+.Bl -tag -width ".Dv TCP_CONNECTIONTIMEOUT"
+.It Dv TCP_NODELAY
Under most circumstances,
.Tn TCP
sends data when it is presented;
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.
-Therefore,
-.Tn TCP
-provides a boolean option,
+The boolean option
.Dv TCP_NODELAY
-(from
-.Aq Pa netinet/tcp.h ,
-to defeat this algorithm.
+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
+.Xr setsockopt 2
call is the protocol number for
.Tn TCP ,
available from
-.Xr getprotobyname 3 .
+.Xr getprotobyname 3 ,
+or
+.Dv IPPROTO_TCP .
+All options are declared in
+.In netinet/tcp.h .
.Pp
Options at the
.Tn IP
.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 [EADDRNOTAVAIL]
+.Bl -tag -width Er
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one;
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
+when an attempt is made to create a
socket with a network address for which no network interface
-exists.
+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 intro 4 ,
-.Xr ip 4
+.Xr inet6 4 ,
+.Xr ip 4 ,
+.Xr ip6 4 ,
+.Xr netintro 4 ,
+.Xr setkey 8
.Sh HISTORY
The
-.Nm
-protocol stack appeared in
+.Tn TCP
+protocol appeared in
.Bx 4.2 .
+.Pp
+The socket option
+.Dv TCP_CONNECTIONTIMEOUT
+first appeared in Mac OS X 10.6.
projects / apple / xnu.git / blobdiff