The
TCP
protocol provides a reliable, flow-controlled, two-way
transmission of data.
It is a byte-stream protocol used to support the
SOCK_STREAM
abstraction.
TCP uses the standard
Internet address format and, in addition, provides a per-host
collection of
“port addresses”.
Thus, each address is composed
of an Internet address specifying the host and network, with
a specific
TCP
port on the host identifying the peer entity.
Sockets utilizing the TCP protocol are either
“active”
or
“passive”.
Active sockets initiate connections to passive
sockets.
By default
TCP
sockets are created active; to create a
passive socket the
listen(2)
system call must be used
after binding the socket with the
bind(2)
system call.
Only passive sockets may use the
accept(2)
call to accept incoming connections.
Only active sockets may use the
connect(2)
call to initiate connections.
Passive sockets may
“underspecify”
their location to match
incoming connection requests from multiple networks.
This technique, termed
“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
INADDR_ANY
must be bound.
The
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.
TCP
supports several socket options which are set with
setsockopt(2)
and tested with
getsockopt(2).
Under most circumstances,
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.
Therefore,
TCP
provides a boolean option,
TCP_NODELAY
(from
‹netinet/tcp.h)›,
to defeat this algorithm.
Set the maximum segment size for this connection.
The maximum segment size can only be lowered.
Use selective acknowledgements for this connection.
See
options(4).
Use TCP MD5 signatures per RFC 2385.
This requires
Security Associations
to be set up, which can be done using
ipsecctl(8).
When a listening socket has
TCP_MD5SIG
set, it accepts connections with MD5 signatures only from sources for which a
Security Association
is set up.
Connections without MD5 signatures are only accepted from sources for which no
Security Association
is set up.
The connected socket only has
TCP_MD5SIG
set if the connection is protected with MD5 signatures.
The option level for the
setsockopt(2)
call is the protocol number for
TCP,
available from
getprotobyname(3).
Options at the
IP
transport level may be used with
TCP;
see
ip(4)
or
ip6(4).
Incoming connection requests that are source-routed are noted,
and the reverse source route is used in responding.
DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
[EISCONN]
when trying to establish a connection on a socket which
already has one;
[ENOBUFS]
when the system runs out of memory for
an internal data structure;
[ETIMEDOUT]
when a connection was dropped
due to excessive retransmissions;
[ECONNRESET]
when the remote peer
forces the connection to be closed;
[ECONNREFUSED]
when the remote
peer actively refuses connection establishment (usually because
no process is listening to the port);
[EADDRINUSE]
when an attempt
is made to create a socket with a port which has already been
allocated;
[EADDRNOTAVAIL]
when an attempt is made to create a
socket with a network address for which no network interface
exists.