Transmission Control Protocol
The TCP protocol provides a reliable, flow-controlled, two-way transmission of
data. It is a byte-stream protocol used to support the
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
system call must be
used after binding the socket with the
system call. Only passive
sockets may use the accept(2)
call to accept incoming connections. Only active sockets may use the
call to initiate
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
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
- 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
to defeat this algorithm.
- By convention, the TCP sender will set the
“push” bit and begin transmission immediately (if permitted)
at the end of every user call to
writev(2). When this option
is set to a non-zero value, TCP will delay sending any data at all until
either the socket is closed, the internal send buffer is filled, or this
option is set to a zero value.
- Set the maximum segment size for this connection. The
maximum segment size can only be lowered.
- Use selective acknowledgements for this connection. See
- 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
call is the
protocol number for TCP, available from
Options at the IP transport level may be used with TCP; see
. Incoming connection
requests that are source-routed are noted, and the reverse source route is
used in responding.
A socket operation may fail with one of the following errors returned:
- when trying to establish a connection on a socket which
already has one;
- when the system runs out of memory for an internal data
- when a connection was dropped due to excessive
- when the remote peer forces the connection to be
- when the remote peer actively refuses connection
establishment (usually because no process is listening to the port);
- when an attempt is made to create a socket with a port
which has already been allocated;
- when an attempt is made to create a socket with a network
address for which no network interface exists.
protocol stack appeared in