= zmq_tcp(7)


== NAME
zmq_tcp - 0MQ unicast transport using TCP


== SYNOPSIS
TCP is an ubiquitous, reliable, unicast transport. When connecting distributed
applications over a network with 0MQ, using the TCP transport will likely be
your first choice.


== ADDRESSING
A 0MQ endpoint is a string consisting of a 'transport'`://` followed by an
'address'. The 'transport' specifies the underlying protocol to use. The
'address' specifies the transport-specific address to connect to.

For the TCP transport, the transport is `tcp`, and the meaning of the
'address' part is defined below.


Assigning a local address to a socket
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When assigning a local address to a socket using _zmq_bind()_ with the 'tcp'
transport, the 'endpoint' shall be interpreted as an 'interface' followed by a
colon and the TCP port number to use.

An 'interface' may be specified by either of the following:

* The wild-card `*`, meaning all available interfaces.
* The primary IPv4 or IPv6 address assigned to the interface, in its numeric
  representation.
* The non-portable interface name as defined by the operating system.

The TCP port number may be specified by:

* A numeric value, usually above 1024 on POSIX systems.
* The wild-card `*`, meaning a system-assigned ephemeral port.

When using ephemeral ports, the caller should retrieve the actual assigned
port using the ZMQ_LAST_ENDPOINT socket option. See xref:zmq_getsockopt.adoc[zmq_getsockopt]
for details.

Unbinding wild-card address from a socket
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When wild-card `*` 'endpoint' was used in _zmq_bind()_, the caller should use
real 'endpoint' obtained from the ZMQ_LAST_ENDPOINT socket option to unbind 
this 'endpoint' from a socket using _zmq_unbind()_.

Connecting a socket
~~~~~~~~~~~~~~~~~~~
When connecting a socket to a peer address using _zmq_connect()_ with the 'tcp'
transport, the 'endpoint' shall be interpreted as a 'peer address' followed by
a colon and the TCP port number to use.
You can optionally specify a 'source_endpoint' which will be used as the source
address for your connection; tcp://'source_endpoint';'endpoint', see the
'interface' description above for details.

A 'peer address' may be specified by either of the following:

* The DNS name of the peer.
* The IPv4 or IPv6 address of the peer, in its numeric representation.

Note: A description of the ZeroMQ Message Transport Protocol (ZMTP) which is 
used by the TCP transport can be found at <http://rfc.zeromq.org/spec:23>


== HWM

For the TCP transport, the high water mark (HWM) mechanism works in conjunction
with the TCP socket buffers handled at OS level.
Depending on the OS and several other factors the size of such TCP buffers will
be different. Moreover TCP buffers provided by the OS will accommodate a varying
number of messages depending on the size of messages (unlike ZMQ HWM settings
the TCP socket buffers are measured in bytes and not messages).

This may result in apparently inexplicable behaviors: e.g., you may expect that
setting ZMQ_SNDHWM to 100 on a socket using TCP transport will have the effect
of blocking the transmission of the 101-th message if the receiver is slow.
This is very unlikely when using TCP transport since OS TCP buffers will typically
provide enough buffering to allow you sending much more than 100 messages.

Of course if the receiver is slow, transmitting on a TCP ZMQ socket will eventually trigger
the "mute state" of the socket; simply don't rely on the exact HWM value.

Obviously the same considerations apply for the receive HWM (see ZMQ_RCVHWM).



== EXAMPLES
.Assigning a local address to a socket
----
//  TCP port 5555 on all available interfaces
rc = zmq_bind(socket, "tcp://*:5555");
assert (rc == 0);
//  TCP port 5555 on the local loop-back interface on all platforms
rc = zmq_bind(socket, "tcp://127.0.0.1:5555");
assert (rc == 0);
//  TCP port 5555 on the first Ethernet network interface on Linux
rc = zmq_bind(socket, "tcp://eth0:5555");
assert (rc == 0);
----

.Connecting a socket
----
//  Connecting using an IP address
rc = zmq_connect(socket, "tcp://192.168.1.1:5555");
assert (rc == 0);
//  Connecting using a DNS name
rc = zmq_connect(socket, "tcp://server1:5555");
assert (rc == 0);
//  Connecting using a DNS name and bind to eth1
rc = zmq_connect(socket, "tcp://eth1:0;server1:5555");
assert (rc == 0);
//  Connecting using a IP address and bind to an IP address
rc = zmq_connect(socket, "tcp://192.168.1.17:5555;192.168.1.1:5555");
assert (rc == 0);
----


== SEE ALSO
* xref:zmq_bind.adoc[zmq_bind]
* xref:zmq_connect.adoc[zmq_connect]
* xref:zmq_pgm.adoc[zmq_pgm]
* xref:zmq_ipc.adoc[zmq_ipc]
* xref:zmq_inproc.adoc[zmq_inproc]
* xref:zmq_vmci.adoc[zmq_vmci]
* xref:zmq.adoc[zmq]


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
