# zmq_tcp(7) Manual Page

## NAME

zmq_tcp - 0MQ unicast transport using TCP

## SYNOPSIS

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

## ADDRESSING

A ØMQ 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 zmq_getsockopt(3) 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

zmq_bind(3) zmq_connect(3) zmq_pgm(7) zmq_ipc(7) zmq_inproc(7) zmq_vmci(7) zmq(7)

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.