zmq_tcp - 0MQ unicast transport using TCP
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.
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:
*, 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.
*, 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 for details.
Unbinding wild-card address from a socket
* '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
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).
// 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 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);
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