# 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 address string consists of two parts as follows: transport`://`endpoint. The transport part specifies the underlying transport protocol to use, and for the TCP transport shall be set to `tcp`. The meaning of the endpoint part for the TCP transport 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 interface name as defined by the operating system.

Note | Interface names are not standardised in any way and should be assumed to be arbitrary and platform dependent. On Win32 platforms no short interface names exist, thus only the primary IP address may be used to specify an interface.

### 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.

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 it’s numeric representation.

## WIRE FORMAT

ØMQ messages are transmitted over TCP in frames consisting of an encoded payload length, followed by a flags field and the message body. The payload length is defined as the combined length in octets of the message body and the flags field.

For frames with a payload length not exceeding 254 octets, the payload length shall be encoded as a single octet. The minimum valid payload length of a frame is 1 octet, thus a payload length of 0 octets is invalid and such frames SHOULD be ignored.

For frames with a payload length exceeding 254 octets, the payload length shall be encoded as a single octet with the value `255` followed by the payload length represented as a 64-bit unsigned integer in network byte order.

The flags field consists of a single octet containing various control flags:

Bit 0 (MORE): More message parts to follow. A value of 0 indicates that there are no more message parts to follow; or that the message being sent is not a multi-part message. A value of 1 indicates that the message being sent is a multi-part message and more message parts are to follow.

Bits 1-7: Reserved. Bits 1-7 are reserved for future expansion and MUST be set to zero.

The following ABNF grammar represents a single frame:

        frame           = (length flags data)
        length          = OCTET / (escape 8OCTET)
        flags           = OCTET
        escape          = %xFF
        data            = *OCTET

The following diagram illustrates the layout of a frame with a payload length not exceeding 254 octets:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Payload length|     Flags     |       Message body        ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Message body ...
    +-+-+-+-+-+-+- ...

The following diagram illustrates the layout of a frame with a payload length exceeding 254 octets:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     0xff      |               Payload length              ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Payload length                      ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Payload length|     Flags     |        Message body       ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Message body ...
    +-+-+-+-+-+-+-+ ...

## 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);

## SEE ALSO

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

## AUTHORS

The ØMQ documentation was written by Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <martin@lucina.net>.