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Henry Winkel
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doc/zmq_pgm.txt
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doc/zmq_pgm.txt
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zmq_pgm(7)
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==========
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NAME
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----
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zmq_pgm - 0MQ reliable multicast transport using PGM
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SYNOPSIS
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--------
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PGM (Pragmatic General Multicast) is a protocol for reliable multicast
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transport of data over IP networks.
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DESCRIPTION
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-----------
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0MQ implements two variants of PGM, the standard protocol where PGM datagrams
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are layered directly on top of IP datagrams as defined by RFC 3208 (the 'pgm'
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transport) and "Encapsulated PGM" or EPGM where PGM datagrams are encapsulated
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inside UDP datagrams (the 'epgm' transport).
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The 'pgm' and 'epgm' transports can only be used with the 'ZMQ_PUB' and
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'ZMQ_SUB' socket types.
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Further, PGM sockets are rate limited by default. For details, refer to the
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'ZMQ_RATE', and 'ZMQ_RECOVERY_IVL' options documented in
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linkzmq:zmq_setsockopt[3].
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CAUTION: The 'pgm' transport implementation requires access to raw IP sockets.
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Additional privileges may be required on some operating systems for this
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operation. Applications not requiring direct interoperability with other PGM
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implementations are encouraged to use the 'epgm' transport instead which does
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not require any special privileges.
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ADDRESSING
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----------
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A 0MQ endpoint is a string consisting of a 'transport'`://` followed by an
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'address'. The 'transport' specifies the underlying protocol to use. The
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'address' specifies the transport-specific address to connect to.
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For the PGM transport, the transport is `pgm`, and for the EPGM protocol the
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transport is `epgm`. The meaning of the 'address' part is defined below.
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Connecting a socket
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~~~~~~~~~~~~~~~~~~~
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When connecting a socket to a peer address using _zmq_connect()_ with the 'pgm'
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or 'epgm' transport, the 'endpoint' shall be interpreted as an 'interface'
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followed by a semicolon, followed by a 'multicast address', followed by a colon
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and a port number.
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An 'interface' may be specified by either of the following:
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* The interface name as defined by the operating system.
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* The primary IPv4 address assigned to the interface, in its numeric
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representation.
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NOTE: Interface names are not standardised in any way and should be assumed to
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be arbitrary and platform dependent. On Win32 platforms no short interface
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names exist, thus only the primary IPv4 address may be used to specify an
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'interface'. The 'interface' part can be omitted, in that case the default one
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will be selected.
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A 'multicast address' is specified by an IPv4 multicast address in its numeric
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representation.
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WIRE FORMAT
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-----------
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Consecutive PGM datagrams are interpreted by 0MQ as a single continuous stream
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of data where 0MQ messages are not necessarily aligned with PGM datagram
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boundaries and a single 0MQ message may span several PGM datagrams. This stream
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of data consists of 0MQ messages encapsulated in 'frames' as described in
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linkzmq:zmq_tcp[7].
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PGM datagram payload
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~~~~~~~~~~~~~~~~~~~~
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The following ABNF grammar represents the payload of a single PGM datagram as
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used by 0MQ:
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....
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datagram = (offset data)
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offset = 2OCTET
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data = *OCTET
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....
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In order for late joining consumers to be able to identify message boundaries,
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each PGM datagram payload starts with a 16-bit unsigned integer in network byte
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order specifying either the offset of the first message 'frame' in the datagram
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or containing the value `0xFFFF` if the datagram contains solely an
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intermediate part of a larger message.
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Note that offset specifies where the first message begins rather than the first
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message part. Thus, if there are trailing message parts at the beginning of
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the packet the offset ignores them and points to first initial message part
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in the packet.
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The following diagram illustrates the layout of a single PGM datagram payload:
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....
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+------------------+----------------------+
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| offset (16 bits) | data |
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+------------------+----------------------+
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....
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The following diagram further illustrates how three example 0MQ frames are laid
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out in consecutive PGM datagram payloads:
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....
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First datagram payload
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+--------------+-------------+---------------------+
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| Frame offset | Frame 1 | Frame 2, part 1 |
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| 0x0000 | (Message 1) | (Message 2, part 1) |
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+--------------+-------------+---------------------+
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Second datagram payload
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+--------------+---------------------+
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| Frame offset | Frame 2, part 2 |
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| 0xFFFF | (Message 2, part 2) |
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+--------------+---------------------+
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Third datagram payload
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+--------------+----------------------------+-------------+
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| Frame offset | Frame 2, final 8 bytes | Frame 3 |
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| 0x0008 | (Message 2, final 8 bytes) | (Message 3) |
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+--------------+----------------------------+-------------+
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....
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CONFIGURATION
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-------------
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The PGM is protocol is capable of multicasting data at high rates (500Mbps+)
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with large messages (1MB+), however it requires setting the relevant ZMQ socket
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options that are documented in linkzmq:zmq_setsockopt[3]:
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* The 'ZMQ_RATE' should be set sufficiently high, e.g. 1Gbps
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* The 'ZMQ_RCVBUF' should be increased on the subscriber, e.g. 4MB
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* The 'ZMQ_SNDBUF' should be increased on the publisher, e.g. 4MB
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It's important to note that the 'ZMQ_RCVBUF' and 'ZMQ_SNDBUF' options are
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limited by the underlying host OS tx/rx buffer size limit. On linux, these can
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be increased for the current session with the following commands:
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....
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# set tx/rx buffers to 4MB (default can also be read as the initial buffer size)
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sudo sysctl -w net.core.rmem_max=4194304
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sudo sysctl -w net.core.wmem_max=4194304
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sudo sysctl -w net.core.rmem_default=4194304
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sudo sysctl -w net.core.wmem_default=4194304
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....
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EXAMPLE
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-------
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.Connecting a socket
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----
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// Connecting to the multicast address 239.192.1.1, port 5555,
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// using the first Ethernet network interface on Linux
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// and the Encapsulated PGM protocol
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rc = zmq_connect(socket, "epgm://eth0;239.192.1.1:5555");
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assert (rc == 0);
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// Connecting to the multicast address 239.192.1.1, port 5555,
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// using the network interface with the address 192.168.1.1
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// and the standard PGM protocol
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rc = zmq_connect(socket, "pgm://192.168.1.1;239.192.1.1:5555");
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assert (rc == 0);
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----
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SEE ALSO
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--------
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linkzmq:zmq_connect[3]
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linkzmq:zmq_setsockopt[3]
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linkzmq:zmq_tcp[7]
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linkzmq:zmq_ipc[7]
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linkzmq:zmq_inproc[7]
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linkzmq:zmq_vmci[7]
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linkzmq:zmq[7]
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AUTHORS
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-------
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This page was written by the 0MQ community. To make a change please
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read the 0MQ Contribution Policy at <http://www.zeromq.org/docs:contributing>.
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