NAME

       zmq_pgm - 0MQ reliable multicast transport using PGM

SYNOPSIS

       PGM (Pragmatic General Multicast) is a protocol for reliable multicast transport of data
       over IP networks.

DESCRIPTION

       0MQ implements two variants of PGM, the standard protocol where PGM datagrams are layered
       directly on top of IP datagrams as defined by RFC 3208 (the pgm transport) and
       "Encapsulated PGM" or EPGM where PGM datagrams are encapsulated inside UDP datagrams (the
       epgm transport).

       The pgm and epgm transports can only be used with the ZMQ_PUB and ZMQ_SUB socket types.

       Further, PGM sockets are rate limited by default. For details, refer to the ZMQ_RATE, and
       ZMQ_RECOVERY_IVL options documented in zmq_setsockopt(3).

           Caution
           The pgm transport implementation requires access to raw IP sockets. Additional
           privileges may be required on some operating systems for this operation. Applications
           not requiring direct interoperability with other PGM implementations are encouraged to
           use the epgm transport instead which does not require any special privileges.

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 PGM transport, the transport is pgm, and for the EPGM protocol the transport is
       epgm. The meaning of the address part is defined below.

   Connecting a socket
       When connecting a socket to a peer address using zmq_connect() with the pgm or epgm
       transport, the endpoint shall be interpreted as an interface followed by a semicolon,
       followed by a multicast address, followed by a colon and a port number.

       An interface may be specified by either of the following:

       •   The interface name as defined by the operating system.

       •   The primary IPv4 address assigned to the interface, in its numeric representation.

           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 IPv4 address may be used to specify an interface. The interface part can
           be omitted, in that case the default one will be selected.

       A multicast address is specified by an IPv4 multicast address in its numeric
       representation.

WIRE FORMAT

       Consecutive PGM datagrams are interpreted by 0MQ as a single continuous stream of data
       where 0MQ messages are not necessarily aligned with PGM datagram boundaries and a single
       0MQ message may span several PGM datagrams. This stream of data consists of 0MQ messages
       encapsulated in frames as described in zmq_tcp(7).

   PGM datagram payload
       The following ABNF grammar represents the payload of a single PGM datagram as used by 0MQ:

           datagram               = (offset data)
           offset                 = 2OCTET
           data                   = *OCTET

       In order for late joining consumers to be able to identify message boundaries, each PGM
       datagram payload starts with a 16-bit unsigned integer in network byte order specifying
       either the offset of the first message frame in the datagram or containing the value
       0xFFFF if the datagram contains solely an intermediate part of a larger message.

       Note that offset specifies where the first message begins rather than the first message
       part. Thus, if there are trailing message parts at the beginning of the packet the offset
       ignores them and points to first initial message part in the packet.

       The following diagram illustrates the layout of a single PGM datagram payload:

           +------------------+----------------------+
           | offset (16 bits) |         data         |
           +------------------+----------------------+

       The following diagram further illustrates how three example 0MQ frames are laid out in
       consecutive PGM datagram payloads:

           First datagram payload
           +--------------+-------------+---------------------+
           | Frame offset |   Frame 1   |   Frame 2, part 1   |
           |    0x0000    | (Message 1) | (Message 2, part 1) |
           +--------------+-------------+---------------------+

           Second datagram payload
           +--------------+---------------------+
           | Frame offset |   Frame 2, part 2   |
           | 0xFFFF       | (Message 2, part 2) |
           +--------------+---------------------+

           Third datagram payload
           +--------------+----------------------------+-------------+
           | Frame offset |   Frame 2, final 8 bytes   |   Frame 3   |
           | 0x0008       | (Message 2, final 8 bytes) | (Message 3) |
           +--------------+----------------------------+-------------+

CONFIGURATION

       The PGM is protocol is capable of multicasting data at high rates (500Mbps+) with large
       messages (1MB+), however it requires setting the relevent ZMQ socket options that are
       documented in zmq_setsockopt(3):

       •   The ZMQ_RATE should be set sufficiently high, e.g. 1Gbps

       •   The ZMQ_RCVBUF should be increased on the subscriber, e.g. 4MB

       •   The ZMQ_SNDBUF should be increased on the publisher, e.g. 4MB

       It’s important to note that the ZMQ_RCVBUF and ZMQ_SNDBUF options are limited by the
       underlying host OS tx/rx buffer size limit. On linux, these can be increased for the
       current session with the following commands:

           # set tx/rx buffers to 4MB (default can also be read as the initial buffer size)
           sudo sysctl -w net.core.rmem_max=4194304
           sudo sysctl -w net.core.wmem_max=4194304
           sudo sysctl -w net.core.rmem_default=4194304
           sudo sysctl -w net.core.wmem_default=4194304

EXAMPLE

       Connecting a socket.

           //  Connecting to the multicast address 239.192.1.1, port 5555,
           //  using the first Ethernet network interface on Linux
           //  and the Encapsulated PGM protocol
           rc = zmq_connect(socket, "epgm://eth0;239.192.1.1:5555");
           assert (rc == 0);
           //  Connecting to the multicast address 239.192.1.1, port 5555,
           //  using the network interface with the address 192.168.1.1
           //  and the standard PGM protocol
           rc = zmq_connect(socket, "pgm://192.168.1.1;239.192.1.1:5555");
           assert (rc == 0);

SEE ALSO

       zmq_connect(3) zmq_setsockopt(3) zmq_tcp(7) zmq_ipc(7) zmq_inproc(7) zmq_vmci(7) zmq(7)

AUTHORS

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