NAME

       xs_pgm - reliable multicast transport via PGM protocol

SYNOPSIS

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

DESCRIPTION

       Crossroads implement 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" where PGM datagrams are encapsulated inside UDP datagrams (the epgm
       transport).

       The pgm and epgm transports can only be used with the XS_PUB and XS_SUB socket types.

       Further, PGM sockets are rate limited by default. For details, refer to the XS_RATE, and
       XS_RECOVERY_IVL options documented in xs_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 Crossroads address string consists of two parts as follows: transport://endpoint. The
       transport part specifies the underlying transport protocol to use. For the standard PGM
       protocol, transport shall be set to pgm. For the "Encapsulated PGM" protocol transport
       shall be set to epgm. The meaning of the endpoint part for both the pgm and epgm transport
       is defined below.

   Connecting a socket
       When connecting a socket to a peer address using xs_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 it’s 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.

       A multicast address is specified by an IPv4 multicast address in it’s numeric
       representation.

WIRE FORMAT

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

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

           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 Crossroads 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) |
           +--------------+----------------------------+-------------+

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 = xs_connect(socket, "epgm://eth0;239.192.1.1:5555");
           assert (rc != -1);
           /* 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 = xs_connect(socket, "pgm://192.168.1.1;239.192.1.1:5555");
           assert (rc != -1);

SEE ALSO

       xs_connect(3) xs_setsockopt(3) xs_tcp(7) xs_ipc(7) xs_inproc(7) xs(7)

AUTHORS

       The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and
       Martin Lucina <martin@lucina.net[2]>.

NOTES

        1. sustrik@250bpm.com
           mailto:sustrik@250bpm.com

        2. martin@lucina.net
           mailto:martin@lucina.net