xs_pgm(7) ========= 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 linkxs: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 linkxs: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 -------- linkxs:xs_connect[3] linkxs:xs_setsockopt[3] linkxs:xs_tcp[7] linkxs:xs_ipc[7] linkxs:xs_inproc[7] linkxs:xs[7] AUTHORS ------- The Crossroads documentation was written by Martin Sustrik and Martin Lucina .