1ZMQ_PGM(7) 0MQ Manual ZMQ_PGM(7)
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6 zmq_pgm - 0MQ reliable multicast transport using PGM
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9 PGM (Pragmatic General Multicast) is a protocol for reliable multicast
10 transport of data over IP networks.
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13 0MQ implements two variants of PGM, the standard protocol where PGM
14 datagrams are layered directly on top of IP datagrams as defined by RFC
15 3208 (the pgm transport) and "Encapsulated PGM" where PGM datagrams are
16 encapsulated inside UDP datagrams (the epgm transport).
17 The pgm and epgm transports can only be used with the ZMQ_PUB and
19 ZMQ_SUB socket types.
20 Further, PGM sockets are rate limited by default and incur a
22 performance penalty when used over a loop-back interface. For details,
23 refer to the ZMQ_RATE, ZMQ_RECOVERY_IVL and ZMQ_MCAST_LOOP options
24 documented in zmq_setsockopt(3).
25 Caution
27 The pgm transport implementation requires access to raw IP sockets.
28 Additional privileges may be required on some operating systems for
29 this operation. Applications not requiring direct interoperability
30 with other PGM implementations are encouraged to use the epgm
31 transport instead which does not require any special privileges.
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34 A 0MQ address string consists of two parts as follows:
35 transport://endpoint. The transport part specifies the underlying
36 transport protocol to use. For the standard PGM protocol, transport
37 shall be set to pgm. For the "Encapsulated PGM" protocol transport
38 shall be set to epgm. The meaning of the endpoint part for both the pgm
39 and epgm transport is defined below.
40 Connecting a socket
42 When connecting a socket to a peer address using zmq_connect() with the
43 pgm or epgm transport, the endpoint shall be interpreted as an
44 interface followed by a semicolon, followed by a multicast address,
45 followed by a colon and a port number.
46 An interface may be specified by either of the following:
48 · The interface name as defined by the operating system.
50 · The primary IPv4 address assigned to the interface, in it’s numeric
52 representation.
53 Note
55 Interface names are not standardised in any way and should be
56 assumed to be arbitrary and platform dependent. On Win32 platforms
57 no short interface names exist, thus only the primary IPv4 address
58 may be used to specify an interface.
59 A multicast address is specified by an IPv4 multicast address in it’s
61 numeric representation.
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64 Consecutive PGM datagrams are interpreted by 0MQ as a single continuous
65 stream of data where 0MQ messages are not necessarily aligned with PGM
66 datagram boundaries and a single 0MQ message may span several PGM
67 datagrams. This stream of data consists of 0MQ messages encapsulated in
68 frames as described in zmq_tcp(7).
69 PGM datagram payload
71 The following ABNF grammar represents the payload of a single PGM
72 datagram as used by 0MQ:
73 datagram = (offset data)
75 offset = 2OCTET
76 data = *OCTET
77 In order for late joining consumers to be able to identify message
79 boundaries, each PGM datagram payload starts with a 16-bit unsigned
80 integer in network byte order specifying either the offset of the first
81 message frame in the datagram or containing the value 0xFFFF if the
82 datagram contains solely an intermediate part of a larger message.
83 The following diagram illustrates the layout of a single PGM datagram
85 payload:
86 +------------------+----------------------+
88 | offset (16 bits) | data |
89 +------------------+----------------------+
90 The following diagram further illustrates how three example 0MQ frames
92 are laid out in consecutive PGM datagram payloads:
93 First datagram payload
95 +--------------+-------------+---------------------+
96 | Frame offset | Frame 1 | Frame 2, part 1 |
97 | 0x0000 | (Message 1) | (Message 2, part 1) |
98 +--------------+-------------+---------------------+
99 Second datagram payload
101 +--------------+---------------------+
102 | Frame offset | Frame 2, part 2 |
103 | 0xFFFF | (Message 2, part 2) |
104 +--------------+---------------------+
105 Third datagram payload
107 +--------------+----------------------------+-------------+
108 | Frame offset | Frame 2, final 8 bytes | Frame 3 |
109 | 0x0008 | (Message 2, final 8 bytes) | (Message 3) |
110 +--------------+----------------------------+-------------+
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113 Connecting a socket.
114 /* Connecting to the multicast address 239.192.1.1, port 5555, */
116 /* using the first Ethernet network interface on Linux */
117 /* and the Encapsulated PGM protocol */
118 rc = zmq_connect(socket, "epgm://eth0;239.192.1.1:5555");
119 assert (rc == 0);
120 /* Connecting to the multicast address 239.192.1.1, port 5555, */
121 /* using the network interface with the address 192.168.1.1 */
122 /* and the standard PGM protocol */
123 rc = zmq_connect(socket, "pgm://192.168.1.1;239.192.1.1:5555");
124 assert (rc == 0);
125
128 zmq_connect(3) zmq_setsockopt(3) zmq_tcp(7) zmq_ipc(7) zmq_inproc(7)
129 zmq(7)
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132 This 0MQ manual page was written by Martin Sustrik
133 <sustrik@250bpm.com[1]> and Martin Lucina <mato@kotelna.sk[2]>.
134
136 1. sustrik@250bpm.com
137 mailto:sustrik@250bpm.com
138 2. mato@kotelna.sk
140 mailto:mato@kotelna.sk
1410MQ 2.1.4 03/30/2011 ZMQ_PGM(7)