= zmq_pgm(7)


== 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
xref:zmq_setsockopt.adoc[zmq_setsockopt]

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
xref:zmq_tcp.adoc[zmq_tcp]


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 relevant ZMQ socket
options that are documented in xref:zmq_setsockopt.adoc[zmq_setsockopt]:

* 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
* xref:zmq_connect.adoc[zmq_connect]
* xref:zmq_setsockopt.adoc[zmq_setsockopt]
* xref:zmq_tcp.adoc[zmq_tcp]
* xref:zmq_ipc.adoc[zmq_ipc]
* xref:zmq_inproc.adoc[zmq_inproc]
* xref:zmq_vmci.adoc[zmq_vmci]
* xref:zmq.adoc[zmq]


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
