The transport protocol URL for UDP multicast and broadcast communication is configured on a PubSubConnection for Publisher and Subscriber. The Address parameter for a PubSubConnection is defined in 6.2.7.3.

The Url field in the Address is used as destination address for NetworkMessages sent as UDP datagram. The Address is also used to receive UDP datagrams from the multicast IP address. All DataSetWriters and DataSetReaders that send to and receive from the multicast IP address shall be configured on one PubSubConnection. The Address parameter for WriterGroup datagram TransportSettings shall be null. If an Address is configured on a WriterGroup, the WriterGroup PubSubState shall be Error. The NetworkInterface field in the Address is required if more than one network interface is available.

The syntax of the UDP transport protocol URL used in the Address has the following form:

opc.udp://<address>[:<port>]

The address is either an multicast or broadcast IP address or a registered name like a domain name that can be resolved to a multicast or broadcast IP address.. It is the destination of the UDP datagram.

The IANA registered OPC UA port for UDP communication is 4840. This is the default and recommended port for broadcast and multicast communication. Alternative ports may be used.

It is recommended to use switches with IGMP and MLD support to limit the distribution of multicast traffic to the interested participants.

Note: The Internet Group Management Protocol (IGMP) is a standard protocol used by hosts to report their IP multicast group memberships for IPv4 and needs to be implemented by any host that wishes to receive IP multicast datagrams. IGMP messages are used by multicast routers to learn which multicast groups have members on their attached networks. IGMP messages are also used by switches capable of supporting “IGMP snooping” whereby the switch listens to IGMP messages and only sends the multicast NetworkMessages to ports that have joined the multicast group. The corresponding protocol for IPv6 is the Multicast Listener Discovery (MLD).There are different versions of IGMP and MLD: - IGMP V1 is defined in IETF RFC 1112. - IGMP V2 is defined in IETF RFC 2236. - IGMP V3 is defined in IETF RFC 3376.

- IGMP V3 and MLD V2 are defined in IETF RFC 4604.

IETF RFC 2236 and IETF RFC 3376 discuss host and router requirements for interoperation with older IGMP versions.If OPC UA devices make extensive use of IP multicast for UDP transport, consistent IGMP and MLD usage by OPC UA devices is essential in order to create well-functioning OPC UA Application networks.

OPC UA Applications shall issue an IGMP membership report message (V1, V2 or V3 as appropriate) for IPv4 or a MLD membership report message for IPv6 when enabling a PubSub connection on which they will receive UDP multicast NetworkMessages.