OPC UA UDP is a simple UDP based protocol that is used to transport UADP NetworkMessages.

A PubSubConnection for UDP shall have a unique Address across all PubSubConnections of an OPC UA Application.

If the Address specifies a domain name then the resolution to an IP address requires access to a domain name resolution service (e.g., the DNS protocol) that maps the domain name onto a usable network address. OPC 10000-7 defines Profiles for different name resolution protocols that Publisher or Subscriber may support.

For OPC UA UDP it is recommended to limit the MaxNetworkMessageSize plus additional headers to a MTU size. The number of frames used for a UADP NetworkMessage influences the probability that UADP NetworkMessages get lost.

Note: The MaxNetworkMessageSize that fits into one MTU is maximum 1472 Byte for IPv4 and 1452 Byte for IPv6. The additional headers have a size of 22 Byte for Ethernet, 20 Byte for IPv4 or 40 Byte for IPv6 and 8 Byte for UDP. This is based on IETF RFC 8200 for IPv6, RFC 791 for IPv4 and RFC 768 for UDP.

For OPC UA UDP the MaxNetworkMessageSize plus additional headers shall be limited to 65535 Byte.

The transport of a UADP NetworkMessage in a UDP packet is defined in Table 170.

Table 170 – UADP message transported over UDP

Name	Description
Frame Header	The frame header.
IP Header	The IP header for the frame contains information like source IP address and destination IP address. IPv4 and IPv6 addresses can be used. The size of the IP header depends on the used version.
UDP Header	The UDP header for the frame contains the source port, destination port, length and checksum. Each field is two byte long. The total size of the UDP header is 8 byte.
UADP NetworkMessage	The UADP NetworkMessage is sent as UDP data.
Frame Footer	The frame footer.

The IANA registered IPv4 multicast address for discovery is 224.0.2.14. It shall only be used for OPC UA discovery purposes. The recommended port for discovery is 4840. Therefore the default DiscoveryAddress has the following form:

opc.udp://224.0.2.14

The default DiscoveryMaxMessageSize for UDP is 4096 bytes.