opc.tcp

What the protocol looks like

The client sends HEL with its buffer sizes, maximum message size and maximum chunk count; the server answers ACK with the values it will accept - the lower of each. Then OPN opens the secure channel, MSG carries service calls, CLO closes it. Messages larger than the negotiated buffer are split into chunks, and exceeding the agreed limits produces BadTcpMessageTooLarge, BadEncodingLimitsExceeded or BadResponseTooLarge.

Why it beats the HTTP-based mappings

One long-lived socket, binary encoding, and a UA Secure Conversation channel that provides signing and encryption independently of TLS. The JSON and OpenAPI profiles have no UA secure channel at all and are advertised with MessageSecurityMode None, relying entirely on TLS. For machine-to-machine industrial traffic opc.tcp is both the more efficient and the more securable option, and the one to standardise on.

URL anatomy

opc.tcp://host:port/path - and the path is part of endpoint identity, not decoration. Omitting a path the server expects returns BadTcpEndpointUrlInvalid, which is a string mismatch masquerading as a network error. The port defaults to 4840 when omitted, but few clients rely on that; write it explicitly so log lines and firewall rules stay unambiguous.

Example

URL anatomy and the TCP message types

opc.tcp://plc01.plant.local:4840/UA/Server
   |          |             |     |
   scheme     host          port  path - part of endpoint identity

HEL / ACK   buffer sizes, max message size, max chunk count
OPN         open or renew the secure channel
MSG         service requests and responses, chunked if large
CLO         close the channel
ERR / RHE   protocol error / reverse hello

Defined in: Part 6 - Mappings, the OPC UA TCP protocol and the uatcp-uasc-uabinary transport profile.

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.