Table 1 defines a recommended usage of priorities for various types of traffic in networks containing UAFX Stations. Combined with the traffic class mapping priority defined in 6.4.3, these recommendations provide consistent, system-wide behaviour for networks that do not currently use the recommended priority values for other purposes.
Table 1 – Recommended PCP/DSCP usage
|
PCP |
DSCP |
Recommended usage |
|
7 |
–1 |
Reserved |
|
6 |
–1 |
Reserved |
|
5 |
48 |
Network control2 (gPTP, LLDP, loop prevention) |
|
4 |
46 |
Green4 (UA PubSub, see 6.5.4) |
|
3 |
–1 |
Reserved |
|
2 |
16 |
Connection establishment (UA Client Server3), remote management2 (NETCONF) |
|
0 |
0 |
Best Effort |
|
1 |
8 |
Background |
|
NOTE 1 The entries in the table are in the order of effective priority of the packets on the network and not in the order of priority values (PCP and DSCP). NOTE 2 IEEE Std 802.1Q2022, I.4 explains the rationale for inverting PCP 0 and 1. PCP of 0 is used for default priority and Best Effort. Tagging a frame with PCP 1 allows explicit indication of traffic that can be treated with a lower priority than Best Effort. This same rationale applies to the default Priority to Traffic Class Mapping in 6.4.3. NOTE 3 The DSCP values for Network Control, Best Effort, and Background have been selected from IETF RFC 4594, Figure 3. NOTE 4 The DSCP value for GREEN is selected from the IANA DSCP Pool 1 Codepoints registry value for Expedited Forwarding defined in IETF RFC 3246. NOTE 5 The default PCP and DSCP values for any type of traffic without explicit priority configuration are equal to Best Effort. |
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|
1 Additional DSCP values may be defined in a future release of this document. 2 Currently, there is no OPC UA means for the assignment of priority information for these services. This is expected to be added in a future release, e.g., through IEC/IEEE 60802. 3 The OPC UA Information Model currently does not allow the assignment of priority information for UA Client Server messages. A future release is expected to allow UA Client Server QoS configuration in addition to UA PubSub. 4 “Green” corresponds to the OPC UA PriorityLabel opc.qos.lbl://green, which this release of the document supports for OPC UA PubSub-based exchange of control data with UAFX. |
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Figure 5 shows an example of an application running on two Controllers using different priorities on a network. One priority carries OPC UA PubSub messages with an elevation to PCP 4, and another carries all other messages in the same lower priority, including OPC UA Client Server.
