1945   Need to add a note about Ethernet Frame Size

Created: 22 Oct 2024

Status: Triage

Part: Part 8-1 (2020; Edition 2.1)

Links:

Page: 179

Clause: C.1

Paragraph: 3

Issue

There is an issue that IEDs may not be able to know the effective MTU of the network (e.g. Single Attached Node) that has no knowledge of the use of PRP or HSR. Additionally, the multiple encapsulated tagging allowed by IEEE 8802-3 is unknown to an IED.

The typical MTU size is 1500 octets. Any APDU that segments to the full 1500 and then the network adds additional octets will be dropped.

Proposal

Add an editorial note that switch MTU size should be set per the recommendations inf IEC 62439-3, clause Frame Size Restriction.

Additionally, a note should be added that unmanaged switches that do not follow the IEC 62439-3 recommendation may cause frames to be dropped.