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.

Discussion Created Status
When we come up with concrete numbers like "1528 octets", we should also clearly explain what is included in this number. In my notes, I find 1526 of 1530 bytes for the maximum Ethernet packet size (depending if a VLAN Tag is included or not). And as Herb mentioned, there could be multiple tags (stacked VLANs or QinQ).
E.g., Wireshark may or may not (depending on the capture hardware) include the FCS (CRC) in the packet length. This is another hint to be very specific to avoid confusion. 		09 Jan 25 Triage
Setting the MTU size can be a vendor-specific solution. The more important point is that Ethernet switches shall forward frames up to the maximum size of 1528 octets. So, the proposal should be reworded:

“Add an editorial note that Ethernet switches shall forward frames up to the maximum size per the recommendations in IEC 62439-3, clause Frame Size Restriction.”

We do not need to mention unmanaged switches. In general, Ethernet switches that do not follow the IEC 62439-3 recommendation may cause frames to be dropped. This is not specific to unmanaged switches. Unmanaged switches are also covered by the first, reworded sentence.

The references to IEC 62439-3 and IEEE 8802-3 in IEC 61850-8-1 need to be updated.

09 Jan 25 Triage

 

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