Re: Link type proposal, IEEE 802.1Qbu Frame Preemption Protocol

[Guy Harris <guy () alum mit edu>]

On Nov 30, 2017, at 1:42 AM, Anton Glukhov <anton.a.glukhov () gmail com> wrote:

> I've been working for IEEE802.1Qbu Frame Preemption Protocol and now it's part of TSN set of protocols. I would like 
> to propose the new link type and sent this proposal to tcpdump-workers () lists tcpdump org, but unfortunately no 
> response.

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> I'm working for Frame Preemption Protocol(IEEE 802.1Qbu), which is part of Time-Sensitive Networking(TSN) set of 
> standards. The protocol has very interesting extension of standard Ethernet frame and I'd like to propose to add the 
> new link-layer type. Here are the reasons:
> - protocol uses non-standard ethernet preamble, user must see the type of frame/fragment

LINKTYPE_ETHERNET frames don't include the preamble, so a preempting frame with a different preamble would look, in an 
LINKTYPE_ETHERNET capture, exactly like a non-preempting frame.

Are you saying that you want a variant of LINKTYPE_ETHERNET, in which the MAC header is preceded by something that 
indicates whether this is a regular or a preempting frame, so that captures with this link-layer header type can 
include both regular and preempting frames?

> - protocol uses specific CRC sum. It's called mCRC and depends on preamble, which means the type of frame/fragment

LINKTYPE_ETHERNET may, but aren't guaranteed to, include the CRC; to properly check the CRC, the additional metadata 
giving the frame type would be required, so it knows whether it's a regular or modified CRC.
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