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

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

On Dec 1, 2017, at 12:56 AM, Anton Glukhov <anton.a.glukhov () gmail com> wrote:

> Correct. I think that CRC(or mCRC for preemption) is also important and I would like to add it too. So finally we 
> have:
> Preamble with SFD | Mac header | Payload | CRC/mCRC

OK, so I guess the idea is that the payload of this link type is what IEEE 802.3br calls an "mPacket" as shown in 
Figure 99-4, always containing, in order:

        either a 7-octet 0x55 preamble followed by a 1-octet 0xD5 SMD-E/SFD or a 6-octet 0x55 preamble followed by 
another SMD value from Table 99-1 followed by a FRAG_COUNT with a value from Table 99-2;

        the MDATA of the mPacket;

        the CRC field, containing either an FCS or an mCRC.

If so, then the first of those contains the metadata necessary to determine what type of mPacket this is.

I also infer that if an "ordinary" Ethernet packet were encapsulated in this fashion, it would look like an "express" 
packet, given that the SFD for "ordinary" packets is 0xD5, which is the same value as an SMD-E, and that the SFD for 
"ordinary" packets is preceded by 7 octets of 0x55 preamble.  This would mean, however, that "ordinary" traffic would 
look like "express" traffic; would that be misleading?
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