Re: Subject: I-D ACTION:draft-clausen-ipdvb-enc-00.txt

[Dr G Fairhurst <gorry@erg.abdn.ac.uk>]

Patrick Cipiere wrote:
> 
> Gorry wrote:
> 
> >> > 0x???? : Bridged Ethernet Frame
> >>
> >> I would recommend 0x6558 (Transparent Ethernet Bridging)
> >>
> 
> > What do you think the FCS should contain?
> 
> If you are talking about the ethernet FCS, this is an interesting
> question, and I think we have to make things clear about that.
> So the question is: does packets with EtherType 0x6558 (Transparent
> Ethernet Bridging) have a 4 bytes FCS?
> I have been chasing this answer for several years, and it looks like
> nobody has the answer.
> So my answer, so far, is no FCS for the encapsulated ethernet packet.
> However we should use the LAN FCS defined in the draft.
> 
> > Should the SNDU include an Ethernet padding (I assume this doesn't matter)?
> 
> I am not sure to understand what you mean. So I guess we do not need
> padding.

Just to be sure... A small payload will normally be padded to the 
Ethernet Minimum frame size - this involves sending padding bytes to
make the frame >= 64B (incl 4B FCS).

IP devices normally ignore this padding.

Some remote bridges don't do IP-level processing, and therefore
forward the payload and also the padding - particularly those devices
which also forward the original LAN FCS.

I guess if the frame has a length field (i.e. LLC) then, it's easy
for bridges to spot the padding, with DIX format frames, this is less easy.

> 
> Patrick.
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