[mpls] Re: Potential MNA technical issue

[Stewart Bryant <[e-mail]>]

Hi Greg

I did not see an attached slide.

The way I have always viewed this is as follows

As specified a PW expects the CW or PW payload immediately after BoS and anything else in there is going to lead to incorrect behaviour. Therefore bad things happen if PSD arrives unannounced.

However if the PW component of the peer PE puts the PSD there it in effect becomes a new PW type and the two PEs will have agreed to that and the egress PE will process it appropriately. In this case the new PW designers can choose where to put the PSD - before or after the CW - so long as they avoid the DPI ECMP problem by appropriate setting of the first nibble. I don’t see why PW imposed end to end PSD would require an ISD indicator since we have PW signalling. 

If we were doing EVPN, then I would assume that we would include an indication of the presence of PSD associated with the payload in the EVPN signalling.

If the PSD was imposed by the delivery service, i.e. it is part of the delivery LSP, then this will be known by the ISD indicator mechanism. This means that the element of the egress PE responsible for LSP disposition needs to process the PSD and logically to recreate a clean packet to hand to the PW processor. In other words it has to remove the ISD indicator and the PSD before handing the packet over for PW label lookup and processing. Of course I doubt that many would implement it by strict removal and reconstruction and that the removal would be a logical removal. This optimisation is of course possible because the receiving PE knows that it just processed the PSD and it therefore knows to change its PW processing to find the CW or payload in a different place.

To make PSD and PWs work together requires engineering, but I see no showstoppers.

Stewart

> On 21 Feb 2025, at 22:05, Greg Mirsky <[e-mail]> wrote:
> 
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> Hi Loa,
> I presented the attached slides on the PSD implications for DetNet and
> BIER. Could you point to your source for alternative interpretations of the
> RFCs that defined them?
> 
> Regards,
> Greg
>