Re: [mpls] mail on ISD encodding

[Tony Li <[e-mail]>]

[WG chair hat: off]

Hi Loa,


> So you mean we should first break what is not broken, then "steal a bit"
> for the U-bit, that was there from the start?


I don’t understand this comment.  

As your original example showed, we currently cannot attach Format D LSEs after Format B LSEs.  I proposed one way of allowing that, which would hopefully make some common cases more efficient as there would be fewer cases where there would be a NOP opcode in the Format B LSE.

You objected to my proposal saying that we should have at least one reserved bit. I’m fine with that, we can simply take one more bit from the Format B data space.


> Or did you mean steal for
> the p-bit which also seems unnecessary, since the R-bit sits there in
> draft-ietf-mpls-mna-hdr, and draft-jags-mpls-ps-mna-hdr nicely allocates
> it as a p-bit? How about just leaving the bit allocation as is?


I cannot follow this at all.

In any case, I take it that you dislike my proposal. That’s fine.


> However, there is one thing about where the allocations are done.
> 
> The p-bit is one of the ISD-flags, it points at OSD, but it is really a
> piece of the ISD. Should be allocated in the draft-ietf-mpls-mna-hdr.


I disagree.  As I mentioned, in the ISD header, that should simply be a reserved bit.  It should not be assigned for PSD.  We may never use PSD. It would be poor engineering to prevent its use in perpetuity.  If we do not use PSD, then that reserved bit might be pressed into service for some other purpose.

Flexiblity in future design would seem to be an overriding principle here.


> Also the instruction (for R-bit), "set to zero when sending and ignore
> when received" is problematic. Let us forget about the R-bit and put the
> p-bit where it belongs. There should be a description of how
> a non-PSD capable node treats a packet with the p-bit set.


I disagree.  That is how we always deal with reserved bits.

There is no point in arguing about PSD and non-PSD nodes until we decide to commit to PSD.

Tony