Re: [mpls] Comment on draft-gandhi-mpls-ioam-sr-05

Hi Stewart,
FYI:
I believe Section 7 in latest revision (06) addresses this comment. Welcome
your feedback on that.
https://datatracker.ietf.org/doc/html/draft-gandhi-mpls-ioam-sr-06

Thanks for your review.
Regards,
Rakesh

On Tue, Jan 12, 2021 at 9:24 AM Stewart Bryant <[email protected]>
wrote:

> There are three problems, how you encode the action and where you place
> the metadata, and how you accommodate anything else after BoS.
>
> This is getting very messy and we need to solve this holistically or we
> will damage the future of the MPLS protocol.
>
> The best place to indicate HxH is at ToS since this is looked at by every
> hop with minimum effort.
>
> This could be via a FEC or an SPL. However I do not like the SPL approach
> because you probably need the EL to neutralise ECMP and that is four labels
> in the stack  before you even get to specify the destination. That is as
> many labels as some LSRs can cope with. You could I suppose have an iOAM
> SPL that specifies the ECMP behaviour but we are loosing our generality if
> we go down that route.
>
> The best place to indicate E2E is at BoS since this is where you handle
> the E2E iOAM. That can be an ordinary label, and since you have to
> advertise E2E iOAM capability it is not much more to specify which label
> you expect to see BoS to indicate its presence.
>
> Now you have two choices to advertise both, - the ToS stuff which you do
> not PHP + the BoS, or one of two different BoS labels. The use of two
> different BoS labels is far less problematic since that is only 2 out of
> million available labels and you need the label in place to indicate that
> the packet carries E2E iOAM and not just plain IP … except that it may not
> carry plain IP...
>
> Now here is what worries me.
>
> What if the LSP is carrying a PW, or is DetNet? What if it is a MS-PW? In
> all these cases there is a CW immediately after BoS. Then there is the
> universal fragmentation idea that is floating about that also wants to
> follow BoS.
>
> How do we fit in 2, possibly 3 or 4 sets of descriptor data after the BoS.
> This has the possibility to get very messy very quickly.
>
> So we need to go up a level and consider how we generalise this, because
> iOAM needs to live with the rest of the MPLS ecosystem, and will not be the
> only candidate for BoS meta data. That is not to say that I have anything I
> want to put there that I am being quiet about, instead it is to point out
> that once the first genie is out of the bottle a whole bunch of their
> friends will soon follow behind. So we have to solve this elegantly such
> that we can deal with the MPLS extensions we have not yet thought of and
> not be tempted to just hack something in that works for the immediate case
> in hand.
>
> - Stewart
>
>
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