Re: I-D ACTION:draft-ayyangar-ccamp-lsp-stitching-00.txt

[Arthi Ayyangar <arthi@juniper.net>]

Hi Dimitri,

Please see my replies (AA--->) inline.

> > ---------> An LSP segment may be created either by configuration or due
> > to arrival of an e2e LSP setup request itself. Similar to an FA, an LSP
> > segment may or may not be advertised as a TE link. But, if pre-created, it
> > could be advertised, in which case other nodes may compute a path over it.
> > Why would you want to or not want to advertise an FA ?
>
> i understand the point on pre-created <-> advertised but this knowledge
> may be useful for nodes part of the same area (not for nodes external to
> this area)
AA -------> Absolutely ...this definitely cannot be advertised outside the
area (domain). I think this has been explicitly mentioned.

so in case a node for inst. advertises three terminating
> links with PSC-2 (one of these being the LSP segment) then a another
> node (part of the same area) receiving an incoming multi-area PSC-2 LSP
> request may start making use of this segment to join the next border,
> therefore advertisement of the LSP segment may create a multi-hop
> condition, but now once used relevance of the existence of the segment
> is not a useful information (for the area) as there is no possibility to
> make re-use of it except when the end-to-end LSP is torn down
AA----------> I understand your point that once an LSP has been admitted
into an LSP segment it is no longer usable by other nodes in that area.
But would you rather stop advertising the link at this point, if you were
previously advertising it ? Don't you think that is a big hammer ? E.g.
how would a head end which has indeed computed a path over that LSP
segment differentiate this event from an LSP segment down event where the
link is deleted from the database ? So, all the document says today is
that you set the unreserved bw on the LSP segment to zero. The idea is to
still let other nodes know that the link exists but is unusable. It is
not different from a FA-LSP being consumed...in that case we don't stop
advertising the FA (if we were doing so previously), right ?

> a more technical point is related to the definition of an FA LSP which
> per LSP-Hierarchy mandates crossing LSP region border: the head-end and
> tail-end switching capability represent the SC of the resulting TE link
> while intermediate node terminates the SC corr. to the switching type of
> the FA-LSP (e.g. creation of a [PSC-1,PSC-1] link throughout a PSC-2
> capable network with first and last link being [PSC-1,PSC-2] and
> [PSC-2,PSC-1], resp.), while in the LSP segment case we would have now
> the creation of a [PSC-1,PSC-1] link with first and last link being
> [PSC-1,PSC-1] and [PSC-1,PSC-1], resp. so there is no region border
> crossing anymore - so here the question is about definition and
> detailing the triggers
AA--------> As far as trigger for setting up an LSP segment is concerned,
I agree that there is no longer the notion of "crossing region
boundaries". I realize that the document doesn't discuss this, especially
given that we are doing other comparisons with FA LSPs. So, I will add
this discussion in the next revision. I think in case of LSP segment the
trigger for LSP segment setup would come from a) successful switching type
and switching capability match and b) some local policy on the node which
dictates the setting up of an LSP segment.

More on a) later.

thanks,
-arthi