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

igor - see in-line

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 ?


IB>> Completley agree with Arthi. Besides, several parallel stitching
segments could be advertised as a single bundle - hence, using the
advertised link by one LSP does not necessarily take away all link's
bandwidth.


you don't understand the question, it is do we have to consider as
default behavior that a pre-provisioned is to be "advertized"


IB>> My point was that I do not see any difference in this respect between
the sticthing FA-LSP (the same layer FA-LSP) and FA-LSP created in the lower
layer. Besides, what do you mean by the default behaviour? The fact whether
to advertise//remove FA TE link is a policy driven carefully thought through
decision, a dnagerous one that could potentially destabilize the network.
I'd say that the default behaviour is "NOT ADVERTISE" in either case.

now beside the fact that there are techniques to do so what would be the
purpose of it ? and what it the overhead that such advertisement would
create - that can be of course decreased by bundling them - 

IB>> The purpose is exactly the same as for any other FA-LSP - add
flexibility in a particular layer.

which flexibility are we expecting here, this "segment" can accommodate exactly one incoming request - additionally only nodes part of the same area can make use of this advertisement - so in fact what it would allow is the possibility to avoid creation of a segment if the edge node receiving the request re-orients its request to the head-end for this advertisement
|
example: ----------D----------
| |
- A ---- Segment 1 ---- B -
| |
----------C----------
|

you would have a segment between A-B that could be reached from C (the node receiving the incoming request) decides to make use of this segment to reach B (so you would have C-A-B) but if this was the best path why not creating directly a segment C-A-B, instead of now having one segment C-A, the pre-provisioned A-B and probably one on top of it C-A-B ?

in case of classical FA-LSP it makes sense to advertize the FA link because it represents a lower region LSP (with usually a given ratio of unreserved bandwidth that makes worth advertizing the FA link) but in case of a segment i do have some difficulties to excatly see where this flexibility would deliver ?

thanks,
- dimitri.

thanks,
- dimitri.


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.



IB>> I have a comment here. LSP-Hierarchy is not a Bible and could be
challenged in many ways. FA LSP is, generally speaking, created on a


layer

boundary rather than on region boundary: nothing prevents creating a VC4 

FA

LSP that starts and stops in the middle of TDM region to carry several 

VC12

LSPs. Furthermore, stitching FA is a special case of FA when it is used 

by

LSPs of the same layer as one where the FA-LSP was created. As for 

triggers,

there could be multiple ones for setting up/tearing down stitching 

FA-LSPs:

configuration, receiving setup request for inter-domain LSP, other 

policies.

Igor





More on a) later.

thanks,
-arthi





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