[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

igor - see in-line

Igor Bryskin wrote:

Dimitri, Arthi,
See my comments in line.

Igor

----- Original Message ----- From: "Arthi Ayyangar" <arthi@juniper.net>
To: <dpapadimitriou@psg.com>; <dimitri.papadimitriou@alcatel.be>
Cc: <ccamp@ops.ietf.org>
Sent: Wednesday, February 16, 2005 9:08 PM
Subject: Re: I-D ACTION:draft-ayyangar-ccamp-lsp-stitching-00.txt



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 ? 

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"

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 -

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





.