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



Igor Bryskin wrote:

dimitri, see in line.

----- Original Message ----- From: "dimitri papadimitriou" <dpapadimitriou@psg.com>
To: "Igor Bryskin" <ibryskin@movaz.com>
Cc: <dimitri.papadimitriou@alcatel.be>; "Arthi Ayyangar"
<arthi@juniper.net>; <ccamp@ops.ietf.org>
Sent: Thursday, February 17, 2005 11:40 AM
Subject: 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 -



IB>> Disagree - the segment could be a component link within a bundle. In
this case stitching FA TE link may accomodate multiple LSPs

 additionally only nodes part of the same

area can make use of this advertisement - 


IB>> Who said that sticthing segments must necessarily terminate on domain
borders? There could be multiple reasons why a network operator could
pre-provision (dynamically or statically) LSP segments inside his network
and advertise them (as bundles or individually) as TE links to be used for
specific TE purposes.


it is exactly these purposes that i am looking for

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 ?



IB>> See my comment above. I might want to use statically provisioned
segments. I might want to use nodes that do not have proper signaling
software.


what does that mean "proper signaling software"

For instance, recall the discussions on P2MP and how we want use legacy LSRs
to be part of P2MP tunnels

but in this case there is a flag telling capabilities of the nodes in order to allow for dynamically trigger the segment

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 ?


IB>> Again, if you imagine that several parallel sticthing segments are
advertised as as single FA, how it would be different from the bandwidth
usage point of view compared to advertising lower layer FA ?

issues are different - FAs are used in manner to preferentially attract over them because - i am still looking for the reason for attracting over a bundle

In fact it would be even more useful, because in case of lower layer FA you need also
to advertise termination/adaptation capabilities, while in case of stitching
FA no addaptation is required.

by the way you don't seem to see the issue that i am pointing out, so probably there is a need to go in more detailed examples before drawing the above confusing conclusion

Igor

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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