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

igor - see in-line:

[snip]

Your point/question is very simple, which is what does it give to
advertise stitching segments as FAs, is that correct?

My answers are very simple either:

1) How else are you going to use statically provisioned segments as part
of dynamically provisioned end-to-end LSPs ?


you ask a different question from the initial issue because advertize a
TE link was possible today without using stitching, however this issue
also depends what do you mean by statical provisioning ?

IB>> I have segments that are provisioned/owned by the management plane

exactly - there is no *LSP stitching* anymore - we discuss about control plane operations (the intermediate segment is an entity owned by the control plane)

note: by the way, operation you are referring to is supported today (without the procedures described as part of this document because this has nothing to do with stitching)

2) How else you can accomplish a transition strategy when some of the
nodes that are going to be involved in end-to-end dynamically provisioned LSPs
do not have proper signaling software?


and i did ask what is the point in applying LSP segment signaling on
segment that "do not have proper signaling software"


IB>> The point is that transit nodes of the segments do not participate and
hence do not have to support e2e signaling

well i think we should then determine the working assumptions here, intermediate nodes part of an LSP segment are GMPLS nodes as any other, otherwise the segment could itself not be setup (stitching is just a way to leave control to the head-end of the segment but there are no specific assumptions made in terms of signaling capabilities for the intermediate nodes, they are assumed to be GMPLS signaling capable nodes as any other)

The specific example is using LSRs that
do not support p2mp signaling as transit nodes in p2mp LSPs. Your point
about advertising of node capabilities is relevant only for routing -
the PCE computing p2mp tree may take in consideration the advertised node
capabilities and assume that stitching segments may be created
dynamically if needed.


just to be clear the PCE discussion is completely outside of the present
discussion - afaik the working group has (still) to provide requirements
for the PCE wg -


IB>> Just to be clear. You mentioned the use of advertised node
capabilities. My point is that these advertisements are useful for path
computation but are of no use for e2e signaling

and so just to be clear if they are not useful for e2e signaling what is the purpose of advertising them for this purpose ?

What if I don't want them to be dynamically created and rather
have them pre-provisioned to spead up the process of p2mp tunnel setup
and decrease blocking probability?


not sure that compared to the global complexity of computing the tree
this is going to be a significant improvement, but if you have the
possibility to determine the bandwidth on the corr. segments i don't see
here how you decrease the blocking probability (afaik propagation of
segment related information is equivalent to the corresponding links
advertisement)

IB>> Dynamic establishment of the stitching segments may fail

not sure to catch your point, if you think a p2p LSP establishment may fail not sure it is wise to start thinking about p2mp LSP establishment - on the other side pre-provisioning of the segment if not guaranteed within certain limits can not guarantee that you would be able to stitch it afterwards (refresh, etc.)

3) What if I want to attract certain LSPs  to use pre-provisioned
segments even if the overall cost of such LSPs would be higher than if they would
take paths computed without consideration of these segments, but it is
desirable for the LSPs to use the segments anyway. Exanple: the segments
are provisioned to satisfy wavelength continuity constraint and I don't have
PCE that can perform path computation with such contraint on path segments
between points of OEO.


as said above it would desirable to leave PCE aspects outside of the
picture, but this application is really interesting because on one side
i thought we had the label set to solve this issue (note: that even if a
path is pre-computed explicit label control can be used to provision the
segment between converting points - don't PCEs for this)


IB>> First, this is just an example of the situation when I want
pre-provisioned segments to be used in e2e LSP provisioning.
Second, satisfying the wavelength continuity constraint takes much more
than just using the label set. You need to have:
a) information about all lambdas in all isles of transparency;

this may be the case but once known explicit label control on each link is enough to assume continuity

b) very complex PCE that is capable of computing optimal path(s) that use
the same wavelength on all links within each OEO-OEO segments satisfying all
other optical constraints.

PCE, again PCE, but the computation of such path was possible well before even the term did exist - you mix the computation with the access to the computation result -

It is much simpler to pre-provision such OEO-OEO segments and advertise them
as TE links, than a simple PCE would do the job.

it is the term "pre-provision" that causes the main issue here, one dimension of the problem is simplified but how many are getting complexified ? and where is the evaluation of this trade-off ?

Igor


on the other hand is how much possible pre-provisioned segments - so
overhead - are now going to be advertised if the traffic demand is
unknown ? and on the other hand, if all traffic matrix is known
beforehand is there any rationale to create these segments instead of
directly provisioning

it is interesting to see that you are populating this discussion with
PCE application - i would be more in favor in leaving the possibility of
advertising segments once a clear set is being identified rather than
the other way around


Igor





----- 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 1:19 PM
Subject: 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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