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

igor,

For the second time you are saying that I don't see the issue that you are
pointed out and this is beginning to annoy me.
> 
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 ?

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"

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 -

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)

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)

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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