Re: Diverse path failure and optimality in draft-dachille-inter-area-path-protection

[Dimitri.Papadimitriou@alcatel.be]

hi, by discussing the proposed method there seems to be three issues 
that make the method questionable in terms of guarantee to deliver what 
it intends to deliver, its usability (the time validity of the path is 
not guaranteed) and its applicability in terms of objective initially 
targeted wrt to the network topology

1) imagine three areas decoupled computation as explained at their
respective ingress, with ARO method; how the third computation element
(tail-end) is aware of srlg's that may affect a link selected in the
head-end area

example: link 1 is selected in area 1 (head-end) with srlg 1, link 2 is
selected in in area 2 with srlg 2, and link 3 in area 3 (tail-end) with
srlg 3 and 1 (since the tail end doesn't know that srlg 1 is associated
to link 1 in addition to its association to link 3 even if it knows that
the link 1 has been selected for the ARO) the problem is that you can
not retrieve such kind of error (except but how practical is it if one
start cumulating all this information between computation points)

2) resource contention, the secondary path may never be established
since the computation point as *no* capability to make any reservation
on it (except from the first segment) since by definition "disjoint" -
it simply becomes a kind of "best effort" secondary path (in the sense
use it if no other reservation are making use of these links)

3) the method seems to raise additional issues when the number of 
potential entry point for the secondary disjoint path increases, at each 
step of the computation (otherwise the method wouldn't provide what it 
intends to deliver)

thanks,
- dimitri.


ugo monaco wrote:

> Dear ccamper, Zafar, Dimitri,
>
> as anticipated in the recent email by Vishal we are addressing several 
> important comments collected at Seoul regarding the joint-selection of 
> diverse paths with ARO, i.e. the approach proposed in 
> <http://www.ietf.org/internet-drafts/draft-dachille-inter-area-path-protection-00.txt>. 
>
>
> As Vishal did I summarized your comments to ensure that we rightly
> understood inputs, and to help people on the ML follow
> and contribute to the discussion.
> Comments from others who have feedback are welcome, and
> much appreciated.
>
> Please let me know if you had any additional comments
> as well. We will take these into account in providing our
> responses, and, later, in updating the document.
>
>
>
> Thanks again for your feedback on our draft during the Seoul meeting.
> Best Regards.
>
> Ugo Monaco
>
>
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 
>
>
>
> Zafar's questions:
>
> i) What happens when the setup of the diverse path fails, or there is a
> failure on it after it has been set up?
>
> ii) Relationship of this scheme to PCS/PCE approach of JP?
>
>
> Dimitri Papadimitriou
>
> i) Your question was about why we are trying for optimality with a joint 
> path selection scheme, when it is not possible, especially as the number 
> of AS's or areas along a path increase, to achieve the *global* optimum.
> Your other comment was that we should mention this somewhere in the 
> document.
>
> Please note that in response to this last question Fabio Ricciato listed 
> many
> advantages of a joint computation (with ARO) of inter-area/AS paths in a 
> prevoius email posted on the ML "About optimality of inter-AS parallel 
> computation in draft-dachille-inter-area-path-protection".
> We hope that Fabio rightly addressed your input and we will appreciate 
> further comments and notes.

--
Papadimitriou Dimitri
E-mail : dimitri.papadimitriou@alcatel.be
E-mail : dpapadimitriou@psg.com
Webpage: http://psg.com/~dpapadimitriou/
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