Ahmed,
Let us consider this "fiction".
RFC4202 section 2.3 defines a shared risk link group as:
"A set of links may constitute a 'shared risk link group' (SRLG) if they
share a resource whose failure may affect all links in the set."
The key work is MAY.
This means that there is some probability that all members of the SRLG
failed, and that there is some other probability that they did not all fail.
In your draft you calculate your repair action on the assumption that
they all failed and assert that this will be congruent with the post
convergence path.
However from the definition of SRLG we can see that they may not have
all failed, in which case some of the not failed elements of the SRLG
will normally remain in service, and may provide a better path than the
path avoiding all elements of the SRLG. If this is the case the
calculated repair path may not be congruent with the post convergence path.
Thus using the standard definition of SRLG this fundamental assertion in
your proposal seems to me to fail.
You could introduce and alternative definition of SRLG:
"A set of links may constitute a 'shared risk link group' (SRLG) if
they share a resource whose failure will effect all links in the set."
However in practice this definition can only be true if on seeing the
failure of a single element of the SRLG you take all elements of the
SRLG out of service. This seems wasteful of resources, but if that is
what you mean your draft needs to specify this definition and action.
Alternatively your draft needs to state that full congruence may not
occur and analyse the consequences of this.
Regards
Stewart
On 15/08/2017 17:23, Ahmed Bashandy (bashandy) wrote:
Stewart
If you think that there are other problems that needs to be addressed,
do not attempt to push it down the throat of our draft. Instead put
out your own proposal. But make sure that it has enough merit to
convince the WG that it is better or more comprehensive instead of
attempting to point out fictitious problems in others' proposals.
Ahmed
On 8/7/2017 1:34 PM, Stewart Bryant wrote:
Ahmed,
The WG decides what is in or out of scope for a WG draft, and it does
this via the rough consensus of the WG, not the view of the authors.
Of course if you wish to refocus this as an independent draft and
submit via the ISE. If you do, you are welcome and I will leave you
to it.
Meanwhile the draft really has to discuss SRLGs are they are in real
life, not as you would wish them to be.
Another type of false SRLG btw is when you are doing node protection
(you normally treat a node as an SRLG), but only a line interface has
failed.
- Stewart
On 07/08/2017 21:04, Ahmed Bashandy (bashandy) wrote:
Stewart
I already replied to Sikhi explaining the concept of the SRLG used
in this draft and the intent to make it even clearer.
IMO the scope of the draft is very clear from the draft itself as
well as the numerous responses during the previous IETF and the
mailing list.
The issue below is **out of scope** of the draft and hence I have no
plans on addressing it.
I hope you don’t insist on pushing out-of-scope topics down the
throat of this draft :)
Ahmed
*From:*Stewart Bryant [mailto:[email protected]]
*Sent:* Monday, August 07, 2017 12:48 PM
*To:* Ahmed Bashandy (bashandy); Stewart Bryant; Sikhivahan Gundu;
[email protected]
*Cc:* [email protected]; [email protected]
*Subject:* Re: I-D Action:
draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
Your answer did not address the issue below, which is one of a class
of issues related to SRLG.
- Stewart
On 07/08/2017 19:23, Ahmed Bashandy (bashandy) wrote:
See my reply to Sikhi
Thanks
Ahmed
On 8/7/2017 2:13 AM, Stewart Bryant wrote:
On 07/08/2017 06:45, Sikhivahan Gundu wrote:
By “ambiguity”, I meant that backup calculation taking
SRLG into
account is based on speculated topology, whereas
computation of
post-convergence path, ie, SPF, is based on actual
topology. This
seems needs reconciling since in TI-LFA the backup is
by definition
the post-convergence path, with a single path-transition
after
link-failure as the intended outcome. Do I understand
correctly that
the draft prefers to relax that expectation for SRLG?
Yes, that is a good point, in the event of an incomplete failure
of an SRLG, there may not be congruence between the
FRR path and the post convergence path. This certainly
needs further study.
*
A--------//---------B
| |
| * | cost 2
C-------------------D
| |
| | cost 100
E-------------------F
AB + CD in same SRLG
TiLFA path is ACEFDB
Post convergence path is ACDB
In this case I think that the impact is just more SR hops in the
repair path than might be needed without the SRLG, but we do
need to
be sure that there are no pathological cases in
topologies that lack the proposed congruence, and as
Sikhivahan notes this effect does need to be clarified in the
text.
- Stewart
_______________________________________________
rtgwg mailing list
[email protected] <mailto:[email protected]>
https://www.ietf.org/mailman/listinfo/rtgwg
_______________________________________________
rtgwg mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/rtgwg
