During the operations directorate early review of
draft-ietf-rtgwg-segment-routing-ti-lfa
Gyan Mishra points to a simple pathological network fragment that I think
deserves wider discussion.
https://datatracker.ietf.org/doc/review-ietf-rtgwg-segment-routing-ti-lfa-11-opsdir-early-mishra-2023-08-25/
I am not aware of any response to the RTGWG by the draft authors concerning the
review comment and I cannot see obvious new text addressing this concern.
The fragment is as follows
CE1 –R1- R2-/-R3-CE2
| |
R4 – R5 -R6
In the pre converged network R4 is ECMP CE2 via R5 (cost 4) and via R1 (cost
also 4).
We can easily build a TI-LFA repair path from R2 under link failure to CE2 (so
long as we remember that R4 is an ECMP path to CE2), but the problem occurs
during convergence. If R1 converges before R4, R4 may ECMP packets addressed to
CE2 back to R1 in a micro loop. Meanwhile since no packets for R3 are reaching
R2 the Ti-LFA repair is not doing anything useful.
The Ti-LFA text leads the reader to conclude that it is a loop-free solution,
but gives no guidance on how to determine when this assumption breaks down.
There is an informational reference to
draft-bashandy-rtgwg-segment-routing-uloop, but this short individual draft
does little in the way of helping the reader determine when loop avoidance
strategy needs to be deployed and the loop-free approach it describes does not
seem to be fully developed.
I am worried that proceeding with the Ti-LFA draft without noting that there is
a real risk that simple network fragments can micoloop, and providing a fully
formed mitigation strategy is a disservice to the operator community given the
industry interest in Ti-LDA and the insidious nature of unexpected micro loop
network transients, I am wondering what the view of the working group is on how
to proceed.
One approach would be for the Ti-LFA draft to incorporate detailed guidance on
how to determine the risk of a micro loop in a specific operator network, and
to provide specific mitigation advice. Another approach would be to reference
a developed loop avoidance strategy and recommending its preemptive deployment.
Another approach would be to make draft-bashandy-rtgwg-segment-routing-uloop a
normative reference and tie the fate of the two drafts. Another approach would
be to elaborate on the risks and their manifestations but declare it a
currently unsolved problem. I am sure there are other options that the WG may
formulate.
What is the opinion of the working group on how we should proceed with
draft-ietf-rtgwg-segment-routing-ti-lfa when considering the possible formation
of micro loops?
- Stewart_______________________________________________
rtgwg mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/rtgwg
