Resolving a Question about Span Restoration: Do Loopbacks Involve a Capacity Penalty?

1. Resolving a Question about Span Restoration:Do Loopbacks Involve a Capacity Penalty? Wayne D. Grover TRLabs and Dept. of Electrical and Computer Engineering, University of Alberta Matthias Scheffel Munich University of Technology, Institute of Communication Networks (Prof. Eberspaecher)

2. Motivation / Background • Span restoration (equivalently, pre-planned span protection) is one of the most longstanding options for survivable mesh networking. • Every year or so, a proposal comes forward (or a new patent) having invented span restoration with “loopback elimination.” • It is assumed obvious that such loopback elimination must save a great amount of spare capacity. • To our knowledge, however, that assumption has never been quantitatively tested. • Moreover, a qualitative argument has been published saying why loopback elimination is probably insignificant. • So, the Question is:  Do loopbacks actually use excess spare capacity in a span restorable mesh network, or not !?

3. Span Restoration Principle • Alternative path segments restore all working channels of the failed span • Local restoration between the end nodes of the failed span • Multiple restoration routes are possible per span • Restoration path segments for different spans can share spare capacity • Mechanisms equivalent to max flow or ksp re-routing

4. Occurrence of Loopbacks • The end-to-end working path is the same except for the local restoration path segment substituting the working link on the failed span • Working path and restoration segment may thus have common spans (or/and transit nodes) Example: Span restoration view Working paths Span restoration paths +

5. Occurrence of Loopbacks • Span restoration is not aware of the end-to-end working paths • In this regard, it is like the mesh correspondent to BLSR rings. • The occurrence of loopbacks depends on the assignment of restoration paths to affected working links Example: End to end path view Restoration without loopbacks Restoration with loopbacks • At first glance, the left restoration configuration seems to be more efficient than the right one

6. The Recurring Idea of “Loopback Elimination” • Nodes along the restoration path may check whether the restoration path and the working path belonging to the affected working link traverse a common span • If so, the node cross-connects the restoration path and the remainder working path • Knowledge about the assignment of restoration paths to working links and the working route requires additional signaling Loopback configuration With loopback elimination

7. Loopback Elimination • However, the loopback capacity cannot necessarily be released from the network • Optimal span restoration minimizes the spare (and working) resources in the network • Spare capacity used to form loopbacks for one span failure is required in order to recover from other span failures, too • Our hypothesis:What appears as loopback capacity for one failure is almost always required in a non loopback way for at least one other failure scenario. Span failure C-F: 10 capacity units appear to be wasted by the loopback Span failure E-H: 10 capacity units are fully utilized by restoration paths

8. Forcer-Based Reasoning and Analysis • Forcer concept • In span restoration, each span s with non-zero spare capacity c has a forcer span f whose failure yields restoration paths on s that utilize the entire spare capacity c • An optimal span restoration design usually involves more than one forcer f (co-forcers) for each span s Failure scenario f1 is a forcer of span s if in the optimal design, its restoration requires use of every spare channel on span s

9. Assessing the Loopback-Revised Spare Capacity • Step 1: Determine the number of protection paths ps,f that traverse span s in case of span failure f for all span pairs (s,f), s≠f • Step 2: Determine the number of working paths ws,f that traverse spans s and f for all span pairs (s,f), s≠f • Step 3: Calculate the number of loopbacks ls,f on span s in case of span failure f by: ls,f = Minimum(ps,f, ws,f) for all span pairs pairs (s,f), s≠f • Step 4: The loopback-revised protection capacity on span s is: cs = Maximum(ps,f1 – ls,f1, ps,f2 - ls,f2, …) over all spans f1, f2, … except span s

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11. Results: “US Network” • 44.6% of all restoration paths involve loopbacks • 1.1% spare capacity reduction in the US network

12. Results: “German network” • 50.2% of all restoration paths involve loopbacks • There is no capacity penalty due to loopbacks

13. Results: “European network” • 74.1% of all restoration paths involve loopbacks • 0.1% spare capacity reduction is possible by loopback elimination

14. Concluding Discussion • Span restoration provides alternative path segments at the end nodes of failed working links • Loopbacks arise if the end-to-end working paths and the assigned protection segment have common spans • Results show loopbacks do occur frequently • A Forcer-based analysis was used to calculate the net capacity penalty due to loopbacks • There are none or only extremely small capacity savings when eliminating loopbacks • Slight changes to working capacity values can make the savings strictly vanish as well. • What is the explanation? • As postulated, it does seem that “what is loopback capacity for one failure scenario is almost invariably needed capacity under one or more other failure scenarios.” • The “forcing” failures in the design will need all the spare capacity present in a non-loopbacked way.