1. Capacity Requirements for Network Recovery from Node Failure with Dynamic Path Restoration Gangxiang Shen and Wayne D. Grover TRLabs and University of Alberta Edmonton, AB, Canada (presented by Jennifer Yates, AT&T Research) OFC 2003, Tuesday March 25 2003, Atlanta, Georgia

2. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 2 Outline Path Restoration & Node Recovery Research Questions Design Models Results Summary of Findings

3. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 3 Background on Path Restoration Long recognized that a path restoration mechanism will “handle node failure as well as span failures.” But this is a statement about the mechanism,not an assurance of adequate spare capacity to permit the mechanism to realize full node recovery.  Question: How much (extra) spare capacity is needed for node recovery via a path restoration mechanism?

4. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 4 Some Background and Points about Node Recovery We consider dynamic adaptive path restoration (not preplanned backup path protection). What is the actual aim in node recovery? –It cannot be the same as in restoration of a span failure, because... –Demands that source / sink at a failed node cannot be restored by network re-routing. –  Seek to restore 100% of transiting flows through a failed node. Aside: (observations about node survivability in general) –In a sense, it is already “too late” when we rely on network re-routing in response to a node failure. –  Good backup power, fire, security, and software are the primary strategies for node survivability

5. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 5 Initial Appreciations about Node Recovery Capacity design to support node recovery has two opposing complexions: 1.It is equivalent to 2 to perhaps 6 simultaneous span failures, depending on node degree:  this suggests a lot of extra spare capacity may be needed. On the other hand, 2.demands terminating at the failed node “disappear from the problem.”  this suggests node failure problems may not be quite as difficult as it seems.  especially if “stub release” applies to the unrestorable demands

6. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 6 Concept of Stub Release Stub release (SR) refers to reuse of capacity on surviving portions of failed paths in the overall restoration effort. It is an option under dynamic path restoration. SR makes the overall response failure-specific and more efficient than using only fully disjoint predefined backup paths However, it requires fault isolation to the respective span (or opaque segment)

7. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 7 Illustrating concept of Stub release Pre-failure demands Span Failure

8. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 8 Illustrating concept of Stub release Possible Restoration / Protection with strictly Disjoint backup paths (no stub release)

9. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 9 Concept of Stub Release Possible Restoration with stub release Failure-specific re-use of surviving path segments (for same or other demands)

10. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 10 Network Recovery from Node Failure With a node failure, not only are terminating demands “not part of the restoration problem,” But in addition, with stub release such failed paths may contribute useful extra “spare” capacity network-wide.

11. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 11 Illustrating Issues in Recovery from Node Failure Pre-failure demands and Node Failure

12. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 12 Illustrating Issues in Recovery from Node Failure (a)Recovery from Node Failure without Stub release: (1)Red demand is not included in the restoration effort (2)Green demand has to take fully disjoint path

13. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 13 Illustrating Issues in Recovery from Node Failure b)Recovery from Node Failure with Stub release: (1)Red demand is (again) not included in the restoration effort (2)Surviving segments of red demand are released as equivalent-to-spare capacity (3)Green demand can take shorter replacement path

14. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 14 Specific Research Questions  What are the maximum levels of node recovery that can be achieved with no more spare capacity than required for 100% span restoration?  Call this the “Intrinsic Node Recovery” Level  How much additional spare capacity is required to guarantee both 100% node and span restoration compared to span restorability only?  How does capacity depend on the mix of services in a multiple Quality of Protection (multi-QoP) context ?  consider a mix of span-failure survivable (Rs) and “node plus span” - failure protected (Rs+n) service assurances.

15. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 15 Optimization Models to Study these Questions 1.Design for 100% span and node failure restoration -- minimize total spare capacity cost while Guaranteeing 100% span failure restoration and 100% transiting flow node failure restoration 2.Design to support Multi-QoP -- Extends the first model to accept a mix of: (1) Best-efforts only (R 0 ) class (2) “Rs” class and (3) “Rs+n” class services 3.Maximal node recovery under spare capacity budget -- accepts a budget total limit on spare capacity -- asserts 100% span restorability (required for feasibility) -- maximizes the node failure restorability given total spare capacity limit Stub Release option: Each model has versions with and without stub release

16. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 16 Test Case Results NetworksARPA2NSFNETSmallNetCost239 Level3 Intrinsic node recovery No stub release91.35%99.84%85.30%78.89%95.59% Stub release88.43%99.60%89.40%82.85%99.998% Redundancy increase (R s+n vs. R s ) No stub release10.0%0.02%2.6%3.4%4.1% Stub release9.7%0.03%2.8%1.4%0.1% Total cost inc. (R s+n vs. R s ) No stub release5.2%0.01%1.7%2.4%2.0% Stub release5.3%0.02%1.9%1.0%0.001% Five test networks Uniform random 1..20 demands on each O-D pair -> lots of transiting flows Costs proportional to distances. Rn of networks designed only for Rs=1 (very high on average) Added % spare capacity to strictly assure both Rs = 1, Rn =1 (very little on average)

17. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 17 Results (2) Node restorability versus total budget allowance for spare capacity (relative to Rs=1 design) In prior table and here we see that SR cases approach Rn =1, more slowly than non –SR case. !!?? …Reason is that non- SR designs for span restorability only had more spare capacity to begin with.

18. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 18 Results (3) Spare capacity increase required to support different percentages of (R s+n ) services -> Depending on network, 30 to 60% of services could be given “Rs+n” service assurance with no extra capacity.

19. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 19 Summary of Findings Very high levels of node recovery are intrinsically feasible in networks using path restoration in networks designed nominally for only span restorability. High levels of premium (“node and span failure assured resilience”) service guarantees can be supported without any penalty in terms of added spare capacity. Stub release is an important advantage of dynamic adaptive path-restorable networks in achieving the highest overall availability if we consider node recovery or multiple span and node/span combined failures.

20. Capacity Requirements for Network Recovery from Node Failure with Dynamic Path Restoration End (Thanks Jennifer ! )

21. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 21 An Example: NSFNET Spare capacity optimized for single span failures

22. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 22 Preliminary Look at counteracting effects (NSFNet) This can give an a priori indication of which node failures may have the most / least severe effects. Demand matrix Demand “relief” Spare capacity loss from node outage

23. Gangxiang Shen & Wayne D. Grover OFC ‘03 Atlanta 23 Understanding why Node Recovery takes so little extra capacity Path Restoration of the span failure Node Failure affecting the same two demands Green path still needs restoration Red path is (necessarily) abandoned