EE 681 Fall 2000 Lecture 16

1. EE 681 Fall 2000 Lecture 16 Mesh-restorable Network Design (3) Introduction to “path restoration”and path-restorable capacity design W. D. Grover, October 31, 2000

2. “path restoration”: what we mean • The set of working paths severed by a span cut are restored by establishing a set of replacement paths end-to-end,simultaneously, between each O-D pair affected. • The replacement paths are formed on-demand using only shared spare capacity (and possibly released working capacity (stub release).) • There is no dedicated reservation of a 1-for-1 backup path for each working path. • Path restoration is equivalent to abandoning the damaged pre-failure paths entirely and rapidly re-provisioning new paths end-to-end. • Path restoration distributes the impact of failures and the recovery effort more widely over the network as a whole and therefore generally permits greater efficiency in sparecapacity design. • The capacity design and real-time restoration problems for path restoration are considerably more complex than span-restoration • the fall-back to each O-D pair creates a capacitated multi-commodity max-flow problem.

3. Pre-failure Comparative illustration of span versus path restoration (b) Net43

4. Failure occurs Comparative illustration of span versus path restoration

5. First look at a span restorationreaction … (1) • example only : Exact routes depend on working and spare capacities Span restoration reaction

6. Loopback / backhaul Demand substitution occurs Loopback / backhaul A span restoration reaction …(2)

7. Failure occurs Now view a path restoration reaction...

8. A path restoration reaction …with “stub release” (1) Stub release

9. A path restoration reaction …with “stub release” (2) restoration

10. Notes about stub release in path restoration • Stub release is an option / issue which does not exist in span restoration. • From a capacity design standpoint it is preferable to have stub-release. • From an operational viewpoint stub release complicates things: • a means of automatic signaling needed to rapidly release the surviving working “stub” capacities, • AIS (Alarm inhibit signal) usually serves nicely for this, however • after physical repair, the reversion process is more complex. • Ironically, without stub release, a reserve network capacitated to support span-restoration may not be restorable under path restoration ! • Class: Can you think why?

11. Optimal Capacity Design for path- restoration Approach that follows is to first develop a “master formulation” that can model joint / non-joint designs and cases with / without stub release, then discuss modifications for each special case.

12. Variations and options within the master formulation for path restoration • The “master formulation” for path-restoration allows for: • modularity • joint optimization of working path routing • stub release or non-stub release • The master formulation requires as inputs: • point-to-point demand • a set of eligible distinct working routes for every (O-D) pair, r • a set of eligible distinct restoration routes for every (O-D) pair, rfor each failure scenario i . • It solves for: • the amount of working flow on each working route for each O-D pair(working flows may be split over several routes) • the working, spare, (and module) capacity totals on each span • the composite restoration path-set for all affected demands in each failure scenario Note: in span restoration this isfor every span, here it is for every OD pair.

13. No. of operatingworking and sparelinks (channels)on span j Set of all point to pointdemand quantities, indexedby r amount of demand on relation r No. of modules oftype m to installon span j for min cost Set of all spans betweenmesh cross-connection points Set of eligible working routes for relation r Encodes routes in= 1 if span j is in qth routefor relation r Parameters and variables in path-restorable capacity design(in the master formulation)* Input data Design output variables Set of eligible restoration routes for relation r upon failure i. Cost of mth modulesize on span j. Capacity of mthmodule size = 1 if span j is in pth routefor relation r upon failure i Intermediate (internal) variables Stub release quantity on span j from failure i Working andrestorationroutingsolutions Amount of demand loston relation r forfailure i N.B. “relation” = “OD pair” * some variables become pre-computable parameters in the variations that follow

14. Orientation to the path restorable design context(variable and parameters) eligible restoration r X i r routes for A-B after = 2 P i the failure of span i, B D C r,p f i span i working route for A-B A = A-B r r,q g r 1 Q =

15. Master formulation for path-restorable capacity design Cost of modules of all sizes placed on all spans S. t. (1) All demands must be routed Working capacity on spans must be adequate (2) Defines the amount of damaged working flow for each relation under each failure scenario (3)

16. With stub release Without stub release Master formulation for path-restorable capacity design (2) Restorability of working flows for each relation (4) Spare capacity on spans must be adequate(see note on stub release) (5) (6) Modularity of installed capacity (7)

17. Understanding how the formulation effects “stub-release” Relation r O D Route q Working flow g r,q Other span j Failure span i Span j enjoys a stub release “credit”of spare capacity = g r,q for any failureon span i such that:

18. Variations and options within the master formulation • If integer (“ideal”) but non-modular capacity is desired: • change objective function to cost-weighted sum of spares (and / or working, if joint) • drop set M (the family of modularities), variables and constraint (7) • If non-joint design is desired: • drop (1), (2), (3), and (6) • pre-compute all and as input parameters based on the pre-defined routing • pre-compute all stub-release quantities according to (6) • If stub-release is not desired: • drop (6), i.e., set all = 0

19. A route-generating method for path restorable design formulation • For each OD pair relation: • generate the set of “all distinct routes” between O-D with effectively unlimited hop limit, e.g, H ~ 3/4 |S| and store in a (large) temporary routes file. • sort the routes by increasing geographical length • If joint formulation: • take first N routes (a budgeted number) as the set of eligible working routes for the formulation. (Do not remove from file). • If non-joint formulation: • take the single shortest route for the working paths between O-D • For both joint or non-joint generate eligible restoration route-sets: • Repeat (for each active O-D pair): • step out one span onto the shortest route, i • find first K routes in routes file which do not include span i as eligible restoration routes • remove chosen routes from file • go to next span along shortest route Until last span in shortest route • merge all routes found as eligible route-set for restoration of relation r.

20. Route generating method for path restorable formulation (2) • What does this achieve ? • This procedure is effective in mediating the following trade-off in populating the route-sets: “All distinct routes” A single budgeted number of distinct routes Insufficient diversity / uniform disjointnessof route-sets found by Depth First Search, infeasibilities arising with even large route-set budgets Far too large DAT file sizesfor realistic AMPL / CPLEX runs project topic: write program to statistically sample the large eligible route space

21. Observed tendency from taking budgeted number of distinct routes of successive length found by DFS ( Leaf end of DFS tree ) O D ( Root end of DFS tree ) Outright infeasibility possible if all ‘budgeted’ routes neck down onto one span in common The preceding process for selecting routes stays within a budget but avoids this problem byrepeatedly pushing out the grey envelope as itproceeds in the direction across the network betweenO-D nodes. Boundary of a network graph

22. Excerpt from a comparative study of span- versus path-restorable network designs Ref: work done in PhD thesis by R. Irashcko, 1996 publication: R.Iraschko, M. MacGregor, W.D. Grover, “Optimal Capacity Placement for path restoration in STM or ATM Mesh Survivable Networks”, IEEE/ACM Transactions on Networking, vol.6, No.3, June 1998, pp. 325-336. (available on web site)

23. joint optimizedrouting makes alargedifference in span restoration • joint-span is aboutas efficient as non-joint path • joint designadds relativelylittle benefit to path restoration Typical result comparing span and path-restorable network designs “non joint” “joint” designs

24. Comment on a current industry issue... “Path restoration” is the latest industry in-vogue thing. Router-vendors (notably Cisco) are claiming to provide implementations of“path restoration” wherein every failed OD pair conducts an independentsimultaneous OSPF-type discovery of a shortest replacement path. We call this “ad-hoc” path restoration as opposed to optimized or coordinated path restoration. Q. Why is this both misleading and technically incorrect? • 1) It is an uncapacitated view of the re-routing problem • - every OD pair may find a logical replacement route, but their is no consideration or knowledge of actual transport capacity on every span along the route. • 2) There is no guarantee of 100% restoration in anythinglike the low spare capacity that a true path restoration mechanism can cope with. • - very careful collective co-ordination of all individual OD pairrestoration pathsets is required to realize these lowcapacity levels.... see example on next slide.

25. Why ad-hoc replacement-route finding is not “path restoration” perse • example shows same span-failure affecting 3 OD-pairs • only spare capacity and restoration paths are shown for simplicity • ad-hoc set of independent replacement paths • -incomplete restoration • collectively co-ordinated set of replacement paths • -complete restoration

26. Other comments on path restoration • (1) A path restorable network inherently provides a response to node-failure and multiple span failure • - 100% restoration not guaranteed • - span restoration needs special extensions to the distributed protocols to respond to these situations as gracefully • (2) Path restoration also copes more gracefully with the multiple logical span failures arising from nodal “bypass” situations. same cable terminated flows express or “bypass” flows

27. Issue of nodal bypass and fault multiplication in span restoration - not a problem for path restoration (A-B) span failure physical picture = B A C logical picturefor span restoration = (A-B) (A-C) simultaneous dual logical span failures B A C generic issue / phenomenon: physical to logical layer fault multiplication • for path restoration, however, • same set of end-node OD pairfailures arise in either case

28. chain higher degree mesh component chain Other comments on path restoration • Recent findings indicate that the capacity benefit of path restoration (over span- restoration) may be considerably less than hoped forin low degree networks..... consider: - intra chain demands - demands that cross only one mesh span or chain----- > can do no better -( spare capacity - wise) than with span restoration ) • for many demand pairs in a low degree network their path restorationsolution is no different than span restoration in the mesh that results from collapsing all degree-2 chains.