E E Module 16 Path-oriented Survivable Mesh Networks W.D. Grover TRLabs & University of Alberta © Wayne D. Grover 2002, 2003

E E Module 15 © Wayne D. Grover 2002, Three basic concepts about end-to-end path protection/restoration: Shared Backup Path Protection –each working path has a (single) fully-disjoint backup route pre-determined at path- provisioning time. –When needed a protection path is cross-connected from spare channels along the backup route. –like “1+1 APS with a shared backup” –same end-node activated reaction regardless of where failure occurs on working path. (“True”) Path Restoration –adaptive, failure-specific, response to failure and network state. –for each failure scenario the set of affected end-nodes are simultaneously restored with an MCMF-like response. has cognizance of the “mutual capacity” issue and global (or self-organized) coordination. –Allows reuse of working capacity on surviving portion of failed paths. –Capacity design to assure 100% restorability to all defined scenarios. GMPLS “mass redial” –completely ad-hoc reliance on mass independent re-provisioning attempts. –no co-ordination, self-organization, or other way to address mutual capacity considerations –inherently unassured, best-efforts, unpredictable outcomes.

E E Module 15 © Wayne D. Grover 2002, Some initial notes on each: Shared Backup Path Protection –most dominant current paradigm for “survivable routing.” –preferred for router-centric control paradigm where “network is dumb, edges are smart.” –high dependency on conventional software, databases, abd current ideas of Internet like global state dissemination, etc. –advantage in fully optical networks is that we don’t need rapid fault location. (“True”) Path Restoration –theoretically most efficient possible scheme. –not currently popular with industry due to perceived complexity. –self-organizing distributed protocol for MCMF-like adaptive performance developed by us. –may return to importance in context of adaptive-second line of defence strategy for ultra- high availability, or, for maximal recovery from arbitrary-attack failure scenarios (9/11 etc.) –true self-organization concepts currently too different from conventional software/ messaging paradigms for “distributed interaction.” GMPLS “mass redial” –industry currently in two camps: those that know this is a disaster waiting to happen if it is positioned as the only survivability mechanism needed. those that don’t understand the issue of mutual capacity yet, or just want to overprovision as much capacity as needed to have a good chance of restoration –characterized in recent M.Sc. Thesis by G. Kaigala (and Globecom 2003 paper).

E E Module 15 © Wayne D. Grover 2002, Shared Backup Path Protection “Protection” scheme (pre-determined fixed backup paths) End-to-end path protection Single protection path for each working path Spare capacity can be shared between disjoint working paths Restoration of path 1 Restoration of path 2 Failure scenario 1 Failure scenario 2 Working path Spare capacity re-use

E E Module 15 © Wayne D. Grover 2002, A slightly more general example of SBPP Green-blue-yellow protection sharing (x3) Green-red sharing (x2)

E E Module 15 © Wayne D. Grover 2002, An ILP Design Model for SBPP (1) Define: D= demand matrix, index r, d r = magnitude of demand on relation r. = 1 if the working-path route for relation r crosses span i (parameter) B r = set of routes that between the end nodes of r that are disjoint from the working route for r = 1 if the route of the bth eligible backup route for relation r crosses span i (parameter) = 1 if route b is chosen as the backup for relation r, zero otherwise (1/0 decision variables)

E E Module 15 © Wayne D. Grover 2002, An ILP Design Model for SBPP (2) Min cost of spare capacity Such that: Pick one backup route only Provide spare capacity needed for all simultaneously activated backup paths

E E Module 15 © Wayne D. Grover 2002, Adding a limit on sharing to SBPP (3) Such that: Don’t let the total number of backup roue choices that use span i exceed F times the spare capacity of span i. new (As before) F = “maximum spare capacity sharing factor”

E E Module 15 © Wayne D. Grover 2002, Cost of Limiting the Maximum Sharing Relationships* Average total capacity increase for F =2: 31.4% Sharing Limit of 3 may be acceptable. Average total capacity increase for F=1: 150.4% Average total capacity increase for F =3: 5.1% Sharing Relationship Limits of 1 and 2 yield total capacity increases that may be unacceptable. * Results for test network family with varying nodal degree based on a 25 nodes - 50 spans master network

E E Module 15 © Wayne D. Grover 2002, Qualitative Appreciation of SBPP vs Span Restoration Multiple restoration paths Characteristics (relative to 1+1 APS) Span Restoration SBPP Effect on capacity efficiency Effect on availability Sharing of spare capacity Localized response Distributed/adaptive restoration Sharing of spare capacity Single backup path End-to-end response Non-adaptive restoration no effect

E E Module 15 © Wayne D. Grover 2002, “True” 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 spare capacity 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.

E E Module 15 © Wayne D. Grover 2002, Comparative illustration of span versus path restoration Pre-failure 3 service paths

E E Module 15 © Wayne D. Grover 2002, Failure occurs Comparative illustration of span versus path restoration All 3 service paths are lost until …

E E Module 15 © Wayne D. Grover 2002, Span restoration reaction First look at a span restoration reaction … (1) Note: example only, exact routes depend on working and spare capacities All 3 service paths are lost until … failed working links on failed span are restored by span restoration

E E Module 15 © Wayne D. Grover 2002, A span restoration reaction …(2) Loopback / backhaul This restoration path could stop here

E E Module 15 © Wayne D. Grover 2002, Now view a path restoration reaction... Same failure occurs

E E Module 15 © Wayne D. Grover 2002, A path restoration reaction …with “stub release” (1) Path restoration action Stub release

E E Module 15 © Wayne D. Grover 2002, 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? Notes about stub release in path restoration

E E Module 15 © Wayne D. Grover 2002, 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.

E E Module 15 © Wayne D. Grover 2002, 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 r for 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 is for every span, here it is for every OD pair. Variations and options within the master formulation for path restoration

E E Module 15 © Wayne D. Grover 2002, * some variables become pre-computable parameters in the variations that follow Input data Intermediate (internal) variables Design output variables Cost of m th module size on span j. Set of all point to point demand quantities, indexed by r amount of demand on relation r Set of all spans between mesh cross-connection points Set of eligible working routes for relation r Encodes routes in = 1 if span j is in q th route for relation r Set of eligible restoration routes for relation r upon failure i. = 1 if span j is in p th route for relation r upon failure i Stub release quantity on span j from failure i Amount of demand lost on relation r for failure i No. of operating working and spare links (channels) on span j No. of modules of type m to install on span j for min cost Capacity of m th module size Working and restoration routing solutions N.B. “relation” = “OD pair” Parameters and variables in path-restorable capacity design (in the master formulation)*

E E Module 15 © Wayne D. Grover 2002, B A f i r,p eligible restoration routes for A-B after the failure of span i, working route for A-B g r,q X i r span i Q r = 1 P i r = 2 r = A-B C D Orientation to the path restorable design context (variable and parameters)

E E Module 15 © Wayne D. Grover 2002, Cost of modules of all sizes placed on all spans S. t. Defines the amount of damaged working flow for each relation under each failure scenario All demands must be routed Working capacity on spans must be adequate (1) (2) (3) Master formulation for path-restorable capacity design

E E Module 15 © Wayne D. Grover 2002, With stub release Without stub release Restorability of working flows for each relation Spare capacity on spans must be adequate (see note on stub release) Modularity of installed capacity (4) (5) (6) (7) Master formulation for path-restorable capacity design (2)

E E Module 15 © Wayne D. Grover 2002, O D Relation r Route q Failure span i Other span j Working flow g r,q Span j enjoys a stub release “credit” of spare capacity = g r,q for any failure on span i such that: Understanding how the formulation effects “stub-release”

E E Module 15 © Wayne D. Grover 2002, joint optimized routing makes a large difference in span restoration Typical result comparing span and path-restorable network designs “non joint” “joint” designs joint-span is about as efficient as non-joint path joint design adds relatively little benefit to path restoration

E E Module 15 © Wayne D. Grover 2002, 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 Variations and options within the master formulation

E E Module 15 © Wayne D. Grover 2002, 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. A route-generating method for path restorable design formulation

E E Module 15 © Wayne D. Grover 2002, 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 Far too large DAT file sizes for realistic AMPL / CPLEX runs Insufficient diversity / uniform disjointness of route-sets found by Depth First Search, infeasibilities arising with even large route-set budgets project topic: write program to statistically sample the large eligible route space Route generating method for path restorable formulation (2)

E E Module 15 © Wayne D. Grover 2002, O D Boundary of a network graph 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 by repeatedly pushing out the grey envelope as it proceeds in the direction across the network between O-D nodes. ( Root end of DFS tree ) ( Leaf end of DFS tree ) Observed tendency from taking budgeted number of distinct routes of successive length found by DFS

E E Module 15 © Wayne D. Grover 2002, 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 Why ad-hoc replacement-route finding (“mass redial”) is not an assured form of path restoration

E E Module 15 © Wayne D. Grover 2002, (1) A path restorable network inherently provides a response to node-failure and multiple span failures - 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. terminated flows express or “bypass” flows same cable Other comments on path restoration

E E Module 15 © Wayne D. Grover 2002, physical picture = logical picture for span restoration = (A-B) (A-C) simultaneous dual logical span failures A C B A C B (A-B) span failure generic issue / phenomenon: physical to logical layer fault multiplication for path restoration, however, same set of end-node OD pair failures arise in either case Issue of nodal bypass and fault multiplication in span restoration - not a problem for path restoration

E E Module 15 © Wayne D. Grover 2002, Recent findings indicate that the capacity benefit of path restoration (over span- restoration) may be considerably less than hoped for in low degree networks..... chain higher degree mesh component 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 restoration solution is no different than span restoration in the mesh that results from collapsing all degree-2 chains. Other comments on path restoration