Resilient Networks 3.2 Resilient Network Design – Restoration & Protection Prepared along: Michal Pioro and Deepankar Medhi - Routing, Flow, and Capacity Design in Communication and Computer Networks, The Morgan Kaufmann Series in Networking, 800 pages, 2004 1
Link-Demand-Path-Identifier-based Notation Notation Summary 2
Restoration and Protection Design New dimension in Network Design Problems (NDP): Failures Adds resilience dimension to network design Restoration Design Problems (RDP) – Designing networks that are robust to failures – Networks are able to carry (possibly decreased) demands also when part of network resources fail temporarily – Re-establishing flows on paths that survived failure situation Different failure situations specified by – availability status of links and nodes – possibly decreased demand volumes requested for particular situation – total or partial failure of links / nodes 3
Restoration and Protection Design - Outline Introduction to Re-establishment Mechanisms Initial model for re-establishment mechanisms (protection and restoration) – Characterization of failure states – Resilience via path diversity Link capacity re-establishment mechanisms – Link restoration with shared capacity – Hot-standby link protection with dedicated capacity Path flow re-establishment mechanisms – Path flow restoration with shared capacity – Hot-standby path protection with dedicated capacity 4
Re-Establishment Mechanisms (1) Network resilience via re-establishment of resources in case of failures – Protection: actions to restore before failure happens, typically used to protect against single link failures – Restoration: actions taken after failure Goal of re-establishment: – Protecting demand volumes or at least certain portion of them Protection / restoration capacity – Needed for both, traffic networks and transport networks – Traffic networks (e.g., Internet): restoration capacity – Transport networks (physical facility providers): protection capacity 5
Re-Establishment Mechanisms (2) Link vs. path re-establishment Link Re-establishment (LR) 5 6 – Link is re-established in case of failure 1 2 – All flows that use link are reconstructed 3 4 together Link Re-establishment Path Re-establishment (PR) 5 6 – End-to-end flows that use failed links are re-established individually 1 2 – PR used at neighboring lower layer of LR 3 4 to reconstruct missing demand capacity units in between end-points of failed link Path Re-establishment 6
Re-Establishment Mechanisms (3) – Paths Path Protection (PP) – Reservation of resources at the time the flow on the path is set up – Restoration is guaranteed as protection (backup) paths are pre-calculated in advance and protection capacity is reserved – Re-establishing of flows on protection paths after failure Path Restoration (PR) – Restoration after path has broken by calculating backup paths using available restoration (spare and released) capacity and re-establishing flows – Network management system has to be invoked after failure Capacity design: No difference between protection and restoration 7
Re-Establishment Mechanisms (4) - Capacity Dedicated vs. shared capacity Dedicated capacity – Spare capacity required to re-establish a link/path that is reserved exclusively for re-establishing this link/path – Spare capacity cannot be used for re-establishing other links/paths – Used in protection schemes, e.g., Automated Protection Switching (APS) – Expensive resource-wise but simple to operate Shared capacity – Common pool of spare resources used for re-establishing broken links/paths – Typically used for restoration schemes, same capacity used for protecting different resources – Restoration schemes can also utilize normal capacity released along broken paths – Requires less spare capacity but much more complicated to control 8
Re-Establishment Mechanisms (5) Hot-standby path protection – 1+1 protection: Data transmitted redundantly via two paths simultaneously – 1:1 protection: Data transmitted via one path, after failure switch to backup paths – 1:N protection: One backup path protects N regular paths Link re-establishment can be entire or partial – If links are modular then only one part of its modules may be re-established (partial re-establishment) Re-establishment may be – Splittable (bifurcated): link or path re-established on several backup paths – Unsplittable (non-bifurcated): link or path is re-established on exactly one backup path 9
Re-Establishment Mechanisms (6) Network Design perspective Whether re-establishment scheme is of protection or restoration type is not important But it is important – whether they use dedicated or shared capacity – and if they can use the released capacity or not 10
Restoration and Protection Design – Naming Naming conventions for restoration and protection problems 1. DR (Restoration Design), D (Dimensioning for the normal state), P (Protection design), D+P (normal Design + Protection design) 2. CF (Continuous Flows), MF (Modular Flows) 3. BR (Bifurcated Routing), NBR (Non-Bifurcated, i.e., single-path Souting) 4. CC (Continuous link Capacity), MC (Modular link Capacity) 5. LIN (LINear cost), MOD (MOdular Cost), CX (ConveX cost), CV (ConcaVe cost), BC (Budget Constraint); we will use mostly the LIN option 6. LR (Link Restoration), LP (Link Protection), HS (Hot Standby), SBP (Single Backup Path), LR + BR (BifuRcated Link Restoration), ... 11
Recapitulation from Last Chapter DP: Minimizing capacity costs 2 Demand 1 3 Network 2 𝑄 31 𝑄 𝑄 32 11 4 𝑄 𝑄 22 12 1 3 𝑄 21 F*=85 12
Restoration and Protection – Initial Model (1) Failure model: – Links become totally unavailable, but do not fail simultaneously – Each failure state consists of one link failure per time (rest is operative) Example: DP – Minimizing capacity costs – Network results in 5 failures states s = 1,2,...,5 corresponding to each link 2 – In state s link e = s is failed, remaining links intact – Normal state (all links available): s = 0 Demand 1 3 Goal : Network 2 – Find cheapest link capacity configuration together with routing and flow allocation so that in all states 𝑄 31 𝑄 𝑄 32 11 4 demand volumes are fully realized 𝑄 𝑄 22 12 1 3 𝑄 21 13
Restoration and Protection – Initial Model (2) State s requires new identifier, e.g., for demand d=1: Six equations instead of one for demand d=1, in general form: Capacity constraints – Since 𝑡 = 1 corresponds to failure of link 𝑓 = 1 , no capacity should be assigned to that link in that state (right side is 0) – Notation 𝛽 𝑓𝑡 set to 1 if link e is up and 0 if it is down in state 𝑡 14
Restoration and Protection – Initial Model (3) Solution for 4-Node Network Example Optimal capacity y * e of link e – Computed as maximum load of link over all states s=0,1,...,5 Optimal cost of F* =245 (in contrast to F* =85 without failure consideration) 15
Restoration and Protection – Initial Model (4) Optimal flow allocation for all flow allocations: all flows are non-bifurcated However, this is not always the case Simple example network with two nodes – Only one demand d=1 – Three links (E=3), three failure situations (S=3) – Demand volume in all situations: h 1s =3, s = 0,1,2,3 – Unit cost of all three links is 1 – In failure situation only one link fails 16
Restoration and Protection – Initial Model (5) (1,5) (1,5) (1,5) (1,5) (1,5) (1,5) (1,5) (1,5) Bifurcated (1,5) (1,5) (1,5) (1,5) Solution 1 1 1 0 1 1 1 0 1 1 1 0 F* =4,5 demand h 10 =3 demand h 11 =3 demand h 12 =3 demand h 13 =3 s=0 s=1 s=2 s=3 (0) (0) (0) (0) (3) (3) (3) (3) Non- Bifurcated (3) (3) (3) (3) Solution 1 1 1 0 1 1 1 0 1 1 1 0 F* =6 demand h 10 =3 demand h 11 =3 demand h 12 =3 demand h 13 =3 s=0 s=1 s=2 s=3 17
Re-establishment Mechanisms – Initial Model (6) Design problems related to network robustness to failures do not follow shortest-path rule that works for normal operating state Number of equations, inequalities, and variables grow with introduction of states to capture different failure situations Restoration problems difficult and time-consuming to solve But restoration problems are important class of DP and can help to find network configurations that are resilient and survivable 18
Characterization of Failure States (1) Capacity /demand protection or capacity /demand restoration mechanism activated when failure situation occurs Failure state (situation) s characterized by vector of link availability coefficients α s = (α 1s , α 2s ,..., α Es ) with 0 ≤ α es ≤ 1 – Each coefficient 𝛽 𝑓𝑡 determines proportion of normal capacity 𝑧 𝑓 of link e, 𝛽 𝑓𝑡 𝑧 𝑓 , available on link e in situation 𝑡 – 𝑡 = 1,2, … , 𝑇 is the predefined list of failure situations – Frequently, we will assume that availability coefficients are binary 𝛽 𝑓𝑡 ∈ {0,1} Multiple link failures at one time possible 19
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