1. Network Survivability, Reliability and Availability: Protection & Restoration
Zilong Ye, Ph.D.

2. Reliability
Reliability is the probability that a system or component will operate without any service-affecting failure for a period of time t
Reliability is a monotonically decreasing probability function
of time, R(t)
A specific reliability number always implies an assumed duration of time
Reliability is about
How soon the next repair expenses might be incurred etc.
but reliability itself does not consider
the repeated cycles of failure
repair time, and
return to service which determine the availability of an ongoing service

3. Availability
Availability is the probability that a system will be found in the operating state at a random time in the future
Availability inherently reflects a statistical equilibrium between
failure processes: mean time to (or between) failure (MTTF/MTBF) and
repair processes: mean time to repair (MTTR)
in maintained repairable systems that are returned to the operating state following any failure
MTTF
Availability =
MTTF + MTTR

4. Quantification of Availability
Percent Availability
N-Nines
Downtime Time Minutes/Year
99%
2-Nines
5,000 Min/Yr
99.9%
3-Nines
500 Min/Yr
99.99%
4-Nines
50 Min/Yr
99.999%
5-Nines
5 Min/Yr %
6-Nines
.5 Min/Yr

5. Survivability Survivability of a network as a whole is
the average fraction of failed working capacity that can be restored by a specified mechanism within the spare capacity provided in a network
A link may be fully survived with 100% capacity or it could be partially survived with <100% capacity

6. Market Drivers for Survivability
Customer Relations
Competitive Advantage
Revenue
Negative - Tariff Rebates
Positive - Premium Services
Business Customers
Medical Institutions
Government Agencies
Impact on Operations
Minimize Liability

7. Failure Types & Other Motivations
Types of failure:
Components: links, nodes, channels in WDM, active components,
software…
Human error: backhaul fiber cut
Fiber inside oil/gas pipelines less likely to be cut
Systems: Entire COs can fail due to catastrophic events
deliberate attacks

8. Network Survivability: drivers
Availability: % (5 nines)  less than 5 min downtime per year
Since a network is made up of several components, the ONLY way to reach 5-nines is to add survivability in the face of failures…
- Survivability = continued services in the presence of failures
Protection switching or restoration: mechanisms used to ensure survivability
- Add redundant capacity, detect faults and automatically re-route traffic around the failure
Protection: fast time-scale: 10s-100s of ms…
- implemented in a distributed manner to ensure fast restoration
Restoration: related term, but slower time-scale

9. Types of Fault-Recovery Mechanisms
Protection
Backup resources (routes and wavelengths) pre- computed and reserved in advance (before a failure occurs) – simple but 50% overhead
Faster recovery time
What if pre-reserved resources also fail?
Restoration
Routes and wavelengths discovered dynamically after
detection of a failure
Resources allocation based current network state info
More resource efficient
Can recover as long as there’re redundant resources
Slower recovery time 10

10. Restoration Path Restoration Link Restoration
Route can be computed after failure
Link Restoration
Path is discovered at the end nodes of the failed link
More practical than path restoration
Advantages & Disadvantages of Restoration
Usually can recover from multiplex element faults
More efficient usage of resource
Complex
Slow: require extra process time to setup path and reserve resource

11. Comparison between Protection & Restoration
Characteristic: Protection -- the resource are reserved before the failure, they may be not used; Restoration -- the resource are reserved and used after the failure
Route: Protection -- predetermined; Restoration -
- can be dynamically computed
Resource Efficiency: Protection -- Low;
Restoration -- High

12. Comparison between Protection &
Restoration (Cont’)
Time used: Protection -- Short; Restoration -- Long
Reliability: Protection -- mainly for single fault;
Restoration -- can survive under multiple faults
Implementation: Protection -- Simple; Restoration
-- Complex

13. Network Survivability Architectures
Restoration
Protection
Self-healing Network
Re-Configurable Network
Protection Switching
Linear Protection Architectures
Ring Protection Architectures
Mesh Restoration
Architectures
Path-based
Link-based
Segment-based

14. Restoration in Mesh Networks
Central Controller
DCS DC
DCS
DC DCS
Self Healing (distributed)
Restoration Architecture
Probing after restoration DC
DCS
DCS
DCS
DCS
Reconfigurable (or Rerouting)
Restoration Architecture (centralized)
DC = Distributed Controller

15. Protection Switching Terminology
1+1 architectures - permanent bridge at the source - select at sink
m:n architectures - m entities provide protection for n working entities where m is less than or equal to n
allows unprotected extra traffic on the m entities
most common - SONET linear 1:1 and 1:n
Coordination Protocol - provides coordination between
controllers in source and sink
Required for all m:n architectures
Not required for 1+1 architectures

16. Basic Ideas: Working and Backup Paths

17. Protection Switching: Terminology
Dedicated vs Shared: working connection assigned dedicated or shared protection bandwidth
1+1 is dedicated, 1:n is shared
Revertive vs Non-revertive: after failure is fixed, traffic is automatically or manually switched back
Shared protection schemes are usually revertive

18. Different protection and restoration schemes
Protection/Restoration Schemes
Protection
Restoration
Path
Ring Protection
Mesh Protection
Ring/Mesh Protection
Link
Restoration
Restoration
Link
Path
Link Path
Protection Protection
Link
Protection
Path
Protection
Protection Protection 21

19. Path vs Link Protection22
Working Path
DCS
DCS
Line or Link Protection
DCS
DCS
DCS
DCS
Protection Path
Control: Centralized or Distributed
Route Calculation: Preplanned or Dynamic
Type of Alternate Routing: Line or Path

20. Types of Fault-Recovery Mechanisms
Path Protection
Two link (node) disjoint paths: primary (working) and
backup (protection) path
Traffic rerouted through a link-disjoint backup route once a link failure occurs on working path
Usually, less resource required (using shorter routes)
Lower end-to-end propagation delay for the recovered
route
Backup path pre-reserved or pre-set up
Backup paths of different connections may or may not
share common wavelengths on common links

21. Types of Fault-Recovery Mechanisms
Dedicated Path Protection
Do not allow sharing among backup paths (resources)
Backup paths pre-configured
No switch configuration necessary along the backup path when a failure occurs
Fast recovery time
Resources not efficiently utilized (100% redundancy)

22. Types of Fault-Recovery Mechanisms
Shared Path Protection
Allow sharing among backup paths subject to certain
constraints
Primary/active paths (AP) are link disjoint  backup paths (BP) may share common link and wavelength
Backup paths configured when a failure occurs since backup paths may be shared; cannot commit resources to a particular primary in advance
Slower recovery time
Resources utilization much better
More signaling required to recover from the failure

23. Shared Path Protection
If and only if two APs are disjoint, their BPs can share backup bandwidth (backup bandwidth) on a common link (i.e., total backup bandwidth = max{w1, w2}).
AP1(w1) S1 D1
BP1
Link L(max{w1,w2})
BP2 S2 D2
AP2(w2)

24. Types of Fault-Recovery Mechanisms
Link Protection
a light-path set up on a primary path
For each link on the primary path, a backup detour is
reserved around the link
No sharing – dedicated-link protection
Wavelength used on backup loop dedicated to specific link to be protected
Shared-link protection
Note, different connections on the same link might have different backup detours for that link

25. Solution 1: Active-Path First
Find an active path (AP) first
Then find a disjoint backup path (BP)
How?
Remove the physical links and resources that the active path travels, and then re- run the routing algorithm to find the backup path. 60

26. Solution 2: Joint Path Selection
Select the active path and backup path in a joint manner.
Joint optimization have better performance compared to active path first schemes in terms of the amount of network resources required
How?
Use Suurballe’s algorithm to compute two link- disjoint paths between (s, d) simultaneously 60

27. Suurballe’s Algorithm
Given a graph G=(V, E), find a pair of edge-disjoint paths from s to t such that the total edge cost of the two paths is minimal among all such path pairs

28. Dedicated-path protection – Heuristic Algorithms
Remove links that do not have free wavelengths
Apply Suurballe’s algorithm to find a pair of paths
Choose the shorter path as the primary path and
the longer path as the backup
Assign a wavelength using First-Fit to each path
Guarantees the minimum total bandwidth (TBW)
= active BW (ABW) + backup BW (backup bandwidth) for this request 30

29. Shared-Path Protection
Heuristic 1
Use Suurballe’s algorithm to generate two routes
Assign wavelengths while trying to share the wavelengths on the backup paths as much as possible
Does not perform well in backup path sharing since routing does not consider wavelength info no backup bandwidth sharing potential

30. A Fast and Efficient Heuristic 3
Challenges
Jointly optimize an AP/BP pair with shared path
protection is NP-hard
using ILP is notoriously time consuming.
also, only guarantee minimal TBW for each request, but not minimal TBW for all requests.
Heuristics such as active path first (APF) can only
achieve sub-optimal results:
does not consider the yet-to-be-incurred backup cost along the BP when selecting a (shortest) AP

31. Potential Backup Cost (PBC)
Uses a shortest path algorithm to find the AP first
But, in selecting the AP, each capable link e (Re≥w) will be assigned a cost of w+e(w), where the second term is the potential backup cost (PBC)
then finds a shortest BP
combines the best of ILP and APF based approaches
See Xu et al, Lightwave Technology, Journal of
Volume: 25 Issue: 8, 2251 – 2259, 2007

32. Protection in SRLG networks

33. Shared Risk Link Group (SRLG)
Widely recognized as an important concept in survivable optical networks
A group of network links that share a common
physical resource (cable, conduit etc.)
Due to layered structure:
Physical layer: Fiber spans (cable, conduit, et
al)
Optical layer: Optical links and nodes (a
subset of the nodes in the physical layer

34. Layered Architecture of Optical Network1 5 e1 e5 e7 e3 2 e2 e4 4 3
(a) Optical
Layer
g8 g7 g6 1 5 g1 g9 g5 2 g2 g4 4 g3 3
(b) Physical
Layer 66

35. Protection in SRLG networks
Finding SRLG-disjoint path pair is more complicated than finding a link/node-disjoint path pair. In fact, the former is a NP-complete problem.
If Backup BandWidth (backup bandwidth) sharing is considered, SRLG protection problem will become even more complicated.

36. 30% of the time statistically) when considering SRLGs
APF and Trap
Active Path First (APF), followed by an SRLG-
disjoint BP
attractive alternative (policy-based routing, optimal AP)
But may fail to find such a BP more frequently (up to
30% of the time statistically) when considering SRLGs
Trap: can’t find an SRLG-disjoint BP
Real Traps: unavoidable, topology-induced
Avoidable Traps: algorithm-induced.
Only a few APF algorithms so far to deal with
avoidable traps.

37. Other APF-based Heuristic
K Shortest Paths (KSP)
Finds the first K shortest paths between the source
and destination as candidate APs, and
then test them in the increasing order of their costs, until a SRLG disjoint BP is found or all of them have been tested.

38. Proposed Trap Avoidance (TA) Algorithm
Similar to KSP: iteratively test candidate APs and find one that has a SRLG-disjoint BP
But TA constructs one AP at a time, and modifies it into a new AP for testing only if necessary
TA uses a more intelligent method to avoid the
most “risky” link when modifying the AP
KSP is oblivious/blind to “bad” links
See Xu et al. IEEE/OSA Journal of Lightwave Technology (JLT), Special Issue on Optical Networks, Vol. 21, No. 11, pp , 2003 70

39. infinity to prevent them from being used by BP.
Find a Candidate BP
All the directed links along AP assigned a cost of
infinity to prevent them from being used by BP.
All the remaining links that share at least one SRLG with any link on AP (including the links along the reversed AP) will be assigned a large value M as cost.
Discourage any shortest-path algorithm to use such “M” links for the candidate BP. But do not forbid.

40. PROtection using MultIple
SEgments (PROMISE)

41. Logically “divide” an AP into several,
Basic idea of PROMISE
Logically “divide” an AP into several,
possibly overlapping sub-path called active segments (AS’s), and then protect each AS with a backup segment (BS)
AS 1
AS 2
BS 1 AP
BS 2

42. - Recall that traps are more likely in SRLG networks
Applications for PROMISE
First proposed for Non-SRLG networks
Particularly effective in dealing with either real or
avoidable traps
- Recall that traps are more likely in SRLG networks

43. Most Bandwidth Efficient: I
Reason: the flexibility it offers in choosing the
appropriate AS’s and corresponding BS’s.
AP1 and AP2 are not link disjoint, so their BPs
cannot share backup bandwidth
But in PROMISE, BS1,1 and BS2,1 can share backup bandwidth
AS 1,1
AS 1,2
AP1 BS 1,1
BS 2,1
AP2 80

44. Most Bandwidth Efficient: II
Inter-Sharing: Sharing between BS’s for
different connections
Intra-Sharing: Sharing between BS’s of the same connection, e.g. BS1 and BS2 share backup bandwidth on link c
AS1 AS2 AS3
b c f
BS2
BS1
BS3 a
e d g 1 2 3 4 5 6

45. Path Protection: 1-(1-0.8 2)2  0.87
Faster Recovery and More Resilient
Faster Recovery: Protects each AS using a shorter BS instead of protecting the entire AP using a longer BP (as in path protection)
More Resilient/Robust: Tolerate more multiple failures than path protection (with the same or lower bandwidth consumption).
Overall Reliability
•Link failure prob:
x = y = p = q =0.8
BS1 p
Path Protection: 1-( )2  0.87
PROMISE: (1-(1-0.8)2)2 0.92 x y s 2 d
BS2 q

46. Other Benefits of PROMISE
Can Succeed When Other Approaches Fail
Routing policies, QoS constraints (e.g., hop limit on the AP
and BP), or just APF
Real/Avoidable Traps in SRLG networks
Readily be applied to MPLS networks by extending the existing protocols for local repair/recovery in MPLS networks

47. Key Challenges in PROMISE
Joint optimization of AP selection and the set of protecting BS's is extremely complex
Even if AP is found first as in APF-based heuristic,
How to optimally divide AP into AS’s (then corresponding BS's)
Harder than modeling the general multi-commodity flow problem: number of BS’s, and the source and destination for each BS are not known beforehand.