Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 1
An introduction to
NETWORK RESILIENCY
Giorgio Ventre & Stefano AvalloneCOMICS Group
Dipartimento di Informatica e SistemisticaUniversità di Napoli Federico II
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 2
ReferencesReferences
Jean-Philippe Vasseur, Mario Pickavet, Piet
Demeester. “Network Recovery, protection and
restoration of optical, SONET-SDH, IP and MPLS”.
Morgan Kaufmann
AA. VV. Building Survivable Networks, Feature Issue
of IEEE Network Magazine, March/April 2004
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 3
Communication Networks RelevanceCommunication Networks Relevance
Communication Networks are becoming fundamental infrastructures:the amount of data carried out by Communication
Networks is considerably grows in the last years;
many social and economic activities depend on Communication Networks;
many safe critical activities depend on Communication Networks.
Reliability is an essential feature of today Communication Networks !
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 4
Network Reliability: definitionNetwork Reliability: definition [1][1]
The (a) ability of a network to maintain or restore an acceptable level of performance during network failures by applying various restoration techniques, and (b) mitigation or prevention of service outages from network failures by applying preventive techniques.
Acronym: Network Survivability.
[1] Alliance for Telecommunications Industry Solutions (ATIS) http://www.atis.org/tg2k/_network_reliability.html
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 5
Network Reliability: related conceptsNetwork Reliability: related concepts
There are many concepts that are related to Network Reliability, for example:network element reliability: the probability of a
network element to be fully operational during a certain period of time;
network element availability: the probability of a network element to be in an up-state at a given instant of time t;
network element fault: the inability of a network element to perform a required action
....
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 6
Which failures may occur Which failures may occur ??
The ability of a network to provide required services may be compromised by different failures:planed or unplanned failures;
internal or external failures;
software or hardware failures;
malicious or casual failures
....
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 7
Accounted FailuresAccounted Failures
Provide actions to address all the failures that may occur on a Communication Network is unfeasible.
Network provider and ISP normally provides actions plain to address the most frequent failures.
These failure are called Accounted Failure
The most common type of Accounted Failure are:single link failure; single node failure.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 8
Failures' ImpactFailures' Impact
In today Communication Networks a single failure may produces a major disruption in network availability.
A single cut in an optical cable may drop thousands of logical network connections.On July 5, 2002 a submarine cable break affected
the Asia Pacific Cable Network (ACPN 2), causing a considerable slowdown in all the network connections among Japan, China, South Korea, etc.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 9
Failures' Impact:Failures' Impact: ATC systems ATC systems Press Releases (
http://www.natca.org/mediacenter/press-release-detail.aspx?id=394) MASSIVE POWER, COMMUNICATIONS FAILURE AT MAJOR AIR
TRAFFIC CONTROL CENTER PUTS CONTROLLERS IN DARK, FLIGHTS IN JEOPARDY
07/19/2006 Bob Marks
PALMDALE, Calif. – A massive power and communications failure late Tuesday at the Los Angeles Air Route Traffic Control Center left scrambling air traffic controllers to deal with a nightmare scenario – how to keep dozens of flights away from each other above a large swath of the Southwestern United States despite the inability to see them, talk to them or relay crucial instructions for 15 excruciatingly long minutes.
Every ounce of skill, heart and determination that controllers bring into the control room every day was put to the test during one of the worst outages to ever hit the facility. It was so bad, controllers say, that the only thing they had of use to aid the situation that actually worked was their cell phones – devices which the Federal Aviation Administration, inexplicably, has barred from control rooms, further impeding the safety of the system.
More details in http://themainbang.typepad.com/blog/2006/07/complete_failur.html
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 10
Network Reliability ParametersNetwork Reliability Parameters
Some parameters that may be used to characterize the reliability of a network may be found in ITU G.911 Recommendation:
“Parameters and Calculation Methodologies for Reliability and Availability of Fibre Optic
Systems”
In the following slides some of the parameters defined in ITU G.911 are introduced
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 11
Failure in Time (FITs) and Maintenance TimeFailure in Time (FITs) and Maintenance Time
Failure in Time:is the number of device's failure occurred in a
specific time interval;
normally is expressed as failures per bilion of device hours.
Maintenance Time:the time interval during which a maintenance
action is performed on an item either manually or automatically, ...
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 12
Mean Time Between Failure (MTBF)Mean Time Between Failure (MTBF)
The Mean Time Between Failures (MTBF) is the steady-state expectation of time between failures
Mathematically the MTBF (in years per failure) is releated to the failure rate F (in FITs per 109 hours) as follows:
MTBF1.14 105
F
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 13
Mean Time To Repair (MTTR)Mean Time To Repair (MTTR)
The Mean Time To Repair (MTTR) is defined as total corrective maintenance time divided by the total number of corrective maintenance actions during a period of time.
Given the definitions of MTBF and MTTR the availability A of an item may be derived as:
A 1MTTRMTBF
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 14
Users, services and reliability requirements Users, services and reliability requirements
Network reliability is a “relative concept”.
The reliability requirements of a communication network depend on:the user type;
the service type.
Different users-services combinations led to divers requirements in terms of MTBF and MTTR.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 15
User classificationUser classification According to their reliability requirements, network
users may be classified in the following categories:
Safety critical users. Users for which service interruption are unacceptable.
Business critical users. Users for which any service interruption bring to a high financial loss.
Low cost users. Users for which service interruption cause only discomfort.
Basic lever users. Users for which service reliability is only a side effect.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 16
Availability: Impact of OutagesAvailability: Impact of Outages
Ref: “Service Applications for SONET DCS Distribution Restoration”, IEEE J. Special Areas in Comm, Jan 94
50 m
s
200 m
s 2 Sec
10 Sec
5 Min
30 M
in
15 M
in
Protection Switching
Range
1st Restoration
Target Range
2nd Restoration
Target Range
3rd Restoration
Target Range
4th Restoration
Target Range
Restoration time after failure detection
Serv
ice
Out
age
Impa
ct
0
Service“Hit””
(Reframes)
Undesirabl
e
Social / B
usiness Im
pact
Unacceptabl
e•Potential voiceband discinnects (<5%)
•Trigger changeover of CSS7 STP signaling links
•Effect cell rerouting process
•May drop voice band calls depending on channel bank vintage
•Drop all circuit switched connections
•PL disconnects
•Potential packet (X.25) disconnects
•Potential data session time-outs
•Packet (X.25) disconnects
•Data session time-outs
•Network congestion
•Minor social/ Business impacts
•Potentially FCC reportable
•Major social/ business impacts
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 17
Market Drivers for SurvivabilityMarket Drivers for Survivability
Customer Relations Competitive Advantage Revenue
Negative - Tariff RebatesPositive - Premium Services
• Business Customers• Medical Institutions• Government Agencies
Impact on Operations Minimize Liability
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 18
Network SurvivabilityNetwork Survivability
Availability: 99.999% (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
Restoration: related term, but slower time-scale
Protection: fast time-scale: 10s-100s of ms…
implemented in a distributed manner to ensure fast restoration
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 19
Failure Types & Other MotivationsFailure Types & Other Motivations
Types of failure:
Components: links, nodes, channels in WDM, active components,
software…
Human error: backhoe fiber cut
• Fiber inside oil/gas pipelines less likely to be cut
Systems: Entire COs can fail due to catastrophic events
Protection allows easy maintenance and upgrades :
Eg: switchover traffic when servicing a link…
Single failure vs multiple concurrent failures…
Goal: mean repair time << mean time between failures…
Protection also depends upon kind of application.
Survivability may hence be provided at several layers
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 20
Network Survivability ArchitecturesNetwork Survivability Architectures
Network Survivability Architectures
Restoration Protection
Protection Switching
Self-healing Network
Re-Configurable
Network
Mesh RestorationArchitectures
Linear ProtectionArchitectures
Ring ProtectionArchitectures
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 21
Network Availability & SurvivabilityNetwork Availability & Survivability
Availability is the probability that an item will be able to
perform its designed functions at the stated
performance level, within the stated conditions and in
the stated environment when called upon to do so.
Reliability
Reliability + Recovery
Availability
=
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 22
Quantification of AvailabilityQuantification 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
99.9999% 6-Nines .5 Min/Yr
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 23
PSTNPSTN Individual elements have an availability of 99.99%
One cut off call in 8000 calls (3 min for average call). Five ineffective calls in
every 10,000 calls.
Facility Facility EntranceEntrance
Facility Facility EntranceEntrance
ANAN
0.01 %0.01 %
0.005 %0.005 % 0.005 %0.005 %
0.02 %0.02 %
0.005 %0.005 % 0.005 %0.005 %
LELELELE
NINININI
LELELELE
NINININI
LDLDLDLD
ANAN
0.01 %0.01 %
PSTN End-2-End Availability 99.94%
NI : Network Interface
LE : Local Exchange
LD : Long Distance
AN : Access Network
Source : http://www.packetcable.com/downloads/specs/pkt-tr-voipar-v01-001128.pdf
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 24
IP Network ExpectationsIP Network Expectations
Service Delay Jitter Loss Availability
Real Time Interactive
(VOIP, Cell Relay ..)L L L H
Layer 2 & Layer 3 VPN’s (FR/Ethernet/AAL5)
M
Internet Service H H M L
Video Services L M M H
HHLL LL
L : Low M : Medium H : HighL : Low M : Medium H : High
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 25
Measuring Availability: The Port MethodMeasuring Availability: The Port Method
• Based on Port count in Network
• Does not take into account the Bandwidth of ports
e.g. OC-192 and 64k are both ports• Good for dedicated Access service because ports are tied to
customers.
(Total # of Ports X Sample Period) - (number of impacted port x outage duration)
(Total number of Ports x sample period) x 100
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 26
The Port Method ExampleThe Port Method Example
• 10,000 active access ports Network
• An Access Router with 100 access ports fails for 30 minutes.– Total Available Port-Hours = 10,000*24 = 240,000– Total Down Port-Hours = 100*.5 = 50– Availability for a Single Day =
(240000-50)/240,000*100 = 99.979166 %
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 27
The Bandwidth MethodThe Bandwidth Method• Based on Amount of Bandwidth available in Network
• Takes into account the Bandwidth of ports
• Good for Core Routers
(Total amount of BW X Sample Period) - (Amount of BE impacted x outage duration)
(Total amount of BW in network x sample period) x 100
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 28
The Bandwidth Method ExampleThe Bandwidth Method Example
• Total capacity of network 100 Gigabits/sec• An Access Router with 1 Gigabits/sec BW fails for 30 minutes.
– Total BW available in network for a day = 100*24 = 2400 Total BW lost in outage = 1*.5 = 0.5
– Availability for a Single Day = ((2400-0.5)/2,400)*100 = 99.979166
%
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 29
Basic Ideas: Working and Protect FibersBasic Ideas: Working and Protect Fibers
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 30
Service classification (1/2)Service classification (1/2)
Communication networks are used to carry many different services.
Different services may have divers reliability requirements.
Reliability requirements of such services are related to QoS parameters:Bit Rate;
Delay;
Jitter;
...
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 31
Service classification (2/2)Service classification (2/2)
Application Bit Rate Bit Rate Variation Delay Sensitivity Need for Recovery
Plain Old Telephone Service31-32 Kbps Constant 5 5Voice Over IP 8-32 Kbps Constant 5 5Video-telephony 256-1920 KbpsHigh High 5 5Videoconferencing at least 256 Kbps High 5 5Teleworking 64 Kbps – 2 Mbps Very High 5 4TV broadcast 2-8 Mbps High 4 3Distance Learning 64 Kbps – 2 Mbps Very High 5 5Movies on Demand 750 Kbps – 4 MbpsHigh 4 5News on Demand 64 Kbps Very High 2 2Internet Access 64 Kbps – 2 Mbps Very High 1 2Teleshopping 64 Kbps – 2 Mbps Very High 2 2
[2] A.Lason, et al., “Network Scenarios and Requirements”, European IST project Layers Internetworking in Optical Network (LION), deliverable D6, Septemper 1999.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 32
How to increase network reliability ?How to increase network reliability ?Prevent network failure:
put network cables deeper in the ground;
more testing for hardware and software;
.....
Duplicate vulnerable network elements:dual homing.
Independently from these measures, network failures still occur.
There is need for network recovery or resilience schemes !
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 33
Network recovery basic ideaNetwork recovery basic idea
Build networks to have alternate paths
Design systems to have alternate entities
Monitor for possible falures
Manage networks proactively
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 34
Network recovery requirementsNetwork recovery requirements
Network recovery imposes several requirements. For example:there should be backup capacity to create a
recovery path;
the backup capacity must be enough to ensure QoS constraints;
single point of failure must be avoided;
.....
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 35
Recovery and reversion cyclesRecovery and reversion cycles
Recovery Cycle
Reversion Cycle
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 36
Recovery mechanismsRecovery mechanisms
A high variety of recovery mechanisms exist.
Every mechanisms has advantages and drawbacks
In the following slides some criteria that may be used to evaluate and classify recovery mechanisms are reported [3, 4].
[3] V. Sharma et al., “Framework for MPLS-based recovery”, RFC 3469, IETF web site, Feb 2003
[4] K. Owens, V. Sharma, M. Oommen, and F. Hellstrand, “Network Survivability Considerations for Traffic Engineered IP Networks”, Internet draft: draft-owens-te-network-survivability-03, May 2002. Available at: www.ietf.org. Accessed July 2005
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 37
Backup CapacityBackup Capacity
Dedicatedone to one relationship between the backup resources
and the working path;
the simplest solution;
an inefficient solution.
Sharedthe backup resources are shared among different
working path;
a more simple solution;
a more efficient solution.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 38
Recovery PathRecovery Path
Preplannedrecovery paths for all accounted failure scenario is
calculated in advance;
allows fast recovery of failure;
lacks flexibility for unaccounted failure scenarios.
Dynamicthe recover path is calculate “on the fly” when the
failure is detected;
may be used to search recovery paths also for unaccounted failure scenarios.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 39
Recovery ApproachesRecovery ApproachesProtection
the recovery paths are preplanned and fully signaled before a failure occurs;
when a failure occurs no additional signaling is needed to establish the recovery path;
is the faster solution.
Restorationthe recovery pat may be preplanned or dynamically
allocated but are not signaled in advance;
when a failure occurs aditional signaling is needed to establish the recovery path;
is a more flexible solution.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 40
Protection Variants (1/2)Protection Variants (1/2)
1+1 Protection (Dedicated Protection) there is exactly one dedicated recovery path for each working
segment;
the traffic is permanently duplicated on both the working path and the recovery path;
is a quite expensive solution.
1:1 Protection (Dedicated Protection with extra traffic) there is exactly one dedicated recovery path for each working
segment;
the traffic is transmitted over only a path at a time;
it is possible to transport extra traffic along the recovery path in failure free condition.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 41
Protection Variants (2/2)Protection Variants (2/2)
1:N (Shared Recovery With Extra Traffic)each recovery entity is used to protect N working
entities;
it is possible use the recovery entities to transport extra traffic in failure free conditions.
M:N (M ≤ N)a set of M recovery entities are used to protect a set of N
working entities;
it is possible use the recovery entities to transport extra
traffic in failure free conditions.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 42
Recovery Extent (1/2)Recovery Extent (1/2)
Local Recoveryin failure condition only the affected network element
are bypassed using the recovery path;
the RHE and RTE are closer to the failure, so they may detect the failure quickly, leading to a smaller recovery time.
in case of failure the route followed by the traffic may be not optimal (e.g the same traffic may cross a link twice !) .
In case of two successive nodes failure will fail
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 43
Recovery Extent (2/2)Recovery Extent (2/2)
Global Recoveryin failure condition the complete working path between
source and destination is bypassed;
the recovery time is greater that that of the local recovery
an optimal recovery path is used in case of failure;
In case of two successive nodes failure could still resolve the problem;
may generate more “state overhead” that the local approach.
An intermediate solution between Local and Global approach may be adopted !!
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 44
Control of Recovery Mechanisms (1/2)Control of Recovery Mechanisms (1/2)
Centralizeda central controller determines the action to take in
case of failure;
the central controller also determine when and where a fault ha occurred;
the central controller is a single point of failure.
is generally an efficient approach;
in principle is a simpler approach, but
the central controller may become a very complex system;
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 45
Control of Recovery Mechanisms (2/2)Control of Recovery Mechanisms (2/2)
Distributedthere is not a centralized controller, all the network
elements are capable to autonomously react to failure;
with this approach there is not a global view of the network condition;
the network elements may have to exchange information to keep a consistent view of the network;
is a more scalable approach.
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 46
Two or more nodes connected to each other with a ring of links
Protection Topologies - Protection Topologies - RingRing
E
W
W
E
W
EW
E
D
L
L
Working Protect
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 47
Protection Topologies - Protection Topologies - MeshMesh Three or more nodes connected to each other
Can be sparse or complete meshes
Spans may be individually protected with linear protection
Overall edge-to-edge connectivity is protected through
multiple paths
Working
Protect
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 48
Protection Switching TerminologyProtection 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
most common - SONET linear 1:1 and 1:n
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 49
1+1 vs 1:n1+1 vs 1:n
Working Protect Working Protect
(1+1) (1:n)
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 50
SONET Linear SONET Linear 1+11+1 APS APS
BR SW
TX
TX
RX
RX
SW
RX
RX
BR
TX
TX
Working
Protection
Working
Protection
TX = TransmitterRX = Receiver
BR = BridgeSW = Switch
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 51
SONET SONET 1:11:1 Linear APS Linear APS
BR SW
TX RX
RX
SW
RX
BR
TX
TX
TX
RX
APS Channel
TX = TransmitterRX = Receiver
BR = BridgeSW = Switch
Protection
Working
Protection
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 52
Protection Switching: TerminologyProtection 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 backShared protection schemes are usually revertive
Uni-directional or bi-directional protection:Uni: each direction of traffic is handled independent of the
other. Fiber cut => only one direction switched over to protection .
Usually done with dedicated protection; no signaling required.
Bi-directional transmission on fiber (full duplex) => requires bi-directional switching & signaling required
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 53
Mesh RestorationMesh Restoration
DCS
DCS DCS
DCS DCS
DCS
Line or Link Restoration
Working Path
Path Restoration
• Control: Centralized or Distributed• Route Calculation: Preplanned or Dynamic• Type of Alternate Routing: Line or Path
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 54
Link vs. Path restoration Link vs. Path restoration Link restorationLink restoration
• Requires the ability to identify the failed link at both ends.
• Can not protect node failureCan not protect node failure.
• Link based
Mesh (generalized loop back) – insensitive to additions to network – scalable;
backup path can be pre-computed – fast recovery; dynamic rerouting
Path restorationPath restoration
More resilient than link restorationMore resilient than link restoration.
Reroutes the traffic from the primary path to a Shared Risk Group (SRG) -disjoint
backup path.
Protect both end-to-end paths and single linksProtect both end-to-end paths and single links. • Preferred: Path BasedPreferred: Path Based
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 55
Link vs. Path restoration Link vs. Path restoration
A
B
C
D
E
F
Flow 1: A-C-D
Flow 2: E-C-D-F
A
B
C
D
E
F
A
B
C
D
E
F
Link (Generalized Loopback) Restoration
Path Restoration
Fault: Link Cut
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 56
Pre-compute vs. Real-timePre-compute vs. Real-time
Pre-computedPre-computed calculates restoration paths before a failure happens. Allows prior availability of reroute information to the nodes where
actions need to be taken after failure is detected. Enables fast restorationEnables fast restoration.
Real-timeReal-time calculates restoration paths after a failure happens. Restoration is slower. Restoration is slower. Enables more efficient capacity utilizationEnables more efficient capacity utilization.
• Preferred: Pre-computedPreferred: Pre-computed
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 57
Centralized vs. DistributedCentralized vs. Distributed Centralized restoration:Centralized restoration:
Computes restoration and primary paths for all demands with up-to-date information
Routes may then be downloaded into nodal databases. Effectiveness?
• More capacity efficiency More capacity efficiency • Possibly slow (but may be executed in the background)Possibly slow (but may be executed in the background)• Scalability in questionScalability in question.
Distributed restorationDistributed restoration Source and destination nodes dynamically search for the protection
wavelengths required to reestablish the disrupted lightpath Since lack of knowledge of sharing database of other OXCs, it may not be able
to determine backup sharability for any given primary path
• Preferred: Preferred: • Central path determinationCentral path determination• Distributed RestorationDistributed Restoration
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 58
Protection Topologies - Protection Topologies - LinearLinear
Two nodes connected to each other with two or
more sets of links
Working Protect Working Protect
(1+1) (1:n)
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 59
Mesh Restoration vs Ring/Linear ProtectionMesh Restoration vs Ring/Linear Protection
Attributes Linear APS Ring PS MeshRestoration
Spare Capacity Needed Most Moderate Least
Fiber Counts Highest Moderate Moderate
Restoration Time <50 ms <50 ms 2-10 seconds
Software Complexity Least Moderate Most
Protection Against MajorFailures
Worst Medium Best
Planning/OperationsComplexity
Least Moderate/least Most
Extracted from: T-H. Wu, Emerging Technologies for Fiber Network Survivability, See References
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 60
IP layer restorationIP layer restoration
IP Layer Restoration (real-time)IP Layer Restoration (real-time)
Achieved by exchanging control messages between adjacent routers
• Re-determine the affected route
• Update routing tables
• Propagate changes (OSPF, BGP-4)
Capable of recovery from multiple faultsCapable of recovery from multiple faults
Slow (10s of seconds to minutes – Fumagalli)Slow (10s of seconds to minutes – Fumagalli) requires online processing upon failure
• Fault discovery:
• Explicitly: ICMP messaging
• Implicitly: Expiring of timers
Guarantees networkwide survivability Guarantees networkwide survivability
Independent of underlying physical networkIndependent of underlying physical network
Physical
Data Link
Network (IP)
Transport
Session
Presentation
Application
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 61
MPLS layer restorationMPLS layer restoration
MPLS Layer ProtectionMPLS Layer ProtectionReal-time or pre-computedReal-time or pre-computedLine or path level protectionLine or path level protectionProtection path is node and link disjointnode and link disjoint from the primary
path. Protection path may be allocated to low-priority trafficallocated to low-priority traffic in
the absence of network failure. Faster than dynamic IP reroutingFaster than dynamic IP rerouting Working LSPs have pre-established node/link disjoint protection Working LSPs have pre-established node/link disjoint protection
pathspaths
Physical
Data Link
Network
Transport
Session
Presentation
Application
MPLS
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 62
Optical layer restorationOptical layer restoration
Optical layer restorationOptical layer restoration
Real-time or pre-computedReal-time or pre-computed
Ring protection or mesh restoration Ring protection or mesh restoration
No visibility into higher layer operations.
May be wasteful use of resourceswasteful use of resources.
• For ring protectionring protection, there is over 100% capacity redundancyover 100% capacity redundancy
• For mesh restoration, 60-80% physical redundancymesh restoration, 60-80% physical redundancy level is typical.
Not recommended for node (or software) failuresNot recommended for node (or software) failures
Faster than higher layer restorations (??)Faster than higher layer restorations (??)
Physical
DWDM (Optical)
Network IP)
Transport
Session
Presentation
Application
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 63
Multilayer Recovery (1/2)Multilayer Recovery (1/2)
In a multilayer network it is possible to imagine a situation in which each layer has its own recovery mechanisms.
Not every failure in a particular layer may be resolved in the same layer.
If a failure may be resolved in several layer uncoordinated actions may produce inefficient results
A coordination among the layers is needed !!
Dipartimento di Informatica e Sistemistica, University of Napoli Federico II – Comics Group 64
Multilayer Recovery (2/2)Multilayer Recovery (2/2)
Sequential Approach[1]
using an hold-off time a chronological order among the recovery mechanisms adopted in different layer is imposed;
alternatively a “token” may used to impose a sequential order among the different layers.
Integrated Approach[1]
there is a recovery scheme that has a full overview of all the layers;
the recovery scheme may decide when and in which layer (layers) the recovery actions must be taken.
[1] D. Colle, et all., “Data-centric optical networks and their survivability”, Selected Areas in Communications, IEEE Journal on Volume 20, Issue 1, Jan. 2002 Page(s):6 - 20