Increasing IP Network Survivability:
An Introduction to Protection Mechanisms
20
October 22, 2000
Jonathan Sadler
Lead Engineer - ONG SE
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Motivation
 There is increasing demand to carry mission critical
traffic, real-time traffic, and other high priority traffic over
the public internet
 Any network that carries critical, high-priority traffic needs
to be resilient to faults
 As network technologies continue to improve and
converge, protection and restoration schemes have
become available at multiple layers
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Protection
 What is it?
 Automated mechanism for recovering traffic path
 Invoked when the current working path fails
 Requirements
 Fast restoration time
 Voice / video / data can tolerate small outages ( 50ms)
 Predictable
 Protection path is pre-determined
 Can be dedicated (1+1) or shared (M:N)
 Can be preemptive
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Protection
 How is protection different from dynamic rerouting?
 Dynamic rerouting develops a new path utilizing current
network state information
 Delay incurred as state updates are flooded through network
 Time to re-converge on new end-to-end path is long
 Therefore time until destinations become re-reachable is long
 Side Effect: State information will be received by nodes that are
not involved in restoration causing unnecessary CPU usage
 While best effort services may tolerate this behavior, new
services will not
 VoIP
 Virtual leased line
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Protection Domains
 Method of dividing up a network into separate sub-
networks in which a protection mechanism will operate
 Cross domain coordination is required
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Protection Topologies
 Within a protection domain, a number of protection
topologies may be used
 Linear
 Ring
 Mesh
 For any topology the following terminology applies:
 Working: The path or span being used to carry live traffic
 Protect:
traffic
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The path or span that will be used to recover live
Protection Topologies - Linear
 Two nodes connected to each other with two or more sets
of links
Working
Protect
(1+1)
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Working
Protect
(1:n)
Protection Topologies - Ring
 Two or more nodes connected to each other with a ring of
links
 Line vs. Drop interfaces
 East vs. West interfaces
W
D
E
L
E
L
W
Working
Protect
W
E
E
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W
Protection Topologies - Mesh
 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
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Protection Mechanisms
 Protection mechanisms are the algorithms which will
restore services carried by a specific network topology
 Typically take advantage of topology characteristics
 Two different approaches exist
 Link oriented
 Multiple links that support end-to-end connectivity can be
individually switched to restore service
 Path oriented
 Two paths exist which can be “globally” switched to restore
service
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Protection Mechanisms - Linear APS
 Two nodes connected to each other with two or more sets
of transmission facilities
 Receiving node will signal source node to change from
working to protect facility via out-of-band communication
“Switchover”
Protect
Working
Working
Protect
“OK”
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Protection Mechanisms - BLSR
 Bi-directional = both directions are handled as one unit
Line Switched = multiple nodes reconfigure line behavior
Ring
 Node that determines need for change will signal out-of-
band to other node. All intermediate nodes on protect
path then “reconfigure”. A
“OK”
?
 Pros: Efficient
 Cons: Not as fast as
other protection
mechanisms
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A-Z
Working
Protect
Z-A
Working
Protect
Z-A
Working
Protect
“Switchover”
A-Z
Working
Protect
?
Z
Protection Mechanisms - BLSR cont’d
 How is this efficient?
 Each node is involved in reconfiguring when a protection
switch is necessary. Consequently, each node knows if the
bandwidth reserved for a service is actually in use.
 If a specific route is declared the “primary route” for the
service, then the protect path will only be used when trying
to restore a failure on the primary route.
 As a result, it is possible to insert a second signal on the
protect path.
 When a protection switch is necessary to handle the higher
priority traffic, then the “Extra Traffic” will be removed by the
nodes as part of the switchover activity.
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Protection Mechanisms - BLSR cont’d
 Why is more time needed for a protection switch?
 Signaling latency
 Traffic cross connect activation / deactivation in
intermediate nodes
 Definitely needed when Extra Traffic is in use
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Protection Mechanisms - UPSR
 Unidirectional = Each traffic direction is independent
Path Switched = Not handled “node-by-node”
Ring
 Source generates two copies of signal
 Destination evaluates both copies and chooses “best
path” signal
A
?
 Pros: Low switch time
 Cons: Not efficient
A-Z
Working
Protect
Z-A
Working
A-Z
Working
Protect
Z-A
Protect
?
Z
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Protection Mechanisms - Mesh
 End-to-End Path Oriented
 Requires:
 Topology Discovery
 Constrained Route Selection (x2)
 Primary route
 Protection route

Resource affinity (diversity)
 Signaling Protocol
 Service setup
 Protection switchover
 No standard solutions (yet)
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Protection Mech. - Revertive Switching
 Once the failed path has been restored, should the traffic
be moved back?
 Non-revertive Switching
 Done when failed path is no longer going to be used with
service (i.e. service rolls)
 Revertive Switching
 Automatic

System determines primary path is acceptable
 Wait to Restore Time
 Manual

Technician determines primary path is acceptable
 Good in cases where the fault is experienced only under load
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Protection Domain Consideration
 What should be the scope of repair?
 Global Repair
 Traffic is restored using facilities within the global network
 Local Repair
 Traffic is restored using the minimum amount of facilities
 Lacks network view, leading to potentially inefficient resource
utilization
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Protection Hierarchy
 Protection functionality is defined for:
 Optical Layer
 SONET
 ATM / Frame Relay
 MPLS / IP
 How should all these layers interact?
They shouldn’t
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Two Layer Recovery Model
 Most providers are adopting a two-layer model, where:
 Very-fast bulk restoration is done as close to the transport
media as possible
 Optical Switching
 SONET where Optical Switching is not available
 Service level restoration is done at the specific service layer
 SONET -- VT1.5, STS-1, STS-3c, STS-12c, STS-48c services
 ATM / FR -- Switched Data Services
 MPLS -- IP Services
 Layers in between are not used for restoration
 Service level restoration timers are set so that transport
restoration can be attempted first
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Two Layer Recovery Model - Why?
 Why have two layers instead of one?
 Optical switching allows for the greatest number of services
to be restored with the least amount of overhead
 Optical switching will find out about physical failures first
 Loss of light
 Optical AIS
 Optical protection domains are typically smaller than
service-level protection domains, reducing signaling time
 Service layers understand service specific performance
requirements best, but may have a large number of services
to restore
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Protection in SONET/SDH
 Topologies / Mechanisms Available
 1+1 Linear APS
 UPSR
 BLSR
 2-fiber

Restoration channels must be reserved, reducing protected capacity
 4-fiber -- two sets of Tx/Rx fibers for each line interface

Span Switch: Can restore by utilizing alternate Tx/Rx fibers
 Ring Switch: Utilizes restoration channels located on a separate ring
 Extra Traffic possible
 APS, BLSR signaling done in K0 / K1 bytes of overhead
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Protection in SONET/SDH (cont’d)
 Failure Criteria
 Loss of Signal (LOS)
 Loss of Frame (LOF)
 Threshold Crossing
 Bit Error Rate (BER)
 Coding Violations (CV)
 Excessive SONET Pointer Justifications
 Alarm Indication Signal (AIS)
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Applying Protection to MPLS
 What does this do for me?
 Provides fast restoration of MPLS services
 Can be done on a service-by-service basis. For example:
 Best effort could be biased to use Extra Traffic links
 Bronze could be put on unprotected, but avoid Extra Traffic
 Silver could be protected 1:n
 Gold could be protected 1+1
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Applying Protection to MPLS - How?
 Perform constraint based route selection for primary path
 Signal creation of working path LSP
 Perform constraint-based route selection for secondary
path, adding a constraint which removes links that do not
meet diversity requirements
 Signal “reservation” of protect
path LSP
Working
Protect
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Applying Protection to MPLS - How?
 Extensions to IS-IS / OSPF
 Utilizes the same Constraint Routing extensions as TE
 New constraint: Shared Resource Link Group (SRLG)
 Used for diversity determination
 Extensions to CR-LDP / RSVP-TE
 Add Protection LSP declaration to ERO
 Add Reverse Notification Tree & Fault Notification Messages
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MPLS Protection - General Mesh Mech?
 End-to-End Path Oriented
 Requires:
 Topology Discovery
 Constrained Route Selection (x2)
 Primary route
 Protection route

OSPF w/ TE
IS-IS w/ TE
Resource affinity (diversity)
 Signaling Protocol
 Service setup
 Protection switchover
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RSVP-TE
CR-LDP
Benefits of a Generalized Control Plane
 Extension of MPLS to non-IP technologies allows for:
 Rapid provisioning of lower layer connections
 Optical trails
 SONET / SDH trails
 Cut-through connections
 Reduces traffic load on core routers
 Extension of IP semantics (i.e. diff-serv)
 Validates services that paid for protection are protected
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Cut-through connection (simplified example)
 Four IP Routers operating over Optical Network
 Initial overlay network connects routers in a hub / spoke
topology
 High traffic load exists between Router A and D
 Router A realizes need for direct path (based on link load
threshold crossing), and signals request for path into network
 New direct path is
B
now used for A-D
traffic
D
A
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C
Summary
 New services require mechanisms to recover working
traffic as fast as possible
 Optical Layer protection tools provide restoration with the
least amount of overhead
 Service Layer protection is also necessary
 MPLS-TE with extensions can provide protection support
for IP Networks
 Can be extended to support any mesh network
 Use of MPLS to integrate Optical and IP control planes
allows IP service semantics to control protection
mechanisms used at lower layers
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Sample Deployment
LATA
Long-Haul Network
Distrib. Router
LATA
LATA Router
LATA Router
Distrib. Router
Interconnection
Point
SONET
Ring
DACS
SONET
Ring
DACS
DACS
SONET
Ring
DACS
ADM
SONET
Ring
Core Router Core Router
OXC
Router
Router
OXC
DACS
DACS
DACS
Distrib. Router
Interconnection
Point
LATA Router
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
Core Router
LATA Router
OXC
Core Router
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
Core Router Core Router
Interconnection
Point
Distrib. Router
Interconnection
Point
Core Router
OXC
Core Router
Core Router
Interconnection
Point
T1
ADM
Core Router
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OXC
/
3w
OC PS
A
/
3w
OC PS
A
T1
DACS
OXC
Router
Router
Sample Deployment - LATA
 SONET Protection in
LATA
Local Loop Network
 IP Mesh Protection in
Distrib. Router
Distribution Network
for IP services
SONET
Ring
 SONET Protection in
Distribution Network
for Private Line services
SONET
Ring
/
3w
OC S
AP
T1
DACS
Router
Router
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DACS
Distrib. Router
LATA Router
DACS
DACS
LATA Router
Sample Deployment - Long-Haul
 Private Line and IP services
Long-Haul Network
are clients of Optical Core
Network
 Optical Core Network is a
OXC
OXC
sparse mesh protected
by MPLS mechanisms
OXC
OXC
OXC
OXC
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OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
OXC
References
 GR-253-CORE, “Synchronous Optical NETwork (SONET) Transport Systems:
Common Generic Criteria,” Issue 2 rev 2, (Bellcore, January 1999)
 GR-1230-CORE, “SONET Bi-directional Line Switched Ring (BLSR) Equipment
Generic Criteria,” Issue 4, (Bellcore, December 1998)
 GR-1400-CORE, “SONET Dual-Fed Unidirectional Path Switched Ring (UPSR)
Equipment Generic Criteria,” Issue 2, (Bellcore, January 1999)
 draft-owens-te-network-survivability-00.txt, “Network Survivability
Considerations for Traffic Engineered IP Networks,” (IETF, March 2000)
 draft-ietf-mpls-recovery-frmwrk-00.txt, “Framework for MPLS-based Recovery,”
(IETF, September 2000)
 draft-chang-mpls-path-protection-01.txt, “A Path Protection / Restoration
Mechanism for MPLS Networks,” (IETF, July 2000)
 draft-chang-mpls-rsvpte-path-protection-ext-00.txt, “Extensions to RSVP-TE for
MPLS Path Protection,” (IETF, June 2000)
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