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Increasing IP Network Survivability:Increasing IP Network Survivability:An Introduction to Protection MechanismsAn Introduction to Protection Mechanisms

Jonathan SadlerJonathan SadlerLead Engineer - ONG SELead Engineer - ONG SE

October 22, 2000

20

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MotivationMotivation

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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ProtectionProtection

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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ProtectionProtection

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 DomainsProtection 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 TopologiesProtection 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: The path or span that will be used to recover live traffic

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Protection Topologies - LinearProtection Topologies - Linear

Two nodes connected to each other with two or more sets of links

Working Protect Working Protect

(1+1) (1:n)

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Two or more nodes connected to each other with a ring of links

Line vs. Drop interfaces

East vs. West interfaces

Protection Topologies - RingProtection Topologies - Ring

E

W

W

E

W

EW

E

D

LL

Working Protect

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Protection Topologies - MeshProtection 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 MechanismsProtection 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 APSProtection 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

Working Protect

Protect Working

“Switchover”

“OK”

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Bi-directional = both directions are handled as one unitLine 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”.

Pros: Efficient

Cons: Not as fast asother protectionmechanisms

Z-AWorking

A-ZProtect

Z-AProtect

A-ZWorking

Z-AWorking

A-ZWorking

Z-AProtect

A-ZProtect

?

Z

A

?

Protection Mechanisms - BLSRProtection Mechanisms - BLSR

“Switchover”

“OK”

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Protection Mechanisms - BLSR cont’dProtection 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’dProtection 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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Unidirectional = Each traffic direction is independentPath Switched = Not handled “node-by-node”Ring

Source generates two copies of signal

Destination evaluates both copies and chooses “best path” signal

Pros: Low switch time

Cons: Not efficient?

Protection Mechanisms - UPSRProtection Mechanisms - UPSR

A-ZProtect

A

Z

Z-AWorking

A-ZWorking

A-ZProtect

A-ZWorking

Z-AProtect

?

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Protection Mechanisms - MeshProtection 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 SwitchingProtection 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 ConsiderationProtection 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 HierarchyProtection 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 ModelTwo 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?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/SDHProtection 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)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 MPLSApplying 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?Applying Protection to MPLS - How?

Perform constraint based route selection for primary path

Working

Protect

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 protectpath LSP

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Applying Protection to MPLS - How?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? MPLS Protection - General Mesh Mech?

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

OSPF w/ TEIS-IS w/ TE

RSVP-TECR-LDP

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Benefits of a Generalized Control PlaneBenefits 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)Cut-through connection (simplified example)

Four IP Routers operating over Optical Network

Initial overlay network connects routers in a hub / spoke topology

A

B

CD

New direct path is now used for A-D traffic

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

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SummarySummary

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 DeploymentSample Deployment

S O N E TR ing

S O N E TR ing

S O N E TR ing

S O N E TR ing

DACS

DACS

DACS

DACS

T1

Router

DACS

DACS

LATA Router

LATA Router

LA TA LA TA

Distrib. Router

Distrib. Router

Distrib. Router

Distrib. Router

Core Router

Core Router

Core Router

Core Router

Core Router

In te r-connec tion

P o in t

In te r-connec tion

P o in t

In te r-connec tion

P o in t

In te r-connec tion

P o in t

ADM

T1

Router

Router

OC

3 w/

AP

S

Core RouterRouter

OC

3 w/

AP

S

OXC

OXC OXCOXC

OXC OXC

OXC

OXC

OXC

OXC

OXC

OXC

OXC

OXC OXC

OXC OXC

OXCOXC

OXC

OXC OXC

OXC OXC

OXC

Long-H au l N etwork

DACS

DACS

LATA Router

LATA Router

Core Router Core Router

In te r-connec tion

P o in t

Core Router Core Router

OXC OXC

ADM

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Sample Deployment - LATASample Deployment - LATA

SONET Protection inLocal Loop Network

IP Mesh Protection inDistribution Networkfor IP services

SONET Protection inDistribution Networkfor Private Line services

S O N E TR ing

S O N E TR ing

DACS

DACS

DACS

DACS

T1 LATA Router

LATA Router

Router

LA TA

Distrib. Router

D istrib. Router

Router

OC

3 w/

AP

S

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Sample Deployment - Long-HaulSample Deployment - Long-Haul

Private Line and IP services are clients of Optical Core Network

Optical Core Network is a sparse mesh protectedby MPLS mechanisms

OXC

OXC OXCOXC

OXC OXC

OXC

OXC

OXC

OXC

OXC

OXC

OXC

OXC

OXC OXC

OXC

OXC OXC

OXCOXC

OXC

OXC OXC

OXC OXC

OXC

Long-Haul Netw ork

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ReferencesReferences

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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