A General approachA General approachto MPLS Path Protectionto MPLS Path Protectionusing Segmentsusing Segments

Ashish GuptaAshish GuptaAshish GuptaAshish Gupta

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Overview

• Intro to MPLS— Difference from IP

• Why Path Protection ?— Existing Schemes

• Segment Based Approach— Its Mechanisms

— Algorithm for segment setup

— Simulation Results

— Detection , Notification and Path Switching

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Label Switched Path (LSP)

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The Need for Path Protection

What happens if fault occurs in a network element ?

For traffic with critical QOS requirements , fast rerouting is required

IP rerouting can take order of seconds

Solution : Protect the path with another backup path

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Existing Schemes

• Global Path Protection

• Local Path Protection

• Link Failure

• Node Failure

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Original LSP

Backup LSP

Drawback : No flexibility in providing path protection for a MPLS network

Segment Based Approach : A General Scheme for Path Segment Based Approach : A General Scheme for Path ProtectionProtection

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Segment Based Approach

•Protect each segment separately : Each segment seen as a single unit of failure

•SSR – Segment Switching router

•Flexibility in creating segments -> flexibility in Path Protection ( delay and backup paths )

•SBPP – Segment Based Path Protection

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Steps in SBPP

• Creation of LSP

• Creation of segments - Greedy Algorithm

• Reservation of Backup Paths— Backup paths as tunnels

• A new combined Algorithm— Advantages

• Label Management in SBPP

• Changes required in LSR

• Label Distribution Mechanisms

• Signaling mechanisms

• Buffering to avoid packet loss and reordering

• Steps in recovery :— Fault Detection and Localization

— Fault Notification– How does it work in MPLS ?

— Switching the path

— Backup Path recovery

• Experimental Results

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Fault Detection , Localization and Notification

• Fault can be detected by periodically sending liveness messages – Absence of response indicates link/node failure

• For faster detection , each node sends periodic messages to its neighbors

• Timing Analysis for Detection and Notification

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Fault Detection , Localization and Notification

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Creation of Segments

• Created according to QOS criteria— Delay , Reliability , Bandwidth

• Just ensure each segment individually meets the criteria

• Example - Bounded Delay on switching— Greedy Algorithm

Some Problems - Experiments

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Issues in Reservation of Backup Paths

• Avoiding Loops

• Sharing of backup paths important— Cases :

– 1. Multiple LSPs , Multiple Segments– 2. Multiple LSPs, Same Segment

— Assumptions : Only one failure at a time

• Problem with the previous approach – see figure

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Loops in Backup Paths

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Problem with Greedy Algorithm

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A New Combined Algorithm

• Possible approaches— Exhaustive search for a suitable path – computationally exhaustive –

need a heuristic

• The Combined Path Setup Algorithm— 1. Setup a primary path ( based on a constraint e.g. min delay)

— 2. Start from egress node and find the largest possible segment which satisfies bounded delay switching time constraint ( call the SSR of this segment S1 )

— 3. Find a backup path for this segment starting from S1

— 4. If no backup path can be found , shrink the segment and try to find the backup path from the new SSR. If no further shrinking is possible then Reject request( or try another primary path - see below)

— 5. Repeat Step 4 until a segment with a backup path is found.

— 6. Repeat from step 2 for creating the next segment

— 7. Do this until the complete LSP is segmented.

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Advantages of this algorithm

• Ensures that if segmentation is possible on the primary path, then it will be performed.

• Here we have multiple starting nodes possible for finding the backup paths , so possibility of finding backup paths is more

• Can add more flexibility for the choice of SSR in forming segments e.g. case of overloaded LSR – won’t be made a SSR

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Description of Simulation Setup

• An MPLS network of was created— 100 Nodes

— 200 Edges

• RTT of each link = 10 ms

• Periodicity of Liveness message = 2 ms

• BW – 50 to 100

• Generated large number of random LSPs requests and observed various parameters

• Results indicate advantages of SBPP

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Segment Size vs BW reserved

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Segment Size vs BW reserved

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Segment Size vs Rejection Rate ( for 250 LSPs )

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No. of Requested LSPs vs Rejection Rate

Effect of Backup Path Effect of Backup Path SharingSharing

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Bandwidth reserved vs No. of LSPs setup

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Crossover - Effects of backup path sharing

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Further Analysis – More possibilities

• End-to-end delay of Backup Path also affects switching time !

• Long backup paths : Higher end-to-end delay : Higher Switching time so have to constrain backup path construction also

• New expression for switching time

— Tp + RTT + (t2-t1) < max. switching delay

• Can help in providing bound in other performance metrics like jitter

Steps in ReroutingSteps in Rerouting

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A Mechanism for Notification

• After a fault is detected, notification needs to be sent to the SSR for switching the traffic

• Some nodes will participate in notification and the SSR will switch the route

• What information will be passed after a fault occurs ?

• What changes do we need in the LSR tables for switching?

• Case of Multiple LSPs : All LSPs using that segment may not pass through the faulty node/link – Only concerned LSPs should be switched

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A Mechanism for Notification

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Other work

• Creating Backup paths using tunnels

• Analysis of Liveness message periodicity

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Future Work

• Label Management and Distribution Issues

• Formal Definition of Protocol and Signaling Mechanisms required for detection, notification and other parts of our scheme

• Use of buffering to reduce packet loss during switchover

• Recovery Issues

• Implementation of our scheme in MPLS emulator.

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Targets specified in Mid-sem

• December 1st 2001— Error detection and notification issues in Segment based

protection (SBP)

— Work out example scenarios using SBP

— An algorithm for SBP

— Label management issues in SBP

• May 1st 2002— Simulations to test performance and resource usage vs.

other schemes

— Explore other issues like Buffering

— Documenting our work

Thank YouThank You