A General approach to MPLS Path Protection using Segments Ashish Gupta Ashish Gupta

1 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

2 MPLS Label Distribution Mapping: 0.40 Request: 47.1 Mapping: 0.50 Request: 47.1

3 Label Switched Path (LSP) IP

4 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

5 Existing Schemes Global Path Protection Local Path Protection Link Failure Node Failure r Original LSP Backup LSP Drawback : No flexibility in providing path protection for a MPLS network Segment Based Approach : A General Scheme for Path Protection

6 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

7 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

8 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

9 Fault Detection, Localization and Notification

10 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

11 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

12 Loops in Backup Paths

13 Problem with Greedy Algorithm

14 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.

15 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

16 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

17 Segment Size vs BW reserved

18 Segment Size vs BW reserved

19 Segment Size vs Rejection Rate ( for 250 LSPs )

20 No. of Requested LSPs vs Rejection Rate

Effect of Backup Path Sharing

22 Bandwidth reserved vs No. of LSPs setup

23 Crossover - Effects of backup path sharing

24 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 —T p + RTT + (t2-t1) < max. switching delay Can help in providing bound in other performance metrics like jitter

Steps in Rerouting

26 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

27 A Mechanism for Notification

28 Other work Creating Backup paths using tunnels Analysis of Liveness message periodicity

29 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.

30 Targets specified in Mid-sem December 1 st 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 1 st 2002 —Simulations to test performance and resource usage vs. other schemes —Explore other issues like Buffering —Documenting our work

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