QoS-Aware Path Protection
in MPLS Networks
Ashish Gupta
Ashish Gupta
Bijendra Jain
Satish Tripathi
Indian Institute of Technology
Delhi
University of California
at Riverside
Overview
• MPLS networks
— Special need for path protection
• Approaches to path protection in MPLS
networks
— Link, node based
— LSP based
• Segment Based Approach
— Mechanisms (detection, notification and path
switching)
— Algorithm for segment identification
— Some simulation results
1
Path protection
1 47.1
3
1
1
3
2
2
47.3 3
47.2
2
BACKUP
PATH
•
Routing algorithms for IP networks take seconds to recompute routes
•
Voice & video are sensitive to “switch-over” time
— require switch over in less than 50 to 100 ms
•
Identify and set-up back-up paths a-priori
2
Path protection in MPLS networks
• Complete LSP by-pass
— allocation of resources along back-up path is
efficient
— time to detect failure and switch over is large
— difficult to identify a node-disjoint path that also
meets the specified QoS constraints
6
Path protection in MPLS networks
• Link by-pass
— allocation of resources for back-up paths is
unlikely to be efficient
— fault detection and switch over can be fast
— cannot be sure about QoS resulting from any
failure
— does not address node failure
7
Path protection in MPLS networks
• Node by-pass
— allocation of resources for back-up paths is
unlikely to be efficient
— fault detection and switch over can be very fast
— cannot be sure about QoS resulting from any
failure
8
Segment Based Protection
The Main Idea
Look at the path as a sequence of segments and
protect each segment separately
9
Segment based protection: a proposal
— flexibility in identifying segments
– schemes to protect LSP, links, or nodes are
special cases
— efficient allocation of resources for back-up paths
— bounds on fault detection and switch over time
— ability to identify back-up paths that meet specified
QoS constraints
10
Focus of our paper
develop algorithms to identify
segments, and back-up paths, such
that
— switch over time (time for which packets are lost
between failure and recovery) is bounded
— path resulting from any single failure continues
to satisfy given QoS constraints
— resources are used efficiently (or more
precisely, the number of segments is
minimized)
11
Fault Detection , Location and Notification
• Faults detected using live-ness messages with
periodicity Ttest
• Notification messages to segment switching routers
(SSR)
13
Analysis
Bound on time during which packets are lost
RTT( Ri , Rj ) + Ttest
15
Identifying Segments: A greedy algorithm
• Identify segments such that
— switch-over delay is bounded (for instance, 60 ms)
— Fewest no of segments
• Example computation:
11 hop LSP, Ttest = 10 ms
RTT + Ttest = 45
RTT + Ttest = 51
Segment 2
R0
R1
10
R2
19
R3
11
R4
8
14
R5
R6
13
Segment 3
R7
11
3
R8
10
R9
15
Segment 1
RTT + Ttest = 58
16
R10
10
R11
Another algorithm to identify segments
• Example: consider network with link RTT = 10 ms, Ttest = 5 ms
— bounded switch-over delay of 40 ms
— with as few segments as possible
— disjoint, loop-free back-up paths exist
1010+ +1010+ +105 += 525= <3540
< 40
10 ms
10 ms
10 ms
10 ms
10 ms
Ingress
router
Segment
switch router
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Other algorithms to Identify Segments
• Algorithms to identify segments such that:
— Switch-over time is bounded
— Fewest no. of segments
— disjoint, loop-free back-up paths exist
— QoS constraints are satisfied in case of ONE failure
– End-to-end delay, Jitter, Drop rate
10 ms
10 ms
10 ms
10 ms
10 ms
Ingress
router
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End to End Delay
In this analysis, we need to consider backup paths also.
Max (T + ( T2 – T1 ) ) < 
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End to End Delay
Finding the backup path
d1
d2
d2 + d3
d1 + d2 + d3
d3
d3
0
Dummy node
- Can use shortest-path approach to find the backup path
- Backup path can land at multiple nodes
21
Description of Simulation Setup
• An MPLS network with
— 50 Nodes
— 82 Edges
• Random LSP that require 20 to 70 units of BW
• RTT of each link = 8 to 12 ms
• BW between 3000 and 10000 units
• Periodicity of liveness messages = 2 ms
• BW: 50 to 100
• Results indicate advantages of segment based
approach
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Description of Simulation Setup
• Topology used
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Simulation Results
• BW reserved for back up vs. number of LSP for
different bound on switch-over time
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Simulation Results
• Reserved BW vs. switch-over time
26
Summary
• Segment based approach offers a range of
schemes for path protection
— From link or nodes to segments, to paths
• The approach permits one to insist that back-up
paths continue to provide committed QoS even
when there is a failure
• The approach ensures that resources are reserved
only to the extent necessary
• Many of our algorithms are good, and provably
correct, but may not be optimal
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