A General approach to MPLS Path Protection using Segments

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A General approachto MPLS Path Protectionusing
Segments

• Ashish 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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MPLS BUILT ON STANDARD IP
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• Destination based forwarding tables as built by
  OSPF, IS-IS, RIP, etc.

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IP FORWARDING USED BY HOP-BY-HOP CONTROL
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MPLS Label Distribution
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Label Switched Path (LSP)
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MPLS ROUTE AT EDGE, SWITCH IN CORE
IP
IP
IP Forwarding
IP Forwarding
LABEL SWITCHING

• Applies concept of VC routing
• Packet forwarding is done based on Label
  Switching
• FEC Destination address prefix, Traffic
  Engineering tunnel, Class of Service.

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WHY MPLS ?

• Ultra fast forwarding
• IP Traffic Engineering
• Constraint-based Routing
• Virtual Private Networks
• Controllable tunneling mechanism
• Voice/Video on IP
• Delay variation QoS constraints

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BEST OF BOTH WORLDS
CIRCUITSWITCHING
PACKETROUTING
HYBRID

• MPLS IP form a middle ground that combines the
  best of IP and the best of circuit switching
  technologies.

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Path Protection and its Purpose

• 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
• 11 Path Protection
• Sharing Modes - 1 1, 1 N, M N

Backup LSP
No flexibility in providing path protection for a
MPLS network
Segment Based Approach A General Scheme for
Path Protection
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Example

• Global Path Protection Large Notification
  Delay
• Local Path Protection Small delay but large
  number of backup paths
• These solutions lie at two extremes.
• Segment Based Approach A General Scheme for
  Path Protection

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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 -gt 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
• Label Distribution Mechanisms
• Signaling mechanisms
• Buffering to avoid packet loss and reordering
• Steps in recovery
• Fault Detection and Location
• Fault Notification How does it work in MPLS ?
• Switching the path
• Backup Path recovery
• Experimental Results

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Steps in SBPP

• For setting up a path request - Local Computation
• Information needed
• Topology
• Link Delays and congestion information
• Bandwidth of each link primary , backup , free
• After computation Label information disseminated

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Fault Detection , Location 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 or Reliability or a combination
• Ensure each segment individually meets the
  criteria
• Example - Bounded Delay on switching
• Greedy Algorithm

Some Problems - Experiments
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Reservation of Backup Paths

• Advantage of SBPP Flexibility in reservation of
  backup paths, not rigid
• Issues
• 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
  wont be made a SSR

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Description of Simulation Setup

• An MPLS network with
• 100 Nodes
• 200 Edges
• RTT of each link 10 ms
• Periodicity of Liveness messages 2 ms
• BW 50 to 100
• Generated large number of random LSP 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
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Effect of Backup Path Sharing
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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 Improvement to Algorithm

• Delay over Backup Path also affects jitter !
• 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) lt max. switching delay
• Improvements to our Algorithm due to this

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Steps 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

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Thank You