Path Protection in MPLS Networks

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Path Protection in MPLS Networks
Design and Evaluation of Fault Tolerance
Algorithms with Performance Constraints

• Ashish Gupta
• Ashish Gupta

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Our Work

• Fault Tolerance in MPLS Networks
• Issues
• QoS Constraints
• Expeditious Path Restoration
• Bandwidth Efficiency
• There is a tradeoff

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QoS Parameters

• Important parameters
• Switch-Over Time
• End-to-End Delay
• Reliability
• Jitter
• Have to minimize bandwidth usage

MPLS
ADVANCED NETWORKING LAB
PATH PROTECTION
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QOS Parameters
Switch-Over Time Switch-Over Time is the time
for which the packets will be dropped in case a
failure along the LSP End-to-End Delay The
transmission time of a packet to reach the
destination node from the source Reliability
The probabilistic measure of reachability of the
destination from the source Jitter Jitter is
the deviation from the ideal timing of receiving
a packet at the destination
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Path Protection

• A disjoint backup path is allocated along with
  the primary path
• Local Path Protection
• Global Path Protection
• Segment Based Approach A General Approach to
  Path Protection

MPLS
ADVANCED NETWORKING LAB
PATH PROTECTION
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Segment Protection

• 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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Optimization Problem
The structure of backup path(s) and its peering
relationship with the primary path affects the
QoS Constrains
The Design of backup LSPs must address both BW
efficiency and expeditious path restoration
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Explanation of QoS Parameters
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Switch-Over Time

• Ensure
• Switch-Over time
• RTT( Si , Si1 ) Ttest lt delta
• Where delta is maximum permissible packet loss
  time

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End-to-End Delay
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End-to-End delay

• Ensure
• Max (T ( t2 t1 ) ) lt EED Bound

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Jitter

• Ensure
• Max Jitter from source to destination over all
  backup paths lt Jitter bound

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Problem Statements
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Theoretical Model

• Let G (R,L) describe the given network where L
  has the following properties ltB,pB,bB,D,pgt
• R set of routers
• L set of links
• B Bandwidth of the Links
• pB Primary Path bandwidth reserved
• bB Backup Path bandwidth reserved
• D Delays of the Links
• P Reliability

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Switch-Over Time

• General Problem Statement
• Input
• A Network N, LSP ltR0,,Rngt and Switch-over time
  bound ?.
• Output
• A set of segment switch routers S lt S0,, Sk gt
• Such that
• S0 R0 , Sk Rn
• In case of a fault, the max packet loss time
  while rerouting is lt ?
• RTT ( Si , Si1 ) Ttest lt ?
• No of segments is minimized.

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Consideration of Backup Paths

• Input
• A network N, a LSP ltR0,,Rngt and a switch-over
  time bound ?
• Output
• A set of segment switch routers S and backup
  paths ltpi0,,pingti0..k-1
• Such that
• S0 R0 , Sk Rn
• In case of a fault, the max packet loss time
  while rerouting is lt ?
• RTT ( Si , Si1 ) Ttest lt ?
• No of segments is minimized.

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End-to-End Delay

• General Problem Statement
• Input
• A network N, a LSP ltR0,,Rngt , switch-over time
  bound ?, end-to-end delay bound ?
• Output
• A set of segment switch routers S and backup
  paths ltpi0,,pingti0..k
• Such that
• S0 R0 , Sk Rn
• In case of a fault, the max packet loss time
  while rerouting is lt ?
• RTT ( Si , Si1 ) Ttest lt ?
• No of segments is minimized.
• Backup path constraints

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Jitter

• General Problem Statement
• Input
• A network N, a LSP ltR0,,Rngt , switch-over time
  bound ?, jitter bound J
• Output
• A set of segment switch routers S and backup
  paths ltpi0,,pingti0..k
• Such that
• S0 R0 , Sk Rn
• In case of a fault, the max packet loss time
  while rerouting is lt ?
• RTT ( Si , Si1 ) Ttest lt ?
• No of segments is minimized.
• Backup path constraints

Jitter
Jitter
Jitter
J
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Jitter

• General Problem Statement
• Input
• A Network G and Packet Loss time bound delta and
  jitter bound deltaj . an ingress Node a and an
  egress node b between which a connection of
  bandwidth y has to be routed.
• Output
• A primary path between a and b , a set of segment
  switch routers S and set of backup paths BP.
• Such that
• S0 a
• In case of a fault, maximum jitter bound is
  deltaj
• Max ( t2 t1 ) lt deltaj
• RTT ( Si , Si1 ) Ttest lt delta
• Bandwidth resources are conserved
• No of segments is minimized or S is minimum(
  Transformation )

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Algorithm
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Reliability

• General Problem Statement
• Input
• A network N, a LSP ltR0,,Rngt , switch-over time
  bound ?, minimum reliability requirement r
• Output
• A set of segment switch routers S and backup
  paths ltpi0,,pingti0..k
• Such that
• S0 R0 , Sk Rn
• In case of a fault, the max packet loss time
  while rerouting is lt ?
• RTT ( Si , Si1 ) Ttest lt ?
• No of segments is minimized.
• Backup path constraints
• Minimum reliability is r

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RELIABILITY - 1

• How Backup Path Improves Reliability

Link Reliability pe n links each in the
primary and backup paths. Reliability from A to B
without a backup path p Reliability from A to B
with backup path 2 p p2
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RELIABILITY - 2
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RELIABILITY - 3

• How Backup Path Improves Reliability

A
B
Link Reliability pe n links each in the
primary and backup paths. Reliability from A to B
without a backup path pn Reliability from A to
B with backup path 2 pn p2n
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RELIABILITY - 4
Total number of links in primary path n Size of
Backup Path Size of Segment Size of Segments
k Assume no sharing of backup paths
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RELIABILITY - 5

• Reliability of a link p
• Reliability of a segment 2pk p2k
• Number of Segments n/k
• Reliability of the path (2pk p2k)n/k

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RELIABILITY 6
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Algorithm

• How to calculate reliability
• Given segment heads, find the most reliable
  backup paths
• Find segment heads

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How to Calculate Reliability?

• NP-Complete problem, even when failure
  probability is same for all links.
• For a graph G with edge reliability pe for edge
  e,

where O is the set of operational states.
Therefore we will have to estimate reliability of
a path by using upper and lower bounds.
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Graph Transformations

• Node to Link Reliability

• Merging
• Serial
• Parallel

pe
pe pf - pe pf
pf
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Approximating Reliability

• Consider a path from link A to B
• Total number of links in primary and backup paths
  n
• Reliability of a link p
• Probability ( failure of k links )
• nck pn-k (1-p)k

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Probability of k links failing
Probability that 0 or 1 or 2 links failed
0.9861819
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Approximating Reliability
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Approximating Reliability

• Number of States with 0 link failure nc0
• Probability of occurrence of this state pn
• Probability that a path exist 1
• Number of States with 1 link failure nc1
• Probability of occurrence of this state
  pn-1(1-p)
• Probability that a path exist 1
• Number of States with 2 link failure nc2
• Probability of occurrence of this state
  pn-2(1-p)2
• Probability that a path exist From
  Simulation(say q)

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Approximating Reliability

• Lower Bound
• nc0 pn 1.0 nc1 pn-1(1-p) 1.0 nc2
  pn-2(1-p)2 q
• Upper Bound
• 1 - nc2 pn-2(1-p)2 (1-q)

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Lower Upper Bounds
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Reliability
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Finding Reliable Backup Paths
Given the segment heads, we can find backup paths
that maximizes reliability of the network.
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Finding Segment Heads

• Approach 1
• Consider all possible segmentations.
• Approach 2
• Find the best possible segmentation without
  considering reliability while segmenting.
• Divide segments to improve reliability till
  reliability becomes greater than required.

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Algorithm

• Which segment to divide first?
• Divide segment with maximum reliability first
• Divide segment with maximum reliability first
• Divide longest segment first
• Random

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Future Work

• Algorithm for protection meeting reliability
  criteria
• Optimization issues Bandwidth , capacity
• Implementation of these algorithms in emulator
  and experimental setup