Path Protection in MPLS Networks
Ashish GuptaAshish Gupta
Design and Evaluation of Fault Tolerance Algorithms with Performance Constraints
Our Work Fault Tolerance in MPLS Networks
Issues QoS Constraints
Expeditious Path Restoration Bandwidth Efficiency There is a tradeoff
QoS Parameters
Important parameters Switch-Over Time End-to-End Delay Reliability Jitter
Have to minimize bandwidth usage
ADVANCED NETWORKING LAB MPLSPATH PROTECTION
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
QOS Parameters
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
ADVANCED NETWORKING LAB MPLSPATH PROTECTION
Segment Protection
• 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
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
Explanation of QoS Parameters
Switch-Over Time
Ensure Switch-Over time
RTT( Si , Si+1 ) + Ttest < delta
Where delta is maximum permissible packet loss time
End-to-End Delay
End-to-End delay
Ensure Max (T + ( t2 – t1 ) ) < EED Bound
Jitter
Ensure Max Jitter from source to destination
over all backup paths < Jitter bound
Problem Statements
Theoretical Model
Let G = (R,L) describe the given network where L has the following properties: <B,pB,bB,D,p>
R = set of routersL = set of linksB = Bandwidth of the LinkspB = Primary Path bandwidth reservedbB = Backup Path bandwidth reservedD = Delays of the LinksP = Reliability
Switch-Over TimeGeneral Problem Statement
InputA Network N, LSP <R0,…,Rn> and Switch-over time bound .
OutputA set of segment switch routers S = < S0,…, Sk >
Such that S0 = R0 , Sk = Rn
In case of a fault, the max packet loss time while rerouting is <
RTT ( Si , Si+1 ) + Ttest <= No of segments is minimized.
Consideration of Backup Paths
Input
A network N, a LSP <R0,…,Rn> and a switch-over time bound
OutputA set of segment switch routers S and backup paths {<pi0,
…,pin>:i=0..k-1}
Such that S0 = R0 , Sk = Rn
In case of a fault, the max packet loss time while rerouting is < RTT ( Si , Si+1 ) + Ttest <=
No of segments is minimized.
End-to-End Delay
General Problem StatementInputA network N, a LSP <R0,…,Rn> , switch-over time bound , end-
to-end delay bound OutputA set of segment switch routers S and backup paths {<pi0,
…,pin>:i=0..k}
Such that S0 = R0 , Sk = Rn
In case of a fault, the max packet loss time while rerouting is < RTT ( Si , Si+1 ) + Ttest <=
No of segments is minimized. Backup path constraints
Jitter
General Problem StatementInputA network N, a LSP <R0,…,Rn> , switch-over time bound , jitter
bound JOutputA set of segment switch routers S and backup paths {<pi0,
…,pin>:i=0..k}
Such that S0 = R0 , Sk = Rn
In case of a fault, the max packet loss time while rerouting is < RTT ( Si , Si+1 ) + Ttest <=
No of segments is minimized. Backup path constraintsJitter JitterJitter J
Algorithm
d1 d2 d3
d1 + d2 + d3
d3
0
d2 + d3
ReliabilityGeneral Problem Statement
InputA network N, a LSP <R0,…,Rn> , switch-over time bound ,
minimum reliability requirement rOutputA set of segment switch routers S and backup paths {<pi0,
…,pin>:i=0..k}
Such that S0 = R0 , Sk = Rn
In case of a fault, the max packet loss time while rerouting is < RTT ( Si , Si+1 ) + Ttest <=
No of segments is minimized. Backup path constraints Minimum reliability is r
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
RELIABILITY - 2
RELIABILITY - 4
Segment Heads
Backup Paths
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
RELIABILITY - 5
Reliability of a link : pReliability of a segment = 2pk – p2k
Number of Segments = n/kReliability of the path = (2pk – p2k)n/k
RELIABILITY – 6
Algorithm
How to calculate reliability
Given segment heads, find the most reliable backup paths
Find segment heads
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.
Graph Transformations
Node to Link Reliability
A
pn
A1 A2
pn
Merging Serial
Parallel
pe pf Pe *pf
pe
pf
pe + pf - pe *pf
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
Probability of k links failing
Probability that 0 or 1 or 2 links failed = 0.9861819
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)
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)
Lower & Upper Bounds
Reliability
Finding Reliable Backup Paths
R1 R5 R6R7
R8 R9 R10 R11 R12R2 R3 R4
r912
r1012 r11
12
1
Given the segment heads, we can find backup paths that maximizes reliability of the network.
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.
Algorithm
Which segment to divide first? Divide segment with maximum
reliability first Divide segment with maximum
reliability first Divide longest segment first Random
Future Work
• Algorithm for protection meeting reliability criteria
• Optimization issues – Bandwidth , capacity
• Implementation of these algorithms in emulator and experimental setup
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