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