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 Packet Loss Time Jitter End-to-End Delay Reliability

Have to minimize bandwidth usage

ADVANCED NETWORKING LAB MPLSPATH PROTECTION

Packet Loss Time : Packet Loss time is the time for which the packets will be dropped in case a failure along the LSP

Jitter : Jitter is the deviation from the ideal timing of receiving a packet at the destination

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

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

Expressions

Ensure Packet Loss time

RTT( Si , Si+1 ) + Ttest < deltaWhere delta is maximum permissible packet

loss time Jitter

t2 – t1 < Jitter Bound ( See diagram ) In worst case user doesn’t receive

packets for Max (RTT( Si , Si+1 ) + Ttest + (t2 – t1) )

End-to-End Delay

End-to-End delay

Ensure Max (T + ( t2 – t1 ) ) < EED Bound

Problem Statements

Theoretical Model

Let G = (R,L,B,pB,bB,D) describe the given network where

R= set of routersL = set of linksB = Bandwidth of the LinkspB = Primary Path bw reservedbB = Backup Path bw reservedD = Delays of the Links

Packet Loss TimeGeneral Problem Statement

InputA Network G and Packet Loss time bound delta. An ingress Node a

and an egress node b between which a connection of bandwidth y has to be routed.

OutputA 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, the max packet loss time while rerouting is <

delta RTT ( Si , Si+1 ) + Ttest <= delta

Bandwidth resources are conserved No of segments is minimized or |S| is minimum( Transformation )

JitterGeneral 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.

OutputA 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 ) < deltaj

RTT ( Si , Si+1 ) + Ttest <= delta

Bandwidth resources are conserved No of segments is minimized or |S| is minimum( Transformation )

End-to-End Delay

General Problem StatementInputA Network G and end-to-end delay bound deltaeed . An ingress

Node a and an egress node b between which a connection of bandwidth y has to be routed.

OutputA 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, EED does not exceed delteeed

Max ( T + (t2 – t1) ) < deltaeed

Bandwidth resources are conserved No of segments is minimized or |S| is minimum

( Transformation )

ReliabilityGeneral Problem Statement

InputA Network G and set of reliabilities of each node and link in G . A

lower bound of acceptable reliability p* , an ingress Node a and an egress node b between which a connection of bandwidth y has to be routed.

OutputA primary path between a and b , a set of segment switch

routers S and set of backup paths BP.Such that S0 = a The reliability of the LSP from a to b is greater than a certain

reliability value p* The bandwidth used is minimum

No of segments is minimized or |S| is minimum ( Transformation )

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 - 3

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 = pn

Reliability from A to B with backup path = 2 pn – p2*n

A B

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

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

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)

Reliability

(Upper Bound + Lower Bound)/2

Lower & Upper Bounds

Maximum Difference between Actual & Approximated Reliability