Path Protection in MPLS Networks Design and Evaluation
- Slides: 38
Path Protection in MPLS Networks Design and Evaluation of Fault Tolerance Algorithms with Performance Constraints Ashish Gupta
Our Work n Fault Tolerance in MPLS Networks Issues n Qo. S Constraints n n n Expeditious Path Restoration Bandwidth Efficiency There is a tradeoff
Qo. S Parameters n Important parameters n n n Switch-Over Time End-to-End Delay Reliability Jitter Have to minimize bandwidth usage ADVANCED NETWORKING LAB MPLS PATH PROTECTION
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
Path Protection A disjoint backup path is allocated along with the primary path n n n Local Path Protection Global Path Protection Segment Based Approach : A General Approach to Path Protection ADVANCED NETWORKING LAB MPLS PATH 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 Qo. S Constrains The Design of backup LSPs must address both BW efficiency and expeditious path restoration
Explanation of Qo. S Parameters
Switch-Over Time Ensure n Switch-Over time RTT( Si , Si+1 ) + Ttest < delta Where delta is maximum permissible packet loss time n
End-to-End Delay
End-to-End delay n Ensure n Max (T + ( t 2 – t 1 ) ) < EED Bound
Jitter n Ensure n Max Jitter from source to destination over all backup paths < Jitter bound
Problem Statements
Theoretical Model n Let G = (R, L) describe the given network where L has the following properties: <B, p. B, b. B, D, p> R L B p. B b. B D P = = = = set of routers set of links Bandwidth of the Links Primary Path bandwidth reserved Backup Path bandwidth reserved Delays of the Links Reliability
Switch-Over Time General Problem Statement Input A Network N, LSP <R 0, …, Rn> and Switch-over time bound . Output A set of segment switch routers S = < S 0, …, Sk > Such that n S 0 = R 0 , Sk = R n n In case of a fault, the max packet loss time while rerouting is < n RTT ( Si , Si+1 ) + Ttest <= n No of segments is minimized.
Consideration of Backup Paths Input A network N, a LSP <R 0, …, Rn> and a switch-over time bound Output A set of segment switch routers S and backup paths {<pi 0, …, pin>: i=0. . k-1} Such that n S 0 = R 0 , S k = R n n In case of a fault, the max packet loss time while rerouting is < n n RTT ( Si , Si+1 ) + Ttest <= No of segments is minimized.
End-to-End Delay General Problem Statement Input A network N, a LSP <R 0, …, Rn> , switch-over time bound , end-to-end delay bound Output A set of segment switch routers S and backup paths {<pi 0, …, pin>: i=0. . k} Such that n S 0 = R 0 , S k = R n n In case of a fault, the max packet loss time while rerouting is < n n n RTT ( Si , Si+1 ) + Ttest <= No of segments is minimized. Backup path constraints
Jitter General Problem Statement Input A network N, a LSP <R 0, …, Rn> , switch-over time bound , jitter bound J Output A set of segment switch routers S and backup paths {<pi 0, …, pin>: i=0. . k} Such that n S 0 = R 0 , S k = R n n In case of a fault, the max packet loss time while rerouting is < n n n RTT ( Si , Si+1 ) + Ttest <= No of segments is minimized. Backup path constraints Jitter J
Algorithm d 1 d 2 + d 3 d 1 + d 2 + d 3 d 3 0
Reliability General Problem Statement Input A network N, a LSP <R 0, …, Rn> , switch-over time bound , minimum reliability requirement r Output A set of segment switch routers S and backup paths {<pi 0, …, pin>: i=0. . k} Such that n S 0 = R 0 , S k = R n n In case of a fault, the max packet loss time while rerouting is < n n RTT ( Si , Si+1 ) + Ttest <= No of segments is minimized. Backup path constraints Minimum reliability is r
RELIABILITY - 1 n 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 – p 2
RELIABILITY - 2
RELIABILITY - 4 Segment Heads Backup Paths Total number of links in primary path = n Size of Backup Path = Size of Segments = k Assume no sharing of backup paths
RELIABILITY - 5 Reliability of a link : p Reliability of a segment = 2 pk – p 2 k Number of Segments = n/k Reliability of the path = (2 pk – p 2 k)n/k
RELIABILITY – 6
Algorithm n n n How to calculate reliability Given segment heads, find the most reliable backup paths Find segment heads
How to Calculate Reliability? n n NP-Complete problem, even when failure probability is same for all links. For a graph G with edge reliability pe for edge e, n where O is the set of operational states. n Therefore we will have to estimate reliability of a path by using upper and lower bounds.
Graph Transformations n Node to Link Reliability pn A n Merging n Serial n Parallel pn A 1 pe pe pf pf A 2 Pe *pf pe + pf - pe *pf
Approximating Reliability n n Consider a path from link A to B Total number of links in primary and backup paths = n n Reliability of a link : p n Probability ( failure of k links ) nc k * pn-k * (1 -p)k
Probability of k links failing Probability that 0 or 1 or 2 links failed = 0. 9861819
Approximating Reliability n Number of States with 0 link failure : nc 0 Probability of occurrence of this state : pn Probability that a path exist : 1 n Number of States with 1 link failure : nc 1 Probability of occurrence of this state : pn-1(1 -p) Probability that a path exist : 1 n Number of States with 2 link failure : nc 2 Probability of occurrence of this state : pn-2(1 -p)2 Probability that a path exist : From Simulation(say q)
Approximating Reliability n Lower Bound n n c 0 * pn * 1. 0 + nc 1 * pn-1(1 -p) * 1. 0 + nc 2 * pn-2(1 -p)2 * q Upper Bound 1 - n c 2 * pn-2(1 -p)2 * (1 -q)
Lower & Upper Bounds
Reliability
Finding Reliable Backup Paths R 1 R 2 R 3 R 4 R 5 R 6 R 7 R 8 R 9 R 10 r 1012 r 912 R 11 R 12 r 1112 1 Given the segment heads, we can find backup paths that maximizes reliability of the network.
Finding Segment Heads Approach #1 n Consider all possible segmentations. Approach #2 n Find the best possible segmentation without considering reliability while segmenting. n Divide segments to improve reliability till reliability becomes greater than required.
Algorithm Which segment to divide first? n Divide segment with maximum reliability first n Divide longest segment first n 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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