1

Maximizing Restorable Throughput in MPLS Networks

Reuven CohenDept. of Computer Science, Technion

Gabi NakiblyNational EW Research Center

Published in Infocom 2008 – mini-conference

2

Motivation

IP networks are required to service real-time applications such as phone conversation

These services demand high availability and reliability, and in particular– Fast restoration– Guaranteed QoS even in the case of failures

IP routing protocols are not able to provide these features MPLS protection mechanisms are able to provide these features

– by pre-establishment of backup LSPs We study the effectiveness of the various MPLS protections schemes

3

Outline

Define the various MPLS protection schemes Define our optimization metric Define four different problem models Present algorithms for the various protections mechanisms and models Present simulations results for the various algorithms

4

The protection schemes we study

1. A Global Recovery scheme (GR)– For each LSP we find a path between

the same (S,D) pair that does not use any link of the LSP

– The backup path can protect against any failure along the LSP

2. A Local Recovery scheme (LR)– For each link A-B we find a path that

starts at A and ends between B and D– Recovery is faster than GR (because

it is initiated by the detecting node)– However, more backup LSPs are

needed for the protection of each LSP

SS DDAA BB

SS DDAA BB

a standard MPLS schemea standard MPLS scheme

5

The protection schemes we study (cont.)

3. A Restricted Local Recovery scheme (RLR)

– The backup path for link A-B is established between A and B

4. A Facility Local Recovery scheme (FLR)

– Same as RLR, except that the new path serves all the LSPs that use the failed link

SS DDAA BB

AA BB

a standard MPLS schemea standard MPLS scheme

6

The protection schemes we study (cont.)

5. An extended k-facility Local Recovery scheme (EkFLR)

– Same as FLR, except that the number of LSPs protected by each backup path is limited to k

– Hence, we can use more backup paths for the failed LSPs

6. An Unrestricted Recovery scheme (UR)

– The backup path for every link can use any route and can protect any number of LSPs

AA BB

SS DDAA BB

7

Our optimization criterion

Most past research aims at minimizing the total bandwidth reserved for the backup LSPs (Spare Capacity Allocation).

– Such models consider a network with unbounded capacities, and a cost function associated with bandwidth usage.

We believe that network operators struggle with a different problem:– They have a network with finite link capacities and seek to maximize the

traffic that can be admitted with protection. Our optimization criterion: constructing primary and backup LSPs

while maximizing throughput.

8

Our four problem models

A capacitated directed network We make the common “single-failure” assumption. A set of source-destination pairs with associated BW demands and

profits.

Splittable Unsplittable

Each flow can be split over several primary or backup paths Each flow can be partially satisfied

One primary LSP and one backup LSP All or nothing

Primary-restricted Primary + Backup

the primary LSPs are given in advance

We also need to establish the primary LSPs (joint optimization)

9

Our results

1. We show that the splittable version of the problem is in P and we offer a polynomial time algorithm for it.

2. We show that the unsplittable version of the problem is NP-complete and has no approximation algorithm with a ratio better than |E|½.

3. We propose an approximation algorithm with that ratio.

4. We present efficient heuristics for the various recovery schemes.

5. We compare the various recovery schemes with respect to our throughput maximization criterion.

We show that UR, GR and, LR differ only marginally in their performance. Since LR has the fastest restoration time of the three schemes, it should

be the scheme of choice. We show that EkFLR with k=2 has almost the same performance as RLR and

should be preferred over it. Due to its lower administrative overhead (fewer backup LSPs).

10

Complexity results - summary

1. S-PRFP (Splitable, Primary restricted)

2. U-PRFP (Unsplitable, Primary restricted)

3. S-RFP (Splitable, joint primary/backup optimization)

4. U-RFP (Unplitable, joint primary/backup optimization)

11

The Splittable Primary-restricted Restorable Flow Problem (S-PRFP)

It is in P for all recovery schemes. We showed it using the following linear program:

– - the fraction of flow f routed over edge e when edge e fails– - the routed fraction of f

Maximize the profit

S-PRFPprimary route is already given

12

LP common constraints

The following constraints are common to all recovery schemes:

(C1) = flow conservation (C2) = capacity constraints (C3) = a flow is routed on its primary LSP as long as there is no failure (C4) = a flow is not routed over a failed link

S-PRFP

13

The recovery-specific LP constraints for LR

This rule ensures that the backup LSP will follow the primary LSP all the way from the source to A.

From node A to the destination node, the backup LSP is not constrained.

S-PRFP

SS DDAA BB

14

The recovery-specific LP constraints for RLR

RLR-1 is similar to LR-1, except that it also ensures that the backup LSP will follow the primary LSP from B to the destination.

S-PRFP

SS DDAA BB

16

The recovery-specific LP constraints for GR

GR-1 ensures that the backup LSPs must be edge disjoint with the primary LSP. GR-2 and GR-3 ensures that the backup LSPs are identical for every failure.

S-PRFP

SS DDAA BB

17

The Splittable Restorable Flow Problem (S-RFP)

Joint primary and backup LSP optimization The same linear program but without the primary LSP constraint (C-3). Can only be applied to RLR scheme.

S-RFP

26

The “penalty of reliability” in the Splittable Primary-restricted model

For each backup scheme we find the ratio OPT_S-PRFP/OPT_S-PFP For all schemes: it is easier to protect when load is lower

optimal algorithm optimal algorithm w/ restorationw/ restoration

optimal algorithm optimal algorithm w/o restorationw/o restoration

As expected, UR is the best As expected, LR is better than RLR The advantage of GR over LR is

interesting

SS DDAA BB

SS DDAA BB LRLR

RLRRLR

SS DDAA BB GRGR

29

The penalty of unsplittable backup LSPs (for primary-restricted)

We see here the penalty ratio for using unsplittable backup

– As a function of the load in the network

Surprisingly, the penalty decreases as the load increases.

Can be explained by the fact that the primary LSPs traverse the shortest-paths.

30

The benefit of joint optimization (primary + backup)

As expected, as the load in the network increases so does the penalty of using primary LSP set in advance.

The penalty increases for network with higher average degree.

31

Conclusions

The first comprehensive study of maximizing restorable throughput in MPLS networks

We considered 4 models of the problem and 6 restoration schemes The splittable versions are in P The unsplittable versions are all NP-complete, and they cannot be

approximated within |E| ½-

LR should be the recovery scheme of choice

SS DDAA BB