Restoration by Path Restoration by Path Concatenation:Concatenation: Fast Recovery of MPLS PathsFast Recovery of MPLS Paths

Anat Bremler-Barr Yehuda Afek Haim Kaplan Tel-Aviv University

Edith Cohen Michael Merritt AT&T Labs-Research

AgendaAgenda

MPLS - quick introduction

A fast restoration scheme for MPLS

MPLS: Multi Protocol MPLS: Multi Protocol Label SwitchingLabel Switching

Fast forwarding (eliminate IP-

lookup)

Traffic Engineering & QoS

Two motivating forces:Two motivating forces:

IP Lookup forwardingIP Lookup forwarding

IP lookup - given an IP address, determine the next hop for reaching that destination

Fast Address lookup key component for high performance routers

1011001101011011001111110101 Destination Address

Prefix NxtHop* 400* 12011101110* 310000001* 3 10110* 3101111* 510110011 * 210110011010* 4

Forwarding Table

Multi Protocol Label Switching

Label– Short, fixed-length packet identifier

–Label swapping (similar to forwarding algorithm used in Frame Relay and ATM)

IP PacketIP PacketMPLS Header

Incoming Label Mapping

In(port, label)

Out(port, label)

(1, 2)(1, 6)(1, 8)

(2, 13)

(2, 17)(2, 21)(4, 7)

(3, 32)

LabelOperation

Swap

SwapSwap

Swap

8IP

7IP

–Incoming Label Mapping (ILM)

Port 3

Port 1

Port 4

Port 2

MPLS Forward Equivalence Class MPLS Forward Equivalence Class (FEC)(FEC) The same label to a stream/flow of IP

packets:

– Forwarded over the same path

– Treated in the same manner

FEC/label binding mechanism

– Currently based on destination IP address

prefix

– Future mappings based on TE-defined policyIP PacketIP Packet

32-bits

MPLS Header

134.5.1.5

200.3.2.7

1 2

2 6

3 5

200.3.2.1

134.5.6.1

FEC Table

Destination Next Hop

134.5/16

200.3.2/24

(2, 84)

(3, 99)

ILM Table

In Out

(1, 99) (2, 56)

ILM Table

In Out

(3, 56) (5, 3)

ILM Table

In Out

(2, 84) (6, 3)

2

3

MPLS Forwarding ExampleMPLS Forwarding Example

134.5.1.5

134.5.1.5

8484134.5.1.5 33134.5.1.5

3

MPLS Label StackMPLS Label Stack

In OutILM Table

(2, Push [12])(1, 21)

(3, 9) (2, Push [12])

In OutILM Table

(6, 3 )(2, 12)

In OutILM Table

(5, Pop )(1, 3)

In OutILM Table

(2, 56)(4, 21)

(4, 9) (5, 7)

3

1

–Each LSR processes the top label

–Stack of labels in the header

IP PacketIP PacketMPLS Label MPLS Label

IP 21

2 2 6 1 5 4 5

2IP 21 12 IP 21 3 IP 21 IP 56

In conclusion:In conclusion: MPLS benefits:

+No IP lookup+Traffic engineering+QoS- Restoration

Fault TeardownCalculate – loop freeEstablish

Restoration by Path Restoration by Path Concatenation:Concatenation: Fast Recovery of MPLS PathsFast Recovery of MPLS Paths

Part IIPart II

Restoration By Path Restoration By Path ConcatenationConcatenation ( (RBPC)RBPC)

Restore by concatenating existing paths

s t

m

Main claim:Main claim: Unweighted case: Any shortest path after k edge failures is a

concatenation of at most k+1 original surviving shortest paths.

Weighted case: k+1 paths and k edges

The basic set of Paths: Either All shortest paths or One shortest path for each pair of routers.

ExampleExample

st

Two edge failures - concatenation of three paths

One edge failure - concatenation of two paths

no

m

Path Concatenation with MPLSPath Concatenation with MPLS• Use the stack of labels mechanism:• source pushes two labels (one fault)

30

8727

Ingress Routing Table (FEC)Destination Next Hop

134.5/16

200.3.2/24

(1, 30)

(2, 87)

1

2

134.5/16

(2, 27|87)

200.3.2/24

No changes in ILM

tables

s t

Concatenation mechanism inConcatenation mechanism in ATM or WDM ATM or WDM

VC Table of S

t

• Need an IP-lookup at m !!!

V30

V87 V27

m

s

Dest label (vci/vpi) port

t V30 1

m V87 2

V87 2

21

The restoration method The restoration method requirementsrequirements

• Global knowledge at Ingress LSR

• Store the global view locally (on a disk)

Limitations of RBPCLimitations of RBPC•Bandwidth reservation: have not yet dealt with

•Non shortest paths: Requires T.E. Algorithms at the source

•Theory does not apply to node failure

•Does not, in general work in directed graphs

s t

v

Main claim:Main claim: Unweighted case: Any shortest path after k edge failures is a

concatenation of at most k+1 original surviving shortest paths.

Weighted case: k+1 paths and k edges

The basic set of Paths: Either All shortest paths or One shortest path for each pair of routers.

Unweighted case: sketch of Unweighted case: sketch of proofproof

Let p be the shortest path after removing k edges. Let bypasses {bp1, bp2, bp3, bp4} be:

s t

s t

e1 e2 e3

Claim: There are at most k bypasses ==> Main claim

e1

p

e2e1 e3 e2

u v x w

Proof by contradiction:

•Assume there are more than k bypasses•Then exists p* (s->t), s.t., p* is shorter than p.

constructing p*:

claim: exists a subset of bypasses, s.t., each removed edge occurs in an even number of bypasses.

s tp

e1 e1 e2 e2

s t

x y x y z w z w

p

e1 e1 e2 e3 e2

e1 e1 e2 e2

s t

Building blocks for the shortest path p*:

p

x y x y z w z w

e1 e1 e2 e2

s tp

x y x y z w z w

P* must exist - Euler st

x

y

z

w

e1 e2 e3

p*

Building blocks for the shortest path p*:

Pre-provisioned methodPre-provisioned method For each link & LSP (label swapping path) going over it maintain (pre-

provision) a restoration path

Similarly, for each two links in an LSP maintain a restoration path

Huge O/H: ILM tables

Not scalable

The restoration method The restoration method benefitsbenefits• Fast restoration

• Static set of paths

• No messages for tearing down and setting up

• Static & Small ILM tables

• Only one router changes the FEC table.

•Speed and simplicity of pre-provisioned restoration paths without the associated overhead.

Empirical resultsEmpirical results

NameName NodesNodes LinksLinks Avg. degreeAvg. degree

ISPISP ~200 ~400 ~3.7

InternetInternet 40,377 101,659 5.035

AS GraphAS Graph 4,746 9,878 4.16

AS

AS Graph

After one link failureAfter one link failureNetwork max ILM Avg ILM. Avg. Concate Length. savings savings s.factor

ISP weighted 12.5% 25.6% 2.05 1.15

ISP unweighted 20.0% 32.3% 2 1.14

Internet 16.7% 22.8% 2 1.08

AS graph 25.0% 32.7% 2 1.19

RBPC ILM table size / pre-provisioned t.s.

After two link failuresAfter two link failures

Network max ILM Avg ILM. Avg. PC length Length. savings savings s.factor

ISP weighted 2.3% 6.1% 2.38 1.77

ISP unweighted 3.6% 8.5% 2.20 1.34

Internet 3.0% 4.7% 2.06 1.15

AS graph 7.1% 16.4% 2.09 1.32

RBPC ILM table size / pre-provisioned t.s.

After one router failureAfter one router failureNetwork max ILM Avg ILM. Avg. PC length Length.

savings savings s.factor

ISP weighted 25.0% 43.7% 2.10 1.38

ISP unweighted 20.0% 36.8% 2.03 1.18

Internet 12.5% 21.1% 2.02 1.08

AS graph 25.0% 38.5% 2.03 1.26

s t

v

RBPC ILM table size / pre-provisioned t.s.

After two router failuresAfter two router failuresNetwork max ILM Avg ILM. Avg. PC length Length.

savings savings s.f.

ISP weighted 5.26% 11.1% 2.43 1.57

ISP unweighted 6.67% 13.3% 2.21 1.44

Internet 2.50% 4.1% 2.23 1.17

AS graph 8.33% 18.5% 2.17 1.31

RBPC ILM table size / pre-provisioned t.s.

End End