luxbg arcs

ARCs

Uni.lu, December 2012

Pascal Thubert (Cisco Systems)

© 2012 Cisco and/or its affiliates. All rights reserved. UnclassifiedUni.lu 2

Agenda

ARCs Concept

oLAF algorithm

Bicasting

fArctals

© 2010 Cisco and/or its affiliates. All rights reserved. 3UnclassifiedBRKEWN-3012

Available Routing Constructs


Arc concept

An Arc is a 2 ended reversible path

Edges are directed outwards; links within are reversible

An arc is resilient to any link or Junction break by returning links

Links are oriented from cursor to edges and returned by moving the cursor.

We build Arcs between Safe Nodes

C

Re

v

Re

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Re

v Re

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EdgeCursor


ARC topology

ARCs form dual or multi-ended structures

• An ARC stitches 2 SPF subpaths together

• ARCs + buttressing ARCs = Comb

• One cursor per ARC / Comb as the water separation line

cursor

cursor

cursor cursor

cursor

cursorcursor

cursor

Omega


Forwarding

In normal operations, traffic flows away from the cursor and cascades

from ARC to ARC along shortest path

cursor

cursor

cursor cursor

cursor

cursorcursor

cursor

Omega


A

Forwarding errors

Are Addressed inside an ARC by returning the incoming link,

In order to exit via the other edge of this ARC

In control plane, this means that the Cursor is placed at the failure location

cursor

cursor

cursor cursor

cursor

cursorcursor

Bcursor

Omega


A

Double breakage

Each ARC is its own domain of fault recovery

cursor

cursor

cursor cursor

cursor

cursor

cursor

Bcursor

Omega


oLAF


Notations for Link types

A is SPF successor of B

B -> A is unresolved for Safe Node S

A B

A B

A B

A B

B is standby alternate on A isolation

Non SPF Link used to join an ARC

A is non shortest path successor of

B

A B

Rev

?-S


LAF (Lowest ARC First)

LAF is a SPF variation that creates ARCs by connecting SPF paths

- The ARCs include the SPF tree

- The algorithm identifies the mono-connected zones

- and provides redundancy inside such zones

R

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B


oLAF Example: Initial topology

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N


Running the modified Algo, Start from R:

A

D

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B

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C

EF

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R

o

o

t

R(A) R(B) A and B are Heir

Since we have a

single root we

create virtual roots

R(A) and R(B)

We note the set

dependent on R(A)

as ?-A for

convenience

?-A

N


Picking A (closest to root), and D, and C:

R

A

D

L

B

K

J

C

EF

GH

I

Then pick

Pick D,

Pick C,

Each time place in

the parent set?-A ?-A

?-A

N


Picking B:R(A)

A

D

L

B

K

J

C

EF

GH

I

Pick K, start

building up B’s

dependent set

?-A ?-A?-B

M

R(B)

?-A ?-B

N


Picking M and J:

A

D

L

B

K

J

C

EF

GH

I

- The dependent

sets grow.

?-A ?-A?-B

?-BM

?-B

R(A) R(B)

?-A ?-B

N


Running the AlgoPicking L and then E:

A

D

L

B

K

J

C

EF

GH

I

?-B

?-B

?-A?-A

?-A ?-A

M

?-B

?-A ?-BR(A) R(B)

N


Picking G:

A

D

L

B

K

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C

EF

GH

I

?-B

?-B

?-A?-A

?-A ?-A

?-A

M

?-B

?-A ?-BR(A) R(B)

N


Picking F; F is a Safe node!

A

D

L

B

K

J

C

EF

G

H I

Examining F’s

neighbors we find J

that is B-dependent

F has 2 non

congruent path to 2

Safe Nodes, though

virtual this time

since they are R(A)

and R(B)

?-B

?-B

?-A?-A

?-A

?-A

?-A

?-A

M

?-B

?-A ?-BR(A) R(B)

N


We can form the first infrastructure ARC!

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B

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We can use F-J to

tie F’s shortest path

to R(A) with J’s

shortest path to

R(B)?-B

?-B

?-A?-A

?-A

?-A

?-A

?-A

M

?-B

?-A ?-B


All nodes along the ARC are Safe

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Rev

Rev

Rev

Rev

Rev

N

M

Nodes along the

ARC are placed

alone in there own

dependent set

(not represented)

All other nodes are

returned to the

original set


Next is D

R

A

D

L

B

K

J

C

EF

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

- D depends on A

- D can reach C

which is in

another set

?-A


D is a collapsed ARC

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A

L

B

K

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C

EF

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

D’s parent A and

D’s preferred

neighbor C are both

Safe NodesD


Next is M

R

A

L

B

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C

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Rev

Rev

Rev

Rev

Rev

N

M

- Same goes for M

?-K

D


M is a collapsed ARC

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Rev

Rev

Rev

Rev

Rev

N

D

M


Picking L

R

A

L

B

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Rev

Rev

Rev

Rev

Rev

N

D

M

?-D

All depend on D at

this point


Picking E

R

A

L

B

K

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C

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Rev

Rev

Rev

Rev

Rev

N

D

M

?-D?-D

All depend on D at

this point


E has links to C and F

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Rev

Rev

Rev

Rev

Rev

N

D

M

?-D?-D

E has links that end

deeper than D’s

collapsed ARC


E adds a buttressing ARC

R

A

L

B

K

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C

F

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

We can form a

buttressing ARC

keeping E’s links

that end deeper

than D’s collapsed

ARC

E->D becomes this

reversible

L returns to the set

D being the Cursor

of the origin ARC is

cursor for the Comb

E

D

Rev


Picking L

R

A

L

B

K

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C

F

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

L forms its own

collapsed ARC

E

D

Rev?-D


Picking N

R

A B

K

J

C

F

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

E

D

L

?-A

Rev


Picking G

R

A B

K

J

C

F

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

E

D

Rev

?-E

L

?-A


Picking H

R

A B

K

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C

F

GH

I

Rev

Rev

Rev

Rev

Rev

N

M

E

D

Rev

?-E

L

?-A ?-E


H adds a buttressing ARC

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A B

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G

I

Rev

Rev

Rev

Rev

Rev

N

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E

D

Rev

L

Rev

H


Picking N and G again

R

A B

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C

F

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I

Rev

Rev

Rev

Rev

Rev

N

M

E

D

Rev

L

Rev

H

?-E?-A


Picking N again

R

A B

K

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C

F

I

Rev

Rev

Rev

Rev

Rev

N

M

E

D

Rev

L

Rev

HG

?-A


Picking N again and then I

R

A B

K

J

C

F

I

Rev

Rev

Rev

Rev

Rev

N

M

E

D

Rev

L

Rev

H

We’re done with the

set

N is still dependent

N’s subgraph is

monoconnected

If N has a

dependent set we

run the algorithm in

that set using N as

root.G

?-A


Original Graph and Classical rev-SPF

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Original Graph and SPF-based DAG

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Only 3 nodes are Safe but in all cases packet end in Single point of failure

waterbasins


Original Graph and resulting construct

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Constructed ARCs

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Bicasting


Adding Bicasting to ARCs

A concept of Left and Right is introduced.

C

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v


Omega

Building L/R indicators

When an ARC is formed, each end is associated to a side.

At least one Right (green) and one Left (orange) per ARC

cursor

cursor

cursor cursor

cursor

cursorcursor

cursor

H1 H2 H3 H4 H5 H6


Building L/R indicators (cont’d)

Nodes between cursor and edge are associated to the edge side

For an edge ending at Omega, the association is free form

cursor

cursor

cursor cursor

cursor

cursorcursor

cursor

H1 H2 H3 H4 H5 H6

Omega



For and ARC ending in another ARC, the end is associated to

the same heir as the node the ARC exits into. This keeps ARC

bicasting routes close to shortest path.

cursor

cursor

cursor cursor

cursor

cursorcursor

cursor

H1 H2 H3 H4 H5 H6

Omega



In case of collision (both ends of an ARC select the same heir)

• One end picks that heir (shortest path)

• The other picks the heir of the other end of the ARC it falls into

cursor

cursor

cursor cursor

cursor

cursorcursor

cursor

H1 H2 H3 H4 H5 H6

Omega


A

Bicasted routing (ex)

cursor

cursor

cursor

cursor

cursorcursor

2 packet copies are colored by the colors of the ARC through which

the original packet is injected

Packet copies exit ARCs by the the edge corresponding to their color.

Below, the black path is shortest whereas the

orange and green paths are Left and Right paths (via H2 and H6)

cursor

Bcursor

H2 H6

Omega


A

Bicasted routing (2nd ex)

L/R Packets are routed away along there E/W tag,

=> independent of the cursor.

L/R tagging is used to prevent re-U-turning in a same ARC.

cursor

cursor

cursor

cursorcursor

cursor

Bcursor

cursor

H2 H5

Omega


A

Bicasted routing (2nd, with breakage)

E/W Packets are routed away along there E/W tag,

=> independent of the cursor.

E/W tagging is used to prevent re-U-turning in a same ARC.

cursor

cursor

cursor

cursorcursor

cursor

Bcursor

cursor

Omega


A

Bicasted reservation (ex)

cursor

cursor

cursor

cursor

cursorcursor

Reservation Packets are routed away along there E/W tag,

For traffic coming back from root (bi-casted, in red)

Collisions are identified and resolved (next slides)

cursor

Bcursor

Omega


Collision type 1

Reservation Packets cross in a same arc from different entry points

Resolution is to prune cross-forwarding along the ARC


Collision type 2

Reservation Packets enter a same arc at a same entry point

This means an incoming ARC faced a coloring collision (orange

below)

Resolution is to return the second reservation packet along its ARC

And prune the u-turn path. Say orange arrived first; green is sent

back.

cursor

Coloring collision: ends in

orange but cannot be

orange


Collision type 2 (cont’d)

The returned reservation packet arrives on the other end with the

wrong color for that end, which is also the color of the other end.

If the packet cannot be forwarded with its original color it is recolored

to any color but that of its copy. Returning the packet for a collision is

equivalent to a breakage or a missing link if the graph is not

biconnected.

cursor

cursorcursor


Collision type 2 (alt)

Resolution is still to return the second reservation along its ARC

And prune the u-turn path. Say orange arrived last and is sent back.

In this example the orange packet does not need to be recolored at

the other end since that terminates in an ARC that has an orange

end.

cursor


Collision type 2 (alt cont’d)

Now we are back to a collision of type 1 which is resolved by pruning

opposite paths along the ARC


fArctals


Laying labels.

Destination

We lay omega ( labels for a destination at the level of the DODAG of

ARCs

Within an ARC, the omega label is encapsulated in a lambda ( label

path that describes the ARC and is independant of the destination. The

lambda label is semected and pushed at the ARC ingress and popped

at the ARC egressLambda

labels

switched

path along

an ARC in

both dir.

Inner omega

label pops for

DODAG level

switching


Main labels along an ARC

We build to Label switch paths along an arc

Left to Right (dark blue) and Right to Left (dark red)

labels are classical MPLS labels

Note the A edge node is selecting one of its exit to build a LSP, this selection

could be balanced upon Omega labels

CB

A

C

D

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Re

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v

Intermediate Junction

edge Junction Exit node

Cursor Junction

1

2 3

4

1

23

4

21

12

13


Backup labels along an ARC

We build to backup Label switch paths along an arc

Left to Right (light red, backup of red) and Right to Left (light blue)

labels are classical MPLS labels

The light label paths are fast reroute backup for the dark labels

The light labels do not have back ups

CB

A

C

D

Re

v

Re

v

Re

v Re

v

Intermediate Junction

edge Junction Exit node

Cursor Junction

21

12

13 1bis

2bis 3bis

4bis

1bis

2bis3bis

4bis


Hierachical Drawing of ARCs in a Ring topology


Drawing ARCs between rings

We start with a ring topology where rings are interconnected at intersections

points (themselves maybe quite complex inside for redundancy)


Hierarchical ARC computation in rings (cont’d)

The Interconnection type (simple, double or more intersections) does not

matter at this level of the hierarchy. All that counts is whether rings connect


Hierarchical ARC computation in rings (cont’d)

Each ring is abstracted as a virtual supernode and a ring connection is

abstracted as a link in the super topology.

Complex connection points can be itemized at that level as well


Computing the Omega label Path

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To reach a destination R we compute an ARC set over the super topology.

ring

connection

between

rings

destination


Result of the OLAF algorithm:

DAG visualization

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L

B

K

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C

EF

GH

I

== ARC visualization

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Rev

Rev

Rev

Rev

Rev

RevMM

NN Rev


(Alt) Result of the COMB algorithm:

Buttressing ARC links

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NRev

RevRev

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NH

Comb vizualization


Computing Omega Labels

Now we have computed an omega

ARC set to destination R. In a Ring,

this determines the egresses and the

cursor location

Again we can lay 4 Omega Labels

per Omega ARC. Omega labels are

per destination but not processed

within the rings

Note that for Combs, the labels follow

the labyrinth logic (Combs and comb

path are discussed in another paper)

Omega Labels are switched at the

connection points. They are invisible

within the Rings (they are

encapsulated in lambda labels).

E

R

A

D

L

B

K

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C

F

G I

M

NH


Omega level ARC (in ARC set of rings)

Dbis

CbisBbis

Abis

B

C D

E

a f

b c

a

b

c

f

Ebis

DbisCbis

Bbis

A

B C

D


Drawing ARCs within a ring (simple connection)

When a Ring has a simple connection to another we draw a single ARC that

goes around this ring to reach the other on both side of the connection


Olympic style rings

Olympic style rings have a two intersection to one another so we draw a pair of

ARCs that interconnect the connection points that lead to the next ring


Multiple interconnections

We draw ARCs between contiguous intersection with another ring.

Each ARC is a redundant path to that next ring


Label encapsulation

When a packet to is injected in a ring ARC,

A MPLS label is added to allow forwarding along the ARC

In ARC, is the destination node in which FEC are defined

ARC label

( )packet

Incoming label

Preferred switched

label

L

Backup switched label Rbis

A table indicates the preferred and the backup labels along the ARC

The preferred is the one that goes away from the cursor

FEC label

( )


In infrastructure Junction e.g. B

Incoming label B Bbis Bbis B

Preferred switch label A Abis Cbis B

Encapsulating ARC

Alternate label Bbis N/A N/A Bbis

A

B

Cbis

Bbis


Operation in infra junction ring

Incoming label B Bbis Bbis B

Preferred switch label A Abis Cbis B

Encapsulating ARC

Alternate label Bbis N/A N/A Bbis

• Packet incoming Ring with label B.

• Table indicates to encaps in ARC towards A’s intersections

• A Label is added to forward the packet away from cursor

• Say the ARC is broken in that direction

• is switch to bis to follow the backup label path towards the other edge

• Say the ARC is broken there too. bis has no backup so it is popped

• Thus forwarding over to A is broken. A is switched to alternate Bbis

• Packet now follows ARC towards ring C over the backup LSP


In edge Junction e.g. A

Incoming label A abis bbis cbis

Preferred switch label a b c Abis

Encapsulating ARC

Alternate label abis bbis cbis Bbis

a

b

c


Operation in edge Junction

Incoming label A abis bbis cbis

Preferred switch label a b c Abis

Encapsulating ARC

Alternate label abis bbis cbis Bbis

• Packet incoming Ring with label A.

• Table indicates to encaps in ARC towards a’s intersections

• A Label is added to forward the packet away from cursor

• Say the ARC is broken in that direction

• is switch to bis to follow the backup label path towards the other edge

• Say the ARC is broken there too. bis has no backup so it is popped

• Thus forwarding over to a is broken. a is switched to alternate abis

• Packet now follows ARC towards ring b over

• All possible exits a, b, c are tried and if all fail packet goes back to B


Parallel rings

B

C D

E F

A

G

IH

B

C

D

E

F

A

G

I

H


Parallel rings (cont’d)

E

A

H

B

C D

E F

A

G

IH

B

C D

E F

A

G

IH

B

C D

E F

A

G

IH


Label encapsulation

When a packet to is injected in a ring ARC,

A MPLS label is added to allow forwarding along the ARC

In ARC, is the destination node in which FEC are defined

ring label

( )packet

ARC to reach the next hop in a Ring

ARC to reach the next ring

ARC to reach the destination

interring label

( )

intra label

( )


Questions?

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