Luxbg arcs

ARCs
Uni.lu, December 2012

Pascal Thubert (Cisco Systems)

Agenda

ARCs Concept
oLAF algorithm
Bicasting
fArctals

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

Available Routing Constructs

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

Arc concept
Cursor Edge
Re C Re
Re v v Re
v v

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


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

cursor
cursor 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

cursor
cursor cursor

Omega

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 A cursor
cursor
B
cursor

cursor
cursor cursor

Omega

Double breakage
Each ARC is its own domain of fault recovery

cursor

cursor A cursor
cursor
B
cursor

cursor
cursor

cursor

Omega

oLAF


Notations for Link types

A B A is SPF successor of B

A B A is non shortest path successor of
B
A ?-S B B -> A is unresolved for Safe Node S

A Rev B B is standby alternate on A isolation

A B Non SPF Link used to join an ARC


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

B
D A

C
K

M
M
L
E
F
J

N G H
I


oLAF Example: Initial topology
R

A B

D C
K
M
L E
F

J

N G
H I


Running the modified Algo, Start from R:
R
o
R(A)
o R(B) A and B are Heir
?-A t
A B Since we have a
single root we
create virtual roots
D C R(A) and R(B)
K We note the set
dependent on R(A)
L as ?-A for
E
F convenience

J

N G
H I


Picking A (closest to root), and D, and C:
R Then pick
?-A Pick D,
A B Pick C,
Each time place in
?-A ?-A the parent set
D C
K

L E
F

J

N G
H I


Picking B:
R(A) R(B) Pick K, start
?-A ?-B building up B’s
A B dependent set
?-A ?-A
?-B
D C
K
M
L E
F

J

N G
H I


Picking M and J:
R(A) R(B)
?-A ?-B
A B
?-A ?-A
?-B
D C - The dependent
sets grow.
K
?-B
M
L ?-B
E
F

J

N G
H I


Running the Algo
Picking L and then E:
R(A) R(B)
?-A ?-B
A B
?-A ?-A
?-B
D C
K
?-A ?-A ?-B
M
L ?-B
E
F

J

N G
H I


Picking G:
R(A) R(B)
?-A ?-B
A B
?-A ?-A
?-B
D C
K
?-A ?-A ?-B
M
L ?-B
E
F
?-A J

N G
H I


Picking F; F is a Safe node!
R(A) R(B) Examining F’s
?-A ?-B
neighbors we find J
A B that is B-dependent
?-A ?-A
?-B F has 2 non
D C congruent path to 2
K Safe Nodes, though
?-A ?-A ?-B virtual this time
?-A
M since they are R(A)
L ?-B
E and R(B)
F
?-A J

N G
H I


We can form the first infrastructure ARC!
R
?-A ?-B We can use F-J to
A B tie F’s shortest path
to R(A) with J’s
?-A ?-A
?-B shortest path to
D C R(B)
K
?-A ?-A ?-B
?-A
M
L ?-B
E
F
?-A J
G
H I


All nodes along the ARC are Safe
R

A B
Nodes along the
Rev ARC are placed
Rev
D C alone in there own
dependent set
K (not represented)
Rev
M
Rev All other nodes are
L E
F returned to the
Rev original set
J

N G
H I


Next is D
R

A B
- D depends on A
?-A Rev - D can reach C
Rev
D C which is in
another set
K
Rev
M
L Rev
E
F
Rev
J

N G
H I


D is a collapsed ARC
R

A B
D’s parent A and
Rev D’s preferred
Rev
D C neighbor C are both
Safe Nodes
K
Rev
M
L Rev
E
F
Rev
J

N G
H I


Next is M
R

A B
- Same goes for M
Rev
Rev
D C
K
Rev ?-K
M
L Rev
E
F
Rev
J

N G
H I


M is a collapsed ARC
R

A B
Rev
Rev
D C
K
Rev
M
L Rev
E
F
Rev
J

N G
H I


Picking L
R

A B
All depend on D at
Rev this point
Rev
D C
K
?-D
Rev
M
L Rev
E
F
Rev
J

N G
H I


Picking E
R

A B
All depend on D at
Rev this point
Rev
D C
K
?-D ?-D
Rev
M
L Rev
E
F
Rev
J

N G
H I


E has links to C and F
R

A B
E has links that end
Rev deeper than D’s
Rev
D C collapsed ARC

K
?-D ?-D
Rev
M
L Rev
E
F
Rev
J

N G
H I


E adds a buttressing ARC
R

A B
We can form a
Rev buttressing ARC
Rev
D C keeping E’s links
that end deeper
K than D’s collapsed
Rev
Rev ARC
M
L Rev
E E->D becomes this
F
Rev reversible
J
L returns to the set
N G
H I D being the Cursor
of the origin ARC is
cursor for the Comb


Picking L
R

A B
L forms its own
Rev collapsed ARC
Rev
D C
K
?-D Rev
Rev
M
L Rev
E
F
Rev
J

N G
H I


Picking N
R

A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
?-A J

N G
H I


Picking G
R

A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
?-A ?-E J

N G
H I


Picking H
R

A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
?-A ?-E ?-E J

N G
H I


H adds a buttressing ARC
R

A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
Rev J

N G
H I


Picking N and G again
R

A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
?-A ?-E Rev J

N G
H I


Picking N again
R

A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
?-A Rev J

N G
H I


Picking N again and then I
R

A B We’re done with the
Rev set
Rev
D C
N is still dependent
K
Rev N’s subgraph is
Rev
M monoconnected
L Rev
E
F
If N has a
Rev
?-A Rev dependent set we
J
run the algorithm in
N G that set using N as
H I root.


Original Graph and Classical rev-SPF
R
R

A B
A B

D C
D C
K
K
M
M
L E
L E F
F
J
J
N G
N G H
H I
I


Original Graph and SPF-based DAG
R
R

A B
A B

D C
D C
K
K
M
M
L E
L E F
F
J
J
N G
N G H
H I
I

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

Original Graph and resulting construct
R
R

A B
A B
Rev
Rev
D C
D C
K
K Rev
Rev
M
M Rev
L E
L E F
F
Rev
Rev J
J
N G
N G H
H I
I


Constructed ARCs
R

B
D A

C
K
M
L
E
F
J

N G H
I


Bicasting


Adding Bicasting to ARCs

Re C Re
Re v v Re
v v

A concept of Left and Right is introduced.


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

cursor
cursor cursor

H1 H2 H3 H4 H5 H6
Omega

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

cursor
cursor 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

cursor
cursor 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

cursor
cursor cursor

H1 H2 H3 H4 H5 H6
Omega

Bicasted routing (ex)
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

cursor A cursor
cursor
B
cursor

cursor
cursor cursor

H2 H6
Omega

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 A cursor
cursor
B
cursor

cursor
cursor cursor

H2 H5
Omega

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 A cursor
cursor
B
cursor

cursor
cursor cursor

Omega

Bicasted reservation (ex)
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

cursor A cursor
cursor
B
cursor

cursor
cursor cursor

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. Coloring collision: ends in
orange but cannot be
orange

cursor


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

cursor
cursor


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.
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
Lambda
at the ARC egress labels
switched
path along
an ARC in
both dir.
Inner omega
label pops for
DODAG level
switching

Destination


Main labels along an ARC
Cursor Junction
edge Junction 3 Exit node
2
4
Re C Re 1

Re B v v C
Re
v 2 3
v
A D
13 1 4
21

12 Intermediate Junction

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

Backup labels along an ARC
Cursor Junction
edge Junction 3bis Exit node
2bis
4bis
Re C Re 1bis

Re B v v C
Re
v 2bis 3bis
v
A D
13 1bis 4bis
21

12 Intermediate Junction

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

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
R destination

A B

ring
D C
K
M
L E connection
F between
rings
J

M G
H I

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

Result of the OLAF algorithm:
R R

A B A B
Rev
Rev
D C D C
K K
Rev
M M
L Rev E Rev
L E
F F
Rev
J J
N G Rev N G
H I H I

DAG visualization == ARC visualization


(Alt) Result of the COMB algorithm:
R R

A B A B
Rev
Rev
D C D
C
K K
Rev
Rev M M
L Rev L
E
F E F
Rev
Rev J J

N G N G I
H Rev I H

Buttressing ARC links  Comb vizualization


Computing Omega Labels
Now we have computed an omega R
ARC set to destination R. In a Ring,
this determines the egresses and the
A B
cursor location

Again we can lay 4 Omega Labels D
per Omega ARC. Omega labels are C
per destination but not processed K
within the rings
M
Note that for Combs, the labels follow L E F
the labyrinth logic (Combs and comb
path are discussed in another paper) J
Omega Labels are switched at the N G I
connection points. They are invisible H
within the Rings (they are
encapsulated in lambda labels).


Omega level ARC (in ARC set of rings)

C D

Cbis E
B Bbis

Abis Dbis

B C
A D
a
Cbis Dbis f
b Bbis
Ebis
a c
f
b c


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 FEC label
packet
( ) ( )

A table indicates the preferred and the backup labels along the ARC
The preferred is the one that goes away from the cursor
Incoming label
Preferred switched L
label
Backup switched label Rbis


In infrastructure Junction e.g. B

B
A
Cbis
Bbis

Incoming label B Bbis Bbis B

Preferred switch label A Abis Cbis B

Encapsulating ARC
Alternate label Bbis N/A N/A 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

a c
b

Incoming label A abis bbis cbis

Preferred switch label a b c Abis

Encapsulating ARC
Alternate label abis bbis cbis Bbis


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
Uni.lu Unclassified
© 2012 Cisco and/or its affiliates. All rights reserved. 77

Parallel rings

B A B

A
D C D
C
E F
E
H F G
G
I H I


Parallel rings (cont’d)

A B A B A B A

C D C D C D

E F E F E F E

G G G

H I H I H I H


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

intra label ring label interring label
packet
() ( ) ( )

ARC to reach the next hop in a Ring
ARC to reach the next ring
ARC to reach the destination


Questions?


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