Presentation on ARC at University of Luxembourg

1. ARCs
Uni.lu, December 2012
Pascal Thubert (Cisco Systems)

2. Agenda
ARCs Concept
oLAF algorithm
Bicasting
fArctals
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3. Available Routing Constructs
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4. 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
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5. 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
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6. 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
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7. 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
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8. Double breakage
Each ARC is its own domain of fault recovery
cursor
cursor A cursor
cursor
B
cursor
cursor
cursor
cursor
Omega
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9. oLAF
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10. 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
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11. 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
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12. oLAF Example: Initial topology
R
A B
D C
K
M
L E
F
J
N G
H I
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13. 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
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14. 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
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15. 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
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16. 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
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17. 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
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18. 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
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19. 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
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20. 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
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21. 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
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22. 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
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23. 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
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24. 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
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25. 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
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26. 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
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27. 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
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28. 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
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29. 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
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30. 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
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31. Picking N
R
A B
Rev
Rev
D C
K
Rev
Rev
M
L Rev
E
F
Rev
?-A J
N G
H I
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32. 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
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33. 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
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34. 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
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35. 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
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36. 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
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37. 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.
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38. 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
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39. 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
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40. 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
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41. Constructed ARCs
R
B
D A
C
K
M
L
E
F
J
N G H
I
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42. Bicasting
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43. Adding Bicasting to ARCs
Re C Re
Re v v Re
v v
A concept of Left and Right is introduced.
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44. 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
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45. 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
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46. Building L/R indicators (cont’d)
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
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47. Building L/R indicators (cont’d)
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
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48. 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
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49. 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
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50. 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
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51. 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
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52. Collision type 1
Reservation Packets cross in a same arc from different entry points
Resolution is to prune cross-forwarding along the ARC
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53. 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
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54. 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
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55. 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
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56. 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
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57. fArctals
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58. 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
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59. 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
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60. 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
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61. Hierachical Drawing of ARCs in a Ring
topology
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62. Drawing ARCs between rings
We start with a ring topology where rings are interconnected at intersections
points (themselves maybe quite complex inside for redundancy)
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63. 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
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64. 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
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65. 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.
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66. 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
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67. (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
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68. 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).
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69. 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
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70. 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
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71. 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
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72. Multiple interconnections
We draw ARCs between contiguous intersection with another ring.
Each ARC is a redundant path to that next ring
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73. 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
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74. 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
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75. 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
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76. 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
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77. 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
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78. Parallel rings
B A B
A
D C D
C
E F
E
H F G
G
I H I
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79. 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
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80. 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
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81. Questions?
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