Is Centralized Really Simpler?Russ White

just about everyone right now…

This was simpler…

This was simpler…

So we built these just to make ourlives more complex… right?

And then recentralized because it was simpler…

We just keep doing this centralize/decentralize thing don’t we?

…let’s consider this for a bit…

The

Prob

lem

Spa

celeast state

least surface optimal util

consistency

accessibility partitioning

CAP

complexity

CAP Theorem

Complexity least state versus optimal util

2001:db8:3e8:10::/64

2001:db8:3e8:11::/64

2001:db8:3e8:12::/64

2001:db8:3e8:23::/64

2001:db8:3e8:20::/64

2001:db8:3e8:21::/64

2001:db8:3e8:22::/64

2001:db8:3e8:13::/64

::/0 == 50% chance of selecting the shorter path

Complexity least state versus optimal util

2001:db8:3e8:10::/64

2001:db8:3e8:11::/64

2001:db8:3e8:12::/64

2001:db8:3e8:23::/64

2001:db8:3e8:20::/64

2001:db8:3e8:21::/64

2001:db8:3e8:22::/64

2001:db8:3e8:13::/64

/60’s == 75% chance of selecting the shorter path

Complexity least state versus optimal util

2001:db8:3e8:10::/64

2001:db8:3e8:11::/64

2001:db8:3e8:12::/64

2001:db8:3e8:23::/64

2001:db8:3e8:20::/64

2001:db8:3e8:21::/64

2001:db8:3e8:22::/64

2001:db8:3e8:13::/64

/64’s == 100% chance of selecting the shorter path

Centralized CP• Separating the control

plane from the forwarding device

• Adds a new interaction surface

• Partitions a database across the network that was previously stored on a single device

• But, merges the control plane onto a single device

• This is not the real world

new interaction surface (API)

newly partitioned database

Centralized CP• In the real world,

you need more than one controller

• The tradeoff is not so clear in this situation

new interaction surface (API)

newly partitioned database

existing partitioned database

Centralized Control Plane•We are adding complexityA new interaction surface between the forwarding device and the controller

•We are adding “distance” in an existing partitionBetween the RIB and the FIB

•We are (probably) reducing the number of copies of another partitionThe routing database

Added Complexity

Centralized Control Plane•What are we gaining?“It’s simpler to implement filters…”“It’s simpler to do traffic engineering…”“It’s simpler to build virtual topologies…”•These are optimizations—…in resource usage…in security…in policy Gain

A Worry•We (seem to be/are) trading operational/policy efficiency against control plane efficiencyThis is what we should expect according to the complexity model

• Is this a good trade?How many balloons fit in a bag?• Is there any way around this?

??

These…are never printed on this…

Does this help?•Policy and reachability intentionally split•Allows different tradeoffs for each• Increases interaction surfaces•Reduces state in both

Real World• Distributed RP

Modified IS-ISCarries IPv6, IPv4, MPLS labels

• Policy OverlayCarries SR label stacks for TE, segmentation, etc.Carries packet filtersModel based publish/subscribe system

Openfabric

Is centralized really simpler?

Conclusion•The wrong questionIs centralized really simpler?•The right questionWhat does it make sense to centralize?