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Complexity and Degrees of Freedom in Network Design

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Complexity and Degrees of Freedom in Network Design

Michael Sinatra

University of California, Berkeley

17 July 2007

Internet2/ESCC Joint Techs

http://www.simonizusa.com/default.asp

- Terry Gray, Scott Sagan, Charles Perrow, Todd LaPorte, Martin Landau
- Poorly-designed networks and network disruption devices
- Greg Bell, Greg Travis and everyone who sent me interesting examples after the 5-9s talk

- Single points of “failure”
- Probabilistic analysis of redundancy
- Redundant components can reduce the chances of failure
- A component with a 10% failure probability can be made redundant with another component with a 10% failure probability and yield a 1% system-failure probability
- But there’s a BIG assumption here!

- Components must be fully redundant! Are they?
- Classic example: aircraft engines
- Can you think of some networking examples?

Outside

FW

FW

Switch

Inside

- You must be this tall to really break the network.

- Nick:You're a rotten driver, either you ought to be more careful or you oughtn't drive at all.
- Jordan: I am careful.
- Nick: No you're not.
- Jordan: Well, other people are.
- Nick: What's that got to do with it?
- Jordan: They'll keep out of my way, It takes two to make an
- accident.
- Nick: Suppose you met somebody just as careless as yourself?
- Jordan: I hope I never will, I hate careless people. That's why
- I like you.

- The problem is, there are too many careless devices on the network!

Net

Firewall

LB

Server

Client

- Adds complexity (but reduces it too!)
- Tightens coupling!

Hosts

To border

Router

Switch/FW

- Demanded by high-reliability systems
- Organizational redundancy
- Change management
- Multiple approval/sign-off

- Organizations can be made redundant in the same way as systems…
- …with many of the same problems
- Common-mode failures
- Non-linear complexity
- And more…

- Not really an analogous concept in physical systems
- Change-management difficulties

- Has to do with the way physical systems are designed and operated
- Does anycast DNS encourage bad behavior?

- How do we deal with all of this?
- Points of failure? Really points of freedom (and that’s a bad thing)
- We need to reduce degrees of freedom in networks, not necessarily increase redundancy!
- Networks need to get simpler, not more complex!

- Risks exist where we may not expect them
- Five-nines mentality
- Virtualization
- Network disruption devices: duh!
- Security

- Maybe we shouldn’t assume that the system can be made fully reliable (Travis)

- Need to recognize trade-offs: In complex systems, “win-win scenarios” are very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very RARE!

Conclusion to the Conclusions