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The Role of Inter-Controller Traffic in SDN Controllers Placement

Tianzhu Zhang, Andrea Bianco, Paolo Giaccone

IEEE NFV-SDN: Palo Alto, CANovember 8th, 2016

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Software Defined Networking

• programmability • enabled by centralized view of the

network state

Advantages

• scalability for large networks

One relevant challenge

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Distributed controllers

• fault-tolerance and resilience• load balancing, thus higher scalability

Motivation

• how the network state is distributed across the controllers to allow a centralized logical view

Critical issue

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Control-plane in distributed controllers

• in-band for large networks (e.g. SD-WAN)• switch-controller (Sw-Ctr) traffic

– standard protocols (e.g. OpenFlow)• controller-controller (Ctr-Ctr) traffic

Switc

h-co

ntro

ller

traffi

c

Inter-controller traffic

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Inter-controller (Crt-Ctr) traffic

• needed to synchronize the controllers’ states – shared data structures

• generated by consistency protocols – ad-hoc protocols – different models for consistency

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Consistency models

• assume a shared table=(key,value)• strong consistency

– any read(key) returns always the same value• eventual consistency

– any read(key) returns eventually the same value• adopted model affects heavily

– the mechanisms to distribute and update the data– the reactivity of the SDN controllers perceived by the network

devices– the correct behavior of the network

• e.g. inconsistent view of the network graph may lead to routing loops

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Consistency in SDN controllers

• eventual consistency• anti-entropy algorithm• network topology• flow rules • flow statistics

• strong consistency• RAFT consensus algorithm• switch-controller mapping• distributed locks

ONOS

• strong consistency• RAFT consensus algorithm• all shared data structures

OpenDaylight

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Data-ownership model• Consistency model affects that controller owner of the

shared data

• Read/write operations always forwarded by the local controller to the data-owner controller

• distributed architecture only for high availability• e.g. strong consistent data structures

Single data-ownership (SDO) model

• Read/write operations are local and then forwarded (asynchronously) to the other controllers

• e.g. eventual consistent data structures

Multiple data-ownership (MDO) model

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Our main contributions

• we provide formulas to estimate the reactivity perceived at the switch due to the Sw-Ctr and Ctr-Ctr interactions

• we show the performance tradeoffs for the controller placement problem

Reactivity for Multi Data-Ownership

Switch S1

Response Update data

Floodupdate

Data ownercontroller

TR TR = Sw-Ctr RTT

Data ownercontroller

Data ownercontroller

Reactivity for Single Data-Ownership Data owner

controller

Switch S1

Raftrequest

Log replicatiom

Response Update data

Log reply

Log commit

ControllerController

Hp: RAFT algorithm for strong consistency

TR

TR = Sw-Ctr RTT+2 Ctr-Ctr RTT

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Optimal controller placements3 controllers in large ISPFor Single Data-Ownership (SDO), 3 possible data owners

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The role of data owner in SDO

Choosing carefully the data owner is very important for the reactivity

Minimum reduction factor = Reactivity (2nd best data owner choice) Reactivity (best data owner choice)

Maximum reduction factor = Reactivity (worst data owner choice) Reactivity (best data owner choice)

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Conclusions

• single/multiple data-ownership models adopted by real SDN controllers affect strongly the performance perceived at the network devices

• reactivity formulas allow to devise the optimal placement of the controllers– the latencies among controllers are crucial in SDO model

• in the SDO model the choice of the data owner is crucial

• accurate validation of the given formulas in a large SD-WAN controlled by OpenDaylight – available on extended version [1] on arxiv