1. Timed Consistent Network Updates in Software-Defined Networks
Speaker: CHAO-YU WANG
Advisor: KE, KAI-WEI

2. Outline Introduction Terminology and Notations Worst-Case Analysis
Evaluation
Discussion

3. Introduction
Network updates, such as policy and routing changes, occur frequently in software-defined networks .
Updates should be performed consistently, preventing temporary disruptions, and should require as little overhead as possible.

4. Introduction (Cont’d)
Ordered Update:
At each phase the controller waits until all the switches have completed their updates, and only then invokes the next phase in the sequence.
Two-Phase Updates:
In the first phase the new configuration is installed in all the middle-stage switches of the network.
In the second phase the ingress switches are instructed to start using a version tag that represents the new configuration.

5. Introduction (Cont’d)
Assume that switches keep local clocks that are synchronized to a central reference clock by a synchronization protocol or by an accurate time source such as GPS.
The controller sends network update messages to switches using an SDN protocol such as OpenFlow. An update message may specify when the corresponding update is scheduled to be performed

6. Introduction (Cont’d)

7. Introduction (Cont’d)
只是特例 kphase

8. Terminology and Notations
The Network Model
System consists of N + 1 nodes: a controller c, and a set of N switches.
It is assumed that every switch maintains a local clock.
Network Updates
We define consistent forwarding based on the per-packet consistency definition of . A packet is consistently forwarded if it is processed by all switches either according to the new configuration, after the update, or according to the old one, but not according to a mixture of the two.

9. Terminology and Notations (Cont’d)
Delay-Related Notations

10. Terminology and Notations (Cont’d)
Explicit Acknowledgment
OpenFlow currently does not support such an acknowledgment mechanism. Hence, one can either use a different SDN protocol that supports explicit acknowledgment or use an update procedure in which the controller waits for a fixed period (Dc) until the switch is guaranteed to complete the update.
Observation :
In typical settings δ<Dc.
Software timeout
ACKnowledgment

11. Worst-Case Analysis Worst-Case Analysis of Untimed Update
Worst-Case Analysis of Timed Updates

12. Worst-Case Analysis (Cont’d)
We use Program Evaluation and Review Technique (PERT) charts to illustrate the worst-case update duration analysis.
A node labeled Cj,i represents the event ‘the controller starts transmitting a phase j update message to switch Si’.
A node labeled Sj,i represents ‘switch Si has completed its phase j update’.

13. Worst-Case Analysis (Cont’d)
Untimed Updates

14. Worst-Case Analysis (Cont’d)
The weight of each edge indicates the maximal delay to complete the transition from one event to another
Cstart and Cfin represent the start and finish times of the update procedure, respectively.

15. Worst-Case Analysis (Cont’d)
Untimed Updates

16. Worst-Case Analysis (Cont’d)
Untimed Updates With Garbage Collection

17. Worst-Case Analysis (Cont’d)
Untimed Updates With Garbage Collection

18. Worst-Case Analysis (Cont’d)
Worst-Case-Based Scheduling

19. Worst-Case Analysis (Cont’d)
Timed update

20. Worst-Case Analysis (Cont’d)
Timed vs. Untimed Updates

21. Worst-Case Analysis (Cont’d)
Timed vs. Untimed Updates
Proof:

22. Worst-Case Analysis (Cont’d)

23. Worst-Case Analysis (Cont’d)
Proof

24. Worst-Case Analysis (Cont’d)
Using Acknowledgments

25. Worst-Case Analysis (Cont’d)

26. Worst-Case Analysis (Cont’d)
We observe that for a sufficiently large value of Dn the timed approach produces a lower update duration.
DgT = Dn + δ < Dn + Dc
< Dn+ (NG2 - 1) * Δ + Dc = DgUT

27. Fine Tuning Consistency
By setting the update times T1, T2,...,Tk, Tg1,...,Tgk, the controller can play with the consistency- scalability tradeoff; the update overhead can be reduced at the expense of some inconsistency, or vice versa.

28. Evaluation
Timed vs. Untimed Updates

30. Evaluation (Cont’d) Fine Tuning Consistency
Constant delay — each link had a constant delay that was configured to the value we computed as described above.
Exponential delay — each link had an exponentially distributed delay.

32. Evaluation (Cont’d)
Simulation: Using ACKs

33. Evaluation (Cont’d)
Simulation: Using ACKs

34. Evaluation (Cont’d)
Simulation: Using ACKs

35. Discussion Failures Security Considerations
If the controller detects a switch failure before an update is scheduled to take place, it can send a cancellation message to all the switches that take part in the scheduled update, thus guaranteeing an all-or-none behavior.
Security Considerations
This problem can be mitigated by using an explicit acknowledgment mechanism

36. Reference
T. Mizrahi, E. Saat and Y. Moses, "Timed Consistent Network Updates in Software-Defined Networks," in IEEE/ACM Transactions on Networking, vol. 24, no. 6, pp , December 2016.