1. A Comparative Study of Extensible Routers Yitzchak Gottlieb and Larry Peterson

2. Extensibility: Why? n New Services – Packet taggers – Filters – Firewalls – Overlay Networks n Old Routers – Fixed

3. Extensibility: How? n Software routers – Easy to change n Rewrite code – Hard to extend n Little guidance n Extensible software routers – Easy to extend n Defined architecture

4. Extensibility: Who? n Scout – Princeton University – SILK (Scout in Linux Kernel) n Click – MIT – Linux Kernel Module n Router Plugins (Crossbow) – Washington U St. L, ETH Zürich, Ascom – BSD Hack

5. How To Choose? n Choose Scout – Bias? n Choose at random n Compare strengths and weaknesses – Does your application fit its model? – Does it provide access to what you need? – Is it hard to program?

6. Modeling Extensible Routers n Basis for comparison – Abstracts implementation n Simple – Few components – Comprehensible n Powerful – Models all three systems

7. Model: The Players n Queues – FIFO n Classifiers – One to many n Schedulers – Many to one n Forwarders – One to one F CS

8. Comparing Extensible Routers n Extensibility – Classifiers – Schedulers – Forwarders n Process Scheduling n Flow Isolation

9. Extensibility: Two approaches n Outside the architecture – Extend the fixed pieces – Why use extensible routers n Within the architecture – Insert – Replace – Program n Parameterize n Add Code

10. Extending Components

11. Forwarders in Click n Small (Null element: 8 lines) n Multiple

12. Forwarders in Crossbow n Bigger (Null plugin: 98 lines) n Restricted placement SC F F F F C F F F SF

13. Forwarders in Scout n Biggest (Null module: 200 lines) n Powerful S FF C

14. Process scheduling n Scout – Classify to path – Schedule Path n Click – Pull from upstream queue – Push until downstream queue n Crossbow – Process at arrival

15. Flow Isolation n Scout – Paths are isolated n Click – Queues isolate subgraphs n Crossbow – No isolation

16. Conclusions n Powerful default classifier – Programmable substructure n Flow isolation – Schedule whole computation n Small components – Good aggregation tools