A Load-Balanced Switch with an Arbitrary Number of Linecards Offense Anwis Das

High Level Goal  Design Scalable, fault-tolerant router  Operate at 100 Tb/s, 40 times more than current standards  Clearly a challenge

Tradeoffs?  Architecture is based upon load-balanced Birkhoff-von Neumann switch – Essentially a load-balancer followed by input buffered switch  Types of switches – Input buffered switches – Output buffered switches – Combined input-output switches (CIOS)  But is throughput all that matters??

Quality of Service  Load balanced BV switch cannot guarantee any rate of service to any flow Providing Guaranteed Rate Services in the Load Balanced Birkhoff-von Neumann switch. Chang et al. Infocom  Such guarantees are required if a router wishes to implement certain classes of QoS such as Expedited Forwarding in DiffServe

Flexibility  Original Frame based scheduling allowed for flow guarantees  In this architecture, everything is fixed  Impossible to guarantee any flow any bandwidth without running scheduling algorithm again

Average Delay and Delay Variability  “frame based scheduling suffers from an important drawback: it often results in large cell delays and large delay variability” – Issac Keslassy in “On Guaranteed Smooth Scheduling for Input-Queued Switches” in Infocom 2003 – Failed to mention packet mis-sequencing problem already solved. Load Balanced Birkhoff-von Neumann Switches, part II: one-stage buffering”, Computer Communications 2002 – Problem is even worse in this paper due to their solution to solve the packet-missequencing problem  Scheduling is not smooth – Average delay high, burstiness, low short-term fairness

Linecards and Delay  Delay is proportional to frame size and and frame size is proportional to number of linecards – Delay is proportional to number of linecards!! – Large groups of linecards=> lots of linecards=> large delay!!

Fault Tolerance  Authors claim that lack of centralized scheduler leads to greater fault tolerance: Agree  Ironically, the paper discusses how to improve fault-tolerance – Linecards, or MEMS switches, or connectors more likely to fail than centralized scheduler  Proposed Solution: – Run algorithm to figure out static MEMS configuration  Too slow!!! (50 seconds vs. 50 milliseconds) – Polynomial algorithm means nothing in practice – Authors partially failed in what they set out to do

Crux of Paper  Outline- Part 1 – L-L, L-G, G-G (“easily deduce”)  Outline- Part 2 – G-G (Interesting, but more about this later) – G-L, L-L (Uninteresting) and similar work already done. “Load Balanced Birkhoff-von Neumann Switches, part I: one-stage buffering”, Computer Communications  Invoking Santa’s principle, not much original work