A Load-Balanced A Load-Balanced Switch with an Switch with an

Arbitrary Number of Arbitrary Number of LinecardsLinecards

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 2003.

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 2002.

Invoking Santa’s principle, not much original work