1. Sizing Router Buffers (Summary)
Isaac Keslassy (Technion)
Guido Appenzeller & Nick McKeown (Stanford)

2. Routers Need Packet Buffers
It’s well known that routers need packet buffers
It’s less clear why and how much
Goal of this work is to answer the question:
How much buffering do routers need?
Given that queueing delay is the only variable part of packet delay in the Internet, you’d think we’d know the answer already!

3. How Much Buffer Does a Router Need?
Source
Router
Destination C 2T
Universally applied rule-of-thumb:
A router needs a buffer size:
2T is the two-way propagation delay (or just 250ms)
C is capacity of bottleneck link

4. The rule-of-thumb is hard to apply
10Gb/s linecard
Requires 300Mbytes of buffering.
Read and write 40 byte packet every 32ns.
Memory technologies
DRAM: require 4 devices, but too slow.
SRAM: require 80 devices, 1kW, $2000.
Problem gets harder at 40Gb/s
Hence RLDRAM, FCRAM, etc.

5. Main Result in This Talk
The rule-of-thumb is wrong for a core router today
Required buffer is instead of

6. Only W=2 packets may be outstanding
How TCP Works
Only W=2 packets may be outstanding
Router
Source
Dest
C’ > C C
TCP Congestion Window controls the sending rate
Sender sends packets, receiver sends ACKs
Sending rate is controlled by Window W,
At any time, only W unacknowledged packets may be outstanding
The sending rate of TCP is

7. Single TCP Flow Buffer size is height of sawtooth
Source
Dest
C’ > C C t
Window size B
Buffer full & packet drop
Buffer empties
Buffer size is height of sawtooth
With single flow: B = 2T x C (rule-of-thumb)

8. Rule-of-thumb Typical backbone link has > 20,000 flows
Does the rule-of-thumb still hold?
Answer:
If flows are perfectly synchronized, then Yes.
If flows are desynchronized then No.

9. Not Synchronized Flows
Many TCP Flows
Synchronized Flows
Not Synchronized Flows B B

10. When are Flows Synchronized?
Small numbers of flows tend to synchronize
Large aggregates of flows are not synchronized
For > 200 flows, synchronization disappears
Measurements in the core give no indication of synchronization

11. Summary Flows in the core are desynchronized
For desynchronized flows, routers need only buffers of

12. Required buffer size
Simulation

13. Conclusion: Impact on Router Design
10Gb/s linecard with 200,000 x 56kb/s flows
Rule-of-thumb: Buffer = 2.5Gbits
Requires external, slow DRAM
Becomes: Buffer = 6Mbits
Can use on-chip, fast SRAM
Completion time halved for short-flows
DSL routers: why pay more for buffers if delays are larger?