Designing Networks with Little or No Buffers or Can Gulliver Survive in Lilliput? Yashar Ganjali...

Designing Networks with Little or No Buffers

orCan Gulliver Survive in Lilliput?

Yashar GanjaliHigh Performance Networking GroupStanford University

[email protected]://www.stanford.edu/~yganjali

Joint work with Prof. Ashish Goel, Prof. Nick McKeown, Prof. Tim Roughgarden

October 21, 2004 - SNRC

mailto:%5Cmailto:[email protected]

October 21, 2004 - SNRC 2

MotivationNetworks with Little or No

Buffers Problem

Internet traffic is doubled every year Disparity between traffic and router growth

(space, power, cost) Possible Solution

All-Optical Networking Consequences

Large capacity large traffic Little or no buffers Gulliver wakes up in Lilliput!


Why do we need buffers? How large are buffers today? How small can buffers be? Summary and conclusions

Outline


Why do we need buffers? Pathological example Contention Congestion

How large are buffers today? How small can buffers be? Summary and conclusions

Outline


Pathological Example

If S1 sends a packet at time t, S2 cannot send any packets at time t, and t+1.

To achieve 100% throughput we need at least one unit of buffering.

1 2

S1

S2

D3 4

Flow 1: S1 D; Load = 50%

Flow 2: S2 D; Load = 50%


Why do we need buffers?

Internet traffic is bursty in nature Starting time Flow length variations Rate changes over time

We need buffers to compensate momentary over-subscriptions Short-term: Contention Long-term: Congestion


Contention Even when links are under-subscribed

contention occurs due to stochastic collision of packets.


Congestion

Internet uses distributed congestion control.

Links are temporarily over-subscribed.

TCP needs buffers to work.


Rule for adjusting W If an ACK is received:W ← W+1/W If a packet is lost: W ← W/2

Congestion

Only W packets may be outstanding


Congestion

Rule for adjusting W If an ACK is received:W ← W+1/W If a packet is lost: W ← W/2

Source Dest

maxW

2maxW

t

Window size

Only W packets may be outstanding


Why do we need buffers? How large are buffers today?

Congestion control Contention resolution

How small can buffers be? Summary and conclusions

Outline


Universally applied rule-of-thumb: A router needs a buffer size:• 2T is the two-way propagation delay• C is capacity of bottleneck link

The buffer needed for contention resolution is dominated by this.

Router Buffer Needed for Congestion Control

CTB 2

CRouterSource Destination

2T


Example

Link capacity 40Gbps Two way propagation delay 100ms Required buffer size 500MB

2,000,000 packets Hard to implement even in

electronics


Why do we need buffers? How large are buffers today? How small can buffers be?

Congestion control• Aggregate flows• No buffering

Contention resolution• Scheduling• Load balancing in time and space

Summary and conclusions

Outline


Buffer Size in the Core

ProbabilityDistribution

B

0

Buffer Size

W


The rule of thumb is true for one flow.

Thousands of flows in the core of the Internet.

Required buffer is instead of n is the number of flows [Appenzeller, Keslassy, McKeown 2004]

Required buffer: 20,000 packets Much less than 2,000,000 packets Still not feasible in optics

Required Buffer Size

CT 2n

CT 2


TCP with ALMOST No Buffers

Utilization of bottleneck link = 75%


TCP with Almost No Buffers

With almost no buffering and just a single flow we lose about 25% of the capacity.

Capacity is not a bottleneck anymore.

Small buffers not a major problem for congestion control.


Throughput with Little BuffersBottleneck Link Utilization vs. Number of Flows

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700 800 900 1000

Number of Flows

Uti

liza

tio

n

Capacity: 100Mbps, Buffer size: 10 packets






Outline


Contention Resolution

Two extreme approaches No scheduling

• Use large buffers Tight scheduling

• Inject packets according to a schedule, so that contention is minimized.

No scheduling

Tight scheduling


Exact Schedule

Hard to find in general Must be distributed Exact measurements of flows needed Extreme synchronization required


Randomization

Randomization Randomly delay packet at injection time

• Imitate a Poisson process Reshape traffic at the core

• Keep Poisson

No scheduling

Tight scheduling

???Randomization


Randomization (cont’d)

21 3 4 5 6 7

21 3 4 5

Time

Input #1

Input #2 6

Before randomization

41 2 3 5 6 7

21 63 4

Time

Input #1

Input #2 5

After randomization


Randomization (cont’d)

Mimic an M/M/1 queue

Under low load, queue size is small with high probability

Loss can be bounded

kkXP

EX

1

11

M/M/1

X

b

P(Q > b)Buffer B

Packet Loss


Example16 Node Network, Tree Shaped Topology, Load = 70%, Loss = 1%


Implications of Randomization

Alleviates short-term contention Simple to implement Guaranteed performance


Preliminary Results

Theorem. We can achieve constant factor throughput (roughly 70-80%) with very

small amount of loss using buffers of size O(log L), where L is the length of the

longest path in the network.

Assumption: No reactive flow control Typical value of L is 3 to 5






Outline


Preliminary Results

Conjecture. Routers in the core of the current Internet only need buffers of

size O(log L) instead of the 2TxC.

Justification Currently maximum window size is 64 and 12

packets for Linux and Windows XP Access links are the bottleneck and limit the

flow Randomization takes care of momentary

collisions


Implications & Considerations

The required buffer size does not depend on the link capacity.

All we need is 10-20 packets worth of buffering (instead of thousands)

Need to study The interaction between randomization and

congestion control Impact of co-existing flows Emerging applications (non-TCP or modified

TCP) which allow much large windows per flow


Summary and conclusions We need buffers. To gain 100% we need relatively LARGE

buffers Not as large as the widely used rule of thumb

At the price of losing some capacity, TCP performs well even with small buffers.

We can address contention by adding randomization.

Gulliver might not have the best life in Lilliput, but he seems to be able to survive!

Many thanks to Guido Appenzeller for providing the flash animations.

Designing Networks with Little or No Buffers or Can Gulliver Survive in Lilliput? Yashar Ganjali...

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