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Routers with Small Buffers
Yashar GanjaliHigh Performance Networking GroupStanford University
[email protected]://yuba.stanford.edu/~yganjali/
Joint work with:
Guido Appenzeller, Mihaela Enachescu, Ashish Goel, Tim Roughgarden, Nick McKeown
Special thanks to: Level 3 Communications
NANOG, October 25, 2005
October 2005 Routers with Small Buffers 2
The Story
)(logO2
2 )2()1( Wn
CTCT
)(logO
22 )2()1( W
n
CTCT
(1) Assume: Large number of desynchronized flows; 100% utilization(2) Assume: Large number of flows; <100% utilization
1,000,000 10,000 20# packetsat 10Gb/s
SawtoothPeak-to-trough
Smoothing of many sawtooths
Non-bursty arrivals
Intuition& Proofs
SimulatedSingle TCP Flow
Simulations, Experiments
Simulated ManyTCP Flows
Evidence
October 2005 Routers with Small Buffers 3
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
Context Mandated in backbone and edge routers. Appears in RFPs and IETF architectural guidelines. Usually referenced to Villamizar and Song: “High Performance
TCP in ANSNET”, CCR, 1994. Already known by inventors of TCP [Van Jacobson, 1988] Has major consequences for router design
CTB 2
CRouterSource Destination
2T
Backbone Router Buffers
October 2005 Routers with Small Buffers 4
Rule for adjusting W If an ACK is received: W ← W+1/W If a packet is lost: W ← W/2
Single TCP Flow
Only W packets may be outstanding
October 2005 Routers with Small Buffers 5
Rule for adjusting W If an ACK is received: W ← W+1/W If a packet is lost: W ← W/2
Single TCP Flow
Only W packets may be outstanding
Source Dest
maxW
2maxW
t
Window size
CT 2
CT 2
October 2005 Routers with Small Buffers 6
Time evolution of a single TCP flow through a router. Buffer is < 2T*C
Time Evolution of a Single TCP FlowTime evolution of a single TCP flow through a router. Buffer is 2T*C
October 2005 Routers with Small Buffers 7
Synchronized Flows
Aggregate window has same dynamics Therefore buffer occupancy has same dynamics Rule-of-thumb still holds.
2maxW
t
max
2
W
maxW
maxW
October 2005 Routers with Small Buffers 10
Real Network Experiments
Stanford University dorm traffic Network Lab (Cisco routers) at University of
Wisconsin Internet2 Operational Internet backbone
October 2005 Routers with Small Buffers 11
Internet Backbone Experiment Buffer sizes 190ms,
10ms, 5ms, 2.5 and 1ms Load balancing High link utilization Long duration (about two
weeks)
Drops, utilization data collected every 30 seconds
Test flows
October 2005 Routers with Small Buffers 12
Packet Drops vs. Link Load
Buffer size = 190ms, 10ms, 5ms
MAX
October 2005 Routers with Small Buffers 14
Relative Link UtilizationUtilization of the link with 1ms buffer /Utilization of the link with 190ms buffer
October 2005 Routers with Small Buffers 16
Theory vs. Practice
M/D/1
Theory (benign conditions) Practice
Typical OC192 router linecard buffers over 1,000,000 packets
Can we make traffic look “Poisson-enough” when it arrives to the routers…?
Can we make traffic look “Poisson-enough” when it arrives to the routers…?
Poisson
BD
B loss
%1loss pkts20 80%, .. Bei
Loss independent of link rate, RTT, number of flows, etc.
5 orders of magnitudedifference!
5 orders of magnitudedifference!
October 2005 Routers with Small Buffers 17
Assume: Buffer size > Distance between consecutive packets of a
single flow S > Limited injection rate
Flows are not synchronized; and Start times picked randomly and independently
We can prove that the packet drop probability is very low.
Paced Injections
Similar results from Cambridge/UCL, UMass and StanfordSee papers in: ACM Computer Communications Review, July 2005
Similar results from Cambridge/UCL, UMass and StanfordSee papers in: ACM Computer Communications Review, July 2005
October 2005 Routers with Small Buffers 18
O(log W) BuffersAssumptions:
Internet core is over-provisioned Example: Load < 80%
There is spacing between packets of the same flow: Natural: Slow access links Artificial: Paced TCP
Result:Traffic is very smooth, and loss rate is very low,
independent of RTT, and number of flows.
With a buffer size of just 10-20 packets we can gain high throughput.
October 2005 Routers with Small Buffers 19
Leaky Bucket – Paced vs. RenoBucket drains with a constant rate. Load is 90% for both cases.
October 2005 Routers with Small Buffers 22
O(log W) BuffersRegular TCPRegular TCP
TCP WithPacing
TCP WithPacing
October 2005 Routers with Small Buffers 24
Ideal Experiment
Highly loaded link, with real/realistic traffic Precisely controlled router buffers Packet traces with precise timestamps Work in progress: Sprint, Verizion, Telcordia, Lucent,
…
Packet Trace Monitor
Packet Trace Monitor
October 2005 Routers with Small Buffers 25
Conclusion and Future Work Theory:
Reducing buffer sizes by a factor of sqrt(N) does not affect the network performance.
Reducing the buffer sizes to O(logW) does not affect the network performance if: The network is over provisioned; and We use Paced TCP; or Have slow access links
Experimental Validation: Thousands of ns2 simulations Stanford dorm, University of Wisconsin Testbed, Internet2, Level
3 Communications, … Ongoing work and need your help
October 2005 Routers with Small Buffers 26
Thanks!Thanks!
More Info?More Info?
[email protected]@stanford.eduhttp://www.stanford.edu/~yganjalihttp://www.stanford.edu/~yganjali
http://yuba.stanford.edu/~yganjali/research/http://yuba.stanford.edu/~yganjali/research/bsizing/bsizing/