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Networks with Very Small Buffers
The current “Rule of Thumb”
•A router needs a buffer size:
Yashar Ganjali, Guido Appenzeller, High Performance Networking Group
Prof. Ashish Goel, Prof. Tim Roughgarden, Prof. Nick McKeown
Sum of congestion windows is Gaussian
1. Date-rate of most flows small fraction of line rate
•For many flows, TCP windows are multiplexed
B  2T  C
• Access links throttle flows to low rate (1-2Mb/s)
•Buffer size is reduced
•2T is the two-way propagation delay
• Core:Access > 1000:1
•Sum of congestion windows is Gaussian
•C is the capacity of bottleneck link
• Today’s TCP’s window size is limited
•Reason: the central limit theorem
•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]
•Width of Gaussian is dependent on square root of number of
flows
2. If packet arrivals were Poisson
•Buffer has to be wide enough to “fit” Gaussian
3. In an all-optical network capacity is plentiful (presumably)
Router
C
B
Arbitrary Injection
Process
Destination
Theory
Any rate > 0
need unbounded
buffers
Probability
Distribution
Our new rule for sizing buffers
2T  C
B
n
Validation
•Theoretical results validated by
•Thousands of ns-2 simulations
•2T is the two-way propagation delay
•Network lab (Cisco routers) at University of Wisconsin
•C is the capacity of bottleneck link
•n is the number of long-lived flows through the router
•Why? What has changed?
•High-end routers serve many unsynchronized flows
•Statistical multiplexing reduces the amount of buffering needed
If Poisson Process with load < 1
Complete Centralized
Control
Buffer Size
2T
•A router needs a buffer size:
• We get 80-90% utilization with 5-10 packet buffers
What we know so far about very small buffers?
W
0
Source
How small buffers can be?
Need buffer size of
approx:
O(logD + logW)
D=#of hops
W=window size
Experiment
TCP Pacing:
Results as good or
better than for
Poisson
Constant fraction
throughput with constant
buffers [Leighton]
[Goel 2004]
Fast access links, TCP Reno vs. Paced TCP
Congested core router
RTT = 100ms;
Each flow limited to 2.5Mb/s
•Stanford University dorm traffic
source
•Internet2 experiments
source
>10Gb/s
•Buffer size is reduced
>10Gb/s
router
•Ongoing work with network operators and router vendors ….
10Gb/s
What determines router buffer size?
server
•TCP needs buffers to fully utilize a link
•Only with maximum congestion window the links is full
Slow access links, lots of flows
•Window scales down just after it reaches maximum
2T  C
n
•The main role of a router buffer is to compensate for the difference
in outstanding packets
source
•TCP window size changes over time (the TCP saw tooth)
•Number of outstanding packets changes over time
Congested core router;
RTT = 100ms
Each flow limited to 2.5Mb/s
source
5Mb/s
5Mb/s
Simulation
•Buffer has to buffer these packets
router
B
•Example: 40 Gb/s linecard with 40,000 x 1Mb/s flows
•Rule-of-thumb: Buffer = 10Gbits
0
t
•New rule: Buffer = 50Mbits
10Gb/s
server