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Transcript
High Performance Networking with
Little or No Buffers
Yashar Ganjali
High Performance Networking Group
Stanford University
Joint work with:
Guido Appenzeller, Ashish Goel, Tim Roughgarden, Nick McKeown
May 5, 2005
[email protected]
http://www.stanford.edu/~yganjali
Motivation
Networks 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
May 5, 2005
2
Which would you choose?
DSL Router 1
DSL Router 2
$50
4 x 10/100 Ethernet
1.5Mb/s DSL connection
$55
4 x 10/100 Ethernet
1.5Mb/s DSL connection
1Mbit of packet buffer
4Mbit of packet buffer
Bigger buffers are better
May 5, 2005
3
What we learn in school

Packet switching is good
 Long
haul links are expensive
 Statistical multiplexing allows efficient sharing
of long haul links
Packet switching requires buffers
 Packet loss is bad
 Use big buffers
 Luckily, big buffers are cheap

May 5, 2005
4
Statistical Multiplexing
Observations
1. The bigger the buffer, the lower the packet loss.
2. If the buffer never goes empty, the outgoing line
is busy 100% of the time.
May 5, 2005
5
What we learn in school
Queueing Theory

M/M/1
EX 
1

1 
P X  k    k
X
Loss
rate
May 5, 2005
Observations
1. Can pick buffer size for a given loss rate.
2. Loss rate falls fast with increasing buffer size.
3. Bigger is better.
Buffer size
6
What we learn in school

Moore’s Law: Memory is plentiful and
halves in price every 18 months.
 1Gbit
memory holds 500k packets
and costs $25.

Conclusion:
 Make
buffers big.
 Choose the $55 DSL router.
May 5, 2005
7
Why bigger isn’t better
Network users don’t like buffers
 Network operators don’t like buffers
 Router architects don’t like buffers
 We don’t need big buffers
 We’d often be better off with smaller ones

May 5, 2005
8
Backbone Router Buffers
Source
Router
Destination
C
2T

Universally applied rule-of-thumb:
 A router needs a buffer size: B  2T  C
• 2T is the two-way propagation delay
• C is capacity of bottleneck line

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
May 5, 2005
9
Review: TCP Congestion Control
Rule for adjusting W
Only W packets
may be outstanding
May 5, 2005


If an ACK is received:
If a packet is lost:
W ← W+1/W
W ← W/2
10
Review: TCP Congestion Control
Rule for adjusting W
Only W packets
may be outstanding
Source


If an ACK is received:
If a packet is lost:
W ← W+1/W
W ← W/2
Dest
Window size
Wmax
Wmax
2
t
May 5, 2005
11
Buffer Size in the Core
W
B
0
Buffer Size
May 5, 2005
Probability
Distribution
12
Backbone router buffers

It turns out that
 The
rule of thumb is wrong for a core routers
today
2T  C
 Required buffer is
instead of 2T  C
n
May 5, 2005
13
Required Buffer Size
2T  C
n
Simulation
May 5, 2005
14
Validation
Theoretical results validated by:
 Thousands
of ns2 simulations
 Network lab (Cisco routers) at University of
Wisconsin
 Stanford University dorm traffic
 Internet2 experiments
Ongoing work with network operators and
router vendors…
May 5, 2005
15
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

40Gb/s linecard with 40,000 x 1Mb/s flows


Rule-of-thumb: Buffer = 10Gbits
Becomes: Buffer = 50Mbits
May 5, 2005
16
How small can buffers be?
Imagine you want to build an all-optical
router for a backbone network…
 …and you can build a few dozen packets in
delay lines.
 Conventional wisdom: It’s a routing
problem (hence deflection routing, burstswitching, etc.)
 Our belief: First, think about congestion
control.

May 5, 2005
17
TCP with ALMOST No Buffers
Utilization of bottleneck link = 75%
May 5, 2005
18
Two Concurrent TCP Flows
May 5, 2005
19
TCP Throughput with Small Buffers
TCP Throughput vs. Number of Flows
0.9
0.8
Throughput
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
May 5, 2005
200
400
600
Number of Flows
800
1000
20
TCP Reno Performance
Buffer Size = 10; Load = 80%
1
0.9
Throughput
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1000
2000
3000
4000
5000
6000
Bottleneck Capacity Mbps
May 5, 2005
21
The chasm between theory and practice
Theory (benign conditions)

Practice
M/M/1
1
X
EX 

 = 50%, EX = 1 packet
 = 75%, EX = 3 packets
1 
P X  k   
k
Typical OC192 router linecard
buffers over 2,000,000 packets
 = 50%, P[X>10] < 10-3
 = 75%, P[X>10] < 0.06
Can we make the traffic arriving at the routers
May 5, 2005 Poisson “enough” to get most of the benefit?
22
Ideal Solution



If packets are spaced out perfectly; and
The starting times of flows are chosen
randomly;
We only need a small buffer for contention
resolution.
May 5, 2005
23
Pacing

We need to break bursts
 Modify
TCP: Instead of sending packets when
your receive ACKS send packets with a fixed
rate of CWND/RTT.
 Rely on network properties:
• Access links throttle the flows to low rate
• Core:Acess > 1000:1
• TCP’s window size is limited today.

If these properties make the flow look like
poisson with only 5-10 packets of buffering
we can get 70-80% throughput.
May 5, 2005
24
What we know so far
about very small buffers
Arbitrary
Injection
Process
If Poisson Process
with load < 1
Theory
Any rate > 0
need unbounded
buffers
Need buffer
size of approx:
O(logD + logW)
i.e. 20-30 pkts
Complete
Centralized
Control
Experiment
TCP Pacing:
Results as good
or better than for
Poisson
Constant fraction
throughput with
constant buffers
[Leighton 1999]
D=#of hops
W=window size
May 5, 2005
25
CWND: Reno vs. Paced TCP
May 5, 2005
26
TCP Reno: Throughput vs. Buffer Size
May 5, 2005
27
Paced TCP: Throughput vs. Buffer Size
May 5, 2005
28
Early results
Congested core router with 10 packet buffers.
Average offered load = 80%
RTT = 100ms; each flow limited to 2.5Mb/s
source
source
>10Gb/s
>10Gb/s
router
10Gb/s
server
May 5, 2005
29
Slow access links, lots of flows,
10 packet buffers
Congested core router with 10 packet buffers.
RTT = 100ms; each flow limited to 2.5Mb/s
source
source
5Mb/s
5Mb/s
router
10Gb/s
server
May 5, 2005
30
Conclusion
We can reduce 1,000,000 packet buffers
to 10,000 today.
 We can probably reduce to 10-20 packet
buffers:

 With
many small flows, no change needed
 With some large flows, need pacing in the
access routers or at the edge devices.

Need more experiments.
May 5, 2005
31
Experiments

Performance measurement with



Metrics:





Small (thousands of packets); and
Tiny (tens of packets) buffers
Link utilization (goodput/throughput)
Drops
Buffer occupancy
Etc.
Data


Gathered for minutes to days
High load (50-70% utilization) is better
May 5, 2005
32
Thank you!
Questions?