One More Bit Is Enough

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• One More Bit Is Enough

Yong Xia, RPI Lakshminarayanan Subramanian,
UCB Ion Stoica, UCB Shivkumar Kalyanaraman,
RPI SIGCOMM05, August 22-26, 2005,
Philadelphia, Pennsylvania, USA
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Goal

• Achieve fair bandwidth allocation and high
  utilization, and minimize packet loss in high
  bandwidth-delay product (BDP) network
• TCP ?
• TCP AQM / ECN?
• XCP ?

VCP
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Why TCP does not scale?

• TCP uses duplicate ACK or timeout no degree of
  congestion and instability

congestion window
Multiplicative Decrease (MD)
time

• AI with a fixed step-size can be very slow for
  large bandwidth

VCP
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TCP does not perform well in high b/w
VCP
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XCP scales
C
spare bandwidth
DATA
x
ACK
?rate
sender
receiver
router

• But, XCP needs multiple bits (128 bits in its
  current IETF draft) to carry the
  congestion-related information from/to network

VCP
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Goal

• Design a TCP-like scheme that
• requires a small amount of congestion information
  (e.g., 2 bits)
• scales across a wide range of network scenarios

VCP
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General idea
degree of explicit information
XCP multiple bits, report spare bandwidth
VCP two bits, low-load, high-load and overload
ECN one bit, underload and overload
TCP no explicit feedback, relay on duplicate
ACKand timeout
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Variable-structure congestion Control Protocol
(VCP)

• Routers signal the level of congestion

• End-hosts adapt the control algorithm accordingly

VCP
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VCP vs. ECN
code
region
control
overload
Multiplicative Decrease (MD)
VCP
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VCP vs. ECN
VCP
utilization
cwnd
time (sec)
VCP
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VCP key ideas and properties

• Use network link load factor as the congestion
  signal

• Decouple efficiency and fairness controls in
  different load regions

• Achieve high efficiency, low loss, and small
  queue
• Fairness model is similar to TCP
• Long flows get lower bandwidth than in XCP
  (proportional vs. max-min fairness)
• Fairness convergence much slower than XCP

VCP
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Major design issues

• At the router
• How to measure and encode the load factor?

• At the end-host
• When to switch from MI to AI?
• What MI / AI / MD parameters to use?
• How to handle heterogeneous RTTs?

VCP
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Design issue 1 measuring and encoding load
factor

• Calculate the link load factor ?

link_bandwidth t?

• The load factor is quantized and encoded into the
  two ECN bits

VCP
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Design issue 2 setting MI / AI / MD parameters
(?, ?, ? )
VCP
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Design issue 2 setting MI / AI / MD parameters
(?, ?, ? )
load factor
MD
100
AI
1.0
TCP ? 1.0 VCP ? 0.06 STCP ? 0.01
k (1 ? ?) / ? where k 0.25 (for
stability)
MI
0
VCP
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Design issue 3 Handling RTT heterogeneity for
MI/AI
VCP
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VCP scales across b/w, rtt, num flows

• Evaluation using extensive ns2 simulations

VCP
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Impact of bottleneck capacity
vary link capacity from 100Kbps to 5Gbps
high utilization, gap comparing toXCP at most 7
at extremely low capacities alpha 1 is too
large for such low capacity
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Impact of feedback delay
average queue lt 5maximal queue lt 15
fix bottleneck capacity at 150Mbps, vary
round-trip propagation delay from 1ms to 1500ms
utilization gt 90
no packet loss
lower utilization due to comparable
smallmeasurement interval, i.e. tp 200ms ltlt RTT
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Impact of number of long-lived flows
small queue length, even outperform XCP
increase the forward FTP flows
high utilization
zero packet drop
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Impact of short-lived traffic
small queue length
arrive according to Poisson process, avg. arrival
rate varying from 1/s to 1k/s,transfer size
obeys Pareto distribution with avg. 30 packets
high utilization
zero packet drop
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Multiple bottlenecks
lt 0.2 buffer size average queue length
typical parking-lot topology
as good as single-bottleneck scenarios
zero packet drop
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Fairness

• same RTT
• small RTT difference
• huge RTT difference

even distribution among all flows in case (a)
and (b)
fairness discrepancy due to high value of MI and
AI, whose value is bound in implementation to
prevent burst
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Convergence behavior
high utilization during the whole period
introduce 5 flows one after another
use router buffer size to scale queue length
axis low queue length during the whole period
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Sudden demand change
high utilization
150 flows join at 80s and leave at 160s
remain much lower than full size
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Conclusions

• With a few minor changes over TCP AQM / ECN,
  VCP is able to approximate the performance of XCP
• High efficiency
• Low persistent bottleneck queue
• Negligible congestion-caused packet loss
• Reasonable (i.e., TCP-like) fairness

VCP
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VCP comparisons

• Compared to TCPAQM/ECN
• Same architecture (end-hosts control, routers
  signal)
• Router congestion detection queue-based ?
  load-based
• Router congestion signaling 1-bit ? 2-bit ECN
• End-host adapts (MI/AI/MD) according to the ECN
  feedback
• End-host scales its MI/AI parameters with its RTT
• Compared to XCP
• Decouple efficiency/fairness control across load
  regions
• Functionality primarily placed at end-hosts, not
  in routers

VCP
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THE END
VCP
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VCP Parameter Setting
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Remark

• Modified from
• www.ecse.rpi.edu/Homepages/ shivkuma/research/pape
  rs/vcp05.ppt