Download Cellular Automata Modeling of TCP/IP Connections

Self-generated Self-similar Traffic
Péter Hága
Péter Pollner
Gábor Simon
István Csabai
Gábor Vattay
Outline
• Motivations
• Self-similarity
• Karn’s Algorithm
• Backoff mechanism & Self-similar traffic
• Virtual loss
• Simulation
• Measurement
• Discussion
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CNL - Network Performance Measurement Group
Motivations
Goal:
• network dynamics: self-similar
• new explanation: RTT fluctuations & self-organization
• self-similarity without the former known reasons:
file size distribution, user interaction, chaos, high packet loss
• separation of the real & virtual losses
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Self-similarity
Hurst exponent: degree of self-similarity
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Self-similarity
• known sources:
• file size distribution
• user interaction
• chaos due to small buffers
• high loss rate
Buffer/No of TCPs high
< Rcrit
=>
forces
TCPs
intodistribution
backoff states
packet
loss =>
backoff
states
heavytailed
heavytailed
file
modem
size
duration
time
self-similar
TCPflow
flow
Self-similar
self-similar
TCP
TCPflow
flow
self-similar
TCP
L.Guo, M.Crovella,
A.Fekete,I.Matta
G.Vattay
2000
2001
A.Feldmann, A.C.Gilbert,
W.Willinger,
T.G.Kurtz
1997
M.Crovella,
A.Bestravos
1997
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CNL - Network Performance Measurement Group
Karn’s Algorithm
• Route: very congested
• TCP: exponential backoff state:
• If packets are lost many times cwnd=1 is reached,
halving is not an option
• TCP waits an TRTT and tries again
• If fails, waits 2 TRTT, 4 TRTT, 8 TRTT,...
• k = 1,…,6 denote backoff states of increasing depth
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Backoff mechanism & Self-similar traffic
Backoff probability distrribution
Effective packet loss ratio
Pk: probability of kth backoff state
Pk+1 = (2p-p2) Pk,
k=0,…,4
where p: packet loss rate felt by the TCP
Pk
peffective
A.Fekete, G.Vattay 2001
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CNL - Network Performance Measurement Group
Backoff mechanism & Self-similar traffic
Backoff probability distrribution
Hurst exponent
packet sending process: ON/OFF process
OFF periods:
inter arrival times of packets » t-(a+1)
Hurst parameter of such an aggregated traffic:
a = log2(1/2p)
H = (3-a)/2,
if a > 2
when 1 < a < 2, or 12.5% < p < 25% => 0.5 < H < 1
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L.Guo, M.Crovella, I.Matta 2000
CNL - Network Performance Measurement Group
Virtual losses
Packet losses
real loss: dropped packets
virtual loss: ACK arrives, but after the
RTO period, so the packet is
retransmitted
Source of packet loss:
• real: at high congested buffers, or at low quality lines
(e.g. radio lines) - solution: simple, by improving
hardware conditions
• virtual: it comes from the heavily fluctuating
background traffic - solution: ??
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Virtual losses
bursty background traffic
heavily fluctuating queuing time
heavily fluctuating round-trip time
If queuing time jumps to a high value due to increased traffic
RTTreal > RTOTCP => virtual loss occurs
(the TCP doesn’t get ACK until RTO expires)
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Simulations
• Network Simulator v2 (NS)
• Small network, but general operation:
• random connections between nodes
• fixed file size (NOT heavytailed distribution)
• big buffers (no real packet loss)
Link bandwidth 1 Mbps
Link delay 1 ms
Buffer size 1000 pkts
File size 1000 pkts
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Simulations
We found self-similarity in the flow:
Hvariance=0.86
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Simulations
the traffic is self-similar, BUT:
the KNOWN SOURCES:
• file size distribution
• user interaction
• chaos due to small buffers
• high loss rate
were NOT ENOUGH:
• fixed file size
• ~ contunious transfer
• big buffers
• no packet loss
What is the cause of self-similarity in our case?
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Simulations
Backoff statistics
a = log2(1/2p)
the cause of the self-similarity
H = (3-a)/2,
if a > 2
Hbackoff = 0.89
( Hvariance = 0.86 )
peffective = 21% =><= preal = 0%
(felt by the TCP)
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Measurement
• modified linux kernel (2.2.x series)
• tcpdump
• congested transcontinental line
• packet inter arrival time and backoff statistics
• separate of real and virtual loss
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Measurement
Self-similarity of the flow, Hurst exponent
Packet inter arrival distribution
H=0.70
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Variance-time plot
H=0.69
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Measurement
Backoff statistics
backoff values - time
k=1,…,15
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backoff probability distribution
ploss=16.5%, Hbackoff=0.70
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Measurement
Packet loss detection and separation:
tcpdump
Real packet loss
p ¼ 6.5%
congested route
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Virtual loss
p ¼ 10 –12%
peffective ¼ 16 – 18%
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Measurement
TCP is backed off, by: • real loss (dropped)
• virtual loss (only delayed and timed out)
• loss ratio from backoff
statistics, p=16.5%
• loss ratio calculated from
tcpdump output: real,
effective (real+virtual) losses
pbackoff = peffective = preal + pvirtual  preal
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Conclusions
Main results:
• new source of the self-similar traffic: RTT fluctuations
• RTT fluctuations generates virtual packet losses,
which induce backoff states with high probability,
and the backoff states cause self-similar traffic
• former sources are avoidable by dimensioning:
file or user quotas, big buffers, high quality lines
• the RTT fluctuations: comes from the confluent random flows and
network dynamics. Solution: dimensioning, protocol modification, etc.?
• self-organizing self-similarity: RTT fluctuations feeds back
into the background traffic
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