Transcript Slide 1

Infocom 2009, April 19-25, Rio de Janeiro
Exploiting the Path Propagation Time Differences
in Multipath Transmission with FEC
Maciej Kurant
EPFL, Switzerland
Acknowledgements:
Patrick Thiran (EPFL), Dan Jurca (DoCoMo, Munich), Pascal Frossard (EPFL)
Multipath transmission with FEC
T
source
destination
relay
protection
(FEC)
data to
send
received
reconstructed
schedule
Requirements:
• few losses
• small delay (no retransmission possible)
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Example
Assumptions:
1. Paths are independent
T=5ms
source
destination
relay
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Example
Assumptions:
1%, 10ms
T=5ms
source
destination
relay
Time t=0
Schedule:
Effective
loss rate
1. Paths are independent
2. Continuous-time Gilbert
model for packet losses:
• average loss rate
• average loss burst length
:
1.000%
0.553%
0.148%
State of the art - IMMEDIATE
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Example
source
destination
relay
Time t=0
Schedule:
Effective
loss rate
∆t >> 0
:
(here assume 50ms)
1.000%
0.553%
0.148%
State of the art - IMMEDIATE
0.113%
We use ∆t to SPREAD the packets on
the green path, so that the total block
delay is not changed.
∆t = 50ms
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Example
source
destination
relay
Time t=0
Schedule:
Effective
loss rate
1.000%
:
The gain over state of
the art is close to one
order of magnitude!
0.553%
0.148%
State of the art - IMMEDIATE
0.113%
We use ∆t to SPREAD the packets on
the green path, so that the total block
delay is not changed.
0.016%
The optimal rates on the paths
may change.
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Contributions
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1. Exact solution of the model
Model:
Assumptions:
source
destination
relay
protection data to send
(FEC)
1. Paths are independent
2. Continuous-time Gilbert model for packet losses:
• average loss rate
• average loss burst length
schedule
Solution (effective loss rate
):
Previous attempts (using approximations):
• L. Golubchik et al, “Multi-path continuous media streaming. what are the benefits?” Performance Evaluation Journal, 2002.
• E. Vergetis, R. Guerin, and S. Sarkar, “Realizing the benefits of user-level channel diversity,” SIGCOMM Comput. Commun. Rev., 2005.
• Y. Li, Y. Zhang, L. Qiu, and S. Lam, “SmartTunnel: Achieving reliability in the internet,” Proc. of INFOCOM’07, 2007.
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•…
2. Observation that ∆t>0
∆t = t2 - t1 [ms]
t1
t2
∆t = max(t2,t3) - t1 [ms]
t1
t2
t3
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3. Proposition of SPREAD
Time t=0
T
IMMEDIATE rules:
•
Send each packet immediately after it is generated at the source
•
Find the best packet rates for each path
•
Assign packets to paths to minimize the loss rate
Usually very good,
but not necessarily optimal
SPREAD rules:
•
•
On each path, spread the packets evenly in time within all the
available time budget
Find the best packet rates for each path
Might schedule a packet departure
before it is generated!
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4. Evaluation: Loss rate improvement
 B* ( IMMEDIATE)

 B* ( SPREAD)
path propagation time difference [ms]
i.e., a relative loss rate improvement over the state of the art
source packet generation period [ms]
number of packets per FEC block
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4. Evaluation: Minimizing delays
Reverse the problem, i.e.:
FEC block delay gain [ms]
“Minimize the FEC block delay of SPREAD,
but keep  B* ( SPREAD)   B* ( IMMEDIATE) .”
path propagation time difference [ms]
Useful, e.g., to minimize the effect of jitter.
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5. Trace-driven evaluation
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Limitations
•
•
•
•
Simple performance metric
Not (yet) adaptive
Assumes path independence
Assumes that we control when packets
are actually sent
• Needs more experimental verification
But the general principle remains.
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Conclusions
• Paths often have different propagation times.
• This can be exploited in multipath FEC
transmission to reduce:
– the effective loss rate
– the total FEC block delay.
• To achieve this, we:
– solved the multipath FEC model,
– proposed and evaluated SPREAD.
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Sponsored article
SPREAD
Space Packets Regularly Exploiting Asymmetry in Delays
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MASTErPIECE
MultipAth Scheduler That Exploits PropagatIon diffErenCEs
bETTER
Exploit Transmission Time diffERences
SPLENDiD
Space Packets evenLy and Exploit Network Delay Differences
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Sponsored article
Sponsored article
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