Exploiting the Path Propagation Time Differencesin Multipath Transmission with FEC

Maciej Kurant EPFL, Switzerland

Infocom 2009, April 19-25, Rio de Janeiro

Acknowledgements:Patrick Thiran (EPFL), Dan Jurca (DoCoMo, Munich), Pascal Frossard (EPFL)

2

T

source destination

relay

Multipath transmission with FEC

Requirements:• few losses• small delay (no retransmission possible)

schedule

data to send

protection (FEC)

received reconstructed

3

destination

relay

source

T=5ms

ExampleAssumptions:1. Paths are independent

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0.553%

0.148%

destination

relay

source

T=5ms

1.000%

1%, 10ms

1%

10ms

Example

Schedule:

Time t =0

Assumptions:1. Paths are independent2. Continuous-time Gilbert model for packet losses: • average loss rate • average loss burst length

Effective loss rate :

State of the art - IMMEDIATE

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1.000%

0.553%

0.148%

0.113%

destination

relay

source

∆t >> 0 (here assume 50ms)

We use ∆t to SPREAD the packets on the green path, so that the total block delay is not changed.

∆t = 50ms

Example

State of the art - IMMEDIATE

Schedule:

Time t =0

slow!

Effective loss rate :

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1.000%

0.553%

0.148%

0.113%

destination

relay

source

Example

We use ∆t to SPREAD the packets on the green path, so that the total block delay is not changed.

The optimal rates on the paths may change.0.016%

The gain over state of the art is close to one order of magnitude!

Schedule:

Time t =0 Effective loss rate :

slow!

State of the art - IMMEDIATE

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Contributions

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1. Exact solution of the model

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.

• …

Solution (effective loss rate ):

destination

relay

source

Assumptions:1. Paths are independent2. Continuous-time Gilbert model for packet losses: • average loss rate • average loss burst length

data to sendprotection (FEC)

Model:

schedule

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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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Time t =0

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

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

3. Proposition of SPREAD

T

Usually very good,

but not necessarily optimal

Might schedule a packet departure

before it is generated!

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i.e., a relative loss rate improvement over the state of the art

path propagation time difference [ms] source packet generation period [ms] number of packets per FEC block

)(

)(*

*

SPREAD

IMMEDIATE

B

B

4. Evaluation: Loss rate improvement

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path propagation time difference [ms]

FE

C b

lock

del

ay g

ain

[ms]

4. Evaluation: Minimizing delays

“Minimize the FEC block delay of SPREAD,

but keep .”)()( ** IMMEDIATESPREAD BB

Reverse the problem, i.e.:

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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SPREADSpace Packets Regularly Exploiting Asymmetry in Delays

MASTErPIECEMultipAth Scheduler That Exploits PropagatIon diffErenCEs

bETTERExploit Transmission Time diffERences

SPLENDiDSpace Packets evenLy and Exploit Network Delay Differences

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Thank you !

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