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Coupled congestion control for RTP media. draft-welzl-rmcat-coupled-cc- 02 Michael Welzl , Safiqul Islam, Stein Gjessing. RMCAT @ 88th IETF Meeting Vancouver, BC, Canada 6 November 2013. FP7 RITE Reducing Internet Transport Latency. Flow State Exchange (FSE). Flow 1. SBD.

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coupled congestion control for rtp media

Coupled congestion controlfor RTP media

draft-welzl-rmcat-coupled-cc-02

Michael Welzl, Safiqul Islam, Stein Gjessing

RMCAT @ 88th IETF Meeting

Vancouver, BC, Canada6 November 2013

FP7 RITEReducing Internet Transport Latency

flow state exchange fse
Flow State Exchange (FSE)

Flow 1

SBD

Hoping to have adraft by the next IETF

Flow 2

Flow 3

Flow 4

FSE

Flow n

previous version only passive
Previous version: only passive
  • Goal: Minimal change to existing CC
    • each time it updates its sending rate (New_CR), the flow calls update (New_CR, New_DR), and gets the new rate
    • Complicates the FSE algorithm and resulting dynamics (e.g. need dampening to avoid overshoot by slowly-reacting flows)

Update_rate()

FSE

Flow 1

Store Information

New_Rate

Update_rate()

Flow 2

Calculate Rates

New_Rate

Update_rate()

Flow n

New_Rate

now added fse active
Now added: FSE - Active
  • Actively initiates communication will all the flows
    • Perhaps harder to use, but simpler algorithm and “nicer” resulting dynamics

Update_rate()

FSE

Flow 1

Store Information

New_Rate

Flow 2

Calculate Rates

New_Rate

Flow n

New_Rate

now added fse active1
Now added: FSE - Active
  • Actively initiates communication will all the flows
    • Perhaps harder to use, but simpler algorithm and “nicer” resulting dynamics

FSE

Flow 1

Store Information

New_Rate

Update_rate()

Flow 2

Calculate Rates

New_Rate

Flow n

New_Rate

active algorithm
Active algorithm
  • Every time the congestion controller of a flow determines a new sending rate CC_R, the flow calls UPDATE
    • Updates the sum of all rates, calculates the sending rates for all the flows and distributes them to all registered flows
  • Essentially all that is left in this version:

for all flows i in FG do

FSE_R(i) = (P(i)*S_CR)/S_P

send FSE_R(i) to the flow I

end for

  • Designed to be as simple as possible
    • Lacks 1 feature (to be included in the next version): immediately using the capacity freed by application-limited flows
dynamic behavior rate adaptation protocol rap rate based aimd
Dynamic behavior: Rate Adaptation Protocol RAP ( = rate-based AIMD)

All simulations in this presentation:Dumbbell topology with bottleneck 10Mb, 1 BDP (13 packets) drop tail queue,RTT = 10 ms, duration 300 seconds

With FSE

Without FSE

dynamic behavior tfrc
Dynamic behavior: TFRC

With FSE

Without FSE

fse goals
FSE goals
  • Charter:“Develop a mechanism for identifying shared bottlenecks between groups of flows, and means to flexibly allocate their rates within the aggregate hitting the shared bottleneck.”(requirement F34 in draft-ietf-rtcweb-use-cases-and-requirements-12)
    • This works perfectly
    • Also did in the previousversion
  • But: because this avoids competition between flows, we expect reduced queuing delay and loss as a side effect

Priority of flow 1 increased over time

what s going on
What’s going on?
  • Queue drains more often without FSE
    • Thought behind expected benefits: coupling emulates one flow
      • But, e.g.: 2 flows with rate X each; one flow halves its rate: 2X  1 ½X
    • When flows synchronize, both halve their rate on congestion, which really halves the aggregate rate: 2X 1X

With FSE

Without FSE

current work
Current work
  • Trying to fix this (proportionally reduce aggregate rate on congestion, but increase by delta/N)
    • Some issues, e.g. slow start
  • Why do we have these problems?
    • Because all papers on RFC2140 etc. did not focus on reducing queuing delay
    • RFC2140 cwnd sharing probably has the same problem
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