REM : Active Queue Management Sanjeewa Athuraliya, Victor H. Li Steven H. Low, Qinghe Yin Presented by Hwangnam Kim

1. Background TCP Reno –Slow Start, Congestion Avoidance, Fast Transmit, Fast Recovery –Congestion indication Retransmission Timeout Duplicate ACKs Explicit Congestion Notification (ECN)

TCP in wireless link –Link errors embedded in congestion –Burst errors Random Early Detection (RED) –Calculates average queue length –Uses –Depends on queue length When congestion, queue length should be long enough to notify congestion Targets either Bandwidth or Delay

–Congestion Measure

2. REM Overview Goals –Decouples congestion from Performance Introduction of Price –Targets at both Bandwidth and Delay –Based on mathematical dual model

TCP congestion model –Sources decide rates based on feedback information End-to-End concept Positive news: ACKs Negative news: Duplicate ACKs, Transmission Timeout, ECN –Simple networks

REM model –Optimization approach Sources adjust rates to maximize the sum of gains Networks calculates prices for given source rates –Treats equally network links and sources

Congestion measure

Marking probability End-to-end marking probability

3. REM Details Optimization Approach –Model Network : a set of link L = {1,…,L} Sources : S={1,…,S}, where s has (L(s),Us,ms,Ms) Is = [ms,Ms], S(l)=a set of sources that use link l –Primal Problem

–Dual Problem Bound primal problem By duality theory, –There exists a dual optimal p* >=0 such that x(p*) is primal optimal

4. Stability

Equations Nonlinear term

Linear equations Coefficient matrix Spectral radius of Q

Eigenvalue of Q Therefore,

5. Performance Single link

Multiple links

Varying propagation delays

Wireless TCP

6. Conclusion Contribution –Matches rate and queue length –Achieves high utilization & negligible loss and delay –Mathematical modeling and stability proof Remarks –How to decide target queue length or target rate ? –How to address packets out of order ? –Errors in the stage of slow start –SACK