1 Auction or Tâtonnement – Finding Congestion Prices for Adaptive Applications Xin Wang Henning Schulzrinne Columbia University

2 Outline Motivations Goal and scope Pricing strategies Congestion pricing Performance studies Conclusion

3 Motivations Two types of congestion pricing models in Internet today Tâtonnement Iteratively update price that aggregate user demand approaches available bandwidth Auction Allocate bandwidth based on users’ bidding price No work comparing their performance, and little work address the practical issues

4 Goal and Scope Compare tâtonnement and auction Develop comparable pricing models Compare the performances Address issues for practical usages Scope Network: periodical price adjustments/resource allocations Users: short-term reservation/demand adaptations

5 Pricing Strategies Holding price and charge: based on cost of blocking other users by holding bandwidth even without sending data Usage price and charge: maximize the provider’s profit, constrained by resource availability Congestion price and charge: drive demand to supply level (tâtonnement or auction)

6 Congestion Pricing Tâtonnement process Congestion charge proportional to excess demand relative to target utilization M-bid auction model User indicates its willingness to pay a premium for different bandwidths under congestion through bids Congestion price: charge highest rejected bid price Features: reduce uncertainty: user can express multiple preferences reduce signaling bursts: user provides bids in advance reduce setup delay: inter-auction admission allowed support periodical auctions

7 Resource Allocations Tâtonnement: User agent: determines the optimal demand based on user preferences, network price, constrained by user budget and application QoS requirements. Auction: Network: selects bids exceeding the auction price; multiple bids of a user can be higher than the auction price, select the one with higher bandwidth (lower price per unit bandwidth). User agents: adapt rate based on allocated bandwidth/QoS from network auctions.

8 Simulation Topologies Bottlenecks Studied Topology 1 Topology 2

9 Tâtonnement vs. auction Tâtonnement can maintain the target utilization (0.9); With similar user benefit, auction has higher utilization Blocking of tâtonnement is 40 times smaller than that of fixed pricing; blocking of auction is almost zero, with fractions of users delayed until next auction.

10 Tâtonnement and auction have comparable total and average user benefit Tâtonnement has higher congestion price, and hence allows for higher network revenue. Tâtonnement vs. auction (cont’d)

11 Impact of target utilization and M Higher throughput of tâtonnement is at the cost of performances: higher blocking probability and lower user benefit Auction performance is robust to the variations of M

12 Performance of topology-2 Similar trends as those of single bottleneck

13 Conclusions Performance comparisons Tâtonnement and auction effectively control congestions have comparable performances function effectively over a range of parameters: control periods, demand elasticity, different numbers of user multiplexing, different network topologies. Auction has higher bandwidth utilization at a given user benefit, but has higher implementation complexity longer setup delay Tâtonnement has higher network revenue Resolve some practical issues for both schemes