Buffer Sizing for Congested Internet Links Chi Yin Cheung Cs 395 Advanced Networking

Introduction Buffers are important for routers Common rule of thumb is to assign buffers based on bandwidth delay product Why is this so? Potential problems?

Related work Stanford Scheme Buffer requirement to achieve ~100% util is given by B = CT(avg) / root(N) N=no. of TCP flows However, focuses only on utilization, does not consider loss rate

Purpose of BSCL Focus on Buffer requirement of a Drop-queue design, given Min utilization Max loss rate Max queuing delay Applicable when 80-90% of traffic of given link belongs to Nb locally bottlenecked flows. Rest can be UDP flows, or short TCP flows

Assumptions Single queue Constant Capacity C, buffer space B bytes Drop-tail Policy At real routers, typically only one queue is used Buffer is structured in terms of bytes, rather than units of cells etc.

Goals Full utilization: avg utilization should be 100% Max loss rate should not be more than 1-2% for a fully saturated link Max queuing delay should not exceed a bound d-hat

Traffic types Locally Bottlenecked Persistent Remotely Bottlenecked Persistent Window-limited Persistent TCP flows Short TCP flows and non-TCP traffic Paper assumes that most of the traffic is generated by LBP flows – if LBP flows is a large proportion, buffer reqs for other flows can be igonred.

Evaluation NS-2 simulator used for simulation Capacity = 50Mbps, loaded with 4 types of traffic, with LBP flows varying from 2 – 400, other two types of flow is minimal (10-20) Simulate traffic by first setting buffer according to rule of thumb, then stanford, then BSCL BSCL works better than other two in terms of maintaining utilization and low loss

Results When Number of flows is small (20-40) BSCL predicts much smaller number of buffers than rule-of-thumb. Stanford approach requires more buffering than BSCL for small number of flows When more flows are simulated, stanford buffer requirements drop quickly, but loss rates increase rapidly

Conclusions BSCL is applicable for traffic where 80-90% of the traffic comes from LBP TCP flows Considers number of heterogeneous RTTs and partial loss synchornization It is better than the stanford scheme in that it maintains high utilization whilst keeping loss rate low. Limitations: work is based solely on simulations