Sizing Router Buffers

Question 1

Discuss the drawbacks to over-buffering routers. If memory is widely available at low cost, why is it a bad idea to use massive buffers to ensure high link utilization?

Answer 1

Using massive buffers in internet routers increases the size, power consumption, and design complexity of routers.

Large buffers are typically implemented in off chip DRAM, where small buffers can be implemented on chip. Additionally, large off chip DRAM is slower to retrieve data than on chip SRAM.

During periods of congestion with a large amount of buffered packets, latency sensitive applications like live streaming and networked video games will suffer.

Using large amounts of cheap memory may eliminate the need to worry about proper buffer sizing, but it induces hardware efficiency issues and presents problems for low latency applications.

Question 2

Under what conditions was the “rule-of-thumb” for buffer size (𝐵= 𝑅𝑇𝑇̅̅̅̅̅̅ 𝑋 𝐶) originally conceived? How does this fundamentally differ from current, real world conditions?

Answer 2

The “rule-of-thumb” is derived from an analysis of a single long lived TCP flow. The rate is designed to maintain buffer occupancy during TCP congestion avoidance, preventing the bottleneck link from going idle.

These conditions are not realistic compared to actual flows in backbone routers.

Question 3

Statistical modeling of desynchronized long lived flows indicates that smaller buffer sizes are sufficient to maintain link utilization as the number of these long lived flows increases. However, not all flows can be expected to be long lived. Discuss why short lived flows (less than 100 packets) do not significantly detract from these findings.

Answer 3

Even when the vast majority of flows across a link are short lived, the flow length distribution remains dominated by the long lived flows on the link. This means that the majority of the packets on the link at any given time belong to long lived flows. Required buffer size in the case of short lived flows depends on actual load on the links and the length of the flows, not the number of flows or propagation delays. This means that roughly the same amount of buffering required for desynchronized long lived flows will also be sufficient for short lived flows as well.