Jonathan Newton over at Vodafone Group made some interesting observations about what happens when two of the optimal congestion controllers interact through a shared FIFO queue:

If we take two flows E and F;  E is 90% of bandwidth and F 10%;  the time for the congestion signal to reach the sender for each flow is dE and dF where dE is 10ms and dF 100ms.   We assume no prioritisation so they must share the same buffer at X, and therefore share the same peak transient delay.

We have an event at t0 where the bandwidth is halved.

For time t0 to t0+dE (the first 10ms), the total rate transmitted by both sources is twice (90%*10%)/50% the output rate, so the component of the peak delay of this part is (2-1)*dE = 10ms

For time t0+dE to t0+Fd (the next 90ms), the total rate transmitted by both sources is (45%+10%)/50% of the output rate, so the component of the peak delay of this part is (1.1-1)*dF-dE = 9ms

Making the peak transient delay 19ms.

This implies that moving some senders closer to the edge (e.g. CDNs) might help reduce lag spikes for everyone. It also implies that slow-responding flows can have a very big impact on the peak transient latency seen by rapidly responding flows. If the bandwidth sharing ratio in the above example is 50/50, then the peak transient delay will be 55 ms, seen by both flows. For flow E that's a big increase from the 10 ms we'd expect if flow E was alone. For C=3 the increase is even worse, with flow E going from 20ms to 100ms when sharing the link with flow F.

- Bjørn