[Cake] A few puzzling Cake results

[Toke Høiland-Jørgensen]

Jonathan Morton <chromatix99 at gmail.com> writes:

>>> I'm saying that there's a tradeoff between intra-flow induced latency and packet loss, and I've chosen 4 MTUs as the operating point.
>> 
>> Is there a reason for picking 4 MTUs vs 2 MTUs vs 2 packets, etc?
>
> To be more precise, I'm using a sojourn time equivalent to 4 MTU-sized
> packets per bulk flow at line rate, as a modifier to existing AQM
> behaviour.
>
> The worst case for packet loss within the AQM occurs when the inherent
> latency of the links is very low but the available bandwidth per flow
> is also low. This is easy to replicate using a test box flanked by
> GigE links to endpoint hosts; GigE has sub-millisecond inherent
> delays. In this case, the entire BDP of each flow exists within the
> queue.
>
> A general recommendation exists for TCP to use a minimum of 4 packets
> in flight, in order to keep the ack-clock running smoothly in the face
> of packet losses which might otherwise trigger an RTO (retransmit
> timeout).  This allows one packet to be lost and detected by the
> triple-repetition ACK method, without SACK.

But for triple-dupack to work you actually need to drop packets (the
first one, to be precise), not let it sit around in a bloated queue and
wait for precisely RTO timeout. If you turn off the AQM entirely for the
first four packets, it is going to activate when the fifth packet
arrives, resulting in a tail loss and... an RTO!

-Toke