[Bloat] Detecting bufferbloat from outside a node

[Jonathan Morton]

Generally, the minimum observed delay will correspond to the case when both
inbound and outbound queues are empty throughout the path. This delay
should correspond to basic propagation and forwarding delays, which can't
be reduced further without altering some aspect of the network.

Higher observed delays than this will tend to correspond to one or both of
the buffers at the bottleneck being persistently filled. To work out which
one, you'll need to estimate the network load in each direction. This is of
course easiest if you can see all or most of the traffic passing the
bottleneck link, or if you yourself are participating in that load, but
it's probably possible in some other situations if you get creative.

To determine that bloat is NOT present, you need to observe delays that are
close to the baseline unloaded condition, while also being fairly sure that
the bottleneck link is saturated in the relevant direction.

The most reliable indication of link saturation is to observe ECN marked
packets, which will only normally be produced by an AQM algorithm
signalling link congestion (where both endpoints of the flow have
negotiated ECN support). A slightly less reliable indication of saturation
is to observe lost packets, either via retransmission or ack patterns,
especially if they occur in bursts or at remarkably regular intervals.

- Jonathan Morton
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