On 4 May 2015, at 11:28, Jonathan Morton <[email protected]> wrote:
> 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. > > Yep, that corresponds to (∆Q|G + ∆Q|S(min packet size)) - note that the composition/de-composition only work on the individual bases - i.e need to deal with the delay in terms of ∆Q|G seperately. We call the ∆Q|G and ∆Q|S the "structural delay" - as you point out needs change in some aspects of the network elements/their arrangement. > Higher observed delays than this will tend to correspond to one or both of > the buffers at the bottleneck being persistently filled. > the ∆Q|V (which can be established by measuring the delay and subtracting the (∆Q|G + ∆Q|S(packet size)) can measure this as an instantaneous value (not just at a persistent filing) - we've got measurements that show queues filling and emptying . > 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 estimate the contention for the common resource you need more than the load - you need the traffic pattern as well > 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. > > Noting that, delay and loss is, of course, a natural consequence of having a shared medium and that (sorry for being a bit contentious) - bloat is a subjective not objective term as > 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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