Jonathan We see the problem as the difference between averages and instantaneous.
Network media is never “average” used - it is either “in-use” or “idle” - what we were seeing (and it was not an ISP but the core of a public service network here in the UK) was that delay can be “high” even when the loading is “low” (in the particular 5minute period the actual offered traffic was <0.01% of the capacity) - it was that the path under examination happened to be the constraining factor for a bulk transfer - the induced delay was high enough to place at risk other real-time applications (as defined by the public service network’s users). The reasoning that you seem to be applying below assumes a time-homogenity that doesn’t correspond to network traffic patterns that occur in the engagements we’ve done over the last 15 years. The graph I was referring to is the one example that we can publicly discuss (all the rest are under NDA!). What you are describing - if I’m understanding it properly - is the “busy period”. I would accept that Network Providers (ISP’s, telcos etc) have a problem in that they are relying on the system becoming idle frequently (the busy periods not accreting into longer and longer periods of non-idleness). However that is a pattern as well as a load dependent phenomena. Neil On 4 May 2015, at 13:17, Jonathan Morton <[email protected]> wrote: > >> On 4 May, 2015, at 14:39, Neil Davies <[email protected]> wrote: >> >>>> Noting that, delay and loss is, of course, a natural consequence of having >>>> a shared medium >>> >>> Not so. Delay and loss are inherent to link oversubscription, not to >>> contention. Without ECN, delay is traded off against loss by the size of >>> the buffer; a higher loss rate keeps the queue shorter and thus the induced >>> delay lower. >> >> Sorry Jonathan - that’s not what we’ve observed. We’ve measured “excessive” >> delay on links that are averagely loaded << 0.1% (as measured over a 15 min >> period) - I can supply pointers to the graphs for that. > > Presumably those would involve oversubscription on short timescales, and a > lot of link idle time between those episodes. > > One ISP I know of charges by data volume per month, currently in units of > 75GB (minimum 2 per month, so 150GB). This is on ADSL lines where the link > rate might reasonably be 15Mbps or so in the relevant direction. At that > speed, it would take 100,000 seconds to exhaust the first two units - which > is not much more than 24 hours. There is therefore roughly a 26-fold > mismatch between the peak rate available to the user and the average rate he > must maintain to stay within the data allowance. > > (I am ignoring small niceties in the calculations here, in favour of > revealing the big picture without too much heavy maths.) > > By your measure, that would mean that the link could only ever be 3.85% > utilised (1/26th) on month-long timescales, and is therefore undersubscribed. > But I can assure you that, during the small percentage of time that the link > is in active use, it will spend some time at 100% utilisation on RTT > timescales, with TCP/IP straining to achieve more than that. That is link > oversubscription which results in high induced delay. > > More precisely, instantaneous link oversubscription results in either > *increasing* induced delay (as the buffer fills) or lost packets (which > *will* happen if the buffer becomes completely full), while instantaneous > link undersubscription results in either *decreasing* induced delay (as the > buffer drains) or link idle periods. Long-timescale measures of link > utilisation are simply averages of these instantaneous measures. > >> A single flow can contend the medium just as much as a multiple ones > > I think here, again, we are using wildly different terminology. > > There is no contention for the medium on the dedicated full-duplex link I > described, only for queue space - and given a single flow, it cannot contend > with itself. > > The same goes for a full-duplex shared-access medium (such as DOCSIS cable) > with only one host active. There is no contention for the medium, because it > is always available when that single host requests it, which it will as soon > as it has at least one packet in its queue. There is a more-or-less fixed > latency for medium access, which becomes part of what you call the structural > delay. The rest is down to over- or under-subscription on short timescales, > as above. > > On a half-duplex medium, such as obsolete bus Ethernet or not-so-obsolete > wifi, then there can be some contention for the medium between forward data > and reverse ack packets. But I was *not* talking about half-duplex. > Full-duplex is an important enough subset of the problem - covering at least > ADSL, cable, VDSL, satellite, fibre - on which most of the important effects > can be observed, including the ones we’re talking about. > > - Jonathan Morton > _______________________________________________ Bloat mailing list [email protected] https://lists.bufferbloat.net/listinfo/bloat
