Why a bigger buffer makes things worse
Queue and AQM
Why a bigger buffer makes things worse, and what active queue management does about it.
50.0 Mbps × 50.0 ms ÷ 8 = 305.2 KiB = 216 × MSS
Capacity times round trip, divided by eight because capacity is quoted in bits and everything else here in bytes. That is how much data can be in transit at once, so it is the amount worth having in flight — and the number the buffer below is measured against. Send less and the wire idles; send more and the extra waits in a queue.
How fast the narrowest link on the path can forward packets — the bottleneckThe slowest link on the path. It is the only one that matters: it sets the speed, and it is where the queue forms..
Distance, as time. Doubled it is the RTTHow long a packet takes to get there and be acknowledged back. This is what you feel as lag.Round-trip time. Base RTT is the wire alone; anything above it is time spent waiting in a queue. — the fastest a round trip can possibly be.
How much may wait at the bottleneck. Read it as a multiple of the BDPHow much data fits inside the wire itself. A fast, long link is a long pipe: you can have a lot in flight before the first byte has arrived anywhere.Bandwidth-delay product: capacity × round-trip time. The right amount to keep in flight.: below 0.5 the link goes idle, above 1 the extra is pure latency.
Packets dropped for reasons other than a full queue — radio interference, a bad cable. Leave at zero to see congestion alone.
Run settings60 s · #4
How long to simulate. Longer runs show more sawtooth cycles and take longer to compute.
Which seedWhich set of random numbers this run used. Same seed, same settings, byte-identical result — so a link you share shows exactly what you saw. this run used. Same settings and same seed give a byte-identical result.
A loss-based sender fills whatever buffer it is given. Push the buffer past one BDP and the extra bytes buy no throughput at all — every one of them is latency. That is bufferbloat, and it is why the queue disciplines below exist.