Packets often wait in queues when several flows contend for an output link. Queueing absorbs short bursts, but persistent excess traffic increases delay and eventually causes packet loss when buffers fill.

A path's bandwidth-delay product (BDP) estimates how much data can be in flight while fully using the path:

$$ \mathrm{BDP}=\mathrm{bandwidth}\times\mathrm{round\mbox{-}trip\ time}. $$

A 100 Mbit/s path with 40 ms RTT has a BDP of about 4 Mbit, or 0.5 MB. A sender restricted to only a few kilobytes in flight cannot fill such a path.

Large buffers do not eliminate the trade-off: excessive queue occupancy can create very high latency, while undersized queues can drop bursts unnecessarily.

Throughput therefore depends not just on link rate, but on delay, in-flight data and queue dynamics.