  Most Internet traffic is carried by capacity-seeking protocols such as TCP
and QUIC. Capacity-seeking protocols probe to find the maximum available
throughput from sender to receiver, and, once they converge, attempt to keep
sending traffic at this maximum rate. Achieving reliable low latency with
capacity-seeking end-to-end methods is not yet entirely solved. We contribute a
theoretical analysis to this ongoing discussion. In this work, we derive an
expression for the minimum size of the spike in latency caused by a sudden drop
in network capacity. Our results highlight a quantifiable and fundamental
constraint on capacity-seeking network traffic. When end-to-end capacity is
suddenly reduced, capacity-seeking traffic inevitably produces a latency spike.
A lower bound on this latency spike can be calculated by multiplying the
round-trip delay from the network bottleneck to the source of capacity-seeking
traffic by the magnitude of the end-to-end capacity reduction. Testbed
experiments show that this bound holds for the DCTCP, BBR, and Cubic congestion
control algorithms. Our results have implications for the design of low-latency
PHY and MAC-layer technologies because we quantify an important transport-layer
consequence of unstable traffic rates.
