FLUID: Slack-based Low-latency Delivery

We introduce FLUID (Fountain LiqUId Delivery), a protocol that uses fountain coding and receiver feedback for low-latency delivery of data blocks over lossy networks. Idealized Automatic Repeat reQuest (ARQ) protocols are bandwidth-optimal in the single-path setting, but must deliver every packet in a block and therefore can require additional rounds under packet loss. FLUID uses a controlled amount of slack to relax this all-packets requirement, allowing delivery to finish once enough encoded packets have been received. This yields substantially tighter delivery latency while remaining deterministically close to the ARQ bandwidth optimum. FLUID is controlled by a slack parameter $\epsilon$. Under the Loss-Product Rule, delivery finishes once the product of packet loss fractions across transmission rounds falls below $\epsilon$. Thus, FLUID can finish delivery in a small number of rounds even when every round experiences packet loss, while $\epsilon$ controls the gap between FLUID and bandwidth-optimal ARQ.