Cascades of Nested Acknowledgments in Multi-Hop MASQUE

MASQUE proxying is a mechanism for tunneling traffic over QUIC, which can help obstruct IP-based tracking when used in a nested multi-hop fashion as, e.g., done with Apple's Private Relay. Yet, multi-hop MASQUE tunneling incurs an encapsulation overhead and we find further performance issues: because tunneled ACKs become ACK-eliciting, nested MASQUE tunnels can trigger a cascade of unnecessary ACKs of ACKs that amplifies with each hop. We theoretically derive that the number of tunneled ACKs increases exponentially with the number of proxies, but that the growth factor can be small when ACK aggregation is used. Our testbed measurements reveal that practical ACK handling flattens the worst-case cascade, but the ACK byte overhead still reaches up to 13 % when using four proxies. To reduce this impact, we finally evaluate two mitigation approaches which decrease overhead with little to no performance impact.