Single-Connection Mixed-Criticality Transport with CATS: Bounded Guarantees, Three Structural Limits, and a QUIC Escape
2026-06-15 • Networking and Internet Architecture
Networking and Internet ArchitectureDistributed, Parallel, and Cluster ComputingOperating SystemsPerformance
AI summaryⓘ
The authors address the problem of sending different types of messages together over one network connection, where important messages can get delayed by less critical bulk data. They propose CATS, a method that assigns priorities and carefully orders messages to ensure critical ones are not delayed and overall fairness is maintained. They explain why common solutions like fair queuing and multiple parallel connections don’t fully solve the problem, especially on a single connection. The authors then suggest using CATS with QUIC’s independent streams to overcome these limitations, supported by simulations and a prototype.
mixed-criticalityTCPpriority schedulinghead-of-line blockingcongestion controlfair queuingQUIClow-latencycredit-based shapingtail latency
Authors
Syed Muhammad Aqdas Rizvi
Abstract
Mixed-criticality applications, such as satellite terminals, industrial telemetry, embedded systems, tactical, and other constrained links, often multiplex a small, latency-critical message class and bulk traffic over a single commodity transport connection. A single FIFO connection can starve the critical class under load. The obvious alternative, opening parallel connections, costs an additional five-tuple (often blocked by carrier-grade NAT, port budgets, and operator policy) and is not always available; when the critical class is light, two connections can also be bandwidth-fair only in aggregate rather than single-flow fair. We present CATS (Conductor-driven Asymmetric Transport Scheme), a sender-side, receiver-transparent transport-layer priority scheme over TCP: a Conductor assigns each message a priority class and just-in-time sequence numbers, using a credit-based shaper. CATS provides the one combination its alternatives cannot: deterministic non-starvation together with single-flow fairness, plus a provable bounded per-class delay. We then show that, crucially, CATS-over-TCP is not a tail-latency mechanism, and why. Three structural barriers bound in-band priority: the in-order sequence space (head-of-line blocking), the shared congestion window (cross-class coupling), and the per-flow granularity of network QoS (in-band priority is invisible to it). These barriers explain why fair-queuing and even the modern low-latency standard L4S cannot help a single connection, and why two parallel connections reduce the latency tail at the cost of an additional flow. We give CATS-over-QUIC as the principled escape: independent streams with per-stream isolation under aggregate-coupled congestion control self-isolate at the endpoint, attaining the guarantees on one fair flow. An ns-3 evaluation and QUIC proof-of-concept support the findings.