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.

翻译：混合关键性应用（如卫星终端、工业遥测、嵌入式系统、战术链路及其他受限链路）常通过单一商品化传输连接复用延迟敏感的小型消息类与海量流量。单FIFO连接在负载下会导致关键类消息饥饿。显见的替代方案——建立并行连接——需额外消耗五元组（常被运营商级NAT、端口预算及运营商策略阻断），且并非始终可用；当关键类流量较轻时，双连接仅在聚合层面而非单流层面实现带宽公平。我们提出CATS（导体驱动非对称传输方案），一种基于TCP的发送端驱动、接收端透明的传输层优先级方案：导体通过基于信用的整形器为每条消息分配优先级类与即时序列号。CATS提供了其替代方案无法实现的唯一组合：确定性非饥饿与单流公平性，以及可证明的有界逐类延迟。我们进一步证明，CATS-over-TCP并非尾部延迟机制，并揭示其本质原因。三类结构性屏障制约了带内优先级：有序序列空间（队头阻塞）、共享拥塞窗口（跨类耦合）以及网络QoS的逐流粒度（带内优先级对其不可见）。这些屏障解释了为何公平队列甚至现代低延迟标准L4S无法辅助单连接，以及为何双并行连接通过额外流的代价降低延迟尾部。我们提出CATS-over-QUIC作为原则性逃逸路径：在聚合耦合拥塞控制下，具有逐流隔离的独立流在端点处实现自隔离，从而在单一公平流上达成保障。ns-3评估与QUIC概念验证结果支持上述发现。