Quality of Service

Quality of Service (QoS) is essential for many backplane applications. While Ethernet through TCP/IP can support millions of individual streams and differentiate traffic by port number and protocol fields, no universally used class of service (CoS) field exists. The RapidIO standard defines up to six flows that can be considered prioritized classes of service. Through the use of Type 9 encapsulation and virtual channels, millions of streams can be differentiated as well.

QoS is also affected by Ethernet’s best-effort service, which commonly manages congestion by dropping packets, leading to latency jitter. Since flow control belongs to upper-layer protocols such as TCP, congestion cannot be promptly managed to prevent packet loss. This lack of short-term, link-level flow control requires larger endpoint receive buffers to avoid overruns. Further exacerbating latency is the fact that error detection and recovery occurs at the system rather than link level. Thus, timeouts exist only at layer 3 and above and are managed by offload hardware in the best case or software in the worst case, resulting in much longer timeouts and significantly increased latency jitter. Additionally, no standards exist for hardware-based recovery such as retries or timeouts, and so Ethernet drops packets for a significant period of time before failure is detected. While there are protocols such as bidirectional forwarding detection that exchange packets to detect failures, these continuously impose bandwidth overhead dependent on the responsiveness required.

Per its spec, all RapidIO networks must provide a minimum level of prioritized service to implement logical layer ordering and deadlock avoidance. This also improves average latency because packets marked with a higher relative priority must make forward progress since they might be responses. Optionally, switches can reorder packets from different flows and offer head-ofline blocking avoidance as well as other QoS features. With multiple flow control mechanisms, RapidIO networks allow congestion to be managed before there is a significant impact on network performance.

The RapidIO standard also defines a link-layer protocol for error recovery and various hardware-based link-to-link and end-to-end timeout mechanisms, enabling virtually all errors to recover at the link level without software intervention, substantially lowering latency jitter. Also, because RapidIO technology links carry valid traffic at all times, a broken link is promptly detected locally at the link level. As a result, failure rates, defined as undetected packet or control symbol errors, are significantly less than the hard failure rate of the devices on either end of the link, depending upon operating conditions.