News and New Products

Spec extends RapidIO to medium-rate data-plane applications

By Nicholas Cravotta -- EDN, 11/13/2003

The RapidIO Trade Association released its Flow Control Extensions Specification to complement the link-based flow-control mechanisms of the spec and extend the reach of RapidIO into medium-rate data-plane applications such as media gateways, radio-network controllers, and routers.

The developers of RapidIO designed it as a control interconnection, managing load and store operations and messaging. Key characteristics of such control traffic are low latency, high bandwidth, and hardware-based guaranteed delivery with error coverage. Data-plane applications require protocol encapsulation; traffic classes that can tolerate lossy transmissions; the ability to handle dynamically changing traffic patterns and priorities; low latency; quality of service; and handshaking, which an upper-layer protocol usually manages.

The new spec addresses network-congestion issues that can occur in large systems. Congestion causes head-of-line blocking in multistage switches, potentially reducing effective aggregate fabric efficiency; that is, congested flows exceeding their link capacity passing through a switch can lead to the blocking of unrelated flows passing through the same switch, even though the flows have different destinations. Flow control allows management of congestion by throttling back the sources of congestion, rather than merely applying back pressure at the point of congestion, thereby affecting unrelated flows.

To accomplish this task, RapidIO has introduced the logical-layer CCP (congestion-control packet), which uses source- and destination-ID fields to identify flows at a congestion point. CCP packets travel at the highest priority, waiting only for packets in transit. An XOFF flag stops all flows with the same priority as and lower than the congested flow.

CRC protects flow-control packets, which obey the same rules as other packets; CRC re-tries them on transmission errors and discards them when a receiver has no open buffers. Switches track issued XOFFs and send corresponding XONs when congestion ceases. Source endpoints count XOFFs and XONs using per-flow counters and then enables the flow when the counter reaches zero. A lost XOFF allows performance degradation to continue. An orphan XOFF flow mechanism, often a time-out counter, handles lost XONs.

The RapidIO Trade Association has approved the specification, which is available for downloading from its Web site.

RapidIO Trade Association, www.rapidio.org.