Occupancy Sampling for Terabit CEE Switches Resolving Output and Fabric-Internal Congestion

One consequential feature of Converged Enhanced Ethernet (CEE) is losslessness, achieved through L2 Priority Flow Control (PFC) and Quantized Congestion Notification (QCN). We focus on QCN and its effectiveness in identifying congestive flows in input-buffered CEE switches and switching fabrics. Our objective is to complement PFC’s coarse per-port/priority granularity with QCN’s per-flow control. By detecting buffer overload early, QCN can drastically reduce PFC’s side effects. We install QCN congestion points at input buffers with virtual output queues and demonstrate that arrival-based marking cannot correctly discriminate between culprit and victim flows.
Our main contributions are the following. First, we propose occupancy sampling (QCN-OS), a novel, QCN-compatible marking scheme, and random occupancy sampling as a practical realization. For non-blocking switching fabrics, QCN-OS at input VOQ buffers is shown to correctly identify culprit flows, improving buffer utilization, switch efficiency, and fairness.
Next we consider blocking network topologies and show that (a) switch-internal blocking may prevent QCNOS at the inputs from identifying flows bottlenecked inside the fabric and (b) QCN-OS combined with fabric-internal reliable delivery correctly identifies internally and/or externally bottlenecked flows. Finally, we propose two-samples, a refinement of QCN-OS that more easily identifies internally congested flows.

By: Fredy D. Neeser, Nikolaos I. Chrysos, Mitch Gusat, Rolf Clauberg, Cyriel Minkenberg, Kenneth M. Valk, Claude Basso

Published in: RZ3849 in 2013


This Research Report is available. This report has been submitted for publication outside of IBM and will probably be copyrighted if accepted for publication. It has been issued as a Research Report for early dissemination of its contents. In view of the transfer of copyright to the outside publisher, its distribution outside of IBM prior to publication should be limited to peer communications and specific requests. After outside publication, requests should be filled only by reprints or legally obtained copies of the article (e.g., payment of royalties). I have read and understand this notice and am a member of the scientific community outside or inside of IBM seeking a single copy only.


Questions about this service can be mailed to reports@us.ibm.com .