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The Scala Switch models a shared-buffer data center Ethernet switch with PFC, ECN, ECMP hashing, and flowlet load balancing for Clos fabrics. Model version 3.7.0 is the version these pages describe.

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What this model simulates

The Scala Switch is a generic shared-memory Ethernet switch with IEEE 802.3 interfaces. A simulation configuration can place it as a Rack (ToR), Fabric, or Spine switch in a Clos topology. Rather than modeling a specific vendor ASIC, it models the mechanisms those ASICs share: a single shared packet buffer, per-traffic-class flow control, congestion marking, and multi-path forwarding. The results you reason about with it are buffer occupancy, PFC pause and resume behavior, ECN marking, packet drops, and how traffic spreads across equal-cost paths. Simulations on the platform carry IPv4 traffic. The platform counts a switch’s bandwidth as the sum of its port speeds, each full-duplex port once, assuming a non-blocking switch. SwitchingCapacity is compared with that sum to show whether the switch is oversubscribed; it does not limit what the switch forwards.

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Features

  • Priority Flow Control (PFC), IEEE 802.1Qbb. Per-traffic-class pause and resume. Each port pauses its upstream sender when a lossless arrival would take its own occupancy in a buffer pool past the Xoff threshold, and absorbs in-flight traffic in per-port headroom. It resumes the sender when a lossless packet that arrived on that port leaves the switch while the port’s headroom is empty and its occupancy is at or below the Xon threshold. A pause frame requests 65535 quanta, and the switch re-sends it every half pause time while the port stays paused.
  • Explicit Congestion Notification (ECN), RFC 3168. RED-style marking: packets carrying ECT(0) or ECT(1) are marked CE with a probability that rises between two thresholds on an exponentially weighted moving average of egress queue occupancy.
  • Shared buffer management. One switch-wide buffer divided into as many as eight pools, with eight traffic classes mapped onto them, separate ingress and egress accounting, and a choice of dynamic or fixed egress queue limits for lossy classes.
  • Lossless and lossy traffic classes. A class with PFC enabled pauses its upstream sender when the port’s occupancy in the class’s pool passes Xoff, and is never dropped at egress. Sizing a lossless class gives the conditions under which it is not dropped at ingress either. A class without PFC is bounded by egress queue limits and pool capacity and is dropped when it exceeds them.
  • Traffic classes from DSCP, RFC 2474. Eight classes, numbered 0 to 7, taken from the packet’s DSCP field divided by 8.
  • Bandwidth-delay product (BDP) queue sizing. Egress queue limits can be set as a multiple of the fabric’s bandwidth-delay product.
  • Equal-cost multi-path (ECMP) forwarding. Flow-based path selection over equal-cost routes, with six hash methods. The default, crc32WithSalt, salts the hash per switch, which de-correlates the choices of adjacent tiers in a fabric.
  • Dynamic load balancing (flowlet switching). With LoadBalancingMethod set to flowlet, the switch moves a flow to the member of its equal-cost set with the least egress occupancy when the flow sends nothing for Gap or longer, and by default prefers members that PFC is not pausing.
  • Statistics. Per-port packet and byte counters, PFC and ECN counters, aggregate buffer occupancy per pool, and a load balancing record for each member of each equal-cost set. Records a simulation writes lists them.

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Architecture

The switch is assembled from a few cooperating components. The switch node hands every admission decision to the shared buffer manager, which owns the buffer and drives the ports.

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How to read these pages

Read them in this order:
  1. Packet handling: how a packet is classified, admitted, queued, scheduled, ECN-marked, paused by PFC, and steered across equal-cost paths, and what a simulation records.
  2. Shared buffer manager: how the buffer is divided into pools and limits at startup, how every byte is accounted for at run time, and how to size a lossless class, with a worked example of the defaults.
  3. Configuration: every attribute a simulation configuration can set, with its type and default, how to change attributes through the platform API, and the configuration rules that stop a simulation.