Architecture Overview

Days is built around a small set of concepts:

• Discrete-event simulation: all progression happens by processing scheduled events in timestamp order.
• Actor model: each simulated component is an actor that owns its state and communicates via message passing.
• Packets carry time: most components avoid querying the global simulation clock by reading the timestamp stored in the message itself.

Days uses the nexosim crate as the simulation engine. Each actor implements nexosim::model::Model and receives messages through connected Output<T> ports and Mailbox<T> mailboxes.

Main components

• Topology builder (src/topos/): parses the TOML config, builds the network graph, instantiates models, wires ports, and runs the simulation.
• Switch (src/switches/): PacketSwitch demultiplexes packets using a per-flow forwarding table (FIB).
• Schedulers (src/schedulers/): model per-port queueing, drop/ECN policies, and transmission delay.
• Flows (src/flows/): model sources/sinks (packet distributions, TCP, DCQCN) and collectives.
• Optional L2 (src/l2/, feature-gated): link-layer frames and PFC (802.1Qbb-style pause).
• Logging/UI (src/utils/): progress UI, concurrency tracing, and CSV report logging.

Message flow (L3/L4-only)

When L2 is disabled (default), each network hop looks like:

PacketSwitch (upstream) -> Scheduler (per edge) -> PacketSwitch (downstream)

For an end-to-end flow, the path is:

PacketSource -> (host) PacketSwitch -> [Scheduler+Switch]* -> PacketSink

Where:

• the switch chooses the next hop using fib[flow_id] for data packets and r_fib[flow_id] for reverse traffic (ACKs and control packets),
• the scheduler models queueing + serialization delay and forwards the packet with an updated timestamp.

Optional L2/PFC pipeline

When link.mode="Pfc" and the binary is built with --features l2_pfc, each edge becomes a small pipeline:

Scheduler -> PfcEgressGate -> Link (serializer) -> PfcIngressPort -> PacketSwitch

This keeps the default path lean: when L2 is disabled, none of the extra models exist.

Reports and observability

Days periodically emits CSV reports (configurable by report_interval) for:

• sources (sources.csv)
• sinks (sinks.csv)
• schedulers (switches.csv)
• optional PFC ports (pfc.csv)
• optional per-event protocol traces for the Lean checker:
  • dcqcn_events.csv (with --features dcqcn,lean)
  • pfc_events.csv (with --features l2_pfc,lean)
  • cubic_events.csv (with --features lean)
  • drr_events.csv (with --features lean)
  • wfq_events.csv (with --features lean)