Flows

Flows are specified in TOML and expanded into source/sink actors attached to host switches.

Core types:

• Flow and FlowType (src/flows/flow.rs)
• TrafficCharacteristics (src/flows/mod.rs)
• PacketSource / PacketSink enums (src/flows/source.rs, src/flows/sink.rs)

Flow types

FlowType determines which source/sink implementation is instantiated:

• PacketDistribution: distribution-driven packet source + basic sink
• TCP: TCP source + TCP sink (ACK-based, congestion control)
• DCQCN: DCQCN source + DCQCN sink (feature dcqcn)

Traffic model

Every flow has TrafficCharacteristics:

• start delay: initial_delay
• termination: either size (bytes) or duration (seconds)
• packet arrival distribution: arr_dist
• packet size distribution: pkt_size_dist

Optional per-flow protocol config:

• TCP: tcp.cc_algorithm (Reno/CUBIC/BBR) and tcp.ecn (send ECN-capable packets)
• DCQCN: dcqcn.* parameters (rate bounds, alpha gain, pacing, CNP settings)

Routing

Paths are computed in Flow::compute_path(...):

• ShortestPath: uses petgraph A* to find a shortest path
• ECMP: finds all equal-cost paths and selects one deterministically per flow
• PathFromConfig: uses an explicit path = [...] from TOML

Computed paths include endpoints:

[source_endpoint_id] + [switch nodes...] + [sink_endpoint_id]

The topology builder then installs forwarding table entries (fib and r_fib) into each switch for each flow ID.

Dependencies

Flows can declare ordering constraints:

• starts_after = [flow_id, ...]
• starts_before = [flow_id, ...]

These are wired by sending FlowFinishMsg when a flow completes (either from the sink for distribution flows, or from the source for TCP/DCQCN flows).

Collectives

Collectives (src/flows/collective.rs) generate sets of flows from higher-level patterns:

• Broadcast, Gather, AllReduce, and RingAllReduce

They are expanded into concrete flows inside Topology::process_collectives (src/topos/topo.rs) before routing happens.