Switches & Queues

Switch configuration is global: it applies to every scheduler instantiated per directed edge in the topology graph (see Topology::connect_neighbours in src/topos/topo.rs).

[switch]
port_rate = 1e9
capacity = 100
discipline = "FIFO"
drop = "TailDrop"

switch.port_rate

Bit rate of each egress port in bits per second.

• Type: f64
• Required: yes
• Used by: all schedulers (serialization delay = size_bits / port_rate)

switch.capacity

Queue capacity at each egress scheduler.

• Type: usize
• Required: yes
• Semantics: currently interpreted as packets, not bytes (Days instantiates schedulers with CapacityUnit::Packets)
• Special value: 0 means “unlimited” for built-in drop/ECN strategies

switch.discipline

Selects the scheduling discipline (which scheduler model is instantiated).

Supported values:

switch.drop

Selects the congestion policy applied on enqueue.

Supported values:

Notes:

• Marking ECN only works if the packet is ECN-capable (Ect0/Ect1). Otherwise the scheduler will drop the packet when asked to “mark”.
• For TCP, enable ECN marking by setting [flow.traffic.tcp] ecn = true so the TCP source emits Ect0 packets (see /docs/configuration/flows).
• DCQCN sources always emit Ect0 packets.

switch.ecn_threshold (optional)

Occupancy fraction used by drop = "ECN_THRESHOLD".

• Type: f64 (fraction in [0, 1])
• Default: 0.8
• Meaning (with packet capacity): if queue_len + 1 > floor(ecn_threshold * capacity), the scheduler returns MarkEcn.

switch.run_batch_size (optional, FIFO/DRR/WRR)

Maximum number of packets the FIFO (Port), DRR (DRRServer), or WRR (WRRServer) schedulers pre-schedule per invocation.

• Type: usize
• Default: 64
• Notes: batching amortizes event queue contention. FIFO always schedules up to this many packets in queue order. DRR batches only consecutive transmissions from the currently serviced class to preserve fairness across classes. WRR batches multiple packets from a single class (up to its weight) so other classes are considered between batches just as in the classic algorithm. Values < 1 are coerced to 1.

switch.weights (required for DRR/WRR/WFQ)

Class weights.

• Type: Vec<usize>
• Required when discipline is DRR, WRR, or WFQ

Class assignment:

• Days uses a simple mapping class_id = flow_id % weights.len().

switch.priorities (required for SP)

Maps class IDs to strict priority levels.

• Type: Vec<usize>
• Required when discipline = "SP"
• Interpretation: larger numbers are served first (the scheduler iterates priorities in descending order).

Class assignment:

• class_id = flow_id % priorities.len()

switch.vticks (required for VirtualClock)

Virtual clock tick values per class.

• Type: Vec<f64>
• Required when discipline = "VirtualClock"
• Interpretation: seconds per bit (i.e., inverse of target rate in bits/s)

Class assignment:

• class_id = flow_id % vticks.len()