Packet

The simulator’s primary message is Packet (src/flows/packet.rs). Packets are plain structs (no payload modeling) that carry:

• Timestamp: Packet.time is the current simulation time associated with the packet.
• Creation time: Packet.creation_time is the time the source first created the packet.
• Size: Packet.size (bytes) determines serialization delay at schedulers/links.
• Identifiers: packet_id is per-flow, flow_id identifies the flow globally.
• Accumulated queueing delay: queueing_delay tracks time spent waiting in scheduler queues.
• Priority (PCP): priority is an 802.1Q priority code point (0–7).
• End-of-flow marker: last_packet is used for dependency wiring and “flow finished” events.

Packets also model minimal transport/control signals:

• TCP acks: ack: Option<TCPAck> is used by the TCP source/sink pair.
• Control packets: control: Option<ControlPacket> currently includes DCQCN’s CNP (DcqcnCnp).
• ECN: ecn: EcnField can be NotEct, Ect0, Ect1, or Ce.
• TCP CWR: cwr: bool is used for ECN-CWR signaling in TCP.

Time updates

Two helper methods encode the intended invariants:

• queueing_delay_update(now) adds now - packet.time to queueing_delay. Call this when a packet leaves a queue.
• departure_update(now) sets packet.time = now. Call this when a packet departs a component and is scheduled for delivery downstream.

Schedulers follow the pattern:

1. Pop packet at time now
2. packet.queueing_delay_update(now)
3. Compute serialization delay timeout = size_bits / rate_bps
4. packet.departure_update(now + timeout)
5. Schedule downstream delivery after timeout

ECN marking

Drop/mark policies can call packet.mark_ce():

• If packet.ecn == NotEct, mark_ce() returns false (cannot be ECN-marked).
• If ECN-capable (Ect0/Ect1), mark_ce() sets ecn = Ce and returns true.

Schedulers that “mark ECN” will drop packets that are not ECN-capable.