Constant DEFAULT_SEND_QUEUE_CAP 

pub const DEFAULT_SEND_QUEUE_CAP: usize = 2048;

Default outbound send queue capacity per face.

Must be large enough to absorb bursts from parallel pipeline tasks that all dispatch to the same face near-simultaneously. When full, outbound packets are dropped (equivalent to a congestion drop at the output queue — consistent with NFD’s GenericLinkService model).

With NDNLPv2 fragmentation, a single Data packet may expand to ~6 fragments, each occupying one queue slot. 2048 slots ≈ ~340 Data packets — enough headroom for sustained bursts over high-throughput links without silent drops.