UDP receive buffers clamp to net.core.rmem_max, whose stock Debian
default (~208 KB) is smaller than one VBV-bounded keyframe burst at
moderate-to-high bitrates. The resulting packet loss damages every
keyframe, drop-on-damage discards them silently, and the receiver
shows a black screen with no journal errors while the sender streams
happily. The install script now raises rmem_max to 4 MB and persists
the sysctl; buffers are allocated lazily, so this costs no RAM at
rest. Wi-Fi-only boards (Pi Zero 2 W) remain bandwidth-bound — the
RUNBOOK points at --bitrate as the tuning knob.
fegger
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