RateLimiter

Session-wide upload/download rate limiting: the live driver for the pure BandwidthManager/BandwidthChannel port.

libtorrent gives session_impl one bandwidth_manager per direction plus a global bandwidth_channel pair, with per-torrent channels layered on top; a peer_connection asks for quota before moving payload and is parked in the manager's queue when the buckets are empty. This class is that wiring on coroutines: acquireUpload/acquireDownload suspend until the requested bytes are granted, and a periodic update (driven by start's ticker, or by a test directly) refills the buckets and releases parked waiters. This is the on_tick → update_quotas cadence.

A channel whose limit is 0 is unlimited and never participates (libtorrent skips zero-throttle channels when assembling a request's channel list), so with no limits configured every acquire returns immediately.

Download limiting works by delaying the receive path after the bytes arrived (back-pressure: a reader that stops draining stalls the sender's TCP window), which is also how libtorrent's receive quota throttles a peer.