The listener swallowed every send failure as a counted loss. A send now splits
into permanent (MessageSize, InvalidArgument, AddressFamilyNotSupported: the
datagram can never be sent as built) and transient (everything else, still a
counted loss). SendSchema throws the SocketException and queues nothing, with
the packets it created withdrawn and their sequences reissued; a permanent
failure after an earlier fragment left ends the session. Inside a poll the
failure ends that peer's session, PeerDisconnected carries a reason naming the
error, and the poll is never failed. Tests pin each path, the classification,
BytesSent after a healthy send and a zero failure count.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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