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acknowledged
A link starts at 1,200 bytes; once an RTT is known it sends a PING padded
to 1,452 (DPLPMTUD, RFC 8899), alone and with the don't-fragment bit,
and uses that size once it is acknowledged. A probe lost is no
congestion; it is tried again a second on, three times, then ten minutes
on. GSO is asked for each burst (a control message), not the socket, so
a packet sent alone is never cut. CUBIC counts in the datagram.
On loopback packets fall by 19-27%; the CPU a record falls 13% at 1 KiB
and 7% at 16 KiB; in the simulator of a 50 Mbit/s path with 1% loss the
goodput rises from 6.1 to 7.0 Mbit/s (5%: 2.3 to 3.0).
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link/link-protocol.h is the protocol with no socket, thread or clock of
its own: packets and time in, packets out. link/link.h carries it over
UDP as before. A sender starts small (8 flights, 16 records, a 4 KiB
ring) so one thread can hold many. The transport's CPU a record is the
same within 2% (five rounds each way, 61 B to 16 KiB).
test-link runs both sides through a network of its own: loss, packets
held back and doubled, failed saves, an idle sweep, and both sides
starting again, every record applied once as sent.
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