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A sender that took its epoch for lost offers epoch 1 again with a new
key. The receiver accepts it in the turn, after the answers; until then
its answers still carried the ACCEPT of the earlier offer, which named
the same epoch. The sender began its epoch from that one, the two sides
held different secrets, and 2 s later it took the epoch for lost again:
a turn of that each time the order fell so. Now an ACCEPT carries the
first four bytes of the key it answers, and a sender begins its epoch
only from one that answers its own.
test-link cuts the path for 3 s the moment the sender begins an epoch,
three times; a sender's counts say how many epochs it took for lost,
and it must be three. Before, it was four.
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nothing past
A sender offers the public half of a pair made for the next epoch
(OFFER) until the receiver accepts with a half of its own (ACCEPT); their
X25519 and the epoch's secret make the next one's, and the private
halves go. The header's low four bits say a packet's epoch; epoch 0 is
the nodes' static secret, as the keys were. The first epoch is offered in
the run's first packet (it begins with the first answer when the
receiver is not busy), then one an hour or GiB. One who later has both
nodes' keys opens only epoch 0, and no record went there after an epoch
began: test-link tries every packet sent under the static keys, with
epochs a second long, loss, reordering, restarts and replays.
The receiver writes the epoch it accepted in its memo (a place is 256
bytes now), so it goes on without an X25519 when it starts again; one
that forgot is not heard from, and its senders, after 2 s and four
probes unanswered, offer again in epoch 0 with no records until an epoch
begins. Keys of four epochs are tried (accepted, now, before, zero), as
two may share the four bits.
An accept costs an X25519 on each side, 220 µs on this Pi: one of a
sender with records costs a token (256 a second, 8 at once) and the
next packet offers again when there is none; one of a sender with
nothing else to send waits for none, one a second for it. The X25519 is
made after the turn's answers, never between a packet and its answer.
serve's report and M2's generator count keys, X25519s and epochs.
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test-link now also plays an attacker: packets changed on the way,
packets of the past sent again from the sender's address and its own,
keys and sources not allowed, and a flood of made-up packets. It found:
- A nonce used twice: a receiver sealed every answer in its one run, and
a sender at rest coming back with the same run got a new slot whose
answers began at number 0 again under the same key. Each slot now
draws the run its answers are sealed in.
- A link stopped for good by one packet of the past: a stream that did
not know its last heard (after a restart or a rest) handed over from
its first record held, and a stale part of an old one never came
whole. It hands over from its first whole record.
- Answers taken away: any packet that opened, however old, moved where
answers went. Only the newest of its run moves it now; a new address
is sent a challenge the sender says back (CHALLENGE, RESPONSE), and
until then gets at most three times the bytes that came from it.
- An answer made by a packet that did not open: a number heard before
was acknowledged before the packet was opened. It is opened first.
- An old answer lowering the sender's credit, or its gap making it send
again: the sender now sends up to the most heard and credit it was
told, and takes a gap only from a word not older than it knows.
- Ten thousand made-up packets made ten thousand keys: keys made for a
run that did not open now cost a token, 256 a second.
Counts now say runs keyed and secrets made.
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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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