This idea requires all relays to implement it, in order to deploy.

It is actually two optimizations at once. One optimization is a cell command
type to signal congestion directly. The other optimization is the ability for
this cell type to also carry end-to-end relay data, if any is available.

The second optimization may have AES synchronization complexity, but if we are
ensure end-to-end RELAY treatment of this cell in the cases where it does,
carry valid relay data, that should be OK. But differentiating when it does
and does not cary valid data may be easier said that done, with a single cell
command.

########################

X. Backward ECN signaling [BACKWARD_ECN]

As an optimization after the RTT deployment, we will deploy an explicit
congestion control signal by allowing relays to modify the
cell_t.command field when they detect congestion, on circuits for which
all relays have support for this signal (as mediated by Tor protocol
version handshake via the client). This is taken from the Options
mail[1], section BACKWARD_ECN_TOR.

To detect congestion in order to deliver this signal, we will deploy a
simplified version of the already-simple CoDel algorithm on each
outbound TLS connection at relays.
   https://queue.acm.org/detail.cfm?id=2209336
   https://tools.ietf.org/html/rfc8289

Each cell will get a timestamp upon arrival at a relay that will allow
us to measure how long it spends in queues, all the way to hitting a TLS
outbuf.

The duration of total circuitmux queue time for each cell will be
compared a consensus parameter 'min_queue_target', which is set to 5% of
min network RTT. (This mirrors the CoDel TARGET parameter).

Additionally, an inspection INTERVAL parameter 'queue_interval' governs
how long queue lengths must exceed 'min_queue_target' before a circuit
is declared congested. This mirrors the CoDel INTERVAL parameter, and it
should default to approximately 50-100% of average network RTT.

As soon as the cells of a circuit spend more than 'min_queue_target'
time in queues for at least 'queue_interval' amount of time, per-circuit
flag 'ecn_exit_slow_start' will be set to 1. As soon as a cell is
available in the opposite direction on that circuit, the relay will flip
the cell_t.command of from CELL_COMMAND_RELAY to
CELL_COMMAND_RELAY_CONGESTION. (We must wait for a cell in the opposite
direction because that is the sender that caused the congestion).

This enhancement will allow endpoints to very quickly exit from
[CONTROL_ALGORITHM] "slow start" phase (during which, the congestion
window increases exponentially). The ability to more quickly exit the
exponential slow start phase during congestion will help reduce queue
sizes at relays.

To avoid side channels, this cell must only be flipped on
CELL_COMMAND_RELAY, and not CELL_COMMAND_RELAY_EARLY. Additionally, all
relays MUST enforce that only *one* such cell command is flipped, per
direction, per circuit. Any additional CELL_COMMAND_RELAY_CONGESTION
cells seen by any relay or client MUST cause those circuit participants
to immediately close the circuit.

As a further optimization, if no relay cells are pending in the opposite
direction as congestion is happening, we can send a zero-filled cell
instead. In the forward direction of the circuit, we can send this cell
without any crypto layers, so long as further relays enforce that the
contents are zero-filled, to avoid side channels.


Y. BACKWARD_ECN signal format

   TODO: We need to specify exactly which byte to flip in cells
         to signal congestion on a circuit.

   TODO: Black magic will allow us to send zero-filled BACKWARD_ECN
         cells in the *wrong* direction in a circuit, towards the Exit -
         ie with no crypto layers at all. If we enforce strict format
         and zero-filling of these cells at intermediate relays, we can
         avoid side channels there, too. (Such a hack allows us to
         send BACKWARD_ECN without any wait, if there are no relay cells
         that are available heading in the backward direction, towards
         the endpoint that caused congestion).