There was a report that by quickly asking a large number of nodes if
they have an ACK object (which the attacker knows but it is injected
into the network by the recipient of the message), it can estimate how
an object propagates through the network, and eventually pinpoint an
originating IP address of the injection, i.e. the IP address of the
message recipient.
This patch mitigates against it by stalling when asked for a nonexisting
object (so that the attacker can't spam requests), and also upon
connection before sending its own inventory list (so that reconnecting
won't help the attacker). It estimates how long a short message takes to
propagate through the network based on how many nodes are in a stream
and bases the stalling time on that. Currently that is about 15 seconds.
Initial connection delay takes into account the time that already passed
since the connection was established.
This basically gives the attacker one shot per a combination of his own
nodes and the nodes he can connect to, and thus makes the attack much
more difficult to succeed.
