In order to test remailer A's reliability, Echolot sends an encrypted
ping through a 1-hop chain every two hours.  Pings are not sent strictly
at 02:00, 04:00 etc, but at 2h*n + f(A, date) % 2h.  ( f() is a function
of the remailer name and the date of the current day (basically md5))

We record the timestamp each outgoing ping, and we record the time it
took to return for incoming pings.

The reliability of a node is the result of received/sent, with the
following weighting applied:

 weight := w1 * w2;

 w1 is a function of a ping's age:
  age:    1   2   3   4   5   6   7   8   9  10  11  12   [days]
  weight 0.5 1.0 1.0 1.0 1.0 0.9 0.8 0.5 0.3 0.2 0.2 0.1

  age is how long ago the ping was sent.  So if a ping was sent 23
  hour ago, it weighs 0.5, if it was sent 2 days ago, its weight is 1.
  Approaching 12 days, the weight approaches 0.0.

 w2 also considers this node's pings' latencies over the last 12 days:

  for pings that already returned, w2 is 1.0.
  otherwise:
    Let mod_age := (now - sent - 15m) * 0.8
    w2 is the fraction of pings returned within mod_age seconds.

   Example:
    Assume a ping was sent 2 hours ago. mod_age is 84 minutes.  If 100%
    of this node's pings were faster than 84 minutes, then w2 = 1.  If
    only 30% were received within 84 minutes of sending out the ping,
    then w2 is 0.3,  If no ping was ever faster than 84 minutes, then w2
    is 0.


The reported latency is the median of all received pings of the last 12
days.



Chain pings are done in a similar fashion:  We ping two-hop chains A, B.
Each chain is pinged once a week (with a similar offset function as
above).

"Interesting chains" are pinged more often - daily.

We report chains as broken if
 - we sent at least 3 pings
  AND
 - received/sent <= rel(A) * rel(B) * 0.3.

rel(X) is remailer X's reliability in single-hop pings.

We define interesting chains as chains
 - where we sent less than 3 pings, without getting any back.
  OR
 - where received/sent <= rel(A) * rel(B) * 0.3  (i.e. the chain is
   reported broken)