document reseeding / load-balancing strategy

1 files changed, 35 insertions, 0 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 66f924f..0275a98 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -146,6 +146,41 @@
  * possibility.  Nonetheless, a pseudorandom distribution of numbers
  * should be useful for a wide range of purposes.
 
+ * Strategy for reseeding the PRNG
+ * ===============================
+ *
+ *  There are a lot of things in this world that depend on adaptive
+ *  load-balancing and resource-sharing.  Examples include:
+ *
+ *   a) The "invisible hand" of microeconomics.  If a resource is
+ *    plentiful it will be cheap, and everybody can use it.  If/when the
+ *    resource is scare, the price goes up, and only those who really
+ *    need it will pay for it.
+ *
+ *   b) The "exponential backoff" algorithm used for the Ethernet
+ *    layer-1 CSMA/CD.  http://en.wikipedia.org/wiki/Exponential_backoff
+ *
+ *   c) The rate of TCP retries, which is another example of exponential
+ *    backoff.  http://www.pcvr.nl/tcpip/tcp_time.htm
+ *
+ *  So, the idea is that if entropy is plentiful, the PRNG can reseed
+ *  itself relatively often.  If entropy is not plentiful, the PRNG
+ *  should wait longer between reseedings.  The number of bits delivered
+ *  by the PRNG between reseedings is an exponential function of how far
+ *  the input pool is below its ceiling.  That's the concept.  The
+ *  implementation goes about it in a somewhat backward way, because it
+ *  is implemented on top of the existing "rsvd" mechanism, and usually
+ *  it is better to use the existing mechanism whenever possible.  So,
+ *  if the PRNG has been reseeded recently, it uses a large reserve
+ *  ("rsvd").  If it has not been reseeded in a long time, the reserve
+ *  goes down, eventually down all the way to zero.
+ *
+ *  The intent is that other processes that need entropy from
+ *  /dev/random will play by the same rules.  That is, when entropy is
+ *  scarce they will use it more sparingly.  This mechanism is voluntary
+ *  not mandatory, but voluntary load- balancing is better than none at
+ *  all.
+
  * Exported interfaces ---- output
  * ===============================
  *