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| -rw-r--r-- | drivers/char/random.c | 35 |
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 * =============================== * |