Exponentially back off on repeated connect retries

Exponentially back off on repeated connect retries

- When the number of retries per remote exceeds a limit
  (hard coded to 5), double the restart pause interval
  for each additional retry per remote.
- Trigger a SIGHUP to reset the retry count when the pause
  interval exceeds 1024 times the base value of restart pause.
  (removed in v2 of the patch)

The base value of restart pause is set using --connect-retry
(5 seconds by default).

v2 changes (based on suggestions from Arne Schwabe <arne@rfc2549.org>)

- Do not throw SIGHUP.
- Add an optional argument to "--connect-retry n [m]" where 'm'
  specifies the max value of restart pause interval (default
  300 sec).
  E.g., "--connect-retry 5 1800" will cause the restart pause to
  scale up starting at 5 until it exceeds 1800 seconds at which
  point it gets capped at 1800.
- If n == m no slow down will occur.
- While at it, fix typos and clarify the description of connect-retry-max
  in the man page and Changes.rst

v3 changes (on further feedback from arne@rfc2549.org):
- Limiting the base value of retry wait interval to 16 bits moved
  to options.c
- Apply backoff only in the udp and tcp-client modes. Backing off on
  tcp-server could be exploited by a client in p2p-mode to maliciously
  slow it down (thanks to Arne Schwabe for pointing this out.
- Fix typo in Changes.rst: "third argument" -> "second argument"

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1467732770-19110-1-git-send-email-selva.nair@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/12050
Signed-off-by: Gert Doering <gert@greenie.muc.de>