[riot-notifications] [RIOT-OS/RIOT] gnrc_ipv6_nib/arsm: ensure proper int width in backoff calculation [backport 2019.01] (#11007)

Martine Lenders notifications at github.com
Tue Feb 12 17:56:50 CET 2019

# Backport of #10369

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Therefore, before describing what your contribution is about, we would like
you to make sure that your modifications are compliant with the RIOT
coding conventions, see https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions.

### Contribution description
Otherwise, the result might flow over on 8/16-bit platforms

### Testing procedure
I'm not sure the address resolution state machine fits on any of our 8/16-bit platforms... But otherwise: 

* flash an application with `gnrc_ipv6_default` an `GNRC_IPV6_NIB_CONF_ARSM == 1` to two 8- or 16-bit boards with networking capabilities
* let them find via neighbor discovery (e.g. by pinging one from the other)
* let the neighbor information go STALE (check with `nib neigh`)
* let one of the nodes become unreachable (e.g. by turning it off)
* try to update stale neighbor information (e.g. by pinging again)
* let the neighbor information go to UNREACHABLE (check with `nib neigh`)
* see (e.g. with wireshark) that the neighbor solicitations go out ad infinitum but with an truncated (at `NDP_MAX_RETRANS_TIMER_MS` == 1min) exponential (base 2) backoff between them.
* run `unittests` with `tests-gnrc_ipv6_nib` on various platforms.

### Issues/PRs references
Pointed out in https://github.com/RIOT-OS/RIOT/pull/10350#discussion_r231905390

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-- Commit Summary --

  * gnrc_ipv6_nib/arsm: ensure proper int width in backoff calculation
  * gnrc_ipv6_nib: expose non-randomized part of NS-backoff calculation
  * tests/unittests: add tests for exponential NS backoff

-- File Changes --

    M sys/net/gnrc/network_layer/ipv6/nib/_nib-arsm.c (15)
    M sys/net/gnrc/network_layer/ipv6/nib/_nib-arsm.h (37)
    M tests/unittests/tests-gnrc_ipv6_nib/tests-gnrc_ipv6_nib-internal.c (54)

-- Patch Links --


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