core,libnm: change default value for ipv4.dad-timeout from 0 to 200ms
Summary
Enable IPv4 duplicate address detection by default.
Purpose
A duplicate address is a serious issue which leads to non-working setups or problems hard to debug. Enable IPv4 duplicate address detection (aka ACD, RFC 5227) by default to detect such problems.
While the RFC recommends a timeout of 9 seconds, a comment in n-acd sources says:
A 9s timeout for successful link setups is not acceptable today.
Hence, we will just go forward and ignore the proposed values. On
both wired and wireless local links round-trip latencies of below
3ms are common. We require the caller to set a timeout multiplier,
where 1 corresponds to a total probe time between 0.5 ms and 1.0
ms. On modern networks a multiplier of about 100 should be a
reasonable default. To comply with the RFC select a multiplier of
9000.
Set a default timeout of 200ms, which is the double of the value suggested in n-acd sources. 200ms sounds quick enough, and gives at least ~100ms to other hosts to reply.
See also the Fedora change proposal: https://fedoraproject.org/wiki/Changes/Enable_IPv4_Address_Conflict_Detection