RFC 2460 was obsoleted by RFC 8200. This PR changes the references around, so we don't reference an obsoleted RFC ;-). Also I'm moving these references from the old-style HTML-like format to the newer-style Markdown-like format.
Fixes #4842. RFC 2460 section 8.1 (https://tools.ietf.org/html/rfc2460#section-8.1) states: > if that computation yields a result of zero, it must be changed to hex > FFFF for placement in the UDP header.