The intent with Happy Eyeballs was to focus on user experience: give IPv6 a slight initial advantage to hopefully only make only an IPv6 connection for the dual stack and not waste the network resources and CGNs bindings.
But at the same time, a short time after the start of the IPv6 connection, the IPv4 connection would also be initiated. Whichever connection wins, tilts the balance towards that address family for the next attempts.
This way the users on a broken or very slow IPv6 with fast IPv4 can still have a good experience, Internet Holy Wars notwithstanding.
Source: I am one of the two coauthors of the RFC6555.
Comments
Actually it is a little bit more subtle.
The intent with Happy Eyeballs was to focus on user experience: give IPv6 a slight initial advantage to hopefully only make only an IPv6 connection for the dual stack and not waste the network resources and CGNs bindings.
But at the same time, a short time after the start of the IPv6 connection, the IPv4 connection would also be initiated. Whichever connection wins, tilts the balance towards that address family for the next attempts.
This way the users on a broken or very slow IPv6 with fast IPv4 can still have a good experience, Internet Holy Wars notwithstanding.
Source: I am one of the two coauthors of the RFC6555.