I have reviewed this document as part of the security directorate's ongoing effort to review all IETF documents being processed by the IESG. These comments were written primarily for the benefit of the security area directors. Document editors and WG chairs should treat these comments just like any other last call comments. From the Introduction, "This document expands on "Happy Eyeballs" [RFC6555], a technique of reducing user-visible delays on dual-stack hosts." It lists a set of steps by which a client can asynchronously perform IPv6 and IPv4 DNS queries, and also semantics on how to handle the replies such that the user delay is minimized. The Security Considerations section simply states "This memo has no direct security considerations.", and I believe this is true. However, I wonder about "indirect" security considerations. RFC 6555 warns several times against breaking a browser's same-origin policy, which seems to me to be an "indirect" security consideration. I realize that browser policies have changed considerably since RFC 6555 was published, and I personally do not know if same-origin is still in general use or whether there are other newer but similar issues of which an implementor should be aware. But if there are, then this section should note them. Otherwise, I consider the document ready to be published.