Race IPv6 and IPv4 connection attempts from one interleaved candidate list

Keep this document to one review outcome: Race IPv6 and IPv4 connection attempts from one interleaved candidate list. Only the official source and traced locations below supply facts. Confirm applicability before acting.

Source fact:

The official RFC 8305 — Happy Eyeballs Version 2: Better Connectivity Using Concurrency from RFC Editor / Internet Engineering Task Force supports the following bounded statements:

• A dual-stack client SHOULD issue AAAA first and A immediately afterward, treat resolution asynchronously, and avoid waiting for both address families before beginning connection establishment. The research record locates this support at Section 3 (Hostname Resolution Query Handling).
• The client MUST first apply destination-address selection, then SHOULD interleave IPv6 and IPv4 candidates; historical address data MUST NOT cross interfaces and SHOULD be flushed after a network change. The research record locates this support at Section 4 (Sorting Addresses).
• Connection attempts SHOULD be staggered but may overlap; after one succeeds, other unfinished attempts SHOULD be canceled, and a subsequent attempt MUST NOT begin within 10 milliseconds of the previous one. The research record locates this support at Section 5 (Connection Attempts).

Only the traced statements above are asserted as source facts. Apply the review to address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains after confirming that the source and deployed context match.

What the source does not establish

This RFC evidence supports only the named network-protocol decision; it does not select vendor settings, topology, capacity, or an acceptable failure mode. A correct source interpretation can still be inapplicable to a particular design. Confirm DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring, ownership, and change authority instead of treating documented behavior as a deployment guarantee.

Applicability questions

• For source statement 1 at Section 3 (Hostname Resolution Query Handling), which observable configuration, record, or test can confirm applicability here?
• For source statement 2 at Section 4 (Sorting Addresses), which observable configuration, record, or test can confirm applicability here?
• For source statement 3 at Section 5 (Connection Attempts), which observable configuration, record, or test can confirm applicability here?
• Within address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, which versions, roles, and configuration states define the review population?
• Could DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring invalidate the test, hide a failure, or change applicability?
• Who owns the decision, and which observation requires stopping, escalation, or rollback?

DSE recommendation:

DSE recommends using the cited source as the evidence anchor for this decision. Begin by recording scope and current state before deciding whether a change is warranted. Record the source location, examined part of address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, observed and expected states, owner, and reason for deviation.

An implementation decision needs an owner, approved window, prechecks, observable outcome, stop authority, and rollback path. Validate DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring before and after the test, and store only sanitized operational evidence.

Verification and evidence

Build a reproducible chain from Section 3 (Hostname Resolution Query Handling); Section 4 (Sorting Addresses); Section 5 (Connection Attempts) to the observed environment. Useful domain evidence includes configuration snapshots, route or neighbor state, packet captures, counters, topology records, and controlled failover results; label every item with scope, timestamp, collector, and stable identifier.

Record the decision even when no change is made, including uncertainty and the next trigger. Use safe testing conditions for disruptive work, preserve rollback proof, and revisit the conclusion after relevant platform, dependency, vendor, or ownership changes.

Official references

• RFC 8305 — Happy Eyeballs Version 2: Better Connectivity Using Concurrency — RFC Editor / Internet Engineering Task Force