Implement parallel Happy Eyeballs as described in RFC 8305

[tniessen]

What is the problem this feature will solve?

autoSelectFamily is enabled by default in Node.js 20. However, it does not properly implement parallel connection attempts, which are an integral part of the Happy Eyeballs algorithm. This can cause timeouts to occur that did not occur in versions prior to Node.js 20. See, for example, this discussion.

What is the feature you are proposing to solve the problem?

Implement the algorithm as described in Section 5 of RFC 8305:

Starting a new connection attempt does not affect previous attempts, as multiple connection attempts may occur in parallel.

What alternatives have you considered?

Disabling autoSelectFamily and restoring the pre-20 behavior, see #47644 (comment).

[ShogunPanda]

I originally went for the proper parallel approach as states in RFC8305. I chose not to implement it as I had issues with the code.

Consider that implementing proper RFC8305 will imply that all of sudden Node.js starts opening multiple connections instead of one in case of multiple IP returned by DNS, which is dangerous.

The unwanted timeouts are not properly handled in #47860, which will be available in Node.js 20 once I fix #48000 and #47822.

We can of course disable the feature completely by default, but it was my understanding that this was longly awaited to help beginners struggling with poorly configured dual stack networks.

[tniessen]

Consider that implementing proper RFC8305 will imply that all of sudden Node.js starts opening multiple connections instead of one in case of multiple IP returned by DNS, which is dangerous.

Is that a fact? Is there absolutely no way to prevent responding to more than a single address with a TCP ACK packet after having received TCP SYN-ACK?

[ShogunPanda]

Nope, we don't control the connection at that level, AFAIK.

@nodejs/libuv Am I wrong?

[silverwind]

I think it should be considered disabling this autoSelectFamily by default. It's breaking stuff all over the place. Stabilize first behind a --experimental-auto-select-family or something, but keep this broken stuff out of the upcoming LTS please.

[ShogunPanda]

After 20.3.0 all issues should be fixed. Can we please wait for this release before rushing into decisions?

[silverwind]

The ENETUNREACH issue mentioned here is still unresolved, right? As I understand it, it can only be fixed with parallel connection attempts.

[ShogunPanda]

Not really. We can add a new feature to opt-out A or AAAA in the list of addresses to try when the machine has no IPv4 and IPv6 connectivity respectively.
Note that ENETUNREACH will happen even in previous versions if the DNS only returns AAAA records.

Additionally, to mitigate the problem, a simple fix would be to raise the minimum attempt timeout to 500ms to reduce the possibility that IPv4 fails.

[HinataKah0]

I was interested so I tried reading the RFC as well.
Just giving my 2 cents of what I understand since I saw the discussion...

TBH I was also confused at first how you can make both "non-simultaneous" and "may occur in parallel" connections.
The first paragraph seems conflicting. 😅

But the key is in 2nd paragraph:

A simple implementation can have a fixed delay for how long to wait before starting the next connection attempt.

So, it's something like this:

Which is "non-simultaneous" but also "may occur in parallel".
An alternative/a more sophisticated delay is by using exponential backoff with min & max delay (described in paragraph 3).

[bnoordhuis]

Is there absolutely no way to prevent responding to more than a single address with a TCP ACK packet after having received TCP SYN-ACK?

Not in a portable fashion. I'm not even sure you could do it reliably if all you cared about was Linux.

The only way I'm aware of is polling getsockopt(TCP_INFO) and checking the tcp_info.tcpi_state field but that's a) timing sensitive, and b) not a viable approach for an asynchronous runtime like Node.js.