There’s a Happy Eyeballs version two, defined in RFC 8305. (The Happy Eyeballs RFC 6555 claims Firefox and Chrome both use a 300 millisecond delay, but I’m not convinced this is actually true in current versions.)īy altering this timeout, you can shift the likeliness of one or the other connect to “win”.Įxample: change the happy eyeballs timeout to the same value said to be used by some browsers (300 milliseconds): curl -happy-eyeballs-timeout-ms 300 Happy Eyeballs++ The default timeout from the moment the first IPv6 connect is issued until the first IPv4 starts, is 200 milliseconds. This makes curl prefer a quick IPv6 connect. Happy Eyeballs Timeoutįor hosts having both IPv6 and IPv4 addresses, curl will first fire off the IPv6 attempt and then after a timeout, start the first IPv4 attempt. Of course, if a host name only has addresses in one IP version, curl will only use that specific version. To complicate matters a little more, a host name can resolve to a list of addresses of both IP versions and if a connect to one of the addresses fails, curl will attempt the next in a way so that IPv4 addresses and IPv6 addresses will be attempted, simultaneously, until one succeeds.Ĭurl races connection attempts against each other. The first of these connects that completes its handshake is considered the winner and the other connection attempt then gets ditched and is forgotten. When curl resolves the host name and gets a list of IP addresses back for it, it will try to connect to the host over both IPv4 and IPv6 in parallel, concurrently. This is the name of a standard way of connecting to a host (a server really in curl’s case) that has both IPv4 and IPv6 addresses. To understand this command line option, I think I should make a quick recap of what “happy eyeballs” is exactly and what timeout in there that this command line option is referring to! Happy Eyeballs This option was added to curl 7.59.0, March 2018 and is very rarely actually needed. We show that lowering the HE timer value to 150 ms gives us a margin benefit of 10% while retaining same preference levels over IPv6.This week’s option has no short option and the long name is indeed quite long:
We show that although absolute TCP connect times (in ms) are not that far apart in both address families, HE with a 300 ms timer value tends to prefer slower IPv6 connections in around 90% of the cases. As such, 99% of these websites prefer IPv6 connections more than 98% of the time. The historical trend shows that only around 1% of the TCP connect times over IPv6 were ever above the HE timer value (300 ms), which leaves around 2% chance for IPv4 to win a HE race towards these websites. As of May 2016, 18% of these websites are faster over IPv6 with 91% of the rest at most 1 ms slower.
Using a 3-years long (2013 - 2016) dataset, we show that TCP connect times to popular websites over IPv6 have considerably improved over time. The happy test measures against ALEXA top 10K websites from 80 SamKnows probes connected to dual-stacked networks representing 58 different ASes.
Using an active test (happy) that measures TCP connection establishment times, we evaluate the effects of the HE algorithm. The Happy Eyeballs (HE) algorithm, for instance, prevents bad user experience in situations where IPv6 connectivity is broken. Abstract: The IETF has developed protocols that promote a healthy IPv4 and IPv6 co-existence.
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