Talk about IPv6 and Happy Eyeballs

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Let's look at a picture first. From the relevant information about IPv6, we can see that major Internet companies are promoting the deployment and support of IPv6, which is inseparable from the vigorous promotion of the Information and Communications Technology Research Institute of the Ministry of Industry and Information Technology.

There is no need to say much about the "benefits" of IPv6. If IPv6 is used, each device can be assigned a fixed address, which makes positioning much more convenient. Just thinking about it is beautiful (kong) and good (bu).

Let’s not talk about the improvement of network facilities and software for now, but talk about how to ensure the connectivity of business access.

When it comes to IPv6, the process of visiting an IPv6 website is like this:

The IPv6 address corresponding to the domain name is obtained through DNS AAAA resolution, and then the website is accessed through the IPv6 address. If the access is unsuccessful, the service will be suspended, resulting in user access failure.

If the URL only supports IPv6, then this is the case. If the URL itself supports dual stack, that is, supports both IPv4 and IPv6, then in some environments, the Happy Eyeballs algorithm will be enabled for quick fallback and access using IPv4.

This algorithm was proposed in 2011, when IPv6 was first promoted, in order to avoid people's concerns about user loss if the network was not accessible after using IPv6.

The following is a brief introduction to Happy Eyeballs. In a network environment that supports dual stacks, when accessing a website, AAAA and A DNS query requests will be initiated at the same time, and the obtained results will be sorted, with IPv6 being prioritized, and then the IPv6 connection attempt will be made. If the connection is not connected after about 200ms, the next IP connection request will be immediately enabled, which is generally IPv4. In this scenario, don't worry about the site being inaccessible due to poor IPv6 network connectivity. Even in the current situation where the IPv6 connectivity of various domestic operators is relatively poor, the most you will experience is a delay of about 200ms, which is acceptable compared to denial of service.

Let's try it. I added an IPv6 resolution to my website. I can't access it by filling in a random IP.

Then test whether the analysis is effective:

We can see that there are resolutions for IPv4 and IPv6. Then we use curl (curl supports Happy Eyeballs) to access the following URL: https://fukun.org/httpget.

It can be seen that during the request process, there is an attempt to use IPv6, and then the access is performed using IPv4.

Then capture the packet to see the network situation.

It can be seen that the DNS query is basically a query of AAAA and A records at the same time. The A record is obtained first, and then the AAAA record is obtained after about 30ms. Even if the A record returns relatively quickly, when making a connection request, a SYN connection request is first initiated for the IPv6 address. After about 200ms, if no response is received, a SYN connection request is initiated using IPv4. The connection is successfully established and the correct result is returned.

In summary, in the case of dual stack, even if the connectivity of IPv6 is poor, the Happy Eyeballs algorithm can be used to quickly fall back to IPv4 for normal access.

However, this algorithm is not effective everywhere and requires the support of corresponding software or operating system. Currently, terminals that support Happy Eyeballs include Chrome browser, Opera 12.10, Firefox version 13, Apple operating system OS X, and cURL command.