Re: [v6ops] An Update to Happy Eyeballs

[Stuart Cheshire <[e-mail]>]

On 15 Mar 2017, at 17:00, 神明達哉 <[e-mail]> wrote:

> I have one quick (and minor) question about the 01 version: in Section
> 6 it states
> 
>   If, during the course of connection establishment, the DNS answers
>   change either by adding resolved addresses, or removing previously
>   resolved addresses (for example, due to expiry of the TTL on that DNS
>   record), the client should react based on its current progress.
> 
> Specifically what kind of scenario does this intend to imply?  For
> example, consider the following scenario:
> 
> 1. start a AAAA query for a destination name
> 2. start an A query for the same name almost at the same time
> 3. get a response to the AAAA query (set of IPv6 addresses)
> 4. start establishing a connection for one of the IPv6 addresses
> 5. get a response to the A query (set of IPv4 addresses)
> 
> At step 5, DNS resolution is fully completed.

No, DNS resolution is not fully completed at step 5. In this asynchronous model (as used in Apple products for many years) DNS resolution is an ongoing process, which continues until the client has successfully made a connection, and only stops when the client tells the DNS engine that it can stop.

Many web sites use DNS records with fairly short TTLs, which allows them to update answers fairly rapidly.

If a server is down or overloaded, they can remove that address from the answer set, and substitute another. Thus the set of answers changes over time. The Apple DNS APIs take this passage of time into account, and notify clients as the set of answers changes, via “add” and “remove” events. If a client has been trying to connect for some time, unsuccessfully, and then another IP address becomes available, that address is added to the set of candidates, and may be the one that actually works.

You can easily see this for yourself, e.g., using www.google.com, or www.microsoft.com.

Here are two examples (using IPv4 only, for readability).

You will see that at 12:47:10 the IP address 172.217.6.68 for Google was withdrawn, and replaced by 216.58.194.164.

At 12:47:16 the IP address 104.82.49.139 for Microsoft was withdrawn, and replaced by 173.222.250.151.

% dns-sd -G v4 www.google.com
DATE: ---Thu 16 Mar 2017---
12:47:02.843  ...STARTING...
Timestamp     A/R Flags if Hostname                     Address
12:47:02.845  Add     2  0 www.google.com.              172.217.6.68
12:47:10.854  Rmv     1  0 www.google.com.              172.217.6.68
12:47:10.855  Add     2  0 www.google.com.              216.58.194.164

% dns-sd -G v4 www.microsoft.com
DATE: ---Thu 16 Mar 2017---
12:46:41.253  ...STARTING...
Timestamp     A/R Flags if Hostname                     Address
12:46:41.256  Add     2  0 e1863.dspb.akamaiedge.net.   104.82.49.139
12:47:16.139  Rmv     1  0 e1863.dspb.akamaiedge.net.   104.82.49.139
12:47:16.139  Add     2  0 e1863.dspb.akamaiedge.net.   173.222.250.151

Stuart Cheshire