ECS is an optional extension to the DNS protocol, published in RFC7871. It allows DNS requests to contain information about the source of the request, and for responses to provide information regarding for whom responses can be cached.
ECS is supported by Google name servers, including Google Public DNS, other public DNS providers, and various DNS software implementations.
Benefits of ECS
Without ECS, when a large number of users share the same resolver, it can be difficult for us to provide a DNS response that sends all user traffic to the Google serving location that is best for them.
Enabling ECS in your resolvers allows us to more accurately understand where end-users are located, and serve low-volume traffic to them from optimal locations. It may also improve accuracy of traffic mapping for other CDNs.
Testing and enabling ECS
To test if ECS is currently available in your resolvers, make the following query:
$ host -t txt o-o.myaddr.google.com
The response should contain the text "edns0-client-subnet".
Before enabling ECS in your resolvers, discuss your planned approach with your DNS software provider. There may be performance implications.
More information is available here.