How to use Etag when showing data through aspx file

Suppose i have a aspx page called customer.aspx. it shows single customer information. a single customer data may change several times in database.

 

suppose a user request a customer data first time by browser clicking on url like  http://mysite.com/customer.aspx?id=1

 

after few days the same customer data has been changed a bit. again the same user  request http://mysite.com/customer.aspx?id=1

 

so how user browser will do....browser load the data from cache or fetch data from server.

 

my objective is how can i return etag from aspx file and based on etag value browser will load data from client cache or from web server.

 

please share relevant guide line. thanks