I have ASP.NET server with client application available in Android,Iphone and Windows. I want to append the application version*(for example 2.1.2 my application version)* in HTTP request headers. Which header should I use for this? BTW, I know about User-Agent.I am asking to include my specific application version.
User-Agent is the correct place to add this information. You can attach multiple product/version combinations into a user-agent header. Your client application is acting as a type of user agent in this case and that is the most appropriate place to add this information. From the HTTP spec
The field can contain multiple product tokens (section 3.8) and comments identifying the agent and any subproducts which form a significant part of the user agent.
User-Agent : foo/1.0 foobar/0.9 foobazbar/2.1