Identification middleware does not have to implement IUpstreamIdentification but any full featured implementation should. Identification middleware that implements this interface is providing a way for other downstream middleware to specify the identification requirements specific to a request.

This interface is part of the upstream communication mechanism that allows downstream middleware to communicate back upstream before the upstream middleware processes the request.

For example a page on your web site might be only available to logged on users. In this case the IUpstreamIdentification interface can be used to tell the Identification middleware that the user can not be anonymous, and the Identification middleware will direct the user to login if they are not logged in already.