httpCookie stickyness is confusing


I find the description of configuring sticky load balancing to be confusing:

That would seem to imply that I can use an existing application cookie (like jsession id?) to use as the basis for hashing to a backend service.

However - if I set that to an existing cookie, istio overwrites my application cookie, which of course makes the application fail.

If I set it to a new cookie name, it all works. Am I interpreting the docs wrong? Can someone clarify?