What is currently known as StaleEpoch exception represents means to ensure that two router roles that communicate with each other (e.g. here) are both up-to-date with whatever information they have cached for a particular collection. Because of this, it should have a more appropriate name such as StaleUpstreamRouter.

The reason for having "upstream" in the name is that if the "downstream" router is staler, there is no reason for exception to be thrown to the upstream one to refresh.