http://teamcity:8111/app/rest/latest URL the latest version is available.
http://teamcity:8111/app/rest/<version> URL, the current version is available and earlier versions CAN be available. Our general policy is to supply TeamCity with at least one previous version.
In e.g. in TeamCity 10.x for <version> you can use "10.0" for the current and "9.1", "9.0", "8.1", "6.0" to get earlier versions of the protocol. Protocol version corresponds to the TeamCity version where it was first introduced.