A RequestHandlerSource (aka a forwarded service) needs methods to query static metadata, such as:

1) The presence or absence of ordering guarantees across the total path
2) Security information about the total path (encryption strength for example)
3) Path information (like a traceroute, for cost as well as diagnostic information; could include link cost, transport type & version, etc)
4) Possibly authentication information about the total path

It should be possible to add new metadata types later on without interfering with backwards compatibility. So lack of a given metadata type must be considered to be possibly indicative of an older version along the path.