It should be possible to set a fixed start and end point when computing roundtrip.

i.e.
A round trip that starts at location A and ends at location B and on the way other locations C, D, E, F, ... should be visited is needed. Currently the trip plugin does not support this. It should.

Implementation detail:

It should work when the distance between A and B is set to zero in the distance table.
This is not planned before osrm v4.8.0