Originally Posted By: Bill S.
We can't assign a rest frame to a photon, the CMBR is composed of photons, travelling at c. In what way can the CMBR be said to have a rest frame?

The CMBR doesn't have a rest frame. It's called a rest frame because a body is considered at rest relative the CMBR if it is receiving the radiation at the same energy level from all directions.

"Time is what prevents everything from happening at once" - John Wheeler