If flow control is propagated backward from congested nodes toward traffic sources, then the flow control mechanism can be used as at least a partial solution to the problem of congestion as well.
A node must avoid discarding bundles for which it has accepted custody, and do so only as a last resort.
Determining when a node should engage in or cease to engage in custody transfers is a resource allocation and scheduling problem of current research interest.
In addition to the bundle layer mechanisms, a DTN node maybe able to avail itself of support from lower-layer protocols in affecting its own resource utilizaiton.