  We carry out large-scale micromagnetic simulations which demonstrate that due
to topological constraints, internal domain walls (Bloch lines) within extended
domain walls are more robust than domain walls in nanowires. Thus, the
possibility of spintronics applications based on their motion channeled along
domain walls emerges. Internal domain walls are nucleated within domain walls
in perpendicularly magnetized media concurrent with a Walker breakdown-like
abrupt reduction of the domain wall velocity above a threshold driving force,
and may also be generated within pinned, localized domain walls. We observe
fast field and current driven internal domain wall dynamics without a Walker
breakdown along pinned domain walls, originating from topological protection of
the internal domain wall structure due to the surrounding out-of-plane domains.
