What should happen when mbufs/mbuf clusters are exhausted?

[Matt Reimer]

What should happen when mbufs/mbuf clusters are exhausted? Are packets
dropped, does the kernel panic, or hang, or ...?

Does RELENG_4 behave differently than RELENG_6 or HEAD when
mbufs/clusters are exhausted? I know that 5.3 and later allocate and
free mbufs/clusters dynamically, but are these newer versions more
robust in the face of allocation failures? I thought I read once that
they were, but I would like to make sure.

Thanks in advance.

Matt