As the comment in the code says, eventually there will be a proper
data structure (e.g NetBSD's struct m_tag) to store chains of
annotations, and mbuf-handling procedures will handle these chains
in the correct way.
Right now, these chains do not exist, and we just use the constants
defined here to implement simple ad-hoc solutions to remove some global
variables used so far to pass around informations about packets
being processed.
Global variables are not only ugly and make the code unreadable, they
also prevent from using parallelism in network stack processing.
(the 3-days MFC only refers to this commit, i.e. the PACKET_TAG_*
constants; the full mechanism will be committed and MFC'ed on a
longer timescale).
MFC after: 3 days
4ad01e18e3