Certain other namespaces and associated protocol families are supported
but not documented yet because they are not often used. PF_NS
refers to the Xerox Network Software protocols. PF_ISO stands
for Open Systems Interconnect. PF_CCITT refers to protocols from
CCITT. socket.h defines these symbols and others naming protocols
not actually implemented.
PF_IMPLINK is used for communicating between hosts and Internet
Message Processors. For information on this and PF_ROUTE, an
occasionally-used local area routing protocol, see the GNU Hurd Manual
(to appear in the future).