Certain other namespaces and associated protocol families are supported
but not documented yet because they are not often used.
refers to the Xerox Network Software protocols.
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
occasionally-used local area routing protocol, see the GNU Hurd Manual
(to appear in the future).