3. LEVELS OF CONFORMANCE

   There are three levels of conformance to this specification:

      Level 0: no support for IP multicasting.

   There is, at this time, no requirement that all IP implementations
   support IP multicasting.  Level 0 hosts will, in general, be
   unaffected by multicast activity.  The only exception arises on some
   types of local network, where the presence of level 1 or 2 hosts may
   cause misdelivery of multicast IP datagrams to level 0 hosts.  Such
   datagrams can easily be identified by the presence of a class D IP
   address in their destination address field; they should be quietly
   discarded by hosts that do not support IP multicasting.  Class D
   addresses are described in section 4 of this memo.

      Level 1: support for sending but not receiving multicast IP
      datagrams.

   Level 1 allows a host to partake of some multicast-based services,
   such as resource location or status reporting, but it does not allow
   a host to join any host groups.  An IP implementation may be upgraded
   from level 0 to level 1 very easily and with little new code.  Only
   sections 4, 5, and 6 of this memo are applicable to level 1
   implementations.

      Level 2: full support for IP multicasting.

   Level 2 allows a host to join and leave host groups, as well as send
   IP datagrams to host groups.  It requires implementation of the
   Internet Group Management Protocol (IGMP) and extension of the IP and
   local network service interfaces within the host.  All of the
   following sections of this memo are applicable to level 2

   implementations.