This appendix is not part of the IP multicasting specification, but
provides background discussion of several issues related to IP host
group addresses.
The binding of IP host group addresses to physical hosts may be
considered a generalization of the binding of IP unicast addresses.
An IP unicast address is statically bound to a single local network
interface on a single IP network. An IP host group address is
dynamically bound to a set of local network interfaces on a set of IP
networks.
It is important to understand that an IP host group address is NOT
bound to a set of IP unicast addresses. The multicast routers do not
need to maintain a list of individual members of each host group.
For example, a multicast router attached to an Ethernet need
associate only a single Ethernet multicast address with each host
group having local members, rather than a list of the members'
individual IP or Ethernet addresses.
Host group addresses have been defined specifically for use in the
destination address field of multicast IP datagrams. However, the
fact that group addresses are location-independent (they are not
statically bound to a single network interface) suggests possible
uses as more general "logical addresses", both in the source as well
as the destination address field of datagrams. For example, a mobile
IP host might have a host group address as its only identity, used as
the source of datagrams it sends. Whenever the mobile host moved
from one network to another, it would join its own group on the new
network and depart from the group on the old network. Other hosts
communicating with the mobile one would deal only with the group
address and would be unaware of, and unaffected by, the changing
network location of the mobile host.
Host group addresses cannot, however, be used to solve all problems
of internetwork logical addressing, such as delivery to the "nearest"
or the "least loaded" network interface of a multi-homed host.
Furthermore, there are hazards in using group addresses in the source
address field of datagrams when the group actually contains more than
one host. For instance, the IP datagram reassembly algorithm relies
on every host using a different source address. Also, errors in a
datagram sent with a group source address may result in error reports
being returned to all members of the group, not just the sender. In
view of these hazards, this memo specifies the use of host group
addresses only in the IP destination address field. However, it is
recommended that datagrams with a group source address, or a group
address as part of a source routing option, be accepted without
complaint, thereby allowing other implementations to experiment with
logical addressing applications of host group addresses.
This memo does not specify how transient group address are allocated.
It is anticipated that different portions of the IP transient host
group address space will be allocated using different techniques.
For example, there may be a number of servers that can be contacted
to acquire a new transient group address. Some higher-level
protocols (such as VMTP, specified in RFC-1045exp) may generate higher-
level transient "process group" or "entity group" addresses which are
then algorithmically mapped to a subset of the IP transient host
group addresses, similarly to the way that IP host group addresses
are mapped to Ethernet multicast addresses. A portion of the IP
group address space may be set aside for random allocation by
applications that can tolerate occasional collisions with other
multicast users, perhaps generating new addresses until a suitably
"quiet" one is found.
In general, a host cannot assume that datagrams sent to any host
group address will reach only the intended hosts, or that datagrams
received as a member of a transient host group are intended for the
recipient. Misdelivery must be detected at a level above IP, using
higher-level identifiers or authentication tokens. Information
transmitted to a host group address should be encrypted or governed
by administrative routing controls if the sender is concerned about
unwanted listeners.
