5.7 Continental aggregation
Another level of hierarchy may also be used in this addressing
scheme to further reduce the amount of routing information
necessary for inter-continental routing. Continental aggregation
is useful because continental boundaries provide natural barriers
to topological connection and administrative boundaries. Thus, it
presents a natural boundary for another level of aggregation of
inter-domain routing information. To make use of this, it is
necessary that each continent be assigned an appropriate subset of
the address space. Providers (both direct and indirect) within
that continent would allocate their addresses from this space.
Note that there are numerous exceptions to this, in which a
service provider (either direct or indirect) spans a continental
division. These exceptions can be handled similarly to multi-
homed routing domains, as discussed above.
Note that, in contrast to the case of providers, the aggregation
of continental routing information may not be done on the
continent to which the prefix is allocated. The cost of inter-
continental links (and especially trans-oceanic links) is very
high. If aggregation is performed on the "near" side of the link,
then routing information about unreachable destinations within
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that continent can only reside on that continent. Alternatively,
if continental aggregation is done on the "far" side of an inter-
continental link, the "far" end can perform the aggregation and
inject it into continental routing. This means that destinations
which are part of the continental aggregation, but for which there
is not a corresponding more specific prefix can be rejected before
leaving the continent on which they originated.
For example, suppose that Europe is assigned a prefix of
<194.0.0.0 254.0.0.0>, such that European routing also contains
the longer prefixes <194.1.0.0 255.255.0.0> and <194.2.0.0
255.255.0.0>. All of the longer European prefixes may be
advertised across a trans-Atlantic link to North America. The
router in North America would then aggregate these routes, and
only advertise the prefix <194.0.0.0 255.0.0.0> into North
American routing. Packets which are destined for 194.1.1.1 would
traverse North American routing, but would encounter the North
American router which performed the European aggregation. If the
prefix <194.1.0.0 255.255.0.0> is unreachable, the router would
drop the packet and send an ICMP Unreachable without using the
trans-Atlantic link.
5.8 Transition Issues
Allocation of IP addresses based on connectivity to TRDs is
important to allow scaling of inter-domain routing to an internet
containing millions of routing domains. However, such address
allocation based on topology implies that in order to maximize the
efficiency in routing gained by such allocation, certain changes
in topology may suggest a change of address.
Note that an address change need not happen immediately. A domain
which has changed service providers may still advertise its prefix
through its new service provider. Since upper levels in the
routing hierarchy will perform routing based on the longest
prefix, reachability is preserved, although the aggregation and
scalability of the routing information has greatly diminished.
Thus, a domain which does change its topology should change
addresses as soon as convenient. The timing and mechanics of such
changes must be the result of agreements between the old service
provider, the new provider, and the domain.
This need to allow for change in addresses is a natural,
inevitable consequence of routing data abstraction. The basic
notion of routing data abstraction is that there is some
correspondence between the address and where a system (i.e., a
routing domain, subnetwork, or end system) is located. Thus if the
system moves, in some cases the address will have to change. If it
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were possible to change the connectivity between routing domains
without changing the addresses, then it would clearly be necessary
to keep track of the location of that routing domain on an
individual basis.
In the short term, due to the rapid growth and increased
commercialization of the Internet, it is possible that the
topology may be relatively volatile. This implies that planning
for address transition is very important. Fortunately, there are a
number of steps which can be taken to help ease the effort
required for address transition. A complete description of address
transition issues is outside of the scope of this paper. However,
a very brief outline of some transition issues is contained in
this section.
Also note that the possible requirement to transition addresses
based on changes in topology imply that it is valuable to
anticipate the future topology changes before finalizing a plan
for address allocation. For example, in the case of a routing
domain which is initially single-homed, but which is expecting to
become multi-homed in the future, it may be advantageous to assign
IP addresses based on the anticipated future topology.
In general, it will not be practical to transition the IP
addresses assigned to a routing domain in an instantaneous "change
the address at midnight" manner. Instead, a gradual transition is
required in which both the old and the new addresses will remain
valid for a limited period of time. During the transition period,
both the old and new addresses are accepted by the end systems in
the routing domain, and both old and new addresses must result in
correct routing of packets to the destination.
During the transition period, it is important that packets using
the old address be forwarded correctly, even when the topology has
changed. This is facilitated by the use of "longest match"
inter-domain routing.