5.4 Multi-homed Routing Domains
The discussions in Section 5.3 suggest methods for allocating IP
addresses based on direct or indirect provider connectivity. This
allows a great deal of information reduction to be achieved for
those routing domains which are attached to a single TRD. In
particular, such routing domains may select their IP addresses
from a space delegated to them by the direct provider. This allows
the provider, when announcing the addresses that it can reach to
other providers, to use a single address prefix to describe a
large number of IP addresses corresponding to multiple routing
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domains.
However, there are additional considerations for routing domains
which are attached to multiple providers. Such "multi-homed"
routing domains may, for example, consist of single-site campuses
and companies which are attached to multiple backbones, large
organizations which are attached to different providers at
different locations in the same country, or multi-national
organizations which are attached to backbones in a variety of
countries worldwide. There are a number of possible ways to deal
with these multi-homed routing domains.
One possible solution is for each multi-homed organization to
obtain its IP address space independently from the providers to
which it is attached. This allows each multi-homed organization
to base its IP assignments on a single prefix, and to thereby
summarize the set of all IP addresses reachable within that
organization via a single prefix. The disadvantage of this
approach is that since the IP address for that organization has no
relationship to the addresses of any particular TRD, the TRDs to
which this organization is attached will need to advertise the
prefix for this organization to other providers. Other providers
(potentially worldwide) will need to maintain an explicit entry
for that organization in their routing tables.
For example, suppose that a very large North American company
"Mega Big International Incorporated" (MBII) has a fully
interconnected internal network and is assigned a single prefix as
part of the North American prefix. It is likely that outside of
North America, a single entry may be maintained in routing tables
for all North American destinations. However, within North
America, every provider will need to maintain a separate address
entry for MBII. If MBII is in fact an international corporation,
then it may be necessary for every provider worldwide to maintain
a separate entry for MBII (including backbones to which MBII is
not attached). Clearly this may be acceptable if there are a small
number of such multi-homed routing domains, but would place an
unacceptable load on routers within backbones if all organizations
were to choose such address assignments. This solution may not
scale to internets where there are many hundreds of thousands of
multi-homed organizations.
A second possible approach would be for multi-homed organizations
to be assigned a separate IP address space for each connection to
a TRD, and to assign a single prefix to some subset of its
domain(s) based on the closest interconnection point. For example,
if MBII had connections to two providers in the U.S. (one east
coast, and one west coast), as well as three connections to
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national backbones in Europe, and one in the far east, then MBII
may make use of six different address prefixes. Each part of MBII
would be assigned a single address prefix based on the nearest
connection.
For purposes of external routing of traffic from outside MBII to a
destination inside of MBII, this approach works similarly to
treating MBII as six separate organizations. For purposes of
internal routing, or for routing traffic from inside of MBII to a
destination outside of MBII, this approach works the same as the
first solution.
If we assume that incoming traffic (coming from outside of MBII,
with a destination within MBII) is always to enter via the nearest
point to the destination, then each TRD which has a connection to
MBII needs to announce to other TRDs the ability to reach only
those parts of MBII whose address is taken from its own address
space. This implies that no additional routing information needs
to be exchanged between TRDs, resulting in a smaller load on the
inter-domain routing tables maintained by TRDs when compared to
the first solution. This solution therefore scales better to
extremely large internets containing very large numbers of multi-
homed organizations.
One problem with the second solution is that backup routes to
multi-homed organizations are not automatically maintained. With
the first solution, each TRD, in announcing the ability to reach
MBII, specifies that it is able to reach all of the hosts within
MBII. With the second solution, each TRD announces that it can
reach all of the hosts based on its own address prefix, which only
includes some of the hosts within MBII. If the connection between
MBII and one particular TRD were severed, then the hosts within
MBII with addresses based on that TRD would become unreachable via
inter-domain routing. The impact of this problem can be reduced
somewhat by maintenance of additional information within routing
tables, but this reduces the scaling advantage of the second
approach.
The second solution also requires that when external connectivity
changes, internal addresses also change.
Also note that this and the previous approach will tend to cause
packets to take different routes. With the first approach, packets
from outside of MBII destined for within MBII will tend to enter
via the point which is closest to the source (which will therefore
tend to maximize the load on the networks internal to MBII). With
the second solution, packets from outside destined for within MBII
will tend to enter via the point which is closest to the
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destination (which will tend to minimize the load on the networks
within MBII, and maximize the load on the TRDs).