5.5 Private Links
The discussion up to this point concentrates on the relationship
between IP addresses and routing between various routing domains
over transit routing domains, where each transit routing domain
interconnects a large number of routing domains and offers a
more-or-less public service.
However, there may also exist a number of links which interconnect
two routing domains in such a way, that usage of these links may
be limited to carrying traffic only between the two routing
domains. We'll refer to such links as "private".
For example, let's suppose that the XYZ corporation does a lot of
business with MBII. In this case, XYZ and MBII may contract with a
carrier to provide a private link between the two corporations,
where this link may only be used for packets whose source is
within one of the two corporations, and whose destination is
within the other of the two corporations. Finally, suppose that
the point-to-point link is connected between a single router
(router X) within XYZ corporation and a single router (router M)
within MBII. It is therefore necessary to configure router X to
know which addresses can be reached over this link (specifically,
all addresses reachable in MBII). Similarly, it is necessary to
configure router M to know which addresses can be reached over
this link (specifically, all addresses reachable in XYZ
Corporation).
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The important observation to be made here is that the additional
connectivity due to such private links may be ignored for the
purpose of IP address allocation, and do not pose a problem for
routing. This is because the routing information associated with
such connectivity is not propagated throughout the Internet, and
therefore does not need to be collapsed into a TRD's prefix.
In our example, let's suppose that the XYZ corporation has a
single connection to a regional, and has therefore uses the IP
address space from the space given to that regional. Similarly,
let's suppose that MBII, as an international corporation with
connections to six different providers, has chosen the second
solution from Section 5.4, and therefore has obtained six
different address allocations. In this case, all addresses
reachable in the XYZ Corporation can be described by a single
address prefix (implying that router M only needs to be configured
with a single address prefix to represent the addresses reachable
over this link). All addresses reachable in MBII can be described
by six address prefixes (implying that router X needs to be
configured with six address prefixes to represent the addresses
reachable over the link).
In some cases, such private links may be permitted to forward
traffic for a small number of other routing domains, such as
closely affiliated organizations. This will increase the
configuration requirements slightly. However, provided that the
number of organizations using the link is relatively small, then
this still does not represent a significant problem.
Note that the relationship between routing and IP addressing
described in other sections of this paper is concerned with
problems in scaling caused by large, essentially public transit
routing domains which interconnect a large number of routing
domains. However, for the purpose of IP address allocation,
private links which interconnect only a small number of private
routing domains do not pose a problem, and may be ignored. For
example, this implies that a single leaf routing domain which has
a single connection to a "public" backbone, plus a number of
private links to other leaf routing domains, can be treated as if
it were single-homed to the backbone for the purpose of IP address
allocation. We expect that this is also another way of dealing
with multi-homed domains.
5.6 Zero-Homed Routing Domains
Currently, a very large number of organizations have internal
communications networks which are not connected to any service
providers. Such organizations may, however, have a number of
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private links that they use for communications with other
organizations. Such organizations do not participate in global
routing, but are satisfied with reachability to those
organizations with which they have established private links.
These are referred to as zero-homed routing domains.
Zero-homed routing domains can be considered as the degenerate
case of routing domains with private links, as discussed in the
previous section, and do not pose a problem for inter-domain
routing. As above, the routing information exchanged across the
private links sees very limited distribution, usually only to the
routing domain at the other end of the link. Thus, there are no
address abstraction requirements beyond those inherent in the
address prefixes exchanged across the private link.
However, it is important that zero-homed routing domains use valid
globally unique IP addresses. Suppose that the zero-homed routing
domain is connected through a private link to a routing domain.
Further, this routing domain participates in an internet that
subscribes to the global IP addressing plan. This domain must be
able to distinguish between the zero-homed routing domain's IP
addresses and any other IP addresses that it may need to route to.
The only way this can be guaranteed is if the zero-homed routing
domain uses globally unique IP addresses.