4.1 Efficiency versus Decentralized Control
If the Internet plans to support a decentralized address
administration [4], then there is a balance that must be sought
between the requirements on IP addresses for efficient routing and
the need for decentralized address administration. A proposal
described in [3] offers an example of how these two needs might be
met.
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The IP address prefix <198.0.0.0 254.0.0.0> provides for
administrative decentralization. This prefix identifies part of the
IP address space allocated for North America. The lower order part of
that prefix allows allocation of IP addresses along topological
boundaries in support of increased data abstraction. Clients within
North America use parts of the IP address space that is underneath
the IP address space of their service providers. Within a routing
domain addresses for subnetworks and hosts are allocated from the
unique IP prefix assigned to the domain.
5. IP Address Administration and Routing in the Internet
The basic Internet routing components are service providers (e.g.,
backbones, regional networks), and service subscribers (e.g., sites
or campuses). These components are arranged hierarchically for the
most part. A natural mapping from these components to IP routing
components is that providers and subscribers act as routing domains.
Alternatively, a subscriber (e.g., a site) may choose to operate as a
part of a domain formed by a service provider. We assume that some,
if not most, sites will prefer to operate as part of their provider's
routing domain. Such sites can exchange routing information with
their provider via interior routing protocol route leaking or via an
exterior routing protocol. For the purposes of this discussion, the
choice is not significant. The site is still allocated a prefix from
the provider's address space, and the provider will advertise its own
prefix into inter-domain routing.
Given such a mapping, where should address administration and
allocation be performed to satisfy both administrative
decentralization and data abstraction? The following possibilities
are considered:
- at some part within a routing domain,
- at the leaf routing domain,
- at the transit routing domain (TRD), and
- at the continental boundaries.
A point within a routing domain corresponds to a subnetwork. If a
domain is composed of multiple subnetworks, they are
interconnected via routers. Leaf routing domains correspond to
sites, where the primary purpose is to provide intra-domain
routing services. Transit routing domains are deployed to carry
transit (i.e., inter-domain) traffic; backbones and providers are
TRDs.
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The greatest burden in transmitting and operating on routing
information is at the top of the routing hierarchy, where routing
information tends to accumulate. In the Internet, for example,
providers must manage the set of network numbers for all networks
reachable through the provider. Traffic destined for other
providers is generally routed to the backbones (which act as
providers as well). The backbones, however, must be cognizant of
the network numbers for all attached providers and their
associated networks.
In general, the advantage of abstracting routing information at a
given level of the routing hierarchy is greater at the higher
levels of the hierarchy. There is relatively little direct benefit
to the administration that performs the abstraction, since it must
maintain routing information individually on each attached
topological routing structure.
For example, suppose that a given site is trying to decide whether
to obtain an IP address prefix directly from the IP address space
allocated for North America, or from the IP address space
allocated to its service provider. If considering only their own
self-interest, the site itself and the attached provider have
little reason to choose one approach or the other. The site must
use one prefix or another; the source of the prefix has little
effect on routing efficiency within the site. The provider must
maintain information about each attached site in order to route,
regardless of any commonality in the prefixes of the sites.
However, there is a difference when the provider distributes
routing information to other providers (e.g., backbones or TRDs).
In the first case, the provider cannot aggregate the site's
address into its own prefix; the address must be explicitly listed
in routing exchanges, resulting in an additional burden to other
providers which must exchange and maintain this information.
In the second case, each other provider (e.g., backbone or TRD)
sees a single address prefix for the provider, which encompasses
the new site. This avoids the exchange of additional routing
information to identify the new site's address prefix. Thus, the
advantages primarily accrue to other providers which maintain
routing information about this site and provider.
One might apply a supplier/consumer model to this problem: the
higher level (e.g., a backbone) is a supplier of routing services,
while the lower level (e.g., a TRD) is the consumer of these
services. The price charged for services is based upon the cost of
providing them. The overhead of managing a large table of
addresses for routing to an attached topological entity
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contributes to this cost.
The Internet, however, is not a market economy. Rather, efficient
operation is based on cooperation. The recommendations discussed
below describe simple and tractable ways of managing the IP
address space that benefit the entire community.