5.3 Administration at the Transit Routing Domain
Two kinds of transit routing domains are considered, direct
providers and indirect providers. Most of the subscribers of a
direct provider are domains that act solely as service subscribers
(they carry no transit traffic). Most of the subscribers of an
indirect provider are domains that, themselves, act as service
providers. In present terminology a backbone is an indirect
provider, while a TRD is a direct provider. Each case is discussed
separately below.
5.3.1 Direct Service Providers
It is interesting to consider whether direct service providers'
routing domains should use their IP address space for assigning IP
addresses from a unique prefix to the leaf routing domains that
they serve. The benefits derived from data abstraction are greater
than in the case of leaf routing domains, and the additional
degree of data abstraction provided by this may be necessary in
the short term.
As an illustration consider an example of a direct provider that
serves 100 clients. If each client takes its addresses from 4
independent address spaces then the total number of entries that
are needed to handle routing to these clients is 400 (100 clients
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times 4 providers). If each client takes its addresses from a
single address space then the total number of entries would be
only 100. Finally, if all the clients take their addresses from
the same address space then the total number of entries would be
only 1.
We expect that in the near term the number of routing domains in
the Internet will grow to the point that it will be infeasible to
route on the basis of a flat field of routing domains. It will
therefore be essential to provide a greater degree of information
abstraction.
Direct providers may give part of their address space (prefixes)
to leaf domains, based on an address prefix given to the provider.
This results in direct providers advertising to backbones a small
fraction of the number of address prefixes that would be necessary
if they enumerated the individual prefixes of the leaf routing
domains. This represents a significant savings given the expected
scale of global internetworking.
Are leaf routing domains willing to accept prefixes derived from
the direct providers? In the supplier/consumer model, the direct
provider is offering connectivity as the service, priced according
to its costs of operation. This includes the "price" of obtaining
service from one or more indirect providers (e.g., backbones). In
general, indirect providers will want to handle as few address
prefixes as possible to keep costs low. In the Internet
environment, which does not operate as a typical marketplace, leaf
routing domains must be sensitive to the resource constraints of
the providers (both direct and indirect). The efficiencies gained
in inter-domain routing clearly warrant the adoption of IP address
prefixes derived from the IP address space of the providers.
The mechanics of this scenario are straightforward. Each direct
provider is given a unique small set of IP address prefixes, from
which its attached leaf routing domains can allocates slightly
longer IP address prefixes. For example assume that NIST is a
leaf routing domain whose inter-domain link is via SURANet. If
SURANet is assigned an unique IP address prefix <198.1.0.0
255.255.0.0>, NIST could use a unique IP prefix of <198.1.0.0
255.255.240.0>.
If a direct service provider is connected to another provider(s)
(either direct or indirect) via multiple attachment points, then
in certain cases it may be advantageous to the direct provider to
exert a certain degree of control over the coupling between the
attachment points and flow of the traffic destined to a particular
subscriber. Such control can be facilitated by first partitioning
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all the subscribers into groups, such that traffic destined to all
the subscribers within a group should flow through a particular
attachment point. Once the partitioning is done, the address space
of the provider is subdivided along the group boundaries. A leaf
routing domain that is willing to accept prefixes derived from its
direct provider gets a prefix from the provider's address space
subdivision associated with the group the domain belongs to. Note
that the advertisement by the direct provider of the routing
information associated with each subdivision must be done with
care to ensure that such an advertisement would not result in a
global distribution of separate reachability information
associated with each subdivision, unless such distribution is
warranted for some other purposes (e.g., supporting certain
aspects of policy-based routing).
5.3.2 Indirect Providers (Backbones)
There does not appear to be a strong case for direct providers to
take their address spaces from the the IP space of an indirect
provider (e.g., backbone). The benefit in routing data abstraction
is relatively small. The number of direct providers today is in
the tens and an order of magnitude increase would not cause an
undue burden on the backbones. Also, it may be expected that as
time goes by there will be increased direct interconnection of the
direct providers, leaf routing domains directly attached to the
backbones, and international links directly attached to the
providers. Under these circumstances, the distinction between
direct and indirect providers may become blurred.
An additional factor that discourages allocation of IP addresses
from a backbone prefix is that the backbones and their attached
providers are perceived as being independent. Providers may take
their long- haul service from one or more backbones, or may switch
backbones should a more cost-effective service be provided
elsewhere. Having IP addresses derived from a backbone is
inconsistent with the nature of the relationship.