4. Address Assignment and Routing Aggregation
- Address Assignment and Routing Aggregation
Classless addressing and routing was initially developed primarily to
improve the scaling properties of routing on the global Internet.
Because the scaling of routing is very tightly coupled to the way
that addresses are used, deployment of CIDR had implications for the
way in which addresses were assigned.
4.1. Aggregation Efficiency and Limitations
The only commonly understood method for reducing routing state on a
packet-switched network is through aggregation of information. For
CIDR to succeed in reducing the size and growth rate of the global
routing system, the IPv4 address assignment process needed to be
changed to make possible the aggregation of routing information along
topological lines. Since, in general, the topology of the network is
determined by the service providers who have built it, topologically
significant address assignments are necessarily service-provider
oriented.
Aggregation is simple for an end site that is connected to one
service provider: it uses address space assigned by its service
provider, and that address space is a small piece of a larger block
allocated to the service provider. No explicit route is needed for
the end site; the service provider advertises a single aggregate
route for the larger block. This advertisement provides reachability
and routeability for all the customers numbered in the block.
There are two, more complex, situations that reduce the effectiveness
of aggregation:
o An organization that is multi-homed. Because a multi-homed
organization must be advertised into the system by each of its
service providers, it is often not feasible to aggregate its
routing information into the address space of any one of those
providers. Note that the organization still may receive its
address assignment out of a service provider's address space
(which has other advantages), but that a route to the
organization's prefix is, in the most general case, explicitly
advertised by all of its service providers. For this reason, the
global routing cost for a multi-homed organization is generally
the same as it was prior to the adoption of CIDR. A more detailed
consideration of multi-homing practices can be found in [RFC4116].
o An organization that changes service provider but does not
renumber. This has the effect of "punching a hole" in one of the
original service provider's aggregated route advertisements. CIDR
handles this situation by requiring that the newer service
provider to advertise a specific advertisement for the re-homed
organization; this advertisement is preferred over provider
aggregates because it is a longer match. To maintain efficiency
of aggregation, it is recommended that an organization that
changes service providers plan eventually to migrate its network
into a an prefix assigned from its new provider's address space.
To this end, it is recommended that mechanisms to facilitate such
migration, such as dynamic host address assignment that uses
[RFC2131]), be deployed wherever possible, and that additional
protocol work be done to develop improved technology for
renumbering.
Note that some aggregation efficiency gain can still be had for
multi-homed sites (and, in general, for any site composed of
multiple, logical IPv4 networks); by allocating a contiguous power-
of-two block address space to the site (as opposed to multiple,
independent prefixes), the site's routing information may be
aggregated into a single prefix. Also, since the routing cost
associated with assigning a multi-homed site out of a service
provider's address space is no greater than the old method of
sequential number assignment by a central authority, it makes sense
to assign all end-site address space out of blocks allocated to
service providers.
It is also worthwhile to mention that since aggregation may occur at
multiple levels in the system, it may still be possible to aggregate
these anomalous routes at higher levels of whatever hierarchy may be
present. For example, if a site is multi-homed to two relatively
small providers that both obtain connectivity and address space from
the same large provider, then aggregation by the large provider of
routes from the smaller networks will include all routes to the
multi-homed site. The feasibility of this sort of second-level
aggregation depends on whether topological hierarchy exists among a
site, its directly-connected providers, and other providers to which
they are connected; it may be practical in some regions of the global
Internet but not in others.
Note: In the discussion and examples that follow, prefix notation is
used to represent routing destinations. This is used for
illustration only and does not require that routing protocols use
this representation in their updates.
4.2. Distributed Assignment of Address Space
In the early days of the Internet, IPv4 address space assignment was
performed by the central Network Information Center (NIC). Class
A/B/C network numbers were assigned in essentially arbitrary order,
roughly according to the size of the organizations that requested
them. All assignments were recorded centrally, and no attempt was
made to assign network numbers in a manner that would allow routing
aggregation.
When CIDR was originally deployed, the central assignment authority
continued to exist but changed its procedures to assign large blocks
of "Class C" network numbers to each service provider. Each service
provider, in turn, assigned bitmask-oriented subsets of the
provider's address space to each customer. This worked reasonably
well, as long as the number of service providers was relatively small
and relatively constant, but it did not scale well, as the number of
service providers grew at a rapid rate.
As the Internet started to expand rapidly in the 1990s, it became
clear that a single, centralized address assignment authority was
problematic. This function began being de-centralized when address
space assignment for European Internet sites was delegated in bit-
aligned blocks of 16777216 addresses (what CIDR would later define as
a /8) to the RIPE NCC ([RIPE]), effectively making it the first of
the RIRs. Since then, address assignment has been formally
distributed as a hierarchical function with IANA, the RIRs, and the
service providers. Removing the bottleneck of a single organization
having responsibility for the global Internet address space greatly
improved the efficiency and response time for new assignments.
Hierarchical delegation of addresses in this manner implies that
sites with addresses assigned out of a given service provider are,
for routing purposes, part of that service provider and will be
routed via its infrastructure. This implies that routing information
about multi-homed organizations (i.e., organizations connected to
more than one network service provider) will still need to be known
by higher levels in the hierarchy.
A historical perspective on these issues is described in [RFC1518].
Additional discussion may also be found in [RFC3221].