Network Working Group Y. Rekhter
Request for Comments: 1518 T.J. Watson Research Center, IBM Corp.
Category: Standards Track T. Li
cisco Systems
Editors
September 1993
An Architecture for IP Address Allocation with CIDR
Status of this Memo
This RFC specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
1. Introduction
This paper provides an architecture and a plan for allocating IP
addresses in the Internet. This architecture and the plan are
intended to play an important role in steering the Internet towards
the Address Assignment and Aggregating Strategy outlined in [1].
The IP address space is a scarce shared resource that must be managed
for the good of the community. The managers of this resource are
acting as its custodians. They have a responsibility to the community
to manage it for the common good.
2. Scope
The global Internet can be modeled as a collection of hosts
interconnected via transmission and switching facilities. Control
over the collection of hosts and the transmission and switching
facilities that compose the networking resources of the global
Internet is not homogeneous, but is distributed among multiple
administrative authorities. Resources under control of a single
administration form a domain. For the rest of this paper, "domain"
and "routing domain" will be used interchangeably. Domains that
share their resources with other domains are called network service
providers (or just providers). Domains that utilize other domain's
resources are called network service subscribers (or just
subscribers). A given domain may act as a provider and a subscriber
simultaneously.
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There are two aspects of interest when discussing IP address
allocation within the Internet. The first is the set of
administrative requirements for obtaining and allocating IP
addresses; the second is the technical aspect of such assignments,
having largely to do with routing, both within a routing domain
(intra-domain routing) and between routing domains (inter-domain
routing). This paper focuses on the technical issues.
In the current Internet many routing domains (such as corporate and
campus networks) attach to transit networks (such as regionals) in
only one or a small number of carefully controlled access points.
The former act as subscribers, while the latter act as providers.
The architecture and recommendations provided in this paper are
intended for immediate deployment. This paper specifically does not
address long-term research issues, such as complex policy-based
routing requirements.
Addressing solutions which require substantial changes or constraints
on the current topology are not considered.
The architecture and recommendations in this paper are oriented
primarily toward the large-scale division of IP address allocation in
the Internet. Topics covered include:
- Benefits of encoding some topological information in IP
addresses to significantly reduce routing protocol overhead;
- The anticipated need for additional levels of hierarchy in
Internet addressing to support network growth;
- The recommended mapping between Internet topological entities
(i.e., service providers, and service subscribers) and IP
addressing and routing components;
- The recommended division of IP address assignment among service
providers (e.g., backbones, regionals), and service subscribers
(e.g., sites);
- Allocation of the IP addresses by the Internet Registry;
- Choice of the high-order portion of the IP addresses in leaf
routing domains that are connected to more than one service
provider (e.g., backbone or a regional network).
It is noted that there are other aspects of IP address allocation,
both technical and administrative, that are not covered in this
paper. Topics not covered or mentioned only superficially include:
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- Identification of specific administrative domains in the
Internet;
- Policy or mechanisms for making registered information known to
third parties (such as the entity to which a specific IP address
or a portion of the IP address space has been allocated);
- How a routing domain (especially a site) should organize its
internal topology or allocate portions of its IP address space;
the relationship between topology and addresses is discussed,
but the method of deciding on a particular topology or internal
addressing plan is not; and,
- Procedures for assigning host IP addresses.
3. Background
Some background information is provided in this section that is
helpful in understanding the issues involved in IP address
allocation. A brief discussion of IP routing is provided.
IP partitions the routing problem into three parts:
- routing exchanges between end systems and routers (ARP),
- routing exchanges between routers in the same routing domain
(interior routing), and,
- routing among routing domains (exterior routing).
4. IP Addresses and Routing
For the purposes of this paper, an IP prefix is an IP address and
some indication of the leftmost contiguous significant bits within
this address. Throughout this paper IP address prefixes will be
expressed as <IP-address IP-mask> tuples, such that a bitwise logical
AND operation on the IP-address and IP-mask components of a tuple
yields the sequence of leftmost contiguous significant bits that form
the IP address prefix. For example a tuple with the value <193.1.0.0
255.255.0.0> denotes an IP address prefix with 16 leftmost contiguous
significant bits.
When determining an administrative policy for IP address assignment,
it is important to understand the technical consequences. The
objective behind the use of hierarchical routing is to achieve some
level of routing data abstraction, or summarization, to reduce the
cpu, memory, and transmission bandwidth consumed in support of
routing.
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While the notion of routing data abstraction may be applied to
various types of routing information, this paper focuses on one
particular type, namely reachability information. Reachability
information describes the set of reachable destinations. Abstraction
of reachability information dictates that IP addresses be assigned
according to topological routing structures. However, administrative
assignment falls along organizational or political boundaries. These
may not be congruent to topological boundaries and therefore the
requirements of the two may collide. It is necessary to find a
balance between these two needs.
Routing data abstraction occurs at the boundary between
hierarchically arranged topological routing structures. An element
lower in the hierarchy reports summary routing information to its
parent(s).
At routing domain boundaries, IP address information is exchanged
(statically or dynamically) with other routing domains. If IP
addresses within a routing domain are all drawn from non-contiguous
IP address spaces (allowing no abstraction), then the boundary
information consists of an enumerated list of all the IP addresses.
Alternatively, should the routing domain draw IP addresses for all
the hosts within the domain from a single IP address prefix, boundary
routing information can be summarized into the single IP address
prefix. This permits substantial data reduction and allows better
scaling (as compared to the uncoordinated addressing discussed in the
previous paragraph).
If routing domains are interconnected in a more-or-less random (i.e.,
non-hierarchical) scheme, it is quite likely that no further
abstraction of routing data can occur. Since routing domains would
have no defined hierarchical relationship, administrators would not
be able to assign IP addresses within the domains out of some common
prefix for the purpose of data abstraction. The result would be flat
inter-domain routing; all routing domains would need explicit
knowledge of all other routing domains that they route to. This can
work well in small and medium sized internets. However, this does
not scale to very large internets. For example, we expect growth in
the future to an Internet which has tens or hundreds of thousands of
routing domains in North America alone. This requires a greater
degree of the reachability information abstraction beyond that which
can be achieved at the "routing domain" level.
In the Internet, however, it should be possible to significantly
constrain the volume and the complexity of routing information by
taking advantage of the existing hierarchical interconnectivity, as
discussed in Section 5. Thus, there is the opportunity for a group of
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RFC 1518 CIDR Address Allocation Architecture September 1993
routing domains each to be assigned an address prefix from a shorter
prefix assigned to another routing domain whose function is to
interconnect the group of routing domains. Each member of the group
of routing domains now has its (somewhat longer) prefix, from which
it assigns its addresses.
The most straightforward case of this occurs when there is a set of
routing domains which are all attached to a single service provider
domain (e.g., regional network), and which use that provider for all
external (inter-domain) traffic. A small prefix may be given to the
provider, which then gives slightly longer prefixes (based on the
provider's prefix) to each of the routing domains that it
interconnects. This allows the provider, when informing other routing
domains of the addresses that it can reach, to abbreviate the
reachability information for a large number of routing domains as a
single prefix. This approach therefore can allow a great deal of
hierarchical abbreviation of routing information, and thereby can
greatly improve the scalability of inter-domain routing.
Clearly, this approach is recursive and can be carried through
several iterations. Routing domains at any "level" in the hierarchy
may use their prefix as the basis for subsequent suballocations,
assuming that the IP addresses remain within the overall length and
structure constraints.
At this point, we observe that the number of nodes at each lower
level of a hierarchy tends to grow exponentially. Thus the greatest
gains in the reachability information abstraction (for the benefit of
all higher levels of the hierarchy) occur when the reachability
information aggregation occurs near the leaves of the hierarchy; the
gains drop significantly at each higher level. Therefore, the law of
diminishing returns suggests that at some point data abstraction
ceases to produce significant benefits. Determination of the point at
which data abstraction ceases to be of benefit requires a careful
consideration of the number of routing domains that are expected to
occur at each level of the hierarchy (over a given period of time),
compared to the number of routing domains and address prefixes that
can conveniently and efficiently be handled via dynamic inter-domain
routing protocols.
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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RFC 1518 CIDR Address Allocation Architecture September 1993
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.
5.1 Administration of IP addresses within a domain
If individual subnetworks take their IP addresses from a myriad of
unrelated IP address spaces, there will be effectively no data
abstraction beyond what is built into existing intra-domain
routing protocols. For example, assume that within a routing
domain uses three independent prefixes assigned from three
different IP address spaces associated with three different
attached providers.
This has a negative effect on inter-domain routing, particularly
on those other domains which need to maintain routes to this
domain. There is no common prefix that can be used to represent
these IP addresses and therefore no summarization can take place
at the routing domain boundary. When addresses are advertised by
this routing domain to other routing domains, an enumerated list
of the three individual prefixes must be used.
This situation is roughly analogous to the present dissemination
of routing information in the Internet, where each domain may have
non-contiguous network numbers assigned to it. The result of
allowing subnetworks within a routing domain to take their IP
addresses from unrelated IP address spaces is flat routing at the
A/B/C class network level. The number of IP prefixes that leaf
routing domains would advertise is on the order of the number of
attached network numbers; the number of prefixes a provider's
routing domain would advertise is approximately the number of
network numbers attached to the client leaf routing domains; and
for a backbone this would be summed across all attached providers.
This situation is just barely acceptable in the current Internet,
and as the Internet grows this will quickly become intractable. A
greater degree of hierarchical information reduction is necessary
to allow continued growth in the Internet.
5.2 Administration at the Leaf Routing Domain
As mentioned previously, the greatest degree of data abstraction
comes at the lowest levels of the hierarchy. Providing each leaf
routing domain (that is, site) with a prefix from its provider's
prefix results in the biggest single increase in abstraction. From
outside the leaf routing domain, the set of all addresses
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reachable in the domain can then be represented by a single
prefix. Further, all destinations reachable within the provider's
prefix can be represented by a single prefix.
For example, consider a single campus which is a leaf routing
domain which would currently require 4 different IP networks.
Under the new allocation scheme, they might instead be given a
single prefix which provides the same number of destination
addresses. Further, since the prefix is a subset of the
provider's prefix, they impose no additional burden on the higher
levels of the routing hierarchy.
There is a close relationship between subnetworks and routing
domains implicit in the fact that they operate a common routing
protocol and are under the control of a single administration. The
routing domain administration subdivides the domain into
subnetworks. The routing domain represents the only path between
a subnetwork and the rest of the internetwork. It is reasonable
that this relationship also extend to include a common IP
addressing space. Thus, the subnetworks within the leaf routing
domain should take their IP addresses from the prefix assigned to
the leaf routing domain.
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.
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).
These solutions also have different effects on policies. For
example, suppose that country "X" has a law that traffic from a
source within country X to a destination within country X must at
all times stay entirely within the country. With the first
solution, it is not possible to determine from the destination
address whether or not the destination is within the country. With
the second solution, a separate address may be assigned to those
hosts which are within country X, thereby allowing routing
policies to be followed. Similarly, suppose that "Little Small
Company" (LSC) has a policy that its packets may never be sent to
a destination that is within MBII. With either solution, the
routers within LSC may be configured to discard any traffic that
has a destination within MBII's address space. However, with the
first solution this requires one entry; with the second it
requires many entries and may be impossible as a practical matter.
There are other possible solutions as well. A third approach is to
assign each multi-homed organization a single address prefix,
based on one of its connections to a TRD. Other TRDs to which the
multi-homed organization are attached maintain a routing table
entry for the organization, but are extremely selective in terms
of which other TRDs are told of this route. This approach will
produce a single "default" routing entry which all TRDs will know
how to reach (since presumably all TRDs will maintain routes to
each other), while providing more direct routing in some cases.
There is at least one situation in which this third approach is
particularly appropriate. Suppose that a special interest group of
organizations have deployed their own backbone. For example, lets
suppose that the U.S. National Widget Manufacturers and
Researchers have set up a U.S.-wide backbone, which is used by
corporations who manufacture widgets, and certain universities
which are known for their widget research efforts. We can expect
that the various organizations which are in the widget group will
run their internal networks as separate routing domains, and most
of them will also be attached to other TRDs (since most of the
organizations involved in widget manufacture and research will
also be involved in other activities). We can therefore expect
that many or most of the organizations in the widget group are
dual-homed, with one attachment for widget-associated
communications and the other attachment for other types of
communications. Let's also assume that the total number of
organizations involved in the widget group is small enough that it
is reasonable to maintain a routing table containing one entry per
organization, but that they are distributed throughout a larger
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internet with many millions of (mostly not widget-associated)
routing domains.
With the third approach, each multi-homed organization in the
widget group would make use of an address assignment based on its
other attachment(s) to TRDs (the attachments not associated with
the widget group). The widget backbone would need to maintain
routes to the routing domains associated with the various member
organizations. Similarly, all members of the widget group would
need to maintain a table of routes to the other members via the
widget backbone. However, since the widget backbone does not
inform other general worldwide TRDs of what addresses it can reach
(since the backbone is not intended for use by other outside
organizations), the relatively large set of routing prefixes needs
to be maintained only in a limited number of places. The addresses
assigned to the various organizations which are members of the
widget group would provide a "default route" via each members
other attachments to TRDs, while allowing communications within
the widget group to use the preferred path.
A fourth solution involves assignment of a particular address
prefix for routing domains which are attached to precisely two (or
more) specific routing domains. For example, suppose that there
are two providers "SouthNorthNet" and "NorthSouthNet" which have a
very large number of customers in common (i.e., there are a large
number of routing domains which are attached to both). Rather than
getting two address prefixes these organizations could obtain
three prefixes. Those routing domains which are attached to
NorthSouthNet but not attached to SouthNorthNet obtain an address
assignment based on one of the prefixes. Those routing domains
which are attached to SouthNorthNet but not to NorthSouthNet would
obtain an address based on the second prefix. Finally, those
routing domains which are multi-homed to both of these networks
would obtain an address based on the third prefix. Each of these
two TRDs would then advertise two prefixes to other TRDs, one
prefix for leaf routing domains attached to it only, and one
prefix for leaf routing domains attached to both.
This fourth solution is likely to be important when use of public
data networks becomes more common. In particular, it is likely
that at some point in the future a substantial percentage of all
routing domains will be attached to public data networks. In this
case, nearly all government-sponsored networks (such as some
current regionals) may have a set of customers which overlaps
substantially with the public networks.
There are therefore a number of possible solutions to the problem
of assigning IP addresses to multi-homed routing domains. Each of
Rekhter & Li [Page 15]RFC 1518 CIDR Address Allocation Architecture September 1993
these solutions has very different advantages and disadvantages.
Each solution places a different real (i.e., financial) cost on
the multi-homed organizations, and on the TRDs (including those to
which the multi-homed organizations are not attached).
In addition, most of the solutions described also highlight the
need for each TRD to develop policy on whether and under what
conditions to accept addresses that are not based on its own
address prefix, and how such non-local addresses will be treated.
For example, a somewhat conservative policy might be that non-
local IP address prefixes will be accepted from any attached leaf
routing domain, but not advertised to other TRDs. In a less
conservative policy, a TRD might accept such non-local prefixes
and agree to exchange them with a defined set of other TRDs (this
set could be an a priori group of TRDs that have something in
common such as geographical location, or the result of an
agreement specific to the requesting leaf routing domain). Various
policies involve real costs to TRDs, which may be reflected in
those policies.
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).
Rekhter & Li [Page 16]RFC 1518 CIDR Address Allocation Architecture September 1993
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
Rekhter & Li [Page 17]RFC 1518 CIDR Address Allocation Architecture September 1993
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.
5.7 Continental aggregation
Another level of hierarchy may also be used in this addressing
scheme to further reduce the amount of routing information
necessary for inter-continental routing. Continental aggregation
is useful because continental boundaries provide natural barriers
to topological connection and administrative boundaries. Thus, it
presents a natural boundary for another level of aggregation of
inter-domain routing information. To make use of this, it is
necessary that each continent be assigned an appropriate subset of
the address space. Providers (both direct and indirect) within
that continent would allocate their addresses from this space.
Note that there are numerous exceptions to this, in which a
service provider (either direct or indirect) spans a continental
division. These exceptions can be handled similarly to multi-
homed routing domains, as discussed above.
Note that, in contrast to the case of providers, the aggregation
of continental routing information may not be done on the
continent to which the prefix is allocated. The cost of inter-
continental links (and especially trans-oceanic links) is very
high. If aggregation is performed on the "near" side of the link,
then routing information about unreachable destinations within
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that continent can only reside on that continent. Alternatively,
if continental aggregation is done on the "far" side of an inter-
continental link, the "far" end can perform the aggregation and
inject it into continental routing. This means that destinations
which are part of the continental aggregation, but for which there
is not a corresponding more specific prefix can be rejected before
leaving the continent on which they originated.
For example, suppose that Europe is assigned a prefix of
<194.0.0.0 254.0.0.0>, such that European routing also contains
the longer prefixes <194.1.0.0 255.255.0.0> and <194.2.0.0
255.255.0.0>. All of the longer European prefixes may be
advertised across a trans-Atlantic link to North America. The
router in North America would then aggregate these routes, and
only advertise the prefix <194.0.0.0 255.0.0.0> into North
American routing. Packets which are destined for 194.1.1.1 would
traverse North American routing, but would encounter the North
American router which performed the European aggregation. If the
prefix <194.1.0.0 255.255.0.0> is unreachable, the router would
drop the packet and send an ICMP Unreachable without using the
trans-Atlantic link.
5.8 Transition Issues
Allocation of IP addresses based on connectivity to TRDs is
important to allow scaling of inter-domain routing to an internet
containing millions of routing domains. However, such address
allocation based on topology implies that in order to maximize the
efficiency in routing gained by such allocation, certain changes
in topology may suggest a change of address.
Note that an address change need not happen immediately. A domain
which has changed service providers may still advertise its prefix
through its new service provider. Since upper levels in the
routing hierarchy will perform routing based on the longest
prefix, reachability is preserved, although the aggregation and
scalability of the routing information has greatly diminished.
Thus, a domain which does change its topology should change
addresses as soon as convenient. The timing and mechanics of such
changes must be the result of agreements between the old service
provider, the new provider, and the domain.
This need to allow for change in addresses is a natural,
inevitable consequence of routing data abstraction. The basic
notion of routing data abstraction is that there is some
correspondence between the address and where a system (i.e., a
routing domain, subnetwork, or end system) is located. Thus if the
system moves, in some cases the address will have to change. If it
Rekhter & Li [Page 19]RFC 1518 CIDR Address Allocation Architecture September 1993
were possible to change the connectivity between routing domains
without changing the addresses, then it would clearly be necessary
to keep track of the location of that routing domain on an
individual basis.
In the short term, due to the rapid growth and increased
commercialization of the Internet, it is possible that the
topology may be relatively volatile. This implies that planning
for address transition is very important. Fortunately, there are a
number of steps which can be taken to help ease the effort
required for address transition. A complete description of address
transition issues is outside of the scope of this paper. However,
a very brief outline of some transition issues is contained in
this section.
Also note that the possible requirement to transition addresses
based on changes in topology imply that it is valuable to
anticipate the future topology changes before finalizing a plan
for address allocation. For example, in the case of a routing
domain which is initially single-homed, but which is expecting to
become multi-homed in the future, it may be advantageous to assign
IP addresses based on the anticipated future topology.
In general, it will not be practical to transition the IP
addresses assigned to a routing domain in an instantaneous "change
the address at midnight" manner. Instead, a gradual transition is
required in which both the old and the new addresses will remain
valid for a limited period of time. During the transition period,
both the old and new addresses are accepted by the end systems in
the routing domain, and both old and new addresses must result in
correct routing of packets to the destination.
During the transition period, it is important that packets using
the old address be forwarded correctly, even when the topology has
changed. This is facilitated by the use of "longest match"
inter-domain routing.
For example, suppose that the XYZ Corporation was previously
connected only to the NorthSouthNet regional. The XYZ Corporation
therefore went off to the NorthSouthNet administration and got an
IP address prefix assignment based on the IP address prefix value
assigned to the NorthSouthNet regional. However, for a variety of
reasons, the XYZ Corporation decided to terminate its association
with the NorthSouthNet, and instead connect directly to the
NewCommercialNet public data network. Thus the XYZ Corporation now
has a new address assignment under the IP address prefix assigned
to the NewCommercialNet. The old address for the XYZ Corporation
would seem to imply that traffic for the XYZ Corporation should be
Rekhter & Li [Page 20]RFC 1518 CIDR Address Allocation Architecture September 1993
routed to the NorthSouthNet, which no longer has any direct
connection with XYZ Corporation.
If the old TRD (NorthSouthNet) and the new TRD (NewCommercialNet)
are adjacent and cooperative, then this transition is easy to
accomplish. In this case, packets routed to the XYZ Corporation
using the old address assignment could be routed to the
NorthSouthNet, which would directly forward them to the
NewCommercialNet, which would in turn forward them to XYZ
Corporation. In this case only NorthSouthNet and NewCommercialNet
need be aware of the fact that the old address refers to a
destination which is no longer directly attached to NorthSouthNet.
If the old TRD and the new TRD are not adjacent, then the
situation is a bit more complex, but there are still several
possible ways to forward traffic correctly.
If the old TRD and the new TRD are themselves connected by other
cooperative transit routing domains, then these intermediate
domains may agree to forward traffic for XYZ correctly. For
example, suppose that NorthSouthNet and NewCommercialNet are not
directly connected, but that they are both directly connected to
the BBNet backbone. In this case, all three of NorthSouthNet,
NewCommercialNet, and the BBNet backbone would need to maintain a
special entry for XYZ corporation so that traffic to XYZ using the
old address allocation would be forwarded via NewCommercialNet.
However, other routing domains would not need to be aware of the
new location for XYZ Corporation.
Suppose that the old TRD and the new TRD are separated by a non-
cooperative routing domain, or by a long path of routing domains.
In this case, the old TRD could encapsulate traffic to XYZ
Corporation in order to deliver such packets to the correct
backbone.
Also, those locations which do a significant amount of business
with XYZ Corporation could have a specific entry in their routing
tables added to ensure optimal routing of packets to XYZ. For
example, suppose that another commercial backbone
"OldCommercialNet" has a large number of customers which exchange
traffic with XYZ Corporation, and that this third TRD is directly
connected to both NorthSouthNet and NewCommercialNet. In this case
OldCommercialNet will continue to have a single entry in its
routing tables for other traffic destined for NorthSouthNet, but
may choose to add one additional (more specific) entry to ensure
that packets sent to XYZ Corporation's old address are routed
correctly.
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Whichever method is used to ease address transition, the goal is
that knowledge relating XYZ to its old address that is held
throughout the global internet would eventually be replaced with
the new information. It is reasonable to expect this to take
weeks or months and will be accomplished through the distributed
directory system. Discussion of the directory, along with other
address transition techniques such as automatically informing the
source of a changed address, are outside the scope of this paper.
Another significant transition difficulty is the establishment of
appropriate addressing authorities. In order not to delay the
deployment of this addressing scheme, if no authority has been
created at an appropriate level, a higher level authority may
allocated addresses instead of the lower level authority. For
example, suppose that the continental authority has been allocated
a portion of the address space and that the service providers
present on that continent are clear, but have not yet established
their addressing authority. The continental authority may foresee
(possibly with information from the provider) that the provider
will eventually create an authority. The continental authority
may then act on behalf of that provider until the provider is
prepared to assume its addressing authority duties.
Finally, it is important to emphasize, that a change of addresses
due to changes in topology is not mandated by this document. The
continental level addressing hierarchy, as discussed in Section
5.7, is intended to handle the aggregation of reachability
information in the cases where addresses do not directly reflect
the connectivity between providers and subscribers.
5.9 Interaction with Policy Routing
We assume that any inter-domain routing protocol will have
difficulty trying to aggregate multiple destinations with
dissimilar policies. At the same time, the ability to aggregate
routing information while not violating routing policies is
essential. Therefore, we suggest that address allocation
authorities attempt to allocate addresses so that aggregates of
destinations with similar policies can be easily formed.
6. Recommendations
We anticipate that the current exponential growth of the Internet
will continue or accelerate for the foreseeable future. In
addition, we anticipate a rapid internationalization of the
Internet. The ability of routing to scale is dependent upon the
use of data abstraction based on hierarchical IP addresses. As
CIDR [1] is introduced in the Internet, it is therefore essential
Rekhter & Li [Page 22]RFC 1518 CIDR Address Allocation Architecture September 1993
to choose a hierarchical structure for IP addresses with great
care.
It is in the best interests of the internetworking community that
the cost of operations be kept to a minimum where possible. In the
case of IP address allocation, this again means that routing data
abstraction must be encouraged.
In order for data abstraction to be possible, the assignment of IP
addresses must be accomplished in a manner which is consistent
with the actual physical topology of the Internet. For example, in
those cases where organizational and administrative boundaries are
not related to actual network topology, address assignment based
on such organization boundaries is not recommended.
The intra-domain routing protocols allow for information
abstraction to be maintained within a domain. For zero-homed and
single-homed routing domains (which are expected to remain zero-
homed or single-homed), we recommend that the IP addresses
assigned within a single routing domain use a single address
prefix assigned to that domain. Specifically, this allows the set
of all IP addresses reachable within a single domain to be fully
described via a single prefix.
We anticipate that the total number of routing domains existing on
a worldwide Internet to be great enough that additional levels of
hierarchical data abstraction beyond the routing domain level will
be necessary.
In most cases, network topology will have a close relationship
with national boundaries. For example, the degree of network
connectivity will often be greater within a single country than
between countries. It is therefore appropriate to make specific
recommendations based on national boundaries, with the
understanding that there may be specific situations where these
general recommendations need to be modified.
6.1 Recommendations for an address allocation plan
We anticipate that public interconnectivity between private
routing domains will be provided by a diverse set of TRDs,
including (but not necessarily limited to):
- backbone networks (Alternet, ANSnet, CIX, EBone, PSI,
SprintLink);
- a number of regional or national networks; and,
Rekhter & Li [Page 23]RFC 1518 CIDR Address Allocation Architecture September 1993
- a number of commercial Public Data Networks.
These networks will not be interconnected in a strictly hierarchical
manner (for example, there is expected to be direct connectivity
between regionals, and all of these types of networks may have direct
international connections). However, the total number of such TRDs
is expected to remain (for the foreseeable future) small enough to
allow addressing of this set of TRDs via a flat address space. These
TRDs will be used to interconnect a wide variety of routing domains,
each of which may comprise a single corporation, part of a
corporation, a university campus, a government agency, or other
organizational unit.
In addition, some private corporations may be expected to make use of
dedicated private TRDs for communication within their own
corporation.
We anticipate that the great majority of routing domains will be
attached to only one of the TRDs. This will permit hierarchical
address aggregation based on TRD. We therefore strongly recommend
that addresses be assigned hierarchically, based on address prefixes
assigned to individual TRDs.
To support continental aggregation of routes, we recommend that all
addresses for TRDs which are wholly within a continent be taken from
the continental prefix.
For the proposed address allocation scheme, this implies that
portions of IP address space should be assigned to each TRD
(explicitly including the backbones and regionals). For those leaf
routing domains which are connected to a single TRD, they should be
assigned a prefix value from the address space assigned to that TRD.
For routing domains which are not attached to any publically
available TRD, there is not the same urgent need for hierarchical
address abbreviation. We do not, therefore, make any additional
recommendations for such "isolated" routing domains. Where such
domains are connected to other domains by private point-to-point
links, and where such links are used solely for routing between the
two domains that they interconnect, again no additional technical
problems relating to address abbreviation is caused by such a link,
and no specific additional recommendations are necessary.
Further, in order to allow aggregation of IP addresses at national
and continental boundaries into as few prefixes as possible, we
further recommend that IP addresses allocated to routing domains
should be assigned based on each routing domain's connectivity to
national and continental Internet backbones.
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6.2 Recommendations for Multi-Homed Routing Domains
There are several possible ways that these multi-homed routing
domains may be handled, as described in Section 5.4. Each of these
methods vary with respect to the amount of information that must be
maintained for inter-domain routing and also with respect to the
inter-domain routes. In addition, the organization that will bear the
brunt of this cost varies with the possible solutions. For example,
the solutions vary with respect to:
- resources used within routers within the TRDs;
- administrative cost on TRD personnel; and,
- difficulty of configuration of policy-based inter-domain routing
information within leaf routing domains.
Also, the solution used may affect the actual routes which packets
follow, and may effect the availability of backup routes when the
primary route fails.
For these reasons it is not possible to mandate a single solution for
all situations. Rather, economic considerations will require a
variety of solutions for different routing domains, service
providers, and backbones.
6.3 Recommendations for the Administration of IP addresses
A companion document [3] provides recommendations for the
administrations of IP addresses.
7. Acknowledgments
The authors would like to acknowledge the substantial contributions
made by the authors of RFC 1237 [2], Richard Colella, Ella Gardner,
and Ross Callon. The significant concepts (and a large portion of
the text) in this document are taken directly from their work.
The authors would like to acknowledge the substantial contributions
made by the members of the following two groups, the Federal
Engineering Planning Group (FEPG) and the International Engineering
Planning Group (IEPG). This document also reflects many concepts
expressed at the IETF Addressing BOF which took place in Cambridge,
MA in July 1992.
We would also like to thank Peter Ford (Los Alamos National
Laboratory), Elise Gerich (MERIT), Steve Kent (BBN), Barry Leiner
(ADS), Jon Postel (ISI), Bernhard Stockman (NORDUNET/SUNET), Claudio
Rekhter & Li [Page 25]RFC 1518 CIDR Address Allocation Architecture September 1993
Topolcic (CNRI), and Kannan Varadhan (OARnet) for their review and
constructive comments.
8. References
[1] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting: an
Address Assignment and Aggregation Strategy", RFC 1338, BARRNet,
cicso, Merit, OARnet, June 1992.
[2] Colella, R., Gardner, E, and R. Callon, "Guidelines for OSI NSAP
Allocation in the Internet", RFC 1237, JuNIST, Mitre, DEC, July
1991.
[3] Gerich, E., "Guidelines for Management of IP Address Space", RFC
1466, Merit, May 1993.
[4] Cerf, V., "IAB Recommended Policy on Distributing Internet
Identifier Assignment and IAB Recommended Policy Change to
Internet "Connected" Status", RFC 1174, CNRI, August 1990.
9. Security Considerations
Security issues are not discussed in this memo.
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10. Authors' Addresses
Yakov Rekhter
T.J. Watson Research Center, IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
EMail: yakov@watson.ibm.com
Tony Li
cisco Systems, Inc.
1525 O'Brien Drive
Menlo Park, CA 94025
EMail: tli@cisco.com
Rekhter & Li [Page 27]
