📄 rfc1519.txt
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Network Working Group V. Fuller
Request for Comments: 1519 BARRNet
Obsoletes: 1338 T. Li
Category: Standards Track cisco
J. Yu
MERIT
K. Varadhan
OARnet
September 1993
Classless Inter-Domain Routing (CIDR):
an Address Assignment and Aggregation Strategy
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.
Abstract
This memo discusses strategies for address assignment of the existing
IP address space with a view to conserve the address space and stem
the explosive growth of routing tables in default-route-free routers.
Table of Contents
Acknowledgements ................................................. 2
1. Problem, Goal, and Motivation ................................ 2
2. CIDR address allocation ...................................... 3
2.1 Aggregation and its limitations ............................. 3
2.2 Distributed network number allocation ....................... 5
3. Cost-benefit analysis ........................................ 6
3.1 Present allocation figures .................................. 7
3.2 Historic growth rates ....................................... 8
3.3 Detailed analysis ........................................... 8
3.3.1 Benefits of new addressing plan ........................... 9
3.3.2 Growth rate projections ................................... 9
4. Changes to inter-domain routing protocols and practices ...... 11
4.1 Protocol-independent semantic changes ....................... 11
4.2 Rules for route advertisement ............................... 11
4.3 How the rules work .......................................... 13
4.4 Responsibility for and configuration of aggregation ......... 14
4.5 Intra-domain protocol considerations ........................ 15
5. Example of new allocation and routing ........................ 15
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RFC 1519 CIDR Address Strategy September 1993
5.1 Address allocation .......................................... 15
5.2 Routing advertisements ...................................... 17
6. Extending CIDR to class A addresses .......................... 18
7. Domain Naming Service considerations ......................... 20
7.1 Procedural changes for class-C "supernets" ................... 20
7.2 Procedural changes for class-A subnetting .................... 21
8. Transitioning to a long term solution ........................ 22
9. Conclusions .................................................. 22
10. Recommendations ............................................. 22
11. References .................................................. 23
12. Security Considerations ..................................... 23
13. Authors' Addresses .......................................... 24
Acknowledgements
The authors wish to express their appreciation to the members of the
ROAD group with whom many of the ideas contained in this document
were inspired and developed.
1. Problem, Goal, and Motivation
As the Internet has evolved and grown over in recent years, it has
become evident that it is soon to face several serious scaling
problems. These include:
1. Exhaustion of the class B network address space. One
fundamental cause of this problem is the lack of a network
class of a size which is appropriate for mid-sized
organization; class C, with a maximum of 254 host
addresses, is too small, while class B, which allows up to
65534 addresses, is too large for most organizations.
2. Growth of routing tables in Internet routers beyond the
ability of current software, hardware, and people to
effectively manage.
3. Eventual exhaustion of the 32-bit IP address space.
It has become clear that the first two of these problems are likely
to become critical within the next one to three years. This memo
attempts to deal with these problems by proposing a mechanism to slow
the growth of the routing table and the need for allocating new IP
network numbers. It does not attempt to solve the third problem,
which is of a more long-term nature, but instead endeavors to ease
enough of the short to mid-term difficulties to allow the Internet to
continue to function efficiently while progress is made on a longer-
term solution.
Fuller, Li, Yu & Varadhan [Page 2]
RFC 1519 CIDR Address Strategy September 1993
The proposed solution is to topologically allocate future IP address
assignment, by allocating segments of the IP address space to the
transit routing domains.
This plan for allocating IP addresses should be undertaken as soon as
possible. We believe that this will suffice as a short term
strategy, to fill the gap between now and the time when a viable long
term plan can be put into place and deployed effectively. This plan
should be viable for at least three (3) years, after which time,
deployment of a suitable long term solution is expected to occur.
This plan is primarily directed at the first two problems listed
above. We believe that the judicious use of variable-length
subnetting techniques should help defer the onset of the last problem
problem, the exhaustion of the 32-bit address space. Note also that
improved tools for performing address allocation in a "supernetted"
and variably-subnetted world would greatly help the user community in
accepting these sometimes confusing techniques. Efforts to create
some simple tools for this purpose should be encouraged by the
Internet community.
Note that this plan neither requires nor assumes that already
assigned addresses will be reassigned, though if doing so were
possible, it would further reduce routing table sizes. It is assumed
that routing technology will be capable of dealing with the current
routing table size and with some reasonably small rate of growth.
The emphasis of this plan is on significantly slowing the rate of
this growth.
Note that this plan does not require domains to renumber if they
change their attached transit routing domain. Domains are encouraged
to renumber so that their individual address allocations do not need
to be advertised.
This plan will not affect the deployment of any specific long term
plan, and therefore, this document will not discuss any long term
plans for routing and address architectures.
2. CIDR address allocation
There are two basic components of this addressing and routing plan:
one, to distribute the allocation of Internet address space and two,
to provide a mechanism for the aggregation of routing information.
2.1 Aggregation and its limitations
One major goal of this addressing plan is to allocate Internet
address space in such a manner as to allow aggregation of routing
Fuller, Li, Yu & Varadhan [Page 3]
RFC 1519 CIDR Address Strategy September 1993
information along topological lines. For simple, single-homed
clients, the allocation of their address space out of a transit
routing domain's space will accomplish this automatically - rather
than advertise a separate route for each such client, the transit
domain may advertise a single aggregate route which describes all of
the destinations connected to it. Unfortunately, not all sites are
singly-connected to the network, so some loss of ability to aggregate
is realized for the non-trivial cases.
There are two situations that cause a loss of aggregation efficiency.
o Organizations which are multi-homed. Because multi-homed
organizations must be advertised into the system by each of
their service providers, it is often not feasible to
aggregate their routing information into the address space
any one of those providers. Note that they still may receive
their address allocation out of a transit domain's address
space (which has other advantages), but their routing
information must still be explicitly advertised by most of
their service providers (the exception being that if the
site's allocation comes out of its least-preferable service
provider, then that service provider need not advertise the
explicit route - longest-match will insure that its
aggregated route is used to get to the site on a backup
basis). For this reason, the routing cost for these
organizations will typically be about the same as it is
today.
o Organizations which change service provider but do not
renumber. This has the effect of "punching a hole" in the
aggregation of the original service provider's advertisement.
This plan will handle the situation by requiring the newer
service provider to advertise a specific advertisement for
the new client, which is preferred by virtue of being the
longest match. To maintain efficiency of aggregation, it is
recommended that organizations which do change service
providers plan to eventually migrate their address
assignments from the old provider's space to that of the new
provider. To this end, it is recommended that mechanisms to
facilitate such migration, including improved protocols and
procedures for dynamic host address assignment, be developed.
Note that some aggregation efficiency gain can still be had for
multi-homed sites (and, in general, for any site composed of
multiple, logical IP network numbers) - by allocating a contiguous
power-of-two block of network numbers to the client (as opposed to
multiple, independent network numbers) the client's routing
information may be aggregated into a single (net, mask) pair. Also,
Fuller, Li, Yu & Varadhan [Page 4]
RFC 1519 CIDR Address Strategy September 1993
since the routing cost associated with assigning a multi-homed site
out of a service provider's address space is no greater than the
current method of a random allocation by a central authority, it
makes sense to allocate all address space out of blocks assigned 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 NSFNET regional
networks both of whom obtain their address space from the NSFNET,
then aggregation by the NSFNET of routes from the regionals will
include all routes to the multi-homed site.
Finally, it should also be noted that deployment of the new
addressing plan described in this document may (and should) begin
almost immediately but effective use of the plan to aggregate routing
information will require changes to some Inter-Domain routing
protocols. Likewise, deploying classless Inter-Domain protocols
without deployment of the new address plan will not allow useful
aggregation to occur (in other words, the addressing plan and routing
protocol changes are both required for supernetting, and its
resulting reduction in table growth, to be effective.) Note,
however, that during the period of time between deployment of the
addressing plan and deployment of the new protocols, the size of
routing tables may temporarily grow very rapidly. This must be
considered when planning the deployment of the two plans.
Note: in the discussion and examples which follow, the network and
mask 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.
2.2 Distributed allocation of address space
The basic idea of the plan is to allocate one or more blocks of Class
C network numbers to each network service provider. Organizations
using the network service provider for Internet connectivity are
allocated bitmask-oriented subsets of the provider's address space as
required.
It is also worthwhile to mention that once inter-domain protocols
which support classless network destinations are widely deployed, the
rules described by this plan generalize to permit arbitrary
super/subnetting of the remaining class A and class B address space
(the assumption being that classless inter-domain protocols will
either allow for non-contiguous subnets to exist in the system or
that all components of a sub-allocated class A/B will be contained
Fuller, Li, Yu & Varadhan [Page 5]
RFC 1519 CIDR Address Strategy September 1993
within a single routing domain). This will allow this plan to
continue to be used in the event that the class C space is exhausted
before implementation of a long-term solution is deployed. This
alternative is discussed further below in section 6.
Hierarchical sub-allocation of addresses in this manner implies that
clients with addresses allocated 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.
The advantages of hierarchical assignment in this fashion are
a) It is expected to be easier for a relatively small number of
service providers to obtain addresses from the central
authority, rather than a much larger, and monotonically
increasing, number of individual clients. This is not to be
considered as a loss of part of the service providers' address
space.
b) Given the current growth of the Internet, a scalable and
delegatable method of future allocation of network numbers has
to be achieved.
For these reasons, and in the interest of providing a consistent
procedure for obtaining Internet addresses, it is recommended that
most, if not all, network numbers be distributed through service
providers. These issues are discussed in much greater length in [2].
3. Cost-benefit analysis
This new method of assigning address through service providers can be
put into effect immediately and will, from the start, have the
benefit of distributing the currently centralized process of
assigning new addresses. Unfortunately, before the benefit of
reducing the size of globally-known routing destinations can be
achieved, it will be necessary to deploy an Inter-Domain routing
protocol capable of handling arbitrary network and mask pairs. Only
then will it be possible to aggregate individual class C networks
into larger blocks represented by single routing table entries.
This means that upon introduction, the new addressing allocation plan
will not in and of itself help solve the routing table size problem.
Once the new Inter-Domain routing protocol is deployed, however, an
immediate drop in the number of destinations which clients of the new
protocol must carry will occur. A detailed analysis of the magnitude
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