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Network Working Group Internet Engineering Steering Group Request for Comments: 1517 R. Hinden, Editor Category: Standards Track September 1993

         Applicability Statement for the Implementation of
               Classless Inter-Domain Routing (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

 As the Internet has evolved and grown in recent years, it has become
 clear that it will soon face several serious scaling problems. These
 include:

Exhaustion of the class-B network address space. One

fundamental cause of this problem is the lack of a network

      class of a size that is appropriate for a 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 to be densely populated.  The result is inefficient
      utilization of class-B network numbers.

Routing information overload. The size and rate of growth of the

routing tables in Internet routers is beyond the ability of

      current software (and people) to effectively manage.

Eventual exhaustion of IP network numbers.

 It has become clear that the first two of these problems are likely
 to become critical in the near term.  Classless Inter-Domain Routing
 (CIDR) ttempts to deal with these problems by defining a mechanism to
 slow the growth of routing tables and reduce the need to allocate 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.

 The IESG, after a thorough discussion in the IETF, in June 1992
 selected CIDR as the solution for the short term routing table

IESG [Page 1] RFC 1517 CIDR Applicability Statement September 1993

 explosion problem [1].

2. Components of the Architecture

 The CIDR architecture is described in the following documents:

"An Architecture for IP Address Allocation with CIDR" [2]

"Classless Inter-Domain Routing (CIDR): An Address Assignment

and Aggregation Strategy" [3]

 The first of these documents presents the overall architecture of
 CIDR; the second describes the specific address allocation scheme to
 be used.

 In addition to these two documents, "Guidelines for Management of IP
 Address Space" [4] provides specific recommendations for assigning IP
 addresses that are consistent with [2] and [3], and "Status of CIDR
 Deployment in the Internet" [5] describes the timetable for deploying
 [4] in the Internet.  Both [4] and [5] should be viewed as
 supporting, rather than defining, documents.

 In addition to the documents mentioned above, CIDR requires that
 inter-domain routing protocols be capable of handling reachability
 information that is expressed solely in terms of IP address prefixes.
 While several inter-domain routing protocols are capable of
 supporting such functionality, this Applicability Statement does not
 mandate the use of a particular one.

 Although Internet routing domains are not required to use routing
 protocols capable of propagating CIDR routes, the topology such
 routing domains can support will be somewhat limited.  In particular,
 the non-CIDR-capable parts of the Internet will need to default
 towards the CIDR-capable parts of the Internet for routes which have
 been aggregated to non-network boundaries.

3. Applicability of CIDR

 The CIDR architecture is applicable to any group of connected domains
 that supports IP version 4 [6] [7].  CIDR does not require all of the
 domains in the Internet to be converted to use CIDR. It assumes that
 some of the existing domains in the Internet will never be able to
 convert.  Despite this, CIDR will still provide connectivity to such
 places, although the optimality of routes to these places may be
 impacted.

 This Applicability Statement requires Internet domains providing
 backbone and/or transit service to fully implement CIDR in order to

IESG [Page 2] RFC 1517 CIDR Applicability Statement September 1993

 ensure that the growth of the resources required by routers to
 provide Internet-wide connectivity will be significantly slower than
 the growth of the number of assigned networks.

 This Applicability Statement strongly recommends that all non-
 backbone/transit Internet domains also implement CIDR because it will
 reduce the amount of routing information inside of these domains.

 Individual domains are free to choose whatever inter-domain and
 intra-domain routing architectures best meet their requirements.
 Specifically, this Applicability Statement does not prevent a domain
 or a group of domains from using addressing schemes which do not
 conform to CIDR.  Subject to the available resources in routers, CIDR
 should be able to co-exist with other addressing schemes without
 adversely impacting overall connectivity.

4. References

 [1] Gross, P., and P. Almquist, "IESG Deliberations on Routing and
     Addressing", RFC 1380, IESG Chair, IESG Internet AD, November
     1992.

 [2] Rekhter, Y., and T. Li, "An Architecture for IP Address
     Allocation with CIDR", RFC 1518, T.J. Watson Research Center, IBM
     Corp., cisco Systems, September 1993.

 [3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
     Domain Routing (CIDR): An Address Assignment and Aggregation
     Strategy", RFC 1519, BARRNet, cisco, Merit, and OARnet, September
     1993.

 [4] Gerich, E., "Guidelines for Management of IP Address Space", RFC
     1466, Merit, May 1993.

 [5] Topolcic, C., "Status of CIDR Deployment in the Internet", RFC
     1467, CNRI, August 1993.

 [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
     Specification", STD 5, RFC 791, USC/Information Sciences
     Institute, September 1981.

 [7] Braden, R., Editor, "Requirements for Internet Hosts --
     Communication Layers", STD 3, RFC 1122, IETF, October 1989.

5. Security Considerations

 Security issues are not discussed in this memo.

IESG [Page 3] RFC 1517 CIDR Applicability Statement September 1993

6. Author's Address

 Robert M. Hinden
 Sun Microsystems
 2550 Garcia Ave, MS MTV5-44
 Mt. View, CA 94043

 Phone: (415) 336-2082
 Fax:   (415) 336-6015

 EMail: hinden@eng.sun.com

IESG [Page 4]