Considerations on the IPv6 Host Density Metric
RFC 4692

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Network Working Group                                          G. Huston
Request for Comments: 4692                                         APNIC
Category: Informational                                     October 2006

             Considerations on the IPv6 Host Density Metric

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo provides an analysis of the Host Density metric as it is
   currently used to guide registry allocations of IPv6 unicast address
   blocks.  This document contrasts the address efficiency as currently
   adopted in the allocation of IPv4 network addresses and that used by
   the IPv6 protocol.  Note that for large allocations there are very
   significant variations in the target efficiency metric between the
   two approaches.

Table of Contents

   1. Introduction ....................................................2
   2. IPv6 Address Structure ..........................................2
   3. The Host Density Ratio ..........................................3
   4. The Role of an Address Efficiency Metric ........................4
   5. Network Structure and Address Efficiency Metric .................6
   6. Varying the HD-Ratio ............................................7
      6.1. Simulation Results .........................................8
   7. Considerations .................................................10
   8. Security Considerations ........................................11
   9. Acknowledgements ...............................................11
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................12
   Appendix A.  Comparison Tables ....................................13

Huston                       Informational                      [Page 1]
RFC 4692                IPv6 Host Density Metric            October 2006

1.  Introduction

   Metrics of address assignment efficiency are used in the context of
   the Regional Internet Registries' (RIRs') address allocation
   function.  Through the use of a common address assignment efficiency
   metric, individual networks can be compared to a threshold value in
   an objective fashion.  The common use of this metric is to form part
   of the supporting material for an address allocation request,
   demonstrating that the network has met or exceeded the threshold
   address efficiency value, and it forms part of the supportive
   material relating to the justification of the allocation of a further
   address block.

   Public and private IP networks have significant differences in
   purpose, structure, size, and technology.  Attempting to impose a
   single efficiency metric across this very diverse environment is a
   challenging task.  Any address assignment efficiency threshold value
   has to represent a balance between stating an achievable outcome for
   any competently designed and operated service platform while without
   setting a level of consumption of address resources that imperils the
   protocol's longer term viability through consequent address scarcity.
   There are a number of views relating to address assignment
   efficiency, both in terms of theoretic analyses of assignment
   efficiency and in terms of practical targets that are part of current
   address assignment practices in today's Internet.

   This document contrasts the address efficiency metric and threshold
   value as currently adopted in the allocation of IPv4 network
   addresses and the framework used by the address allocation process
   for the IPv6 protocol.

2.  IPv6 Address Structure

   Before looking at address allocation efficiency metrics, it is
   appropriate to summarize the address structure for IPv6 global
   unicast addresses.

   The general format for IPv6 global unicast addresses is defined in
   [RFC4291] as follows (Figure 1).

    |         64 - m bits    |   m bits  |       64 bits              |
    +------------------------+-----------+----------------------------+
    | global routing prefix  | subnet ID |       interface ID         |
    +------------------------+-----------+----------------------------+

                          IPv6 Address Structure

                                 Figure 1

Huston                       Informational                      [Page 2]
RFC 4692                IPv6 Host Density Metric            October 2006

   Within the current policy framework for allocation of IPv6 addresses
   in the context of the public Internet, the value for 'm' in the
   figure above, referring to the subnet ID, is commonly a 16-bit field.
   Therefore, the end-site global routing prefix is 48 bits in length,