Internet Engineering Task Force                                 J. Palet
Internet-Draft                                                 C. Olvera
Expires: December 28, 2006                                       M. Diaz
                                                           June 26, 2006

   Guidelines for Numbering IPv6 Point-to-Point Links and Easing the
                            Addressing Plans

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   Copyright (C) The Internet Society (2006).


   This document analyzes the rational for using /64 for numbering IPv6
   point-to-point links and provides some guidelines to simplify the
   addressing plans.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Rational for using /64  . . . . . . . . . . . . . . . . . . . . 3
   3.  Numbering Interfaces  . . . . . . . . . . . . . . . . . . . . . 4
   4.  Routing Aggregation of the Point-to-Point Links . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

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1.  Introduction

   There are different alternatives for numbering IPv6 point-to-point
   links, and from an operational perspective, they may have different
   advantages or disadvantages that need to be taken in consideration
   under the scope of each specific network architecture design.

   However, as a general rule, this document suggest the approach of
   using /64 in order to ensure not only compliance with standards, and
   consequently facilitate interoperability, but also in order to ensure
   avoiding possible future issues and simplifying the addressing plans.

   The use of /64 also facilitates an easier way for routing the shorter
   aggregated prefix into the point-to-point link.  Consequently it
   simplifies the "view" of a more unified addressing plan, providing an
   easier path for following up any issue when operating IPv6 networks.

   The proposed approach is suitable for those point-to-point links
   connecting ISP to Customers, but not limited to this case, and in
   fact, has been tried in real scenarios for different cases.  In that
   sense, this document can be read as guidelines for one of the
   possible choices available, not as a generic guideline for all the
   possible ways to address this.

   There is another well known approach, which use two different address
   pools, one for the numbering the point-to-point links and another one
   for delegating the prefixes at the end of the point-to-point link.
   This document approaches for a more unified and aggregated addressing

2.  Rational for using /64

   The IPv6 Addressing Architecture [1] specifies that all the Interface
   Identifiers for all the unicast addresses (except for 000/3) are
   required to be 64 bits long and to be constructed in Modified EUI-64
   format.  As a consequence it is forbidden to use prefixes longer than

   The same document also mandates the usage of the predefined subnet-
   router anycast address, which has cleared to zero all the bits that
   do not form the subnet prefix.

   Moreover, [2] describes de problems of using /127 especially on
   point-to-point links between routers.  This document also describes
   different choices for the point-to-point links and actually, without
   advocating for any specific prefix length, shows that /64 is the best
   solution from different perspectives, including operational

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   Consequently, we shall conclude that /64 should be used for numbering
   point-to-point links.

3.  Numbering Interfaces

   Often, in point-to-point links, hardware tokens are not available, or
   there is the need to keep certain bits (u, g) cleared, so the links
   can be manually numbered sequentially with most of the bits cleared
   to zero.  This numbering makes as well easier to remember the
   interfaces, which typically will become numbered as 1 (with 63
   leading zero bits) for the provider side and 2 (with 63 leading zero
   bits) for the customer side.

   Using interface identifies as 1 and 2 is only a very simple suggested
   example, and other different choices can as well be used as required
   in each case.

   On the other hand, using the EUI-64, makes it more difficult to
   remember and handle the interfaces, but provides an additional degree
   of protection against port (actually address) scanning as described
   at [3].

4.  Routing Aggregation of the Point-to-Point Links

   Following this approach and assuming that a shorter prefix is
   typically delegated to a customer, in general a /48 [4], it is
   possible to simplify the routing aggregation of the point-to-point
   links.  Towards this, the point-to-point link may be numbered using
   the first /64 of a given /48.

   Let's see a practical example:

   o  A service provider uses the prefix 2001:db8::/32 and is using
      2001:db8:aaaa::/48 for a given customer.

   o  Instead of allocating the point-to-point link from a different
      addressing pool, it may use 2001:db8:aaaa::/64 (which is the first
      /64 subnet from the 2001:db8:aaaa::/48) to number the link.

   o  This means that, in the case the non-EUI-64 approach is used, the
      point-to-point link may be numbered as 2001:db8:aaaa::1/64 for the
      provider side and 2001:db8:aaaa::2/64 for the customer side.

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   o  Note that using the first /64 and interface identifiers 1 and 2 is
      only a very simple example, and other values may be chosen
      according to each case specific needs.

   In this way, as the same address pool is being used for both the
   prefix and the point-to-point link, one of the advantages of this
   approach is to make very easy remembering the point-to-point links
   that belong to a given customer prefix, or in the other way around,
   remember the prefix that is linked by a given point-to-point link.

   For example, making a trace-route to debug any issue to a given
   address in the provider network, will show a straight view, and there
   will not be need to check a database that related an address pool for
   the point-to-point links and the customer prefixes, as all they are
   the same.

   Moreover, it is possible to use the shorter prefix as the provider
   side numbering for the point-to-point link and keep the /64 for the
   customer side.  In our example, it will become:

   o  Point-to-point link at provider side: 2001:db8:aaaa::1/48

   o  Point-to-point link at customer side: 2001:db8:aaaa::2/64

   This provides one additional advantage as in some platforms the
   configuration may be easier saving one step for the route of the
   delegated prefix (no need for two routes to be configured, one for
   the prefix, one for the point-to-point link).  It is possible because
   the longest-prefix-match rule.

   The behavior of this type of configuration has been successfully
   tested in different commonly available implementations with different
   routing protocols, including RIP, BGP, IS-IS, OSPF, along static
   routing, and has been used in several scenarios for a few months
   without any failures having been reported.

   As stated in [5], "the requesting router MUST NOT assign any
   delegated prefixes or subnets from the delegated prefix(es) to the
   link through which is received the DHCP message from the delegating
   router", however the approach described in this document may still be
   useful in other DHCPv6 scenarios or non-DHCPv6 scenarios.  Moreover,
   [6] has no explicit requirement that avoids the approach described in
   this document.  Furthermore, this has been tested in DHCPv6-PD
   implementations and worked well, so we must say that it may be

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5.  Security Considerations

   No security concerns seem to be related to this proposal.

6.  IANA Considerations

   This document does not have any specific IANA considerations.

7.  Acknowledgements

   The authors would like to acknowledge the inputs/comments from Alain
   Durand, Chip Popoviciu, Daniel Roesen, Fred Baker, Gert Doering, Olaf
   Bonness, Ole Troan, Pekka Savola, Vincent Jardin and the Spanish
   Ministry of Industry support in the co-funding of the Eureka PlaNetS
   project, where this work is being developed.

8.  References

8.1.  Normative References

   [1]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 4291, February 2006.

8.2.  Informative References

   [2]  Savola, P., "Use of /127 Prefix Length Between Routers
        Considered Harmful", RFC 3627, September 2003.

   [3]  Chown, T., "IPv6 Implications for TCP/UDP Port Scanning",
        draft-chown-v6ops-port-scanning-implications-02 (work in
        progress), October 2005.

   [4]  IAB and IESG, "IAB/IESG Recommendations on IPv6 Address
        Allocations to Sites", RFC 3177, September 2001.

   [5]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host
        Configuration Protocol (DHCP) version 6", RFC 3633,
        December 2003.

   [6]  Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
        Delegation", RFC 3769, June 2004.

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Authors' Addresses

   Jordi Palet Martinez
   Molino de la Navata, 75
   La Navata - Galapagar - Madrid
   E-28420 - Spain

   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98

   Cesar Olvera Morales
   Molino de la Navata, 75
   La Navata - Galapagar - Madrid
   E-28420 - Spain

   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98

   Miguel Angel Diaz Fernandez
   Molino de la Navata, 75
   La Navata - Galapagar - Madrid
   E-28420 - Spain

   Phone: +34 91 151 81 99
   Fax:   +34 91 151 81 98

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