Discovering PREF64 in Router Advertisements
RFC 8781

Document Type RFC - Proposed Standard (April 2020; No errata)
Last updated 2020-04-24
Replaces draft-pref64folks-6man-ra-pref64
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Stream WG state Submitted to IESG for Publication
Document shepherd Bob Hinden
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IESG IESG state RFC 8781 (Proposed Standard)
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Send notices to Bob Hinden <bob.hinden@gmail.com>
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Internet Engineering Task Force (IETF)                        L. Colitti
Request for Comments: 8781                                    J. Linkova
Category: Standards Track                                         Google
ISSN: 2070-1721                                               April 2020

              Discovering PREF64 in Router Advertisements

Abstract

   This document specifies a Neighbor Discovery option to be used in
   Router Advertisements (RAs) to communicate prefixes of Network
   Address and Protocol Translation from IPv6 clients to IPv4 servers
   (NAT64) to hosts.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8781.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Terminology
   2.  Use Cases for Communicating the NAT64 Prefix to Hosts
   3.  Why Include the NAT64 Prefix in Router Advertisements?
   4.  Option Format
     4.1.  Scaled Lifetime Processing
   5.  Usage Guidelines
     5.1.  Handling Multiple NAT64 Prefixes
     5.2.  PREF64 Consistency
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   NAT64 [RFC6146] with DNS Extensions for Network Address Translation
   from IPv6 clients to IPv4 servers (DNS64) [RFC6147] is a widely
   deployed mechanism to provide IPv4 access on IPv6-only networks.  In
   various scenarios, the host must be aware of the NAT64 prefix in use
   by the network.  This document specifies a Neighbor Discovery
   [RFC4861] option to be used in Router Advertisements (RAs) to
   communicate NAT64 prefixes to hosts.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   PREF64 (or NAT64 prefix):  An IPv6 prefix used for IPv6 address
      synthesis [RFC6146];

   NAT64:  Network Address and Protocol Translation from IPv6 clients to
      IPv4 servers [RFC6146];

   Router Advertisement (RA):  A message used by IPv6 routers to
      advertise their presence together with various link and Internet
      parameters [RFC4861];

   DNS64: a mechanism for synthesizing AAAA records from A records
   [RFC6147];

2.  Use Cases for Communicating the NAT64 Prefix to Hosts

   On networks employing NAT64, it is useful for hosts to know the NAT64
   prefix for several reasons, including the following:

   *  Enabling DNS64 functions on end hosts.  In particular:

      -  Local DNSSEC validation (DNS64 in stub-resolver mode).  As
         discussed in [RFC6147], Section 2, the stub resolver in the
         host "will try to obtain (real) AAAA RRs, and in case they are
         not available, the DNS64 function will synthesize AAAA RRs for
         internal usage."  Therefore, to perform the DNS64 function, the
         stub resolver needs to know the NAT64 prefix.  This is required
         in order to use DNSSEC on a NAT64 network.

      -  Trusted DNS server.  AAAA synthesis is required for the host to
         be able to use a DNS server not provided by the network (e.g.,
         a DNS-over-TLS [RFC7858] or DNS-over-HTTPS [RFC8484] server
         with which the host has an existing trust relationship).

      -  Networks with no DNS64 server.  Hosts that support AAAA
         synthesis and are aware of the NAT64 prefix in use do not need
         the network to perform the DNS64 function at all.

   *  Enabling NAT64 address-translation functions on end hosts.  For
      example:
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