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rfc:rfc2874

Network Working Group M. Crawford Request for Comments: 2874 Fermilab Category: Standards Track C. Huitema

                                                 Microsoft Corporation
                                                             July 2000
 DNS Extensions to Support IPv6 Address Aggregation and Renumbering

Status of this Memo

 This document 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" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

 This document defines changes to the Domain Name System to support
 renumberable and aggregatable IPv6 addressing.  The changes include a
 new resource record type to store an IPv6 address in a manner which
 expedites network renumbering and updated definitions of existing
 query types that return Internet addresses as part of additional
 section processing.
 For lookups keyed on IPv6 addresses (often called reverse lookups),
 this document defines a new zone structure which allows a zone to be
 used without modification for parallel copies of an address space (as
 for a multihomed provider or site) and across network renumbering
 events.

Crawford, et al. Standards Track [Page 1] RFC 2874 IPv6 DNS July 2000

Table of Contents

 1.  Introduction ...............................................  2
 2.  Overview ...................................................  3
     2.1.  Name-to-Address Lookup ...............................  4
     2.2.  Underlying Mechanisms for Reverse Lookups ............  4
         2.2.1.  Delegation on Arbitrary Boundaries .............  4
         2.2.2.  Reusable Zones .................................  5
 3.  Specifications .............................................  5
     3.1.  The A6 Record Type ...................................  5
         3.1.1.  Format .........................................  6
         3.1.2.  Processing .....................................  6
         3.1.3.  Textual Representation .........................  7
         3.1.4.  Name Resolution Procedure ......................  7
     3.2.  Zone Structure for Reverse Lookups ...................  7
 4.  Modifications to Existing Query Types ......................  8
 5.  Usage Illustrations ........................................  8
     5.1.  A6 Record Chains .....................................  9
         5.1.1.  Authoritative Data .............................  9
         5.1.2.  Glue ........................................... 10
         5.1.3.  Variations ..................................... 12
     5.2.  Reverse Mapping Zones ................................ 13
         5.2.1.  The TLA level .................................. 13
         5.2.2.  The ISP level .................................. 13
         5.2.3.  The Site Level ................................. 13
     5.3.  Lookups .............................................. 14
     5.4.  Operational Note ..................................... 15
 6.  Transition from RFC 1886 and Deployment Notes .............. 15
     6.1.  Transition from AAAA and Coexistence with A Records .. 16
     6.2.  Transition from Nibble Labels to Binary Labels ....... 17
 7.  Security Considerations .................................... 17
 8.  IANA Considerations ........................................ 17
 9.  Acknowledgments ............................................ 18
 10.  References ................................................ 18
 11.  Authors' Addresses ........................................ 19
 12.  Full Copyright Statement .................................. 20

1. Introduction

 Maintenance of address information in the DNS is one of several
 obstacles which have prevented site and provider renumbering from
 being feasible in IP version 4.  Arguments about the importance of
 network renumbering for the preservation of a stable routing system
 and for other purposes may be read in [RENUM1, RENUM2, RENUM3].  To
 support the storage of IPv6 addresses without impeding renumbering we
 define the following extensions.

Crawford, et al. Standards Track [Page 2] RFC 2874 IPv6 DNS July 2000

 o  A new resource record type, "A6", is defined to map a domain name
    to an IPv6 address, with a provision for indirection for leading
    "prefix" bits.
 o  Existing queries that perform additional section processing to
    locate IPv4 addresses are redefined to do that processing for both
    IPv4 and IPv6 addresses.
 o  A new domain, IP6.ARPA, is defined to support lookups based on
    IPv6 address.
 o  A new prefix-delegation method is defined, relying on new DNS
    features [BITLBL, DNAME].
 The changes are designed to be compatible with existing application
 programming interfaces.  The existing support for IPv4 addresses is
 retained.  Transition issues related to the coexistence of both IPv4
 and IPv6 addresses in DNS are discussed in [TRANS].
 This memo proposes a replacement for the specification in RFC 1886
 [AAAA] and a departure from current implementation practices.  The
 changes are designed to facilitate network renumbering and
 multihoming.  Domains employing the A6 record for IPv6 addresses can
 insert automatically-generated AAAA records in zone files to ease
 transition.  It is expected that after a reasonable period, RFC 1886
 will become Historic.
 The next three major sections of this document are an overview of the
 facilities defined or employed by this specification, the
 specification itself, and examples of use.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [KWORD].  The key word
 "SUGGESTED" signifies a strength between MAY and SHOULD: it is
 believed that compliance with the suggestion has tangible benefits in
 most instances.

2. Overview

 This section provides an overview of the DNS facilities for storage
 of IPv6 addresses and for lookups based on IPv6 address, including
 those defined here and elsewhere.

Crawford, et al. Standards Track [Page 3] RFC 2874 IPv6 DNS July 2000

2.1. Name-to-Address Lookup

 IPv6 addresses are stored in one or more A6 resource records.  A
 single A6 record may include a complete IPv6 address, or a contiguous
 portion of an address and information leading to one or more
 prefixes.  Prefix information comprises a prefix length and a DNS
 name which is in turn the owner of one or more A6 records defining
 the prefix or prefixes which are needed to form one or more complete
 IPv6 addresses.  When the prefix length is zero, no DNS name is
 present and all the leading bits of the address are significant.
 There may be multiple levels of indirection and the existence of
 multiple A6 records at any level multiplies the number of IPv6
 addresses which are formed.
 An application looking up an IPv6 address will generally cause the
 DNS resolver to access several A6 records, and multiple IPv6
 addresses may be returned even if the queried name was the owner of
 only one A6 record.  The authenticity of the returned address(es)
 cannot be directly verified by DNS Security [DNSSEC].  The A6 records
 which contributed to the address(es) may of course be verified if
 signed.
 Implementers are reminded of the necessity to limit the amount of
 work a resolver will perform in response to a client request.  This
 principle MUST be extended to also limit the generation of DNS
 requests in response to one name-to-address (or address-to-name)
 lookup request.

2.2. Underlying Mechanisms for Reverse Lookups

 This section describes the new DNS features which this document
 exploits.  This section is an overview, not a specification of those
 features.  The reader is directed to the referenced documents for
 more details on each.

2.2.1. Delegation on Arbitrary Boundaries

 This new scheme for reverse lookups relies on a new type of DNS label
 called the "bit-string label" [BITLBL].  This label compactly
 represents an arbitrary string of bits which is treated as a
 hierarchical sequence of one-bit domain labels.  Resource records can
 thereby be stored at arbitrary bit-boundaries.
 Examples in section 5 will employ the following textual
 representation for bit-string labels, which is a subset of the syntax
 defined in [BITLBL].  A base indicator "x" for hexadecimal and a
 sequence of hexadecimal digits is enclosed between "\[" and "]".  The
 bits denoted by the digits represent a sequence of one-bit domain

Crawford, et al. Standards Track [Page 4] RFC 2874 IPv6 DNS July 2000

 labels ordered from most to least significant.  (This is the opposite
 of the order they would appear if listed one bit at a time, but it
 appears to be a convenient notation.)  The digit string may be
 followed by a slash ("/") and a decimal count.  If omitted, the
 implicit count is equal to four times the number of hexadecimal
 digits.
 Consecutive bit-string labels are equivalent (up to the limit imposed
 by the size of the bit count field) to a single bit-string label
 containing all the bits of the consecutive labels in the proper
 order.  As an example, either of the following domain names could be
 used in a QCLASS=IN, QTYPE=PTR query to find the name of the node
 with IPv6 address 3ffe:7c0:40:9:a00:20ff:fe81:2b32.
  \[x3FFE07C0004000090A0020FFFE812B32/128].IP6.ARPA.
  \[x0A0020FFFE812B32/64].\[x0009/16].\[x3FFE07C00040/48].IP6.ARPA.

2.2.2. Reusable Zones

 DNS address space delegation is implemented not by zone cuts and NS
 records, but by a new analogue to the CNAME record, called the DNAME
 resource record [DNAME].  The DNAME record provides alternate naming
 to an entire subtree of the domain name space, rather than to a
 single node.  It causes some suffix of a queried name to be
 substituted with a name from the DNAME record's RDATA.
 For example, a resolver or server providing recursion, while looking
 up a QNAME a.b.c.d.e.f may encounter a DNAME record
                      d.e.f.     DNAME     w.xy.
 which will cause it to look for a.b.c.w.xy.

3. Specifications

3.1. The A6 Record Type

 The A6 record type is specific to the IN (Internet) class and has
 type number 38 (decimal).

Crawford, et al. Standards Track [Page 5] RFC 2874 IPv6 DNS July 2000

3.1.1. Format

 The RDATA portion of the A6 record contains two or three fields.
         +-----------+------------------+-------------------+
         |Prefix len.|  Address suffix  |    Prefix name    |
         | (1 octet) |  (0..16 octets)  |  (0..255 octets)  |
         +-----------+------------------+-------------------+
 o  A prefix length, encoded as an eight-bit unsigned integer with
    value between 0 and 128 inclusive.
 o  An IPv6 address suffix, encoded in network order (high-order octet
    first).  There MUST be exactly enough octets in this field to
    contain a number of bits equal to 128 minus prefix length, with 0
    to 7 leading pad bits to make this field an integral number of
    octets.  Pad bits, if present, MUST be set to zero when loading a
    zone file and ignored (other than for SIG [DNSSEC] verification)
    on reception.
 o  The name of the prefix, encoded as a domain name.  By the rules of
    [DNSIS], this name MUST NOT be compressed.
 The domain name component SHALL NOT be present if the prefix length
 is zero.  The address suffix component SHALL NOT be present if the
 prefix length is 128.
 It is SUGGESTED that an A6 record intended for use as a prefix for
 other A6 records have all the insignificant trailing bits in its
 address suffix field set to zero.

3.1.2. Processing

 A query with QTYPE=A6 causes type A6 and type NS additional section
 processing for the prefix names, if any, in the RDATA field of the A6
 records in the answer section.  This processing SHOULD be recursively
 applied to the prefix names of A6 records included as additional
 data.  When space in the reply packet is a limit, inclusion of
 additional A6 records takes priority over NS records.
 It is an error for an A6 record with prefix length L1 > 0 to refer to
 a domain name which owns an A6 record with a prefix length L2 > L1.
 If such a situation is encountered by a resolver, the A6 record with
 the offending (larger) prefix length MUST be ignored.  Robustness
 precludes signaling an error if addresses can still be formed from
 valid A6 records, but it is SUGGESTED that zone maintainers from time
 to time check all the A6 records their zones reference.

Crawford, et al. Standards Track [Page 6] RFC 2874 IPv6 DNS July 2000

3.1.3. Textual Representation

 The textual representation of the RDATA portion of the A6 resource
 record in a zone file comprises two or three fields separated by
 whitespace.
 o  A prefix length, represented as a decimal number between 0 and 128
    inclusive,
 o  the textual representation of an IPv6 address as defined in
    [AARCH] (although some leading and/or trailing bits may not be
    significant),
 o  a domain name, if the prefix length is not zero.
 The domain name MUST be absent if the prefix length is zero.  The
 IPv6 address MAY be be absent if the prefix length is 128.  A number
 of leading address bits equal to the prefix length SHOULD be zero,
 either implicitly (through the :: notation) or explicitly, as
 specified in section 3.1.1.

3.1.4. Name Resolution Procedure

 To obtain the IPv6 address or addresses which belong to a given name,
 a DNS client MUST obtain one or more complete chains of A6 records,
 each chain beginning with a record owned by the given name and
 including a record owned by the prefix name in that record, and so on
 recursively, ending with an A6 record with a prefix length of zero.
 One IPv6 address is formed from one such chain by taking the value of
 each bit position from the earliest A6 record in the chain which
 validly covers that position, as indicated by the prefix length.  The
 set of all IPv6 addresses for the given name comprises the addresses
 formed from all complete chains of A6 records beginning at that name,
 discarding records which have invalid prefix lengths as defined in
 section 3.1.2.
 If some A6 queries fail and others succeed, a client might obtain a
 non-empty but incomplete set of IPv6 addresses for a host.  In many
 situations this may be acceptable.  The completeness of a set of A6
 records may always be determined by inspection.

3.2. Zone Structure for Reverse Lookups

 Very little of the new scheme's data actually appears under IP6.ARPA;
 only the first level of delegation needs to be under that domain.
 More levels of delegation could be placed under IP6.ARPA if some
 top-level delegations were done via NS records instead of DNAME
 records, but this would incur some cost in renumbering ease at the

Crawford, et al. Standards Track [Page 7] RFC 2874 IPv6 DNS July 2000

 level of TLAs [AGGR].  Therefore, it is declared here that all
 address space delegations SHOULD be done by the DNAME mechanism
 rather than NS.
 In addition, since uniformity in deployment will simplify maintenance
 of address delegations, it is SUGGESTED that address and prefix
 information be stored immediately below a DNS label "IP6".  Stated
 another way, conformance with this suggestion would mean that "IP6"
 is the first label in the RDATA field of DNAME records which support
 IPv6 reverse lookups.
 When any "reserved" or "must be zero" bits are adjacent to a
 delegation boundary, the higher-level entity MUST retain those bits
 in its own control and delegate only the bits over which the lower-
 level entity has authority.
 To find the name of a node given its IPv6 address, a DNS client MUST
 perform a query with QCLASS=IN, QTYPE=PTR on the name formed from the
 128 bit address as one or more bit-string labels [BITLBL], followed
 by the two standard labels "IP6.ARPA".  If recursive service was not
 obtained from a server and the desired PTR record was not returned,
 the resolver MUST handle returned DNAME records as specified in
 [DNAME], and NS records as specified in [DNSCF], and iterate.

4. Modifications to Existing Query Types

 All existing query types that perform type A additional section
 processing, i.e. the name server (NS), mail exchange (MX), and
 mailbox (MB) query types, and the experimental AFS data base (AFSDB)
 and route through (RT) types, must be redefined to perform type A, A6
 and AAAA additional section processing, with type A having the
 highest priority for inclusion and type AAAA the lowest.  This
 redefinition means that a name server may add any relevant IPv4 and
 IPv6 address information available locally to the additional section
 of a response when processing any one of the above queries.  The
 recursive inclusion of A6 records referenced by A6 records already
 included in the additional section is OPTIONAL.

5. Usage Illustrations

 This section provides examples of use of the mechanisms defined in
 the previous section.  All addresses and domains mentioned here are
 intended to be fictitious and for illustrative purposes only.
 Example delegations will be on 4-bit boundaries solely for
 readability; this specification is indifferent to bit alignment.
 Use of the IPv6 aggregatable address format [AGGR] is assumed in the
 examples.

Crawford, et al. Standards Track [Page 8] RFC 2874 IPv6 DNS July 2000

5.1. A6 Record Chains

 Let's take the example of a site X that is multi-homed to two
 "intermediate" providers A and B.  The provider A is itself multi-
 homed to two "transit" providers, C and D.  The provider B gets its
 transit service from a single provider, E.  For simplicity suppose
 that C, D and E all belong to the same top-level aggregate (TLA) with
 identifier (including format prefix) '2345', and the TLA authority at
 ALPHA-TLA.ORG assigns to C, D and E respectively the next level
 aggregate (NLA) prefixes 2345:00C0::/28, 2345:00D0::/28 and
 2345:000E::/32.
 C assigns the NLA prefix 2345:00C1:CA00::/40 to A, D assigns the
 prefix 2345:00D2:DA00::/40 to A and E assigns 2345:000E:EB00::/40 to
 B.
 A assigns to X the subscriber identification '11' and B assigns the
 subscriber identification '22'.  As a result, the site X inherits
 three address prefixes:
 o  2345:00C1:CA11::/48 from A, for routes through C.
 o  2345:00D2:DA11::/48 from A, for routes through D.
 o  2345:000E:EB22::/48 from B, for routes through E.
 Let us suppose that N is a node in the site X, that it is assigned to
 subnet number 1 in this site, and that it uses the interface
 identifier '1234:5678:9ABC:DEF0'.  In our configuration, this node
 will have three addresses:
 o  2345:00C1:CA11:0001:1234:5678:9ABC:DEF0
 o  2345:00D2:DA11:0001:1234:5678:9ABC:DEF0
 o  2345:000E:EB22:0001:1234:5678:9ABC:DEF0

5.1.1. Authoritative Data

 We will assume that the site X is represented in the DNS by the
 domain name X.EXAMPLE, while A, B, C, D and E are represented by
 A.NET, B.NET, C.NET, D.NET and E.NET.  In each of these domains, we
 assume a subdomain "IP6" that will hold the corresponding prefixes.
 The node N is identified by the domain name N.X.EXAMPLE.  The
 following records would then appear in X's DNS.
        $ORIGIN X.EXAMPLE.
        N            A6 64 ::1234:5678:9ABC:DEF0 SUBNET-1.IP6
        SUBNET-1.IP6 A6 48 0:0:0:1::  IP6
        IP6          A6 48 0::0       SUBSCRIBER-X.IP6.A.NET.
        IP6          A6 48 0::0       SUBSCRIBER-X.IP6.B.NET.

Crawford, et al. Standards Track [Page 9] RFC 2874 IPv6 DNS July 2000

 And elsewhere there would appear
      SUBSCRIBER-X.IP6.A.NET. A6 40 0:0:0011:: A.NET.IP6.C.NET.
      SUBSCRIBER-X.IP6.A.NET. A6 40 0:0:0011:: A.NET.IP6.D.NET.
      SUBSCRIBER-X.IP6.B.NET. A6 40 0:0:0022:: B-NET.IP6.E.NET.
      A.NET.IP6.C.NET. A6 28 0:0001:CA00:: C.NET.ALPHA-TLA.ORG.
      A.NET.IP6.D.NET. A6 28 0:0002:DA00:: D.NET.ALPHA-TLA.ORG.
      B-NET.IP6.E.NET. A6 32 0:0:EB00::    E.NET.ALPHA-TLA.ORG.
      C.NET.ALPHA-TLA.ORG. A6 0 2345:00C0::
      D.NET.ALPHA-TLA.ORG. A6 0 2345:00D0::
      E.NET.ALPHA-TLA.ORG. A6 0 2345:000E::

5.1.2. Glue

 When, as is common, some or all DNS servers for X.EXAMPLE are within
 the X.EXAMPLE zone itself, the top-level zone EXAMPLE must carry
 enough "glue" information to enable DNS clients to reach those
 nameservers.  This is true in IPv6 just as in IPv4.  However, the A6
 record affords the DNS administrator some choices.  The glue could be
 any of
 o  a minimal set of A6 records duplicated from the X.EXAMPLE zone,
 o  a (possibly smaller) set of records which collapse the structure
    of that minimal set,
 o  or a set of A6 records with prefix length zero, giving the entire
    global addresses of the servers.
 The trade-off is ease of maintenance against robustness.  The best
 and worst of both may be had together by implementing either the
 first or second option together with the third.  To illustrate the
 glue options, suppose that X.EXAMPLE is served by two nameservers
 NS1.X.EXAMPLE and NS2.X.EXAMPLE, having interface identifiers
 ::1:11:111:1111 and ::2:22:222:2222 on subnets 1 and 2 respectively.
 Then the top-level zone EXAMPLE would include one (or more) of the
 following sets of A6 records as glue.

Crawford, et al. Standards Track [Page 10] RFC 2874 IPv6 DNS July 2000

 $ORIGIN EXAMPLE.            ; first option
 X               NS NS1.X
                 NS NS2.X
 NS1.X           A6 64 ::1:11:111:1111 SUBNET-1.IP6.X
 NS2.X           A6 64 ::2:22:222:2222 SUBNET-2.IP6.X
 SUBNET-1.IP6.X  A6 48 0:0:0:1::       IP6.X
 SUBNET-2.IP6.X  A6 48 0:0:0:2::       IP6.X
 IP6.X           A6 48 0::0            SUBSCRIBER-X.IP6.A.NET.
 IP6.X           A6 48 0::0            SUBSCRIBER-X.IP6.B.NET.
 $ORIGIN EXAMPLE.            ; second option
 X               NS NS1.X
                 NS NS2.X
 NS1.X           A6 48 ::1:1:11:111:1111 SUBSCRIBER-X.IP6.A.NET.
                 A6 48 ::1:1:11:111:1111 SUBSCRIBER-X.IP6.B.NET.
 NS2.X           A6 48 ::2:2:22:222:2222 SUBSCRIBER-X.IP6.A.NET.
                 A6 48 ::2:2:22:222:2222 SUBSCRIBER-X.IP6.B.NET.
 $ORIGIN EXAMPLE.            ; third option
 X               NS NS1.X
                 NS NS2.X
 NS1.X           A6 0  2345:00C1:CA11:1:1:11:111:1111
                 A6 0  2345:00D2:DA11:1:1:11:111:1111
                 A6 0  2345:000E:EB22:1:1:11:111:1111
 NS2.X           A6 0  2345:00C1:CA11:2:2:22:222:2222
                 A6 0  2345:00D2:DA11:2:2:22:222:2222
                 A6 0  2345:000E:EB22:2:2:22:222:2222
 The first and second glue options are robust against renumbering of
 X.EXAMPLE's prefixes by providers A.NET and B.NET, but will fail if
 those providers' own DNS is unreachable.  The glue records of the
 third option are robust against DNS failures elsewhere than the zones
 EXAMPLE and X.EXAMPLE themselves, but must be updated when X's
 address space is renumbered.
 If the EXAMPLE zone includes redundant glue, for instance the union
 of the A6 records of the first and third options, then under normal
 circumstances duplicate IPv6 addresses will be derived by DNS
 clients.  But if provider DNS fails, addresses will still be obtained
 from the zero-prefix-length records, while if the EXAMPLE zone lags
 behind a renumbering of X.EXAMPLE, half of the addresses obtained by
 DNS clients will still be up-to-date.
 The zero-prefix-length glue records can of course be automatically
 generated and/or checked in practice.

Crawford, et al. Standards Track [Page 11] RFC 2874 IPv6 DNS July 2000

5.1.3. Variations

 Several more-or-less arbitrary assumptions are reflected in the above
 structure.  All of the following choices could have been made
 differently, according to someone's notion of convenience or an
 agreement between two parties.
    First, that site X has chosen to put subnet information in a
    separate A6 record rather than incorporate it into each node's A6
    records.
    Second, that site X is referred to as "SUBSCRIBER-X" by both of
    its providers A and B.
    Third, that site X chose to indirect its provider information
    through A6 records at IP6.X.EXAMPLE containing no significant
    bits.  An alternative would have been to replicate each subnet
    record for each provider.
    Fourth, B and E used a slightly different prefix naming convention
    between themselves than did A, C and D.  Each hierarchical pair of
    network entities must arrange this naming between themselves.
    Fifth, that the upward prefix referral chain topped out at ALPHA-
    TLA.ORG.  There could have been another level which assigned the
    TLA values and holds A6 records containing those bits.
 Finally, the above structure reflects an assumption that address
 fields assigned by a given entity are recorded only in A6 records
 held by that entity.  Those bits could be entered into A6 records in
 the lower-level entity's zone instead, thus:
              IP6.X.EXAMPLE. A6 40 0:0:11::   IP6.A.NET.
              IP6.X.EXAMPLE. A6 40 0:0:22::   IP6.B.NET.
              IP6.A.NET.     A6 28 0:1:CA00:: IP6.C.NET.
              and so on.
 Or the higher-level entities could hold both sorts of A6 records
 (with different DNS owner names) and allow the lower-level entities
 to choose either mode of A6 chaining.  But the general principle of
 avoiding data duplication suggests that the proper place to store
 assigned values is with the entity that assigned them.
 It is possible, but not necessarily recommended, for a zone
 maintainer to forego the renumbering support afforded by the chaining
 of A6 records and to record entire IPv6 addresses within one zone
 file.

Crawford, et al. Standards Track [Page 12] RFC 2874 IPv6 DNS July 2000

5.2. Reverse Mapping Zones

 Supposing that address space assignments in the TLAs with Format
 Prefix (001) binary and IDs 0345, 0678 and 09AB were maintained in
 zones called ALPHA-TLA.ORG, BRAVO-TLA.ORG and CHARLIE-TLA.XY, then
 the IP6.ARPA zone would include
             $ORIGIN IP6.ARPA.
             \[x234500/24]   DNAME   IP6.ALPHA-TLA.ORG.
             \[x267800/24]   DNAME   IP6.BRAVO-TLA.ORG.
             \[x29AB00/24]   DNAME   IP6.CHARLIE-TLA.XY.
 Eight trailing zero bits have been included in each TLA ID to reflect
 the eight reserved bits in the current aggregatable global unicast
 addresses format [AGGR].

5.2.1. The TLA level

 ALPHA-TLA's assignments to network providers C, D and E are reflected
 in the reverse data as follows.
            \[xC/4].IP6.ALPHA-TLA.ORG.   DNAME  IP6.C.NET.
            \[xD/4].IP6.ALPHA-TLA.ORG.   DNAME  IP6.D.NET.
            \[x0E/8].IP6.ALPHA-TLA.ORG.  DNAME  IP6.E.NET.

5.2.2. The ISP level

 The providers A through E carry the following delegation information
 in their zone files.
             \[x1CA/12].IP6.C.NET.  DNAME  IP6.A.NET.
             \[x2DA/12].IP6.D.NET.  DNAME  IP6.A.NET.
             \[xEB/8].IP6.E.NET.    DNAME  IP6.B.NET.
             \[x11/8].IP6.A.NET.    DNAME  IP6.X.EXAMPLE.
             \[x22/8].IP6.B.NET.    DNAME  IP6.X.EXAMPLE.
 Note that some domain names appear in the RDATA of more than one
 DNAME record.  In those cases, one zone is being used to map multiple
 prefixes.

5.2.3. The Site Level

 Consider the customer X.EXAMPLE using IP6.X.EXAMPLE for address-to-
 name translations.  This domain is now referenced by two different
 DNAME records held by two different providers.

Crawford, et al. Standards Track [Page 13] RFC 2874 IPv6 DNS July 2000

         $ORIGIN IP6.X.EXAMPLE.
         \[x0001/16]                    DNAME   SUBNET-1
         \[x123456789ABCDEF0].SUBNET-1  PTR     N.X.EXAMPLE.
         and so on.
 SUBNET-1 need not have been named in a DNAME record; the subnet bits
 could have been joined with the interface identifier.  But if subnets
 are treated alike in both the A6 records and in the reverse zone, it
 will always be possible to keep the forward and reverse definition
 data for each prefix in one zone.

5.3. Lookups

 A DNS resolver looking for a hostname for the address
 2345:00C1:CA11:0001:1234:5678:9ABC:DEF0 would acquire certain of the
 DNAME records shown above and would form new queries.  Assuming that
 it began the process knowing servers for IP6.ARPA, but that no server
 it consulted provided recursion and none had other useful additional
 information cached, the sequence of queried names and responses would
 be (all with QCLASS=IN, QTYPE=PTR):
 To a server for IP6.ARPA:
 QNAME=\[x234500C1CA110001123456789ABCDEF0/128].IP6.ARPA.
      Answer:
      \[x234500/24].IP6.ARPA. DNAME IP6.ALPHA-TLA.ORG.
 To a server for IP6.ALPHA-TLA.ORG:
 QNAME=\[xC1CA110001123456789ABCDEF0/104].IP6.ALPHA-TLA.ORG.
      Answer:
      \[xC/4].IP6.ALPHA-TLA.ORG. DNAME IP6.C.NET.
 To a server for IP6.C.NET.:
 QNAME=\[x1CA110001123456789ABCDEF0/100].IP6.C.NET.
      Answer:
      \[x1CA/12].IP6.C.NET. DNAME IP6.A.NET.
 To a server for IP6.A.NET.:
 QNAME=\[x110001123456789ABCDEF0/88].IP6.A.NET.
      Answer:
      \[x11/8].IP6.A.NET. DNAME IP6.X.EXAMPLE.
 To a server for IP6.X.EXAMPLE.:
 QNAME=\[x0001123456789ABCDEF0/80].IP6.X.EXAMPLE.

Crawford, et al. Standards Track [Page 14] RFC 2874 IPv6 DNS July 2000

      Answer:
      \[x0001/16].IP6.X.EXAMPLE. DNAME SUBNET-1.IP6.X.EXAMPLE.
      \[x123456789ABCDEF0/64].SUBNET-1.X.EXAMPLE. PTR N.X.EXAMPLE.
 All the DNAME (and NS) records acquired along the way can be cached
 to expedite resolution of addresses topologically near to this
 address.  And if another global address of N.X.EXAMPLE were resolved
 within the TTL of the final PTR record, that record would not have to
 be fetched again.

5.4. Operational Note

 In the illustrations in section 5.1, hierarchically adjacent
 entities, such as a network provider and a customer, must agree on a
 DNS name which will own the definition of the delegated prefix(es).
 One simple convention would be to use a bit-string label representing
 exactly the bits which are assigned to the lower-level entity by the
 higher.  For example, "SUBSCRIBER-X" could be replaced by "\[x11/8]".
 This would place the A6 record(s) defining the delegated prefix at
 exactly the same point in the DNS tree as the DNAME record associated
 with that delegation.  The cost of this simplification is that the
 lower-level zone must update its upward-pointing A6 records when it
 is renumbered.  This cost may be found quite acceptable in practice.

6. Transition from RFC 1886 and Deployment Notes

 When prefixes have been "delegated upward" with A6 records, the
 number of DNS resource records required to establish a single IPv6
 address increases by some non-trivial factor.  Those records will
 typically, but not necessarily, come from different DNS zones (which
 can independently suffer failures for all the usual reasons).  When
 obtaining multiple IPv6 addresses together, this increase in RR count
 will be proportionally less -- and the total size of a DNS reply
 might even decrease -- if the addresses are topologically clustered.
 But the records could still easily exceed the space available in a
 UDP response which returns a large RRset [DNSCLAR] to an MX, NS, or
 SRV query, for example.  The possibilities for overall degradation of
 performance and reliability of DNS lookups are numerous, and increase
 with the number of prefix delegations involved, especially when those
 delegations point to records in other zones.
 DNS Security [DNSSEC] addresses the trustworthiness of cached data,
 which is a problem intrinsic to DNS, but the cost of applying this to
 an IPv6 address is multiplied by a factor which may be greater than
 the number of prefix delegations involved if different signature
 chains must be verified for different A6 records.  If a trusted
 centralized caching server (as in [TSIG], for example) is used, this
 cost might be amortized to acceptable levels.  One new phenomenon is

Crawford, et al. Standards Track [Page 15] RFC 2874 IPv6 DNS July 2000

 the possibility that IPv6 addresses may be formed from a A6 records
 from a combination of secure and unsecured zones.
 Until more deployment experience is gained with the A6 record, it is
 recommended that prefix delegations be limited to one or two levels.
 A reasonable phasing-in mechanism would be to start with no prefix
 delegations (all A6 records having prefix length 0) and then to move
 to the use of a single level of delegation within a single zone.  (If
 the TTL of the "prefix" A6 records is kept to an appropriate duration
 the capability for rapid renumbering is not lost.)  More aggressively
 flexible delegation could be introduced for a subset of hosts for
 experimentation.

6.1. Transition from AAAA and Coexistence with A Records

 Administrators of zones which contain A6 records can easily
 accommodate deployed resolvers which understand AAAA records but not
 A6 records.  Such administrators can do automatic generation of AAAA
 records for all of a zone's names which own A6 records by a process
 which mimics the resolution of a hostname to an IPv6 address (see
 section 3.1.4).  Attention must be paid to the TTL assigned to a
 generated AAAA record, which MUST be no more than the minimum of the
 TTLs of the A6 records that were used to form the IPv6 address in
 that record.  For full robustness, those A6 records which were in
 different zones should be monitored for changes (in TTL or RDATA)
 even when there are no changes to zone for which AAAA records are
 being generated.  If the zone is secure [DNSSEC], the generated AAAA
 records MUST be signed along with the rest of the zone data.
 A zone-specific heuristic MAY be used to avoid generation of AAAA
 records for A6 records which record prefixes, although such
 superfluous records would be relatively few in number and harmless.
 Examples of such heuristics include omitting A6 records with a prefix
 length less than the largest value found in the zone file, or records
 with an address suffix field with a certain number of trailing zero
 bits.
 On the client side, when looking up and IPv6 address, the order of A6
 and AAAA queries MAY be configurable to be one of: A6, then AAAA;
 AAAA, then A6; A6 only; or both in parallel.  The default order (or
 only order, if not configurable) MUST be to try A6 first, then AAAA.
 If and when the AAAA becomes deprecated a new document will change
 the default.
 The guidelines and options for precedence between IPv4 and IPv6
 addresses are specified in [TRANS].  All mentions of AAAA records in
 that document are henceforth to be interpreted as meaning A6 and/or
 AAAA records in the order specified in the previous paragraph.

Crawford, et al. Standards Track [Page 16] RFC 2874 IPv6 DNS July 2000

6.2. Transition from Nibble Labels to Binary Labels

 Implementations conforming to RFC 1886 [AAAA] perform reverse lookups
 as follows:
    An IPv6 address is represented as a name in the IP6.INT domain by
    a sequence of nibbles separated by dots with the suffix
    ".IP6.INT". The sequence of nibbles is encoded in reverse order,
    i.e. the low-order nibble is encoded first, followed by the next
    low-order nibble and so on. Each nibble is represented by a
    hexadecimal digit. For example, a name for the address
    2345:00C1:CA11:0001:1234:5678:9ABC:DEF0 of the example in section
    5.3 would be sought at the DNS name "0.f.e.d.c.b.a.9.-
    8.7.6.5.4.3.2.1.1.0.0.0.1.1.a.c.1.c.0.0.5.4.3.2.ip6.int."
 Implementations conforming to this specification will perform a
 lookup of a binary label in IP6.ARPA as specified in Section 3.2.  It
 is RECOMMENDED that for a transition period implementations first
 lookup the binary label in IP6.ARPA and if this fails try to lookup
 the 'nibble' label in IP6.INT.

7. Security Considerations

 The signing authority [DNSSEC] for the A6 records which determine an
 IPv6 address is distributed among several entities, reflecting the
 delegation path of the address space which that address occupies.
 DNS Security is fully applicable to bit-string labels and DNAME
 records.  And just as in IPv4, verification of name-to-address
 mappings is logically independent of verification of address-to-name
 mappings.
 With or without DNSSEC, the incomplete but non-empty address set
 scenario of section 3.1.4 could be caused by selective interference
 with DNS lookups.  If in some situation this would be more harmful
 than complete DNS failure, it might be mitigated on the client side
 by refusing to act on an incomplete set, or on the server side by
 listing all addresses in A6 records with prefix length 0.

8. IANA Considerations

 The A6 resource record has been assigned a Type value of 38.

Crawford, et al. Standards Track [Page 17] RFC 2874 IPv6 DNS July 2000

9. Acknowledgments

 The authors would like to thank the following persons for valuable
 discussions and reviews:  Mark Andrews, Rob Austein, Jim Bound, Randy
 Bush, Brian Carpenter, David Conrad, Steve Deering, Francis Dupont,
 Robert Elz, Bob Fink, Olafur Gudmundsson, Bob Halley, Bob Hinden,
 Edward Lewis, Bill Manning, Keith Moore, Thomas Narten, Erik
 Nordmark, Mike O'Dell, Michael Patton and Ken Powell.

10. References

 [AAAA]    Thomson, S. and C. Huitema, "DNS Extensions to support IP
           version 6, RFC 1886, December 1995.
 [AARCH]   Hinden, R. and S. Deering, "IP Version 6 Addressing
           Architecture", RFC 2373, July 1998.
 [AGGR]    Hinden, R., O'Dell, M. and S. Deering, "An IPv6
           Aggregatable Global Unicast Address Format", RFC 2374, July
           1998.
 [BITLBL]  Crawford, M., "Binary Labels in the Domain Name System",
           RFC 2673, August 1999.
 [DNAME]   Crawford, M., "Non-Terminal DNS Name Redirection", RFC
           2672, August 1999.
 [DNSCLAR] Elz, R. and R. Bush, "Clarifications to the DNS
           Specification", RFC 2181, July 1997.
 [DNSIS]   Mockapetris, P., "Domain names - implementation and
           specification", STD 13, RFC 1035, November 1987.
 [DNSSEC]  Eastlake, D. 3rd and C. Kaufman, "Domain Name System
           Security Extensions", RFC 2535, March 1999.
 [KWORD]   Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RENUM1]  Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
           1900, February 1996.
 [RENUM2]  Ferguson, P. and H. Berkowitz, "Network Renumbering
           Overview:  Why would I want it and what is it anyway?", RFC
           2071, January 1997.
 [RENUM3]  Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
           Behaviour Today", RFC 2101, February 1997.

Crawford, et al. Standards Track [Page 18] RFC 2874 IPv6 DNS July 2000

 [TRANS]   Gilligan, R. and E. Nordmark, "Transition Mechanisms for
           IPv6 Hosts and Routers", RFC 1933, April 1996.
 [TSIG]    Vixie, P., Gudmundsson, O., Eastlake, D. 3rd and B.
           Wellington, "Secret Key Transaction Authentication for DNS
           (TSIG)", RFC 2845, May 2000.

11. Authors' Addresses

 Matt Crawford
 Fermilab
 MS 368
 PO Box 500
 Batavia, IL 60510
 USA
 Phone: +1 630 840-3461
 EMail: crawdad@fnal.gov
 Christian Huitema
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052-6399
 EMail: huitema@microsoft.com

Crawford, et al. Standards Track [Page 19] RFC 2874 IPv6 DNS July 2000

12. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Crawford, et al. Standards Track [Page 20]

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