Internet DRAFT - draft-ietf-ipngwg-dns-lookups


IPng Working Group                                         Matt Crawford
Internet Draft                                                  Fermilab
                                                       Christian Huitema
                                                           Susan Thomson
                                                            May 17, 2000

     DNS Extensions to Support IPv6 Address Aggregation and Renumbering

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC 2026.  Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF), its
    areas, and its working groups.  Note that other groups may also
    distribute working documents as Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet- Drafts as
    reference material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at

1.  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.

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Status of this Memo ...............................................    1

1.  Abstract ......................................................    1

2.  Introduction ..................................................    3

3.  Overview ......................................................    4
    3.1.  Name-to-Address Lookup ..................................    4
    3.2.  Underlying Mechanisms for Reverse Lookups ...............    4
        3.2.1.  Delegation on Arbitrary Boundaries ................    4
        3.2.2.  Reusable Zones ....................................    5

4.  Specifications ................................................    6
    4.1.  The A6 Record Type ......................................    6
        4.1.1.  Format ............................................    6
        4.1.2.  Processing ........................................    6
        4.1.3.  Textual Representation ............................    7
        4.1.4.  Name Resolution Procedure .........................    7
    4.2.  Zone Structure for Reverse Lookups ......................    8

5.  Modifications to Existing Query Types .........................    8

6.  Usage Illustrations ...........................................    9
    6.1.  A6 Record Chains ........................................    9
        6.1.1.  Authoritative Data ................................   10
        6.1.2.  Glue ..............................................   10
        6.1.3.  Variations ........................................   12
    6.2.  Reverse Mapping Zones ...................................   13
        6.2.1.  The TLA level .....................................   13
        6.2.2.  The ISP level .....................................   14
        6.2.3.  The Site Level ....................................   14
    6.3.  Lookups .................................................   14
    6.4.  Operational Note ........................................   15

7.  Transition from RFC 1886 and Deployment Notes .................   16
    7.1.  Transition from AAAA and Coexistence with A Records .....   17
    7.2.  Transition from Nibble Labels to Binary Labels ..........   17

8.  Security Considerations .......................................   18

9.  IANA Considerations ...........................................   18

10.  Acknowledgments ..............................................   18

11.  References ...................................................   19

12.  Authors' Addresses ...........................................   20

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2.  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 documents cited here as
    [RENUM].  To support the storage of IPv6 addresses without impeding
    renumbering we define the following extensions.

    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
    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",
    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.

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3.  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.

3.1.  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 multiples 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.

3.2.  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.

3.2.1.  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

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    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 6 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 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

    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.



3.2.2.  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.

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4.  Specifications

4.1.  The A6 Record Type

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

4.1.1.  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.

4.1.2.  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

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    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 a A6 record with prefix length L1 > 0 to refer to
    a domain name which owns a 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.

4.1.3.  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

    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

    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 4.1.1.

4.1.4.  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 4.1.2.

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    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.

4.2.  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
    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

5.  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

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    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.

6.  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

6.1.  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

    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:

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    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

6.1.1.  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.

    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::

6.1.2.  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

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    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.

    $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

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    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.

6.1.3.  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

    Finally, the above structure reflects an assumption that address
    fields assigned by a given entity are recorded only in A6 records

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    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.

6.2.  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].

6.2.1.  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.

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6.2.2.  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.

6.2.3.  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.

            $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.

6.3.  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):

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    To a server for IP6.ARPA:

         \[x234500/24].IP6.ARPA. DNAME IP6.ALPHA-TLA.ORG.

    To a server for IP6.ALPHA-TLA.ORG:

         \[xC/4].IP6.ALPHA-TLA.ORG. DNAME IP6.C.NET.

    To a server for IP6.C.NET.:

         \[x1CA/12].IP6.C.NET. DNAME IP6.A.NET.

    To a server for IP6.A.NET.:

         \[x11/8].IP6.A.NET. DNAME IP6.X.EXAMPLE.

    To a server for IP6.X.EXAMPLE.:

         \[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.

6.4.  Operational Note

    In the illustrations in section 6.1, 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

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    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.

7.  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 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.

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7.1.  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 4.1.4).  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.

7.2.  Transition from Nibble Labels to Binary Labels

    Implementations conforming to RFC 1886 perform reverse lookups as

        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

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        2345:00C1:CA11:0001:1234:5678:9ABC:DEF0 of the example in
        section 6.3 would be sought at the DNS name "0.f.e.d.c.b.a.9.-"

    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.

8.  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

    With or without DNSSEC, the incomplete but non-empty address set
    scenario of section 4.1.4 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.

9.  IANA Considerations

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

10.  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.

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11.  References

    [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. V., "Domain names - implementation and
        specification", 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," RFC 2119.

    [RENUM] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
        1900, February 1996.

        Ferguson, P. and H. Berkowitz, "Network Renumbering Overview:
        Why would I want it and what is it anyway?", RFC 2071, January

        Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
        Behaviour Today", RFC 2101, February 1997.

    [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

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        (TSIG)", work in progress.

12.  Authors' Addresses

    Matt Crawford          Christian Huitema      Susan Thomson
    Fermilab               Telcordia              Telcordia
    MS 368                 MCC 1J236B             MCC 1C259B
    PO Box 500             445 South Street       445 South Street
    Batavia, IL 60510      Morristown, NJ 07960   Morristown, NJ 07960
    USA                    USA                    USA

    +1 630 840-3461        +1 201 829-4266        +1 201 829-4514

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