DNSIND Working Group                                       Matt Crawford
Internet Draft                                                  Fermilab
                                                             May 5, 1999

                  Binary Labels in the Domain Name System

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026. Internet-Drafts are working
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1.  Introduction and Terminology

    This document defines a ``Bit-String Label'' which may appear within
    domain names.  This new label type compactly represents a sequence
    of ``One-Bit Labels'' and enables resource records to be stored at
    any bit-boundary in a binary-named section of the domain name tree.

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in [KWORD].

2.  Motivation

    Binary labels are intended to efficiently solve the problem of
    storing data and delegating authority on arbitrary boundaries when
    the structure of underlying name space is most naturally represented
    in binary.

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3.  Label Format

    Up to 256 One-Bit Labels can be grouped into a single Bit-String
    Label.  Within a Bit-String Label the most significant or "highest
    level" bit appears first.  This is unlike the ordering of DNS labels
    themselves, which has the least significant or "lowest level" label
    first.  Nonetheless, this ordering seems to be the most natural and
    efficient for representing binary labels.

    Among consecutive Bit-String Labels, the bits in the first-appearing
    label are less significant or "at a lower level" than the bits in
    subsequent Bit-String Labels, just as ASCII labels are ordered.

3.1.  Encoding

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2     . . .
      |0 1|    ELT    |     Count     |           Label ...         |

    (Each tic mark represents one bit.)

    ELT       000001 binary, the six-bit extended label type [EDNS0]
              assigned to the Bit-String Label.

    Count     The number of significant bits in the Label field.  A
              Count value of zero indicates that 256 bits are
              significant.  (Thus the null label representing the DNS
              root cannot be represented as a Bit String Label.)

    Label     The bit string representing a sequence of One-Bit Labels,
              with the most significant bit first.  That is, the One-Bit
              Label in position 17 in the diagram above represents a
              subdomain of the domain represented by the One-Bit Label
              in position 16, and so on.

              The Label field is padded on the right with zero to seven
              pad bits to make the entire field occupy an integral
              number of octets.  These pad bits MUST be zero on
              transmission and ignored on reception.

    A sequence of bits may be split into two or more Bit-String Labels,
    but the division points have no significance and need not be
    preserved.  An excessively clever server implementation might split

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    Bit-String Labels so as to maximize the effectiveness of message
    compression [DNSIS].  A simpler server might divide Bit-String
    Labels at zone boundaries, if any zone boundaries happen to fall
    between One-Bit Labels.

3.2.  Textual Representation

    A Bit-String Label is represented in text -- in a zone file, for
    example -- as a <bit-spec> surrounded by the delimiters "\[" and
    "]".  The <bit-spec> is either a dotted quad or a base indicator and
    a sequence of digits appropriate to that base, optionally followed
    by a slash and a length.  The base indicators are "b", "o" and "x",
    denoting base 2, 8 and 16 respectively.  The length counts the
    significant bits and MUST be between 1 and 32, inclusive, after a
    dotted quad, or between 1 and 256, inclusive, after one of the other
    forms.  If the length is omitted, the implicit length is 32 for a
    dotted quad or 1, 3 or 4 times the number of binary, octal or
    hexadecimal digits supplied, respectively, for the other forms.

    In augmented Backus-Naur form [ABNF],

      bit-string-label =  "\[" bit-spec "]"

      bit-spec         =  bit-data [ "/" length ]
                        / dotted-quad [ "/" slength ]

      bit-data         =  "x" 1*64HEXDIG
                        / "o" 1*86OCTDIG
                        / "b" 1*256BIT

      dotted-quad      =  decbyte "." decbyte "." decbyte "." decbyte

      decbyte          =  1*3DIGIT

      length           =  NZDIGIT *2DIGIT

      slength          =  NZDIGIT [ DIGIT ]

      OCTDIG           =  %x30-37

      NZDIGIT          =  %x31-39

    If a <length> is present, the number of digits in the <bit-data>
    MUST be just sufficient to contain the number of bits specified by
    the <length>.  If there are insignificant bits in a final
    hexadecimal or octal digit, they MUST be zero.  A <dotted-quad>
    always has all four parts even if the associated <slength> is less

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    than 24, but, like the other forms, insignificant bits MUST be zero.

    Each number represented by a <decbyte> must be between 0 and 255,

    The number represented by <length> must be between 1 and 256

    The number represented by <slength> must be between 1 and 32

    When the textual form of a Bit-String Label is generated by machine,
    the length SHOULD be explicit, not implicit.

3.2.1.  Examples

    The following four textual forms represent the same Bit-String


    The following represents two consecutive Bit-String Labels which
    denote the same relative point in the DNS tree as any of the above
    single Bit-String Labels.


3.3.  Canonical Representation and Sort Order

    Both the wire form and the text form of binary labels have a degree
    of flexibility in their grouping into multiple consecutive Bit-
    String Labels.  For generating and checking DNS signature records
    [DNSSEC] binary labels must be in a predictable form.  This
    canonical form is defined as the form which has the fewest possible
    Bit-String Labels and in which all except possibly the first (least
    significant) label in any sequence of consecutive Bit-String Labels
    is of maximum length.

    For example, the canonical form of any sequence of up to 256 One-Bit
    Labels has a single Bit-String Label, and the canonical form of a
    sequence of 513 to 768 One-Bit Labels has three Bit-String Labels of
    which the second and third contain 256 label bits.

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    The canonical sort order of domain names [DNSSEC] is extended to
    encompass binary labels as follows.  Sorting is still label-by-
    label, from most to least significant, where a label may now be a
    One-Bit Label or a standard (code 00) label.  Any One-Bit Label
    sorts before any standard label, and a 0 bit sorts before a 1 bit.
    The absence of a label sorts before any label, as specified in

    For example, the following domain names are correctly sorted.


4.  Processing Rules

    A One-Bit Label never matches any other kind of label.  In
    particular, the DNS labels represented by the single ASCII
    characters "0" and "1" do not match One-Bit Labels represented by
    the bit values 0 and 1.

5.  Discussion

    A Count of zero in the wire-form represents a 256-bit sequence, not
    to optimize that particular case, but to make it completely
    impossible to have a zero-bit label.

6.  IANA Considerations

    This document defines one Extended Label Type, termed the Bit-String
    Label, and requests registration of the code point 000001 binary in
    the space defined by [EDNS0].

7.  Security Considerations

    All security considerations which apply to traditional ASCII DNS
    labels apply equally to binary labels.  he canonicalization and
    sorting rules of section 3.3 allow these to be addressed by DNS
    Security [DNSSEC].

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8.  References

    [ABNF]  D. Crocker, Ed., P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 2234.

    [DNSIS] P.V. Mockapetris, "Domain names - implementation and
            specification", RFC 1035.

    [DNSSEC]D. Eastlake, 3rd, C. Kaufman, "Domain Name System Security
            Extensions", RFC 2065.

    [EDNS0] P. Vixie, "Extension mechanisms for DNS (EDNS0)", Currently

    [KWORD] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels," RFC 2119.

9.  Author's Address

    Matt Crawford
    Fermilab MS 368
    PO Box 500
    Batavia, IL 60510

    Phone: +1 630 840-3461

    EMail: crawdad@fnal.gov

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