RFC 2673 (rfc2673) - Page 2 of 7

Binary Labels in the Domain Name System

RFC 2673        Binary Labels in the Domain Name System      August 1999


   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
   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  surrounded by the delimiters "\[" and "]".
   The  is either a dotted quad or a base indicator and a
   sequence of digits appropriate to that base, optionally followed by a



Crawford                    Standards Track