Binary Labels in the Domain Name System
RFC 2673
Document | Type |
RFC - Historic
(August 1999; Errata)
Obsoleted by RFC 6891
Updates RFC 1035
|
|
---|---|---|---|
Author | Matt Crawford | ||
Last updated | 2020-01-21 | ||
Stream | Internent Engineering Task Force (IETF) | ||
Formats | plain text html pdf htmlized (tools) htmlized with errata bibtex | ||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 2673 (Historic) | |
Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group M. Crawford Request for Comments: 2673 Fermilab Category: Standards Track August 1999 Binary Labels in the Domain Name System 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 (1999). All Rights Reserved. 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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. 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. Crawford Standards Track [Page 1] 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 <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 Crawford Standards Track [Page 2] RFC 2673 Binary Labels in the Domain Name System August 1999 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],Show full document text