INTERNET-DRAFT                                                Peter Koch
Expires: December 1999                            Universitaet Bielefeld
Updates: 1035, 1183, 2163, 2168, 2535                          June 1999

            A New Scheme for the Compression of Domain Names

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 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-

   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

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   Comments should be sent to the author or the DNSIND WG mailing list


   The compression of domain names in DNS messages was introduced in
   [RFC1035].  Although some remarks were made about applicability to
   future defined resource record types, no method has been deployed yet
   to support interoperable DNS compression for RR types specified since

   This document summarizes current problems and proposes a new
   compression scheme to be applied to future RR types which supports
   interoperability.  Also, suggestions are made how to deal with RR
   types defined so far.

1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in [RFC2119].

   Domain names herein are for explanatory purposes only and should not
   be expected to lead to useful information in real life [RFC2606].

2. Background

   Domain name compression was introduced in [RFC1035], section 4.1.4,
   as an optional protocol feature and later mandated by [RFC1123],
   section  The intent was to reduce the message length,
   especially that of UDP datagrams, by avoiding repetition of domain
   names or even parts thereof.

   A domain name is internally represented by the concatenation of label
   strings, where the first octet denotes the string length, not
   including itself.  The null string, consisting of a single octet of
   zeroes, is the representation of the root domain name and also
   terminates every domain name.

   As labels may be at most 63 characters long, the two most significant
   bits in the length octet will always be zero. Compression works by
   overloading the length octet with a second meaning. If the two MSB
   have the value '1', the remainder of the length octet and the next
   octet form a compression pointer, which denotes the position of the
   next label of the current domain name in the message:

          | 1  1|                OFFSET                   |

   It is important that these pointers always point backwards.

   Compression may occur in several places. First, the owner name of an
   RR may be compressed. The compression target may be another owner
   name or a domain name in the RDATA section of a previous RR.  Second,
   any domain name within the RDATA section may be compressed and the
   target may be part of the same RR, being the owner name or another
   domain name in the RDATA section, or it may live in a previous RR,
   either as its owner or as a domain name in its RDATA section.  In
   fact, due to the chaining feature, combinations of the above may

3. Problems

   While implementations shall use and must understand compressed domain
   names in the RDATA section of "well known" RR types (those initially
   defined in [RFC1035]), there is no interoperable way of applying

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   compression to the RDATA section of newer RRs:

   Quote from [RFC1123], section
        Compression relies on knowledge of the format of data inside a
        particular RR.  Hence compression must only be used for the
        contents of well-known, class-independent RRs, and must never be
        used for class-specific RRs or RR types that are not well-known.
        The owner name of an RR is always eligible for compression.

   DNS records in messages may travel through caching resolvers not
   aware of the particular RR's type and format. These caches cannot
   rearrange compression pointers in the RDATA section simply because
   they do not recognize them. Handing out these RRs in a different
   context later will lead to confusion if the target resolver tries to
   uncompress the domain names using wrong information.  This is not
   restricted to intermediate caching but affects any modification to
   the order of RRs in the DNS message.

4. Local Compression

   We often observe a certain locality in the domain names used as owner
   and occuring in the RDATA section, e.g. in MX or NS RRs but also in
   newer RR types [RFC1183]:  RP

   So, to still profit from compression without putting interoperability
   at risk, a new scheme is defined which limits the effect of
   compression to a single RR.

   In contrast to the usual method of using offsets relative to the
   start of a DNS packet we start counting at the RR owner or calculate
   pointers relative to the start of the RDATA to avoid context
   sensitivity.  We use an additional compression indicator for a two
   octet local pointer:

          | 1  0|                OFFSET                   |

   The "10" bits will indicate the use of local compression and
   distinguish it from conventional compression, plain labels and EDNS
   label codes [EDNS0].  Two types of pointers need to be specified:
   those pointing into the owner name and those pointing into RDATA.

   A) Pointers into the owner name are interpreted as the ordinal label
      number (starting at 0 for the topmost label, the TLD). This way we
      avoid the need for extra decompression of the owner name during

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      message composition or decomposition.

      The highest possible value of a compression pointer pointing into
      the owner name is 254. The value 255 is reserved for future use.

   B) Pointers into the RDATA section start at the fixed value 256 for
      the first octet and have a maximum value of 16383 limiting
      possible targets to the first 16128 octets. The actual offset
      relative to the start of RDATA is determined by subtracting 256
      from the value of the pointer.

   Local pointers MUST point to a previous occurence of the same name in
   the same RR.  Even domain names in another RR of the same type cannot
   serve as compression targets since the order of RRs in an RRSet is
   not necessarily stable.  The length of the compressed name(s) MUST be
   used in the length calculation for the RDLENGTH field.


   Consider a DNS message containing two resource records, one CNAME RR
   and one XMPL RR, undefined and meaningless so far, with an RDATA
   section consisting of two domain names:  IN  CNAME  bar.example
      bar.example     IN  XMPL  foo.example

   In a message this appears as follows (randomly starting at octet 12):

       12 |           2           |           a           |
       14 |           b           |           3           |
       16 |           f           |           o           |
       18 |           o           |           7           |
       20 |           e           |           x           |
       22 |           a           |           m           |
       24 |           p           |           l           |
       26 |           e           |           0           |

       10 octets skipped (TYPE, CLASS, TTL, RDLENGTH)

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       38 |           3           |           b           |
       40 |           a           |           r           |
       42 | 1  1|                 19                      |

   The XMPL RR with local compression applied:

       44 | 1 1 |                 38                      |

       10 octets skipped (TYPE, CLASS, TTL, RDLENGTH)

       56 |           1           |           a           |
       58 |           3           |           f           |
       60 |           o           |           o           |
       62 | 1  0|                 0                       |
       64 | 1  0|               258                       |

   The first local pointer at position 62 points to the topmost label
   "example" of the XMPL RR's owner.

   The second local pointer at position 64 represents the "foo.example"
   and points backwards into the RDATA section, third octet, at absolute
   position 58.  Note that with conventional compression this example
   message would have occupied less space.

5. Interaction with DNSSEC

   The security extensions to DNS [RFC2535] mandate that domain names in
   RDATA be signed only in expanded, lower case format. For RR types
   using local compression the specification is changed as follows:

      Resource Records subject to local compression MUST be stored,
      signed, transmitted and verified in locally compressed form.  Name
      expansion or canonicalization MUST NOT be performed on the RDATA
      section for signing or verification.

   This way RR type transparency can be achieved, since domain names in

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   the RDATA section are treated as arbitrary data and can be cached and
   verified by resolvers not aware of the particular RR type. Old RR
   types subject to conventional or no compression are not affected by
   this change.

   Wildcard owners may serve as compression targets only in their fixed
   part.  Even if a particular query asks for a domain name which could
   be used to compress the RDATA part more efficiently, this MUST NOT be
   done. Otherwise signatures would be invalidated.

   Currently slave servers store zones in text format and re-encode the
   data into wire format, e.g. after a restart. This encoding must be
   unique to ensure signature validity. To achieve this, local
   compression MUST be applied optimally, i.e. every domain name must be
   compressed as far as possible and each local compression pointer must
   point to the earliest available target (including the owner).

6. Interaction with Binary Labels

   When constructing local compression pointers into the owner name,
   every one-bit label is counted as a label. This way the compression
   and decompression is independent of the actual bit-string

   For local compression pointers into the RDATA section, only bit-
   string labels may serve as targets, not single one-bit labels. Bit-
   string labels may be adjusted to increase compression efficiency
   [BINLABELS, section 3.1]

   The internal representation of a domain name has a maximum length of
   255 [RFC 1035].  Any label consists of at least two octets, leading
   to at most 127 labels per domain name plus the terminating zero
   octet, which does not qualify as a compression target. With the
   introduction of binary labels a domain name can consist of up to 1904
   labels (all one-bit labels). This document restricts the possible
   compression targets in an owner name to the topmost 255 labels. This
   limit was chosen to be consistent with [RFC2535], section 4.1.3.

7. Old RR types and deployment

   Although differences in RDATA sections by class have not yet been
   reported and the concept of classes did not really spread, we are
   just considering the IN class here.

   The following RR types with domain names in the RDATA section have
   been defined since [RFC1035] (Standards Track, Experimental and
   Informational RFCs, ignoring withdrawn types):  RP [RFC1183], AFSDB
   [RFC1183], RT [RFC1183], SIG [RFC2535], PX [RFC2163], NXT [RFC2535],

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   SRV [RFC2052], NAPTR [RFC2168], KX [RFC2230].  Some specifications do
   not mention DNS compression at all, others explicitly suggest it and
   only in part identify interoperability issues.  Only the KX and SRV
   RR types are safe as their specifications prohibit compression.

   The specification of RP, AFSDB, RT, PX, and NAPTR is hereby changed
   in that domain names in the RDATA section MUST NOT be compressed and
   MUST NOT be compression targets.

   Local compression MUST NOT be used for owner names and it MUST NOT be
   applied to domain names in RDATA sections of any RR type defined so

   The specification of future RR types should explicitly select the use
   of local compression or forbid RDATA domain name compression at all.

8. Security Considerations

   The usual caveats for using unauthenticated DNS apply. This scheme is
   believed not to introduce any new security problems.  However,
   implementors should be aware of problems caused by blindly following
   compression pointers of any kind. [RFC1035] and this document limit
   compression targets to previous occurences and this MUST be followed
   in constructing and decoding messages. Otherwise applications might
   be vulnerable to denial of service attacks launched by sending DNS
   messages with infinite compression pointer loops. In addition,
   pointers should be verified to really point to the start of a label
   (for conventional and local RDATA pointers) and not beyond the end of
   the domain name (for local owner name pointers).

   The maximum length of 255 applies to domain names in uncompressed
   wire format, so care must be taken during decompression not to exceed
   this limit to avoid buffer overruns.

9. Acknowledgements

   The author would like to thank Andreas Gustafsson, Paul Vixie, Bob
   Halley, Mark Andrews and Thomas Narten for their review and
   constructive comments.

10. References

   [RFC1034]     Mockapetris,P., "Domain Names - Concepts and Facilities",
                 RFC 1034, STD 13, November 1987

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   [RFC1035]     Mockapetris,P., "Domain Names - Implementation and
                 Specification", RFC 1035, STD 13, November 1987

   [RFC1123]     Braden,R., "Requirements for Internet Hosts -- Application
                 and Support", RFC 1123, STD 3, October 1989

   [RFC1183]     Everhart,C., Mamakos,L., Ullmann,R., Mockapetris,P., "New
                 DNS RR Definitions", RFC 1183, October 1990

   [RFC2052]     Gulbrandsen,A., Vixie,P. "A DNS RR for specifying the
                 location of services (DNS SRV)", RFC 2052, October 1996

   [RFC2119]     Bradner,S., "Key words for use in RFCs to Indicate
                 Requirement Levels", RFC 2119, BCP 14, March 1997

   [RFC2163]     Allocchio,C., "Using the Internet DNS to Distribute MIXER
                 Conformant Global Address Mapping (MCGAM)", RFC 2163,
                 January 1998

   [RFC2168]     Daniel,R., Mealling,M., "Resolution of Uniform Resource
                 Identifiers using the Domain Name System", RFC 2168, June

   [RFC2230]     Atkinson,R., "Key Exchange Delegation Record for the DNS",
                 RFC 2230, November 1997

   [RFC2535]     Eastlake,D., "Domain Name System Security Extensions", RFC
                 2535, March 1999

   [RFC2606]     Eastlake,D., Panitz,A., "Reserved Top Level DNS Names",
                 RFC 2606, BCP 32, June 1999

   [EDNS0]       Vixie,P., "Extension mechanisms for DNS (EDNS0)", draft-
                 ietf-dnsind-edns0-XX.txt, work in progress

   [BINLABELS]   Crawford,M., "Binary Labels in the Domain Name System",
                 draft-ietf-dnsind-binary-labels-XX.txt, work in progress

11. Author's Address

   Peter Koch
   Universitaet Bielefeld
   Technische Fakultaet
   Postfach 10 01 31
   D-33501 Bielefeld

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   +49 521 106 2902

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