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Network Working Group                                         R. Austein
draft-ietf-dnsext-edns0dot5-00.txt               InterNetShare.com, Inc.
                                                           H. Alvestrand
                                                          EDB MaXware AS
                                                           February 2000

         A Proposed Enhancement to the EDNS0 Version Mechanism

Status of this document

   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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   Distribution of this document is unlimited.  Please send comments to
   the Namedroppers mailing list <namedroppers@ops.ietf.org>.

Motivation and Scope

   EDNS0 [EDNS0] specifies a general framework for extending the packet
   format used by the Domain Name System protocols.  The framework
   includes a simple version numbering scheme to allow the parties in a
   DNS protocol exchange to determine which extension features the other
   party understands.  While having the advantage of simplicity, the
   version numbering scheme as specified has drawbacks:

   - It provides no way to deprecate a protocol feature;

   - It provides no way to deploy experimental protocol features.

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   This note proposes to replace the monolithic version numbering
   mechanism with a mechanism for listing an explicit set of protocol
   features that a particular implementation supports.  We retain
   version numbering as a way of abbreviating the feature sets that we
   expect to see in common use.


   Our revised extension model for EDNS0 is designed with three goals in

   - We want the protocol to be as simple as possible for the common
     case of a client or server that implements "mainstream standard

   - We want to provide a safe way to experiment with new protocol
     features, both inside and outside the deployed DNS;

   - We want to provide a safe way to deprecate protocol features.

   Our revised extension model has two parts, both of which are carried
   in the OPT pseudo-RR: the VERSION, which stored in the second octet
   of the TTL field of the OPT RR, and a variable-length list of
   FEATURES, stored in the variable part of the OPT RR.

   All FEATUREs are extensions of the DNS.  We reserve FEATURE numbers 1
   to 100 for describing features of the original RFC 1034/1035 DNS
   specification that we might eventually chose to deprecate.

   Any query/response pair can be described as using a set of DNS
   FEATUREs.  Such features might for instance be:

   - Domain binary labels according to [BINARY-LABELS];

   - Extended RCODEs (the general principle, not specific values);

   - Multi-packet UDP response;

   - Increased maximum UDP payload size;

   - Character set identification in DNS labels;

   - SIG record parsing and checking;

   FEATURE numbers are handed out by IANA on a first-come-first-served
   basis.  Any revised specification of a format or function should have
   its own FEATURE number; in the IETF process, any significantly
   changed Internet-Draft should have a new FEATURE number assigned for

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   An assigned VERSION number names a set of FEATUREs.  VERSION numbers
   are assigned by the IETF through a standards action.

   Normally, any VERSION number encompasses every FEATURE of all lower
   VERSION numbers, but the possibility of removing FEATUREs exists for
   two reasons:

   - To remove the need for supporting FEATUREs that turned out to be a
     Really Bad Idea;

   - To allow replacing a badly specified FEATURE with a better
     specified FEATURE performing the same function that has a new
     FEATURE number.


   We propose to transport explicit feature sets as lists of integers
   carried in the variable RDATA portion of the EDNS0 OPT pseudo-RR.


   The OPTION-DATA for FEATURES is an ordered list of "feature numbers";
   a feature number is represented as a big-endian 16-bit unsigned
   integer, and the list is sorted into numerically increasing order.

   Each feature number names a particular protocol feature that is
   supported by the implementation that generated this OPT pseudo-RR.


   When composing a query message, the client includes an OPT record
   indicating the set of FEATUREs that:

   - Allows the client to express this query;

   - Allows the server to express all responses that the client is
     prepared to accept for this query.

   This set is expressed as a VERSION and any additional FEATURES

   The server will include an OPT pseudo-RR that indicates:

   - The highest VERSION numbers supported by the responder (for
     information only -- the client takes no action based on this);

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   - The set of FEATUREs not encompassed by the VERSION that are
     necessary to express the response.

   No FEATURE may be included or used that is not within the set of
   FEATUREs that the query originator indicated.

   As a special case, if a client explicitly queries for the OPT RR of
   the root zone, the server returns an OPT record including all
   FEATUREs that the server supports.  This functionality is provided
   strictly for diagnostic purposes.

Life Cycle

   We expect the life cycle of new features to proceed as follows:

   - VERSION X is defined and deployed.

   - A new FEATURE is defined and experimentally implemented.  All
     clients and servers part of the experiment use FEATURE to indicate

   - Community consensus is reached that this FEATURE is genuinely

   - VERSION X+1 is defined, encompassing all FEATUREs from VERSION X,
     plus the new FEATURE (and perhaps others).

   - The next generation of DNS software supports VERSION X+1, and never
     use FEATURE.


   While we have tried to provide the ability to deprecate old bad
   protocol features, such an ability should be used only rarely, if at
   all, since by any realistic estimate it takes years (decades?)  to
   upgrade all the DNS implementations already in the field.

   A flexible extension mechanism of this type increases the risk that
   some implementors might chose to deploy features designed to hinder
   interoperability (so-called "labeled noninteroperability").

Security Considerations

   We do not believe that this protocol enhancement adds any new
   security risks, but we do believe that it would be helpful in getting
   complicated DNS extensions such as [DNSSEC] deployed more quickly.

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IANA Considerations

   IANA will need to allocate an EDNS0 option code for FEATURES.

   IANA will need to create a new registry of feature numbers.


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

   [DNS-CONCEPTS]  Mockapetris, P., "Domain names - concepts and
        facilities", RFC 1034, November 1987.

   [DNS-IMPLEMENTATION]  Mockapetris, P., "Domain names - implementation
        and specification", RFC 1035, November 1987.

   [EDNS0]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
        August 1999.

   [BINARY-LABELS]  Crawford, M., "Binary Labels in the Domain Name
        System", RFC 2673 August 1999.

Author's addresses:

      Rob Austein
      InterNetShare.com, Inc.
      505 West Olive Ave., Suite 321
      Sunnyvale, CA 94086


      Harald Tveit Alvestrand
      EDB MaXware AS
      N-7486 Trondheim

      +47 73 54 57 97

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