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dnsop                                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Best Current Practice                    March 19, 2018
Expires: September 20, 2018


    DNS Scoped Data Through '_Underscore' Naming of Attribute Leaves
                      draft-ietf-dnsop-attrleaf-03

Abstract

   Formally, any DNS resource record may occur for any domain name.
   However some services have defined an operational convention that
   applies to DNS leaf nodes that are under a DNS branch that has one or
   more reserved node names that begin with an underscore.  The
   underscore naming construct defines a semantic scope for DNS records
   that are associated with the parent domain, above the underscored
   branch.  This specification explores the nature of this DNS usage and
   defines the "DNS Global Underscore Scoped Entry Registry" with IANA.
   The purpose of the Underscore registry is to avoid collisions
   resulting from the use of the same underscore-based name, for
   different services.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on September 20, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  _Underscore Scoping . . . . . . . . . . . . . . . . . . .   3
     1.2.  Scaling Benefits for TXT, SRV, and URI Resource Records .   4
   2.  DNS Underscore Scoped Entry Registries Function . . . . . . .   4
     2.1.  DNS Underscore Global Scoped Entry Registry Definition  .   5
     2.2.  DNS Common Second-Level Underscore Scoped Entry Registry
           Definition  . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  DNS Underscore Global Scoped Entry Registry . . . . . . .   7
     3.2.  DNS Common Second-Level Underscore Scoped Entry
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     5.2.  References -- Informative . . . . . . . . . . . . . . . .  11
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The core Domain Name System (DNS) technical specifications assign no
   semantics to domain names or their parts, and no constraints upon
   which resource records (RRs) are permitted to be associated with
   particular names.[RFC1035] Over time, some leaf node names, such as
   "www" and "ftp" have come to imply support for particular services,
   but this is a matter of operational convention, rather than defined
   protocol semantics.  This freedom in the basic technology has
   permitted a wide range of administrative and semantic policies to be
   used -- in parallel.  DNS data semantics have been limited to the
   specification of particular resource records, on the expectation that
   new ones would be added as needed.  Unfortunately, the addition of
   new resource records has proved extremely challenging, over the life
   of the DNS, with significant adoption and use barriers.








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1.1.  _Underscore Scoping

   As an alternative to defining new RRs, some DNS service enhancements
   call for using an existing resource record, but specify a restricted
   scope for its occurrence.  That scope is a leaf node, within which
   the uses of specific resource records can be formally defined and
   constrained.  The leaf occurs in a branch having a distinguished
   naming convention: At the top of the branch -- beneath the parent
   domain name to which the scope applies -- one or more reserved DNS
   node names begin with an underscore ("_").  Because the DNS rules for
   a "host" (host name) are not allowed to use the underscore character,
   this distinguishes the underscore name from all legal host names
   [RFC1035].  Effectively, this convention for leaf node naming creates
   a space for the listing of 'attributes' -- in the form of resource
   records -- that are associated with the parent domain, above the
   underscore sub-branch.

   The scoping feature is particularly useful when generalized resource
   records are used -- notably "TXT", "SRV", and "URI"
   [RFC1035],[RFC2782],[RFC7553].  It provides efficient separation of
   one use of them from others.  Absent this separation, an
   undifferentiated mass of these "RR"s is returned to the DNS client,
   which then must parse through the internals of the records in the
   hope of finding ones that are relevant.  Worse, in some cases the
   results are ambiguous because the records do not adequately self-
   identify.  With underscore-based scoping, only the relevant "RR"s are
   returned.

   A simple example is DKIM [RFC6376] , which uses "_domainkeys" for
   defining a place to hold a "TXT" record containing signing
   information for the parent domain.

   This specification formally defines how underscore labels are used as
   "attribute" enhancements for their parent domain names.  For example,
   domain name "_domainkey.example." acts as attribute of parent domain
   name "example."  To avoid collisions resulting from the use of the
   same underscore-based labels for different applications, this
   document establishes DNS Underscore Global Scoped Entry IANA Registry
   for the highest-level reserved names that begin with _underscore;
   _underscore-based names that are farther down the hierarchy are
   handled within the scope of the highest-level _underscore name.

   Discussion Venue:    Discussion about this draft should be directed
      to the dnsop@ietf.org [1] mailing list.

      NOTE TO RFC EDITOR:    Please remove "Discussion Venue" paragraph
         prior to publication.




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1.2.  Scaling Benefits for TXT, SRV, and URI Resource Records

   Some resource records are generic and support a variety of uses.
   Each additional use defines its own rules and, possibly, its own
   internal syntax and node-naming conventions to distinguish among
   particular types.  The "TXT", "SRV", and "URI" records are notable
   examples.  Their use can scale poorly, particularly when the same
   "RR" can be present in the same leaf node, but with different uses.

   An increasingly-popular approach, with excellent scaling properties,
   place the RR undr a node wit an underscore-based name, at a defined
   place in the DNS tree, so as to constrain to the use of particular
   "RR"s farther down the branch using that name.  This means that a
   direct lookup produces only the desired records, at no greater cost
   than a typical DNS lookup.

   The definition of a underscore global registry, provided in this
   specification, primarily attends to the "upper-level" names used for
   RRs; that is the _underscore "global" names.  For efficiency, a
   single, subordinate _underscore second-level table also is defined,
   for use with a common set of applications.

2.  DNS Underscore Scoped Entry Registries Function

   A global registry for DNS nodes names that begin with an _underscore
   is defined here.  The names are used to define scope of use for
   specific resource records, associated with the domain name that is
   the "parent" to the branch defined by the _underscore naming.

      A given name defines a specific, constrained context for one or
      more RR records, in which use of such records MUST conform to the
      defined constraints.  Within this scope, other resource records
      that are not specified MAY be used.

   The purpose of the Underscore Global Registry is to avoid collisions
   resulting from the use of the same _underscore-based name, for
   different applications.

   Structurally, the registry is defined as a single, flat table of
   names that begin with _underscore.  In some cases, such as for use of
   an "SRV" record, the full scoping name might be multi-part, as a
   sequence of underscore names.  Semantically, that sequence represents
   a hierarchical model and it is theoretically reasonable to allow re-
   use of a subordinate underscore name in different underscore context;
   that is, a subordinate name is meaningful only within the scope of
   the first (top-level) underscore name.  Therefore they are ignored by
   this DNS Underscore Global Scoped Entry Registry.  This registry is




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   for the definition of highest-level -- ie, global -- underscore node
   name used.

                      +----------------------------+
                      |                       NAME |
                      +----------------------------+
                      |                  _service1 |
                      |         ._protoB._service2 |
                      |          _protoB._service3 |
                      |          _protoC._service3 |
                      |    _useX._protoD._service4 |
                      | _protoE._region._authority |
                      +----------------------------+

                        Example of Underscore Names

   Only the right-most names are registered in the IANA Underscore
   Global table.  Definition and registration of the subordinate names
   is the responsibility of the specification that creates the highest-
   level (right-most) registry entry.

   For convenience, an Underscore Common Second-Level Names table is
   also defined, to cover some popular cases involving the subordinate
   name used with two-level _underscore naming.  In particular, this
   table covers uses of second-level names that scope SRV RRs use.

2.1.  DNS Underscore Global Scoped Entry Registry Definition



      Additions/Removals/Changes:    Please post to the list or send the
         author direct email, that indicates the exact details of
         changes needed to this table.  If a reference needs to be added
         or changed, the xml for this would be ideal.  Thanks. /d .

         NOTE TO RFC EDITOR:    Please remove "Additions/Removals/
            Changes" paragraph prior to publication.

   A registry entry contains:



      ID:    Specifies a textual name for a scoped portion of the DNS.
         The name will usually be taken from the specification cited in
         the "Purpose" column and is intended for use in discussions
         about the entry.





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      _Node Name:    Specifies a single _underscore name that defines a
         reserved name; this name is the "global" entry name for the
         scoped resource records that are associated with that name.

      Constraints:    Specifies any restrictions on use of the DNS
         Label.

      RR(s):    Lists the RRs that are defined for use within this
         scope.

      References  Lists specifications that define the records and their
         use under this Name.

      Purpose:    Specifies the particular purpose/use for specific
         "RR"(s), defined for use within the scope of the registered
         underscore name.

2.2.  DNS Common Second-Level Underscore Scoped Entry Registry
      Definition

   A registry entry contains:



      ID:    Specifies a textual name for a scoped portion of the DNS,
         occurring under a 'global' -- right-most -- _underscore node
         name.  The name will usually be taken from the specification
         cited in the "Purpose" column and is intended for use in
         discussions about the entry.

      _Node Name:    Specifies a single _underscore name that defines a
         reserved name; this name is the "second-level" entry name for
         the scoped resource records that are associated with that name.

      Constraints:    Specifies any restrictions on use of the name.

      RR(s):    Lists the RRs that are defined for use within this
         scope.

      References  Lists specifications that define the records and their
         use under this Name.

      Purpose:    Specifies the particular purpose/use for specific
         "RR"(s), defined for use within the scope of the registered
         underscore name.






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3.  IANA Considerations

   Per [RFC8126], IANA is requested to establish two registries:

   1.  DNS Underscore Global Scoped Entry Registry

   2.  DNS Underscore Common Second-Level Scoped Entry Registry

   This section describes actions requested of IANA.  The guidance in
   [IANA] is used.

3.1.  DNS Underscore Global Scoped Entry Registry

   The DNS Global Underscore Scoped Entry Registry is for DNS node names
   that begin with the underscore character (_) and occur at the "top"
   of a DNS branch -- ie, are right-most -- under a "parent" domain
   name.

      This registry is to operate under the IANA rules for "First Come
      First Served" registration.

   The contents of each entry in the Global registry are defined in
   Section 2.1.



      Additions/Removals/Changes:    Please post to the list or send the
         author direct email, that indicates the exact details of
         changes needed to this table.  If a reference needs to be added
         or changed, the xml for this would be ideal.  Thanks. /d .

         NOTE TO RFC EDITOR:    Please remove "Additions/Removals/
            Changes" paragraph prior to publication.

   Initial entries in the registry are:
















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   +-------+------------+-----+------------+---------------------------+
   | ID    | _NODE NAME | RR  | REFERENCE  | PURPOSE                   |
   +-------+------------+-----+------------+---------------------------+
   | SRV   | _tcp       | SRV | [RFC2782]  | Use of SRV for a TCP-     |
   | TCP   |            |     |            | based service             |
   | SRV   | _udp       | SRV | [RFC2782]  | Use of SRV for a UDP-     |
   | UDP   |            |     |            | based service             |
   | SPF   | _spf       | TXT | [RFC7208]  | Authorized IP addresses   |
   |       |            |     |            | for sending mail          |
   | DKIM  | _domainkey | TXT | [RFC6376]  | Public key for verifying  |
   |       |            |     |            | DKIM signature.           |
   | VBR   | _vouch     | TXT | [RFC5518]  | Vouch-by-refererence      |
   |       |            |     |            | domain assertion          |
   +-------+------------+-----+------------+---------------------------+

           Table 1: Underscore Global Registry (initial entries)

3.2.  DNS Common Second-Level Underscore Scoped Entry Registry

   A DNS Common Second-Level Underscore Scoped Entry Registry is for DNS
   node names that begin with the underscore character (_) and occur
   immediately below a Global ("top-level") node name beginning with an
   _underscore.

      This registry is to operate under the IANA rules for "First Come
      First Served" registration.

   The contents of each entry in the Common, Second-Level registry are
   defined in Section 2.2.

   Initial entries in the registry are:

   +-------------+--------------+---------+----------+-----------------+
   | ID          | _NODE NAME   | RR      | REFERENC | PURPOSE         |
   |             |              |         | E        |                 |
   +-------------+--------------+---------+----------+-----------------+
   | LDAP        | _ldap        | SRV     | [RFC2782 | LDAP server     |
   |             |              |         | ]        |                 |
   | SIP         | _sip         | NAPTR   | [RFC3263 | Locating SIP    |
   |             |              |         | ] [RFC60 | Servers and UA  |
   |             |              |         | 11]      | configuration   |
   | PKI LDAP    | _PKIXREP     | SRV     | [RFC4386 | PKI Repository  |
   |             |              |         | ]        |                 |
   | DDDS        | --???!--     | SRV     | [RFC3404 | Mapping DDDS    |
   |             |              |         | ]        | query to DNS    |
   |             |              |         |          | records         |
   | SOAP BEEP   | _soap-beep   | SRV     | [RFC4227 | SOAP over BEEP  |
   |             |              |         | ]        | lookup, when no |



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   |             |              |         |          | port specified  |
   | XMLRPC BEEP | _xmlrpc-beep | SRV     | [RFC3529 | Resolve url for |
   |             |              |         | ]        | XML-RPC using   |
   |             |              |         |          | BEEP            |
   | Diameter    | _diameter    | SRV     | [RFC6733 | Diameter        |
   |             |              |         | ]        | rendezvous      |
   | Tunnel      | _tunnel      | SRV     | [RFC3620 | Finding the     |
   |             |              |         | ]        | appropriate     |
   |             |              |         |          | address for     |
   |             |              |         |          | tunneling into  |
   |             |              |         |          | a particular    |
   |             |              |         |          | domain          |
   | SLP         | _slpda       | SRV     | [RFC3832 | Discovering     |
   |             |              |         | ]        | desired         |
   |             |              |         |          | services in     |
   |             |              |         |          | given DNS       |
   |             |              |         |          | domains         |
   | Msg Track   | _mtqp        | SRV     | [RFC3887 | Assist in       |
   |             |              |         | ]        | determining the |
   |             |              |         |          | path that a     |
   |             |              |         |          | particular      |
   |             |              |         |          | message has     |
   |             |              |         |          | taken through a |
   |             |              |         |          | messaging       |
   |             |              |         |          | system          |
   | XMPP Client | _xmpp-client | SRV     | [RFC6120 | XMPP client     |
   |             |              |         | ]        | lookup of       |
   |             |              |         |          | server          |
   | XMPP Server | _xmpp-server | SRV     | [RFC6120 | XMPP server-    |
   |             |              |         | ]        | server lookup   |
   | DDDS SRV    | _???         | SRV     | [RFC3958 | Map domain      |
   |             |              | (and    | ]        | name,           |
   |             |              | NAPTR?) |          | application     |
   |             |              |         |          | service name,   |
   |             |              |         |          | and application |
   |             |              |         |          | protocol        |
   |             |              |         |          | dynamically to  |
   |             |              |         |          | target server   |
   |             |              |         |          | and port        |
   | Kerberos    | _kerberos    | SRV     | [RFC4120 | purpose         |
   |             |              |         | ]        |                 |
   | PKI         | _pkixrep     | SRV     | [RFC4386 | Enables         |
   |             |              |         | ]        | certificate-    |
   |             |              |         |          | using systems   |
   |             |              |         |          | to locate PKI   |
   |             |              |         |          | repositories    |
   | Certificate | _certificate | SRV     | [RFC4387 | Obtain          |
   | s           | s            |         | ]        | certificates    |



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   |             |              |         |          | and certificate |
   |             |              |         |          | revocation      |
   |             |              |         |          | lists (CRLs)    |
   |             |              |         |          | from PKI        |
   |             |              |         |          | repositories    |
   | PGP Key     | _pgpkeys     | SRV     | [RFC4387 | Obtain          |
   | Store       |              |         | ]        | certificates    |
   |             |              |         |          | and certificate |
   |             |              |         |          | revocation      |
   |             |              |         |          | lists (CRLs)    |
   |             |              |         |          | from PKI        |
   |             |              |         |          | repositories    |
   | MSRP Relay  | _msrp        | SRV     | [RFC4976 | purpose         |
   | Locator     |              |         | ]        |                 |
   | Mobile IPv6 | _mip6        | SRV     | [RFC5026 | Bootstrap       |
   | Bootstrap   |              |         | ] [RFC55 | Mobile IPv6     |
   |             |              |         | 55]      | Home Agent      |
   |             |              |         |          | information     |
   |             |              |         |          | from non-       |
   |             |              |         |          | topological     |
   |             |              |         |          | information     |
   | Digital     | _dvbservdsc  | SRV     | [RFC5328 | Discover non-   |
   | Video Broad |              |         | ]        | default DVB     |
   | casting     |              |         |          | entry points    |
   |             |              |         |          | addresses       |
   | CAPWAP AC   | _capwap-     | rrs     | [RFC5415 | Discover the    |
   |             | control      |         | ]        | CAPWAP AC       |
   |             |              |         |          | address(es)     |
   | IEEE 802.21 | _mihis       | NAPTR,  | [RFC5679 | Discovering     |
   | Mobility    |              | SRV     | ]        | servers that    |
   |             |              |         |          | provide IEEE    |
   |             |              |         |          | 802.21-defined  |
   |             |              |         |          | Mobility        |
   |             |              |         |          | Services        |
   | STUN Client | _stun        | SRV     | [RFC5389 | Find a STUN     |
   | /Server     |              |         | ]        | server          |
   | TURN        | _turn        | SRV     | [RFC5766 | Control the     |
   |             |              |         | ] [RFC59 | operation of a  |
   |             |              |         | 28]      | relay to bypass |
   |             |              |         |          | NAT             |
   | STUN NAT    | _stun-       | SRV     | [RFC5780 | Discover the    |
   | Behavior    | behavior     |         | ]        | presence and    |
   | Discovery   |              |         |          | current         |
   |             |              |         |          | behavior of     |
   |             |              |         |          | NATs and        |
   |             |              |         |          | firewalls       |
   |             |              |         |          | between the     |
   |             |              |         |          | STUN client and |



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   |             |              |         |          | the STUN server |
   | Sieve       | _sieve       | SRV     | [RFC5804 | Manage Sieve    |
   | Management  |              |         | ]        | scripts on a    |
   |             |              |         |          | remote server   |
   | AFS VLDB    | _afs3-vlserv | SRV     | [RFC5864 | Locate services |
   |             | er           |         | ]        | for the AFS     |
   |             |              |         |          | distributed     |
   |             |              |         |          | file system     |
   | AFS PTS     | _afs3-prserv | SRV     | [RFC5864 | Locate services |
   |             | er           |         | ]        | for the AFS     |
   |             |              |         |          | distributed     |
   |             |              |         |          | file system     |
   | Mail MSA    | _submission  | SRV     | [RFC6186 | Locate email    |
   | Submission  |              |         | ]        | services        |
   | IMAP        | _imap        | SRV     | [RFC6186 | Locate email    |
   |             |              |         | ]        | services        |
   | POP         | _pop3        | SRV     | [RFC6186 | Locate email    |
   |             |              |         | ]        | services        |
   | POP TLS     | _pop3s       | SRV     | [RFC6186 | Locate email    |
   |             |              |         | ]        | services        |
   +-------------+--------------+---------+----------+-----------------+

          Table 2: Underscore 2d-Level Registry (initial entries)

4.  Security Considerations

   This memo raises no security issues.

5.  References

5.1.  Normative References

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", RFC 8126,
              June 2017.

5.2.  References -- Informative

   [IANA]     M. Cotton, B. Leiba, and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", I-D
              draft-leiba-cotton-iana-5226bis-11, 2017.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.



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   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263, June
              2002.

   [RFC3404]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Four: The Uniform Resource Identifiers (URI)
              Resolution Application", RFC 3404, October 2002.

   [RFC3529]  Harold, W., "Using Extensible Markup Language-Remote
              Procedure Calling (XML-RPC) in Blocks Extensible Exchange
              Protocol (BEEP)", RFC 3529, April 2003.

   [RFC3620]  New, D., "The TUNNEL Profile", RFC 3620, October 2003.

   [RFC3832]  Columbia University, Columbia University, Sun
              Microsystems, IBM, and IBM, "Remote Service Discovery in
              the Service Location Protocol (SLP) via DNS SRV",
              RFC 3832, July 2004.

   [RFC3887]  "Message Tracking Query Protocol", RFC 3887, September
              2007.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.

   [RFC4120]  USC-ISI, MIT, MIT, and MIT, "The Kerberos Network
              Authentication Service (V5)", RFC 4120, July 2005.

   [RFC4227]  O'Tuathail, E. and M. Rose, "Using the Simple Object
              Access Protocol (SOAP) in Blocks Extensible Exchange
              Protocol (BEEP)", RFC 4227, January 2006.

   [RFC4386]  Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
              Infrastructure: Repository Locator Service", RFC 4386,
              February 2006.

   [RFC4387]  Gutmann, P., Ed., "Internet X.509 Public Key
              Infrastructure Operational Protocols: Certificate Store
              Access via HTTP", RFC 4387, February 2006.

   [RFC4976]  Jennings, C., Mahy, R., and Roach, "Relay Extensions for
              the Message Session Relay Protocol (MSRP)", RFC 4976,
              September 2007.

   [RFC5026]  Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
              "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
              October 2007.



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   [RFC5328]  Adolf, A. and P. MacAvock, "A Uniform Resource Name (URN)
              Namespace for the Digital Video Broadcasting Project
              (DVB)", RFC 5328, September 2008.

   [RFC5389]  Rosenberg, Mahy, Matthews, and Wing, "Session Traversal
              Utilities for NAT (STUN)", RFC 5389, October 2008.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415, March 2009.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, April 2009.

   [RFC5555]  Soliman, H., Ed., "Mobile IPv6 Support for Dual Stack
              Hosts and Routers", RFC 5555, June 2009.

   [RFC5679]  Bajko, G., "Locating IEEE 802.21 Mobility Services Using
              DNS", RFC 5679, December 2009.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities for NAT (STUN)",
              RFC 5780, May 2010.

   [RFC5804]  Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, July 2010.

   [RFC5864]  Allbery, R., "NS SRV Resource Records for AFS", RFC 5864,
              April 2010.

   [RFC5928]  Petit-Huguenin, M., "Traversal Using Relays around NAT
              (TURN) Resolution Mechanism", RFC 5928, August 2010.

   [RFC6011]  Lawrence, S., Ed. and J. Elwell, "Session Initiation
              Protocol (SIP) User Agent Configuration", RFC 6011,
              October 2010.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186, March 2011.





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   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", RFC 6376, Sept 2011.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in E-Mail, Version 1",
              RFC 7208, April 2014.

   [RFC7553]  Falstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record", RFC 7553,
              ISSN 2070-1721, June 2015.

5.3.  URIs

   [1] mailto:dnsop@ietf.org

Appendix A.  Acknowledgements

   Thanks go to Bill Fenner, Tony Hansen, Peter Koch, Olaf Kolkman, and
   Andrew Sullivan for diligent review of the (much) earlier drafts.
   For the later enhancements, thanks to: Stephane Bortzmeyer, Bob
   Harold, John Levine, Joel Jaeggli, Petr Špaček, Ondřej
   Surř, Tim Wicinski, and Paul Wouters.

   Special thanks to Ray Bellis for more than 12 years of persistent
   encouragement to continue this effort, as well as the suggestion for
   an essential simplification to the registration model.

Author's Address

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale, CA  94086
   USA

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net
   URI:   http://bbiw.net/










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