Internet Engineering Task Force (IETF) D. Crocker
Request for Comments: 8553 Brandenburg InternetWorking
BCP: 222 March 2019
Updates: 2782, 3263, 3529, 3620, 3832,
 3887, 3958, 4120, 4227, 4386,
 4387, 4976, 5026, 5328, 5389,
 5415, 5518, 5555, 5617, 5679,
 5766, 5780, 5804, 5864, 5928,
 6120, 6186, 6376, 6733, 6763,
 7208, 7489, 8145
Category: Best Current Practice
ISSN: 2070-1721
 DNS AttrLeaf Changes:
 Fixing Specifications That Use Underscored Node Names
Abstract
 Using an underscore for a prefix creates a space for constrained
 interoperation of resource records. Original uses of an underscore
 character as a domain node name prefix were specified without the
 benefit of an IANA registry. This produced an entirely uncoordinated
 set of name-creation activities, all drawing from the same namespace.
 A registry for these names has now been defined by RFC 8552.
 However, the existing specifications that use underscored naming need
 to be modified in order to be in line with the new registry. This
 document specifies those changes. The changes preserve existing
 software and operational practice, while adapting the specifications
 for those practices to the newer underscore registry model.
Status of This Memo
 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 BCPs is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8553.
Copyright Notice
 Copyright (c) 2019 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
 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
 2. Underscored RRset Use in Specifications . . . . . . . . . . . 3
 2.1. TXT RRset . . . . . . . . . . . . . . . . . . . . . . . . 4
 2.2. SRV RRset . . . . . . . . . . . . . . . . . . . . . . . . 5
 2.3. URI RRset . . . . . . . . . . . . . . . . . . . . . . . . 6
 3. Underscored Template Specifications . . . . . . . . . . . . . 7
 3.1. SRV Specification Changes . . . . . . . . . . . . . . . . 7
 3.2. URI Specification Changes . . . . . . . . . . . . . . . . 8
 3.3. DNSSEC Signaling Specification Changes . . . . . . . . . 10
 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
 6.1. Normative References . . . . . . . . . . . . . . . . . . 11
 6.2. Informative References . . . . . . . . . . . . . . . . . 12
 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
 Original uses of an underscore character as a domain node name
 [RFC1035] prefix, which creates a space for constrained
 interpretation of resource records, were specified without the
 benefit of an IANA registry [IANA-reg]. This produced an entirely
 uncoordinated set of name-creation activities, all drawing from the
 same namespace. A registry has now been defined (see Section 4 of
 [RFC8552]); the RFC that defined it discusses the background for the
 use of underscored domain names [RFC8552].
 The basic model for underscored name registration, as specified in
 [RFC8552], is to have each registry entry be unique in terms of the
 combination of a resource record type and a "global" (highest-level)
 underscored node name; that is, the node name beginning with an
 underscore that is the closest to the DNS root.
 The specifications describing the existing uses of underscored naming
 do not reflect the existence of this integrated registry. For the
 new reader or the new editor of one of those documents, there is
 currently nothing signaling that the underscored name(s) defined in
 the document are now processed through an IANA registry. This
 document remedies that, by marking such a published document with an
 update that indicates the nature of the change.
 Further, the documents that define the SRV [RFC2782] and URI
 [RFC7553] DNS resource records provide a meta-template for
 underscored name assignments, partially based on separate registries
 [RFC6335]. For the portion that selects the global (highest-level)
 underscored node name, this perpetuates uncoordinated assignment
 activities by separate technical specifications, out of the same
 namespace. This document remedies that by providing detail for
 revisions to the SRV and URI specifications to bring their use in
 line with the single, integrated "Underscored and Globally Scoped DNS
 Node Names" registry.
 The result of these changes preserves existing software and
 operations practices while adapting the technical specifications to
 the newer underscore registry model.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
2. Underscored RRset Use in Specifications
 The use of underscored node names is specific to each RR TYPE that is
 being scoped. Each name defines a place but does not define the
 rules for what appears underneath that place, either as additional
 underscored naming or as a leaf node with resource records. Details
 for those rules are provided by specifications for individual RR
 TYPEs. The sections below describe the way that existing underscored
 names are used with the RR TYPEs that they name.
2.1. TXT RRset
 NOTE - Documents falling into this category include: [RFC5518],
 [RFC5617], [RFC6120], [RFC6376], [RFC6763], [RFC7208], and
 [RFC7489].
 This section provides a generic approach for changes to existing
 specifications that define straightforward use of underscored node
 names when scoping the use of a TXT RRset. The approach provides the
 information needed for adapting such specifications to the use of the
 IANA "Underscored and Globally Scoped DNS Node Names" registry
 [RFC8552]. Hence, the approach is meant both as an update to these
 existing specifications and as guidance for changes when those
 documents are revised.
 For any document that specifies the use of a TXT RRset under one or
 more underscored names, the global node name is expected to be
 registered in the IANA "Underscored and Globally Scoped DNS Node
 Names" registry [RFC8552]. An effort has been made to locate
 existing documents that do this, to register the global underscored
 node names, and to list them in the initial set of names added to the
 registry.
 If a public specification defines use of a TXT RRset and calls for
 the use of an underscored node name, here is a template of suggested
 text for registering the global underscored node name -- the one
 closest to the root -- that can be used through the IANA
 Considerations section of the specification:
 "Per [RFC8552], please add the following entry to the "Underscored
 and Globally Scoped DNS Node Names" registry:"
 +--------+----------------+-----------------------------------------+
 | RR | _NODE NAME | Reference |
 | Type | | |
 +--------+----------------+-----------------------------------------+
 | TXT | _{DNS node | {citation for the document making the |
 | | name} | addition} |
 +--------+----------------+-----------------------------------------+
 Table 1: Entry for the "Underscored and Globally Scoped DNS
 Node Names" Registry for TXT RR Use
2.2. SRV RRset
 NOTE - Documents falling into this category include:
 [RFC3263], [RFC3529], [RFC3620], [RFC3832], [RFC3887],
 [RFC3958], [RFC4120], [RFC4227], [RFC4386], [RFC4387],
 [RFC4976], [RFC5026], [RFC5328], [RFC5389], [RFC5415],
 [RFC5555], [RFC5679], [RFC5766], [RFC5780], [RFC5804],
 [RFC5864], [RFC5928], and [RFC6186].
 Specification of the SRV resource record [RFC2782] provides a
 template for use of underscored node names. The global node name is
 characterized as referencing the 'protocol' that is associated with
 SRV RRset usage.
 This section provides a generic approach for changes to existing
 specifications that define the use of an SRV RRset. The approach
 provides the information needed for adapting such specifications to
 the use of the IANA "Underscored and Globally Scoped DNS Node Names"
 registry [RFC8552]. Hence, the approach is meant both as an update
 to these existing specifications and as guidance for changes when
 those documents are revised.
 For any document that specifies the use of an SRV RRset, the global
 ('protocol') underscored node name is expected to be registered in
 the IANA "Underscored and Globally Scoped DNS Node Names" registry
 [RFC8552]. An effort has been made to locate existing documents that
 do this, to register the global underscored node names, and to list
 them in the initial set of names added to the registry.
 If a public specification defines use of an SRV RRset and calls for
 the use of an underscored node name, here is a template of suggested
 text for registering the global underscored node name -- the one
 closest to the root -- that can be used through the IANA
 Considerations section of the specification:
 "Per [RFC8552], please add the following entry to the "Underscored
 and Globally Scoped DNS Node Names" registry:
 +--------+----------------------+-----------------------------------+
 | RR | _NODE NAME | Reference |
 | Type | | |
 +--------+----------------------+-----------------------------------+
 | SRV | _{DNS 'protocol' | {citation for the document making |
 | | node name} | the addition} |
 +--------+----------------------+-----------------------------------+
 Table 2: Entry for the "Underscored and Globally Scoped DNS Node
 Names" Registry for SRV RR Use
2.3. URI RRset
 Specification of the URI resource record [RFC7553] provides a
 template for use of underscored node names. The global node name is
 characterized as naming the 'protocol' that is associated with URI RR
 usage or by reversing an Enumservice sequence [RFC6117].
 This section provides a generic approach for changes to existing
 specifications that define use of a URI RRset. The approach provides
 the information needed for adapting such specifications to the use of
 the IANA "Underscored and Globally Scoped DNS Node Names" registry
 [RFC8552]. Hence, the approach is meant both as an update to these
 existing specifications and as guidance for changes when those
 documents are revised.
 For any document that specifies the use of a URI RRset, the global
 ('protocol' or highest-level Enumservice) underscored node name is
 expected to be registered in the IANA "Underscored and Globally
 Scoped DNS Node Names" registry [RFC8552]. An effort has been made
 to locate existing documents that do this, to register the global
 underscored node names, and to list them in the initial set of names
 added to the registry.
 If a public specification defines use of a URI RRset and calls for
 the use of an underscored node name, here is a template of suggested
 text for registering the global underscored node name -- the one
 closest to the root -- that can be used through the IANA
 Considerations section of the specification:
 "Per [RFC8552], please add the following entry to the "Underscored
 and Globally Scoped DNS Node Names" registry:
 +-------+----------------------------+------------------------------+
 | RR | _NODE NAME | Reference |
 | Type | | |
 +-------+----------------------------+------------------------------+
 | URI | _{DNS 'protocol' or | {citation for the document |
 | | Enumservice node name} | making the addition} |
 +-------+----------------------------+------------------------------+
 Table 3: Entry for the "Underscored and Globally Scoped DNS Node
 Names" Registry for URI RR Use
3. Underscored Template Specifications
3.1. SRV Specification Changes
 The specification for a domain name, under which an SRV resource
 record [RFC2782] appears, provides a template for use of underscored
 node names. The global underscored node name is characterized as
 indicating the 'protocol' that is associated with SRV RR usage.
 The text of [RFC2782] is changed as described below. In addition,
 note that a normative reference to RFC 8552 is added to the
 References section of RFC 2782.
 OLD:
 The format of the SRV RR
 Here is the format of the SRV RR, whose DNS type code is 33:
 _Service._Proto.Name TTL Class SRV Priority Weight Port Target
 ...
 Proto
 The symbolic name of the desired protocol, with an underscore
 (_) prepended to prevent collisions with DNS labels that occur
 in nature. _TCP and _UDP are at present the most useful values
 for this field, though any name defined by Assigned Numbers or
 locally may be used (as for Service). The Proto is case
 insensitive.
 NEW:
 The format of the SRV RR
 Here is the format of the SRV RR, whose DNS type code is 33:
 "_Service._Proto.Name TTL Class SRV Priority Weight Port
 Target"
 _..._
 Proto
 The symbolic name of the desired protocol with an underscore
 (e.g., "_name") prepended to prevent collisions with DNS
 labels that occur in nature. _TCP and _UDP are at present
 the most useful values for this field. The Proto is case
 insensitive.
 The SRV RRset 'protocol' (global) underscored node name
 SHOULD be registered in the IANA "Underscored and Globally
 Scoped DNS Node Names" registry [RFC8552].
3.2. URI Specification Changes
 Specification for the domain name (under which a URI resource record
 [RFC7553] occurs) is similar to that for the SRV resource record
 [RFC2782], although the text refers only to 'service' name, rather
 than distinguishing 'service' from 'protocol'. Further, the URI RR
 specification permits alternative underscored naming schemes:
 One matches what is used for SRV, with the global underscored node
 name called 'protocol'.
 The other is based on a reversing of an Enumservice [RFC6117]
 sequence.
 Text of [RFC7553] is changed as described below. In addition, a
 normative reference to RFC 8552 is added to the References section of
 RFC 7553.
 OLD:
 4.1. Owner Name, Class, and Type
 The URI owner name is subject to special conventions.
 Just like the SRV RR [RFC2782], the URI RR has service information
 encoded in its owner name. In order to encode the service for a
 specific owner name, one uses service parameters. Valid service
 parameters are those registered by IANA in the "Service Name and
 Transport Protocol Port Number Registry" [RFC6335] or as "Enumservice
 ---
 Registrations [RFC6117]. The Enumservice Registration parameters are
 reversed (i.e., subtype(s) before type), prepended with an underscore
 (_), and prepended to the owner name in separate labels. The
 underscore is prepended to the service parameters to avoid collisions
 with DNS labels that occur in nature, and the order is reversed to
 make it possible to do delegations, if needed, to different zones
 (and therefore providers of DNS).
 For example, suppose we are looking for the URI for a service with
 ENUM Service Parameter "A:B:C" for host example.com. Then we would
 query for (QNAME,QTYPE)=("_C._B._A.example.com","URI").
 As another example, suppose we are looking for the URI for a service
 with Service Name "A" and Transport Protocol "B" for host
 example.com. Then we would query for
 (QNAME,QTYPE)=("_A._B.example.com","URI").
 NEW:
 4.1. Owner Name, Class, and Type
 The URI owner name is subject to special conventions.
 As for the SRV RRset [RFC2782], the URI RRset global (highest-
 level) underscored node name SHOULD be registered in the IANA
 "Underscored and Globally Scoped DNS Node Names" registry
 [RFC8552].
 Just like the SRV RRset, the URI RRset has service information
 encoded in its owner name. In order to encode the service for
 a specific owner name, one uses service parameters. Valid
 service parameters are:
 + Those registered by IANA in the "Service Name and Transport
 Protocol Port Number Registry" [RFC6335]. The underscore is
 prepended to the service parameters to avoid collisions with
 DNS labels that occur in nature, and the order is reversed
 to make it possible to do delegations, if needed, to
 different zones (and therefore providers of DNS).
 + Those listed in "Enumservice Registrations" [RFC6117]. The
 Enumservice Registration parameters are reversed (i.e.,
 subtype(s) before type), prepended with an underscore (e.g.,
 "_name"), and prepended to the owner name in separate
 labels. The highest-level (global) underscored Enumservice
 name becomes the global name per RFC 8552 to register.
 For example, suppose we are looking for the URI for a service
 with ENUM Service Parameter "A:B:C" for host example.com. Then
 we would query for
 (QNAME,QTYPE)=("_C._B._A.example.com","URI").
 As another example, suppose we are looking for the URI for a
 service with Service Name "A" and Transport Protocol "B" for
 host example.com. Then we would query for
 (QNAME,QTYPE)=("_A._B.example.com","URI").
3.3. DNSSEC Signaling Specification Changes
 "Signaling Trust Anchor Knowledge in DNS Security Extensions
 (DNSSEC)" [RFC8145] defines a use of DNS node names that effectively
 consumes all names beginning with the string "_ta-" when using the
 NULL RR in the query.
 Text of Section 5.1, "Query Format", of RFC 8145 is changed as
 described below. In addition, a normative reference to RFC 8552 is
 added to the References section of RFC 8145.
 OLD:
 For example, a validating DNS resolver ...
 QNAME=_ta-4444.
 NEW:
 For example, a validating DNS resolver ... "QNAME=_ta-4444".
 Under the NULL RR, an entry is registered in the IANA
 "Underscored and Globally Scoped DNS Node Names" registry
 [RFC8552] for all node names beginning with "_ta-".
4. IANA Considerations
 Although this document makes reference to IANA registries, it
 introduces no new IANA registries or procedures.
5. Security Considerations
 This memo raises no security issues.
6. References
6.1. Normative References
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/info/rfc2119>.
 [RFC6117] Hoeneisen, B., Mayrhofer, A., and J. Livingood, "IANA
 Registration of Enumservices: Guide, Template, and IANA
 Considerations", RFC 6117, DOI 10.17487/RFC6117, March
 2011, <https://www.rfc-editor.org/info/rfc6117>.
 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
 Cheshire, "Internet Assigned Numbers Authority (IANA)
 Procedures for the Management of the Service Name and
 Transport Protocol Port Number Registry", BCP 165,
 RFC 6335, DOI 10.17487/RFC6335, August 2011,
 <https://www.rfc-editor.org/info/rfc6335>.
 [RFC7553] Faltstrom, P. and O. Kolkman, "The Uniform Resource
 Identifier (URI) DNS Resource Record", RFC 7553,
 DOI 10.17487/RFC7553, June 2015,
 <https://www.rfc-editor.org/info/rfc7553>.
 [RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust
 Anchor Knowledge in DNS Security Extensions (DNSSEC)",
 RFC 8145, DOI 10.17487/RFC8145, April 2017,
 <https://www.rfc-editor.org/info/rfc8145>.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8552] Crocker, D., "Scoped Interpretation of DNS Resource
 Records through "Underscored" Naming of Attribute Leaves",
 RFC 8552, DOI 10.17487/RFC8552, March 2019,
 <https://www.rfc-editor.org/info/rfc8552>.
6.2. Informative References
 [IANA-reg]
 IANA, "Protocol Registries",
 <https://www.iana.org/protocols>.
 [RFC1035] Mockapetris, P., "Domain names - implementation and
 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
 November 1987, <https://www.rfc-editor.org/info/rfc1035>.
 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
 specifying the location of services (DNS SRV)", RFC 2782,
 DOI 10.17487/RFC2782, February 2000,
 <https://www.rfc-editor.org/info/rfc2782>.
 [RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
 Protocol (SIP): Locating SIP Servers", RFC 3263,
 DOI 10.17487/RFC3263, June 2002,
 <https://www.rfc-editor.org/info/rfc3263>.
 [RFC3529] Harold, W., "Using Extensible Markup Language-Remote
 Procedure Calling (XML-RPC) in Blocks Extensible Exchange
 Protocol (BEEP)", RFC 3529, DOI 10.17487/RFC3529, April
 2003, <https://www.rfc-editor.org/info/rfc3529>.
 [RFC3620] New, D., "The TUNNEL Profile", RFC 3620,
 DOI 10.17487/RFC3620, October 2003,
 <https://www.rfc-editor.org/info/rfc3620>.
 [RFC3832] Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C., and
 W. Jerome, "Remote Service Discovery in the Service
 Location Protocol (SLP) via DNS SRV", RFC 3832,
 DOI 10.17487/RFC3832, July 2004,
 <https://www.rfc-editor.org/info/rfc3832>.
 [RFC3887] Hansen, T., "Message Tracking Query Protocol", RFC 3887,
 DOI 10.17487/RFC3887, September 2004,
 <https://www.rfc-editor.org/info/rfc3887>.
 [RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
 Service Location Using SRV RRs and the Dynamic Delegation
 Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
 January 2005, <https://www.rfc-editor.org/info/rfc3958>.
 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
 Kerberos Network Authentication Service (V5)", RFC 4120,
 DOI 10.17487/RFC4120, July 2005,
 <https://www.rfc-editor.org/info/rfc4120>.
 [RFC4227] O'Tuathail, E. and M. Rose, "Using the Simple Object
 Access Protocol (SOAP) in Blocks Extensible Exchange
 Protocol (BEEP)", RFC 4227, DOI 10.17487/RFC4227, January
 2006, <https://www.rfc-editor.org/info/rfc4227>.
 [RFC4386] Boeyen, S. and P. Hallam-Baker, "Internet X.509 Public Key
 Infrastructure Repository Locator Service", RFC 4386,
 DOI 10.17487/RFC4386, February 2006,
 <https://www.rfc-editor.org/info/rfc4386>.
 [RFC4387] Gutmann, P., Ed., "Internet X.509 Public Key
 Infrastructure Operational Protocols: Certificate Store
 Access via HTTP", RFC 4387, DOI 10.17487/RFC4387, February
 2006, <https://www.rfc-editor.org/info/rfc4387>.
 [RFC4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
 for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
 DOI 10.17487/RFC4976, September 2007,
 <https://www.rfc-editor.org/info/rfc4976>.
 [RFC5026] Giaretta, G., Ed., Kempf, J., and V. Devarapalli, Ed.,
 "Mobile IPv6 Bootstrapping in Split Scenario", RFC 5026,
 DOI 10.17487/RFC5026, October 2007,
 <https://www.rfc-editor.org/info/rfc5026>.
 [RFC5328] Adolf, A. and P. MacAvock, "A Uniform Resource Name (URN)
 Namespace for the Digital Video Broadcasting Project
 (DVB)", RFC 5328, DOI 10.17487/RFC5328, September 2008,
 <https://www.rfc-editor.org/info/rfc5328>.
 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
 "Session Traversal Utilities for NAT (STUN)", RFC 5389,
 DOI 10.17487/RFC5389, October 2008,
 <https://www.rfc-editor.org/info/rfc5389>.
 [RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
 Ed., "Control And Provisioning of Wireless Access Points
 (CAPWAP) Protocol Specification", RFC 5415,
 DOI 10.17487/RFC5415, March 2009,
 <https://www.rfc-editor.org/info/rfc5415>.
 [RFC5518] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
 Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
 <https://www.rfc-editor.org/info/rfc5518>.
 [RFC5555] Soliman, H., Ed., "Mobile IPv6 Support for Dual Stack
 Hosts and Routers", RFC 5555, DOI 10.17487/RFC5555, June
 2009, <https://www.rfc-editor.org/info/rfc5555>.
 [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
 "DomainKeys Identified Mail (DKIM) Author Domain Signing
 Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, August
 2009, <https://www.rfc-editor.org/info/rfc5617>.
 [RFC5679] Bajko, G., "Locating IEEE 802.21 Mobility Services Using
 DNS", RFC 5679, DOI 10.17487/RFC5679, December 2009,
 <https://www.rfc-editor.org/info/rfc5679>.
 [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,
 DOI 10.17487/RFC5766, April 2010,
 <https://www.rfc-editor.org/info/rfc5766>.
 [RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
 Using Session Traversal Utilities for NAT (STUN)",
 RFC 5780, DOI 10.17487/RFC5780, May 2010,
 <https://www.rfc-editor.org/info/rfc5780>.
 [RFC5804] Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
 Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
 July 2010, <https://www.rfc-editor.org/info/rfc5804>.
 [RFC5864] Allbery, R., "DNS SRV Resource Records for AFS", RFC 5864,
 DOI 10.17487/RFC5864, April 2010,
 <https://www.rfc-editor.org/info/rfc5864>.
 [RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT
 (TURN) Resolution Mechanism", RFC 5928,
 DOI 10.17487/RFC5928, August 2010,
 <https://www.rfc-editor.org/info/rfc5928>.
 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
 Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
 March 2011, <https://www.rfc-editor.org/info/rfc6120>.
 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
 Submission/Access Services", RFC 6186,
 DOI 10.17487/RFC6186, March 2011,
 <https://www.rfc-editor.org/info/rfc6186>.
 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
 "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
 RFC 6376, DOI 10.17487/RFC6376, September 2011,
 <https://www.rfc-editor.org/info/rfc6376>.
 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
 <https://www.rfc-editor.org/info/rfc6763>.
 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
 Authorizing Use of Domains in Email, Version 1", RFC 7208,
 DOI 10.17487/RFC7208, April 2014,
 <https://www.rfc-editor.org/info/rfc7208>.
 [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
 Message Authentication, Reporting, and Conformance
 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
 <https://www.rfc-editor.org/info/rfc7489>.
Acknowledgements
 Thanks go to Bill Fenner, Dick Franks, Tony Hansen, Peter Koch, Olaf
 Kolkman, and Andrew Sullivan for diligent review of the (much)
 earlier draft versions. For the later enhancements, thanks to Tim
 Wicinski, John Levine, Bob Harold, Joel Jaeggli, Ondrej Sury, and
 Paul Wouters.
 Special thanks to Ray Bellis for his 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
 United States of America
 Phone: +1.408.246.8253
 Email: dcrocker@bbiw.net
 URI: http://bbiw.net/