Network Working Group                                            R. Mahy
Request for Comments: 5333                                  Unaffiliated
Category: Standards Track                                   B. Hoeneisen
                                                                Swisscom
                                                            October 2009


       IANA Registration of Enumservices for Internet Calendaring

Abstract

   This document registers Enumservices for Internet calendaring.
   Specifically, this document focuses on Enumservices for scheduling
   with iMIP (iCalendar Message-Based Interoperability Protocol) and for
   accessing Internet calendaring information with CalDAV (Calendaring
   Extensions to WebDAV).

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.

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1.  Introduction

   ENUM (E.164 Number Mapping, RFC 3761 [1]) is a system that uses DNS
   (Domain Name System, RFC 1034 [2]) to translate telephone numbers,
   such as '+12025550100', into URIs (Uniform Resource Identifiers, RFC
   3986 [3]), such as 'mailto:user@example.com'.  ENUM exists primarily
   to facilitate the interconnection of systems that rely on telephone
   numbers with those that use URIs to identify resources.  The ENUM
   registration here could be used to allow phones, for example, to
   check the free/busy status of a user in their address book or propose
   a meeting with him or her from the user's phone number.

   The Guide to Internet Calendaring [10] describes the relationship
   between various Internet calendaring specifications like this:
   "iCalendar [4] is the language used to describe calendar objects.
   iTIP [5] [iCalendar Transport-Independent Interoperability Protocol]
   describes a way to use the iCalendar language to do scheduling.  iMIP
   [6] [iCalendar Message-Based Interoperability Protocol] describes how
   to do iTIP scheduling via e-mail".

   Recently, another Standards Track protocol for calendar and
   scheduling access has appeared.  CalDAV (Calendaring Extensions to
   WebDAV) [7] is a WebDAV (Web-based Distributed Authoring and
   Versioning) [8] based mechanism for manipulating Internet calendars,
   viewing free/busy lists, and via a planned scheduling extension [15],
   could be used for proposing calendar events as well in the future.

   The existing 'mailto:' URI scheme (defined in RFC 3986 [3]) is
   already used to address iMIP compatible Calendar Services.  Likewise,
   the existing 'http:' and 'https:' URI schemes (defined in RFC 2616
   [11] and RFC 2818 [12]) are already used to address CalDAV compatible
   Calendar Services.

   This document registers Enumservices for scheduling and accessing
   Internet calendaring information associated with an E.164 number.

2.  Enumservice Registrations

   As defined in RFC 3761 [1], the following templates cover the
   information needed for the registration of the Enumservices specified
   in this document:

   Enumservice Name:
      "ical-sched"
   Enumservice Type:
      "ical-sched"
   Enumservice Subtypes:
      "mailto"
   URI scheme(s):
      'mailto:'
   Functional Specification:
      This Enumservice indicates that the resource identified can be
      addressed by the associated URI used for scheduling using Internet
      calendaring via Internet mail with the iMIP [6] protocol.
   Security considerations:
      See Section 4.
   Intended usage:
      COMMON
   Author:
      Rohan Mahy (rohan@ekabal.com)

   Enumservice Name:
      "ical-access"
   Enumservice Type:
      "ical-access"
   Enumservice Subtypes:
      "http"
   URI scheme(s):
      'http:'
   Functional Specification:
      This Enumservice indicates that the resource identified can be
      addressed by the associated URI in order to access a user's
      calendar (for example free/busy status) using the CalDAV [7]
      protocol for Internet calendaring.
   Security considerations:
      See Section 4.
   Intended usage:
      COMMON
   Author:
      Rohan Mahy (rohan@ekabal.com)

   Enumservice Name:
      "ical-access"
   Enumservice Type:
      "ical-access"
   Enumservice Subtypes:
      "https"
   URI scheme(s):
      'https:'
   Functional Specification:
      This Enumservice indicates that the resource identified can be
      addressed by the associated URI in order to access a user's
      calendar (for example free/busy status) using the CalDAV [7]
      protocol for Internet calendaring.
   Security considerations:
      See Section 4.
   Intended usage:
      COMMON
   Author:
      Rohan Mahy (rohan@ekabal.com)

   Note: These Enumservices use a dash "-" in the Type strings.  To
   allow for hierarchical concepts (as required in this case), some kind
   of boundary needs to be in place.  Neither RFC 3761 [1] nor its
   intended successor [17] foresee the concept of sub-subtyping.  The
   natural solution to address this requirement is the usage of dash "-"
   in Type strings, which is slightly contradictory to RFC 3761 [1].
   However, its intended successors [16] [17] clearly allow a dash "-"
   in Type strings, so that using "-" is seen as a practical way
   forward.

3.  Examples

   $ORIGIN 3.2.1.0.5.5.5.2.1.2.1.e164.arpa.
   @ NAPTR 10 100 "u" "E2U+ical-access:https" \
      "!^.*$!https://cal.example.com/home/alice/calendars/!" .

   $ORIGIN 3.2.1.0.5.5.5.2.1.2.1.e164.arpa.
   @ NAPTR 20 100 "u" "E2U+ical-sched:mailto" \
      "!^.*$!mailto:alice@example.com!" .

4.  Security Considerations

   The Domain Name System (DNS) does not make policy decisions about
   which records it provides to a DNS resolver.  All DNS records must be
   assumed to be available to all inquirers at all times.  The
   information provided within an ENUM record set must therefore be
   considered open to the public -- which is a cause for some privacy
   considerations.

   Revealing a calendaring URI by itself is unlikely to introduce many
   privacy concerns, although, depending on the structure of the URI, it
   might reveal the full name or employer of the target.  The use of
   anonymous URIs mitigates this risk.

   As ENUM uses DNS, which in its current form is an insecure protocol,
   there is no mechanism for ensuring that the answer returned to a
   query is authentic.  An analysis of threats specific to the
   dependence of ENUM on the DNS is provided in RFC 3761 [1], and a
   thorough analysis of threats to the DNS itself is covered in RFC 3833
   [14].  Many of these problems are prevented when the resolver
   verifies the authenticity of answers to its ENUM queries via DNSSEC
   (DNS Security, RFC 4035 [9]) in zones where it is available.

   More serious security concerns are associated with potential attacks
   against an underlying calendaring system (for example, unauthorized
   modification or viewing).  For this reason, iTIP discusses a number
   of security requirements (detailed in RFC 2446 [5]) that call for
   authentication, integrity and confidentiality properties, and similar
   measures to prevent such attacks.  Any calendaring protocol used in
   conjunction with a URI scheme currently meets these requirements.
   The use of CalDAV with the 'https:' scheme makes use of TLS
   (Transport Layer Security, RFC 5246 [13]) to provide server
   authentication, confidentiality, and message integrity.

   Unlike a traditional telephone number, the resource identified by an
   calendaring URI is often already guessable, and it often requires
   that users provide cryptographic credentials for authentication and
   authorization before calendar data can be exchanged.  Despite the
   public availability of ENUM records, the use of this information to
   reveal an unprotected calendaring resource is unlikely in practice.

5.  IANA Considerations

   This document requests registration of the "ical-sched" and "ical-
   access" Enumservices according to the definitions in Section 2 of
   this document and RFC 3761 [1].

6.  References

6.1.  Normative References

   [1]   Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
         Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
         Application (ENUM)", RFC 3761, April 2004.

   [2]   Mockapetris, P., "Domain names - concepts and facilities",
         STD 13, RFC 1034, November 1987.

   [3]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.

   [4]   Dawson, F. and Stenerson, D., "Internet Calendaring and
         Scheduling Core Object Specification (iCalendar)", RFC 2445,
         November 1998.

   [5]   Silverberg, S., Mansour, S., Dawson, F., and R. Hopson,
         "iCalendar Transport-Independent Interoperability Protocol
         (iTIP) Scheduling Events, BusyTime, To-dos and Journal
         Entries", RFC 2446, November 1998.

   [6]   Dawson, F., Mansour, S., and S. Silverberg, "iCalendar
         Message-Based Interoperability Protocol (iMIP)", RFC 2447,
         November 1998.

   [7]   Daboo, C., Desruisseaux, B., and L. Dusseault, "Calendaring
         Extensions to WebDAV (CalDAV)", RFC 4791, March 2007.

   [8]   Dusseault, L., "HTTP Extensions for Web Distributed Authoring
         and Versioning (WebDAV)", RFC 4918, June 2007.

   [9]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Protocol Modifications for the DNS Security Extensions",
         RFC 4035, March 2005.

6.2.  Informative References

   [10]  Mahoney, B., Babics, G., and A. Taler, "Guide to Internet
         Calendaring", RFC 3283, June 2002.

   [11]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.

   [12]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [13]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.2", RFC 5246, August 2008.

   [14]  Atkins, D. and R. Austein, "Threat Analysis of the Domain Name
         System (DNS)", RFC 3833, August 2004.

   [15]  Daboo, C. and B. Desruisseaux, "CalDAV Scheduling Extensions to
         WebDAV", Work in Progress, August 2009.

   [16]  Bradner, S., Conroy, L., and K. Fujiwara, "The E.164 to Uniform
         Resource Identifiers (URI) Dynamic Delegation Discovery System
         (DDDS) Application (ENUM)", Work in Progress, May 2009.

   [17]  Hoeneisen, B., Mayrhofer, A., and J. Livingood, "IANA
         Registration of Enumservices: Guide, Template and IANA
         Considerations", Work in Progress, September 2009.

Appendix A.  Acknowledgments

   Thanks to Lisa Dusseault and Alexander Mayrhofer for reviewing this
   document.

Authors' Addresses

   Rohan Mahy
   Unaffiliated

   EMail: rohan@ekabal.com


   Bernie Hoeneisen
   Swisscom
   CH-8000 Zuerich
   Switzerland

   EMail: bernie@ietf.hoeneisen.ch (bernhard.hoeneisen@swisscom.com)
   URI:   http://www.swisscom.com/