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Network Working Group                                        J. PetersonRequest for Comments: 3764                                       NeuStarCategory: Standards Track                                     April 2004enumservice registration for Session Initiation Protocol (SIP)Addresses-of-RecordStatus 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.Copyright Notice   Copyright (C) The Internet Society (2004).  All Rights Reserved.Abstract   This document registers an Electronic Number (ENUM) service for the   Session Initiation Protocol (SIP), pursuant to the guidelines inRFC3761.  Specifically, this document focuses on provisioning SIP   addresses-of-record in ENUM.Table of Contents1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . . .22.  ENUM Service Registration . . . . . . . . . . . . . . . . . . .23.  Addresses-of-record in SIP. . . . . . . . . . . . . . . . . . .34.  The 'E2U+SIP' enumservice . . . . . . . . . . . . . . . . . . .55.  Example of E2U+SIP enumservice. . . . . . . . . . . . . . . . .56.  Security Considerations . . . . . . . . . . . . . . . . . . . .67.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .68.  References. . . . . . . . . . . . . . . . . . . . . . . . . . .68.1.  Normative References. . . . . . . . . . . . . . . . . . .68.2.  Informative References. . . . . . . . . . . . . . . . . .79.  Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . .710. Author's Address. . . . . . . . . . . . . . . . . . . . . . . .711. Full Copyright Statement. . . . . . . . . . . . . . . . . . . .8Peterson                    Standards Track                     [Page 1]

RFC 3764                    SIP enumservice                   April 20041.  Introduction   ENUM (E.164 Number Mapping,RFC 2916 [6]) is a system that uses DNS   (Domain Name Service, STD 13,RFC 1034 [3]) to translate telephone   numbers, like '+12025332600', into URIs (Uniform Resource   Identifiers,RFC 2396 [4]), like 'sip:egar@example.com'.  ENUM exists   primarily to facilitate the interconnection of systems that rely on   telephone numbers with those that use URIs to route transactions.   This document applies to the revised version of ENUM described inRFC3761.   SIP (Session Initiation Protocol,RFC 3261 [2]) is a text-based   application protocol that allows endpoints on the Internet to   discover one another in order to exchange context information about a   session they would like to share.  Common forms of communication that   are set up by SIP include Internet telephony, instant messaging,   video, Internet gaming and other forms of real-time communications.   SIP is a multi-service protocol capable of initiating sessions   involving different forms of real-time communications simultaneously.   SIP is a protocol that finds the best way for parties to communicate.2.  ENUM Service Registration   As defined in [1], the following is a template covering information   needed for the registration of the enumservice specified in this   document.      Enumservice Name: "E2U+SIP"      Type(s): "SIP"      Subtype(s): N/A      URI Scheme(s): "sip:", "sips:"      Functional Specification: seeSection 4      Security considerations: seeSection 6      Intended usage: COMMON      Author: Jon Peterson (jon.peterson@neustar.biz)      Any other information that the author deems interesting: SeeSection 3Peterson                    Standards Track                     [Page 2]

RFC 3764                    SIP enumservice                   April 20043.  Addresses-of-record in SIP   This document specifies an enumservice field that is appropriate for   SIP addresses-of-record URIs.  Various other types of URIs can be   present in SIP requests.  A URI that is associated with a particular   SIP user agent (for example, a SIP phone) is commonly known as a SIP   contact address.   The difference between a contact address and an address-of-record is   like the difference between a device and its user.  While there is no   formal distinction in the syntax of these two forms of addresses,   contact addresses are associated with a particular device, and may   have a very device-specific form (like sip:10.0.0.1, or   sip:edgar@ua21.example.com).  An address-of-record, however,   represents an identity of the user, generally a long-term identity,   and it does not have a dependency on any device; users can move   between devices or even be associated with multiple devices at one   time while retaining the same address-of-record.  A simple URI,   generally of the form 'sip:egdar@example.com', is used for an   address-of-record.   When a SIP request is created by a user agent, it populates the   address-of-record of its target in its To header field and   (generally) Request-URI.  The address-of-record of the user that is   sending the request populates the From header field of the message;   the contact address of the device from which the request is sent is   listed in the Contact header field.   By sending a registration to a registrar on behalf of its user, a SIP   device (i.e., a user agent) can temporarily associate its own contact   address with the user's address-of-record.  In so doing, the device   becomes eligible to receive requests that are sent to the address-   of-record.  Upon receiving the registration request, the registrar   modifies the provisioning data in a SIP location service to create a   mapping between the address-of-record for the user and the device   where the user can currently be reached.  When future requests arrive   at the administrative domain of this location service for the user in   question, proxy servers ask the location service where to find the   user, and will in turn discover the registered contact address(es).   A SIP-based follow-me telephony service, for example, would rely on   this real-time availability data in order to find the best place to   reach the end user without having to cycle through numerous devices   from which the user is not currently registered.  Note that   addresses-of-record can be registered with other addresses-of-record;   for example, while at home, a user might elect to register the   address-of-record they use as their personal identity under theirPeterson                    Standards Track                     [Page 3]

RFC 3764                    SIP enumservice                   April 2004   work address-of-record in order to direct requests for their work   identity to whatever devices they might have associated with their   home address-of-record.   When a SIP entity (be it a user agent or proxy server) needs to make   a forwarding decision for a Request-URI containing an address-of-   record, it uses the mechanisms described in the SIP specification   (RFC 3263) to locate the proper resource in the network.  Ordinarily,   this entails resolving the domain portion of the URI (example.com in   the example above) in order to route the call to a proxy server that   is responsible for that domain.   SIP user agents have specific communications capabilities (such as   the ability to initiate voice communications with particular codecs,   or support for particular SIP protocol extensions).  Because an   address-of-record does not represent any particular device or set of   devices, an address-of-record does not have capabilities as such.   When a SIP user agent sends a request to an address-of-record, it   begins a phase of capability negotiation that will eventually   discover the best way for the originator to communicate with the   target.  The originating user agent first expresses capabilities of   its own in the request it sends (and preferences for the type of   session it would like to initiate).  The expression of these   capabilities may entail the usage of SDP [8] to list acceptable types   of media supported and favored by the client, the inclusion of   Required/Supported headers to negotiate compatibility of extensions,   and possibly the usage of optional SIP extensions, for example using   callee capabilities [7] to communicate request handling dispositions.   Proxy servers or endpoints subsequently return responses that allow a   rich bidirectional capability negotiation process.   The process by which SIP endpoints negotiate capabilities can overlap   with the primary service provided by NAPTR records: permitting the   originating client to select a particular URI for communications   based on an ordered list of enumservices.  However, ENUM's capability   management mechanism is decidedly one-way - the administrator of the   telephone number expresses capabilities (in the form of protocol   names) and preferences that the client must evaluate without   negotiation.  Moreover, listing available protocols is not comparable   to agreement on session media (down to the codec/interval level) and   protocol extension support - it would be difficult to express, in the   level of detail necessary to arrange a desired session, the   capabilities of a SIP device within a NAPTR service field.   Provisioning contact addresses in ENUM rather than addresses-of-   record would compromise the SIP capability negotiation and discovery   process.  Much of the benefit of using a URI comes from the fact thatPeterson                    Standards Track                     [Page 4]

RFC 3764                    SIP enumservice                   April 2004   it represents a logical service associated with a user, rather than a   device - indeed, if ENUM wished to target particular devices,   'E2IPv4' would be a more appropriate resolution service to define   than 'E2U'.   SIP addresses-of-record may use the SIP URI scheme or the SIPS URI   scheme.  The SIPS URI scheme, when used in an address-of-record,   indicates that the user it represents can only be reached over a   secure connection (using TLS).4.  The 'E2U+SIP' enumservice   Traditionally, the services field of a NAPTR record (as defined in   [5]) contains a string that is composed of two subfields: a   'protocol' subfield and a 'resolution service' subfield.  ENUM in   particular defines an 'E2U' (E.164 to URI) resolution service.  This   document defines an 'E2U+SIP' enumservice for SIP.   The scheme of the URI that will appear in the regexp field of a NAPTR   record using the 'E2U+SIP' enumservice may either be 'SIP' or 'SIPS'.   This enumservice is best suited to SIP addresses-of-record.   When a SIP address-of-record appears in the regexp field of a NAPTR   record, there is no need to further qualify the enumservice field   with any capability data, since addresses-of-record do not have   capabilities.   There is also generally no need to have more than one NAPTR record   under a single telephone number that points to a SIP address-of-   record.   Note that the user portion of a SIP URI may contain a telephone   number (e.g., 'sip:+1442079460148@example.com').  Clients should be   careful to avoid infinite loops when recursively performing ENUM   queries on URIs that result from an ENUM lookup.5.  Example of E2U+SIP enumservice   The following is an example of the use of the enumservice registered   by this document in a NAPTR resource record.$ORIGIN 8.4.1.0.6.4.9.7.0.2.4.4.e164.arpa.   IN NAPTR 10 100 "u" "E2U+sip"    "!^.*$!sip:edgar@example.com!"     .Peterson                    Standards Track                     [Page 5]

RFC 3764                    SIP enumservice                   April 20046.  Security Considerations   A SIP address-of-record is a canonical address by which a user is   known - placing this address in ENUM is comparable to placing an   email address or a similar URI in the DNS.   DNS does not make policy decisions about the records that it shares   with an inquirer.  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 to be open to the public -   which is a cause for some privacy considerations.   Unlike a traditional telephone number, the resource identified by a   SIP URI may require that callers provide cryptographic credentials   for authentication and authorization before a user is alerted.  In   this respect, ENUM in concert with SIP can actually provide far   greater protection from unwanted callers than the existing PSTN,   despite the public availability of ENUM records.  An analysis of   threats specific to the dependence of ENUM on the DNS, and the   applicability of DNSSEC [9] to these, is provided in [1].7.  IANA Considerations   This document registers the 'E2U+SIP' enumservice under the   enumservice registry described in the IANA considerations inRFC3761.  Details of the registration are given inSection 2.8.  References8.1.  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]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:        Session Initiation Protocol",RFC 3261, May 2002.   [3]  Mockapetris, P., "Domain Names - Concepts and Facilities", STD        13,RFC 1034, November 1987.   [4]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource        Identifiers (URI): Generic Syntax",RFC 2396, August 1998.   [5]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part        Three: The Domain Name System (DNS) Database",RFC 3403, October        2002.Peterson                    Standards Track                     [Page 6]

RFC 3764                    SIP enumservice                   April 20048.2.  Informative References   [6]  Faltstrom, P., "E.164 number and DNS",RFC 2916, September 2000.   [7]  Rosenberg, J., Schulzrinne, H. and P. Kyzviat, "Indicating User        Agent Capabilities in the Session Initiation Protocol (SIP)",        Work in Progress.   [8]  Handley, M. and V. Jacobson, "SDP: Session Description        Protocol",RFC 2327, April 1998.   [9]  R. Arends, et al., "Protocol Modifications for the DNS Security        Extensions", Work in Progress.9.  Acknowledgements   Thanks to Richard Shockey for comments on the initial draft of this   document, and to Allison Mankin for valuable review comments.10.  Author's Address   Jon Peterson   NeuStar, Inc.   1800 Sutter St   Suite 570   Concord, CA  94520   USA   Phone: +1 925/363-8720   EMail: jon.peterson@neustar.biz   URI:http://www.neustar.biz/Peterson                    Standards Track                     [Page 7]

RFC 3764                    SIP enumservice                   April 200411.  Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Peterson                    Standards Track                     [Page 8]