RFC3764: enumservice registration for Session Initiation Protocol (SIP) Addresses-of-Record

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Network Working Group                                        J. Peterson
Request for Comments: 3764                                       NeuStar
Category: Standards Track                                     April 2004

     enumservice registration for Session Initiation Protocol (SIP)

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.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document registers an Electronic Number (ENUM) service for the
   Session Initiation Protocol (SIP), pursuant to the guidelines in RFC
   3761.  Specifically, this document focuses on provisioning SIP
   addresses-of-record in ENUM.

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  ENUM Service Registration . . . . . . . . . . . . . . . . . . . 2
   3.  Addresses-of-record in SIP. . . . . . . . . . . . . . . . . . . 3
   4.  The 'E2U+SIP' enumservice . . . . . . . . . . . . . . . . . . . 5
   5.  Example of E2U+SIP enumservice. . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
       8.1.  Normative References. . . . . . . . . . . . . . . . . . . 6
       8.2.  Informative References. . . . . . . . . . . . . . . . . . 7
   9.  Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . 7
   10. Author's Address. . . . . . . . . . . . . . . . . . . . . . . . 7
   11. Full Copyright Statement. . . . . . . . . . . . . . . . . . . . 8

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1.  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 in RFC

   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

      Enumservice Name: "E2U+SIP"

      Type(s): "SIP"

      Subtype(s): N/A

      URI Scheme(s): "sip:", "sips:"

      Functional Specification: see Section 4

      Security considerations: see Section 6

      Intended usage: COMMON

      Author: Jon Peterson (jon.peterson@neustar.biz)

      Any other information that the author deems interesting: See
      Section 3

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3.  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:, 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

   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 their

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   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 that

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   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

   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-

   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.

   IN NAPTR 10 100 "u" "E2U+sip"    "!^.*$!sip:edgar@example.com!"     .

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6.  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 in RFC
   3761.  Details of the registration are given in Section 2.

8.  References

8.1.  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

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8.2.  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

   Phone: +1 925/363-8720
   EMail: jon.peterson@neustar.biz
   URI:   http://www.neustar.biz/

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11.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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