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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

Peterson Standards Track [Page 1] RFC 3764 SIP enumservice April 2004

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

Peterson Standards Track [Page 2] RFC 3764 SIP enumservice April 2004

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  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 '', 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

Peterson 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 ( 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

Peterson 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
 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., '').  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"    "!^.*$!!"     .

Peterson Standards Track [Page 5] RFC 3764 SIP enumservice April 2004

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

Peterson Standards Track [Page 6] RFC 3764 SIP enumservice April 2004

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

Peterson Standards Track [Page 7] RFC 3764 SIP enumservice April 2004

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

Intellectual Property

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 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
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 The IETF invites any interested party to bring to its attention any
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 this standard.  Please address the information to the IETF at ietf-


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 Internet Society.

Peterson Standards Track [Page 8]

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