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SIP Working Group                                              J. Elwell
Internet-Draft                                                   Siemens
Intended status:  Informational                        February 15, 2008
Expires:  August 18, 2008

                      SIP E.164 Problem Statement

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   SIP has long supported the use of both email-style addresses
   (user@host) and telephone-style addresses (number@host) in the
   "From:" address.  A significant number of SIP deployments use the
   latter style with E.164 numbers.  This document describes the
   problems that occur when such E.164 numbers are used in SIP.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     2.1.  Domain part of a SIP URI  . . . . . . . . . . . . . . . . . 3
     2.2.  E.164 numbers as From: URIs . . . . . . . . . . . . . . . . 4
     2.3.  Authenticating a From: URI  . . . . . . . . . . . . . . . . 5
     2.4.  Using a received From: URI  . . . . . . . . . . . . . . . . 5
   3.  Summary of problem  . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Informative References  . . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9

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1.  Introduction

   The use of phone numbers with SIP was introduced with the TEL URL
   scheme [RFC3966].  In particular, this covered the use of E.164
   numbers [ITU.E164.1991], as used in the public switched telephone
   network (PSTN).  In RFC 3966, domain names were not used with fully-
   qualified E.164 phone numbers.

   SIP URIs always have domain names.  In SIP [RFC3261], a translation
   between SIP URIs and TEL URLs was described.  When translating from a
   SIP URI to a TEL URL, the domain name from the SIP URI is simply
   dropped.  When translating in the other direction (or simply
   generating a SIP URI from an E.164 number) it is not clear how to
   populate the domain name.

2.  Discussion

2.1.  Domain part of a SIP URI

   When an E.164 number is represented as a SIP URI, it includes a
   domain part.  Unfortunately there is no clear definition of what the
   domain part should contain.  On the one hand, it is clear that, in
   common with any SIP URI, the domain denoted by the domain part should
   at least be able to route the request onwards towards the
   destination.  Therefore if a SIP URI containing an E.164 number is
   the target of a SIP request, the request can be routed to the domain
   given in the domain part and that domain will be able to route the
   request onwards.

   However, this still leaves scope for putting different values into
   the domain part, subject to the identified domain being able to route
   requests onwards towards their correct destinations.  It has been
   suggested that the domain part should be the domain that "owns" the
   E.164 number, but the concept of ownership is unclear.  Does an
   enterprise domain "own" the E.164 numbers assigned to it?  Does a
   public service provider "own" the E.164 numbers assigned to an
   enterprise that it serves?  What if the enterprise obtains services
   associated with these numbers from multiple public service providers?
   Does a service provider "own" E.164 numbers assigned to an end user?
   Who is the "owner" in number portability situations?

   In practice, for a given E.164 number, different domain names tend to
   be used, although such use is perhaps not the original intent of
   [RFC3261].  For example, a service provider might always use its own
   domain name, regardless of whether the URI represents a number
   assigned to one of its users, a number assigned to a different
   service provider but served by that first service provider, or some

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   other number.  A service provider that hosts enterprise users might
   use the service provider's own domain name rather than that of the
   hosted enterprise.  An ENUM look-up on an E.164 number might yield a
   SIP URI with a domain via which the user of that number can be
   reached, but not necessarily the end domain of the user.  When E.164
   numbers are represented as SIP URIs in fields of SIP messages, the
   domain part often changes as the message progresses through different
   domains.  These considerations have a number of consequences.

2.2.  E.164 numbers as From: URIs

   When a UA receives an E.164 number represented as a SIP URI in a From
   header field, what does this say about the source of the request?  Of
   course, it should indicate that the request originated at a user who
   has a right to use that E.164 number and who can be reached by
   submitting a request targeted at that E.164 number.  However, the
   domain part means very little.  At the most it means that the request
   has, at some stage, passed through the domain, and that a return
   request to that E.164 number can be routed via that domain.  The
   request did not necessarily originate at that domain, but could
   simply have transited that domain.  For example, a request could
   originate with the following From URI:


   The request then passes through the service provider domain
   example2.com, which changes it to:


   Furthermore the domain in the received From URI is not necessarily
   the "owner" of that E.164 number.

   Similar considerations apply to E.164 numbers received as SIP URIs in
   the P-Asserted-Identity header field [RFC3325].

   In the PSTN world there is a concept of user-provided and network-
   provided caller numbers.  The two can differ when the user-provided
   number has been provided by an enterprise network (PBX) and denotes
   the particular enterprise user, whereas the network-provided number
   (the only one that the public network is able to authenticate) is the
   default number for the enterprise.  It depends on the particular
   operator whether a network delivers the user-provided number, the
   network-provided number of both to the called user.  The user-
   provided number is more useful for making a return call.  If a SIP
   service provider modifies the From URI to provide the equivalent of a
   network-provided number, the domain part might no longer reflect the
   true identity of the originator of the request.

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2.3.  Authenticating a From: URI

   SIP Identity [RFC4474] provides a means of authenticating a SIP URI
   in the From header field.  The Identity header field contains a
   signature that can be generated only by the domain that appears in
   the SIP URI in the From header field.  This means the request must
   have originated at or passed through that domain.  If a domain
   changes the From URI, any existing Identity signature will be
   invalidated and should be removed, but of course that domain can
   insert its own Identity signature, signing the new From URI.

   Note that although changing the From URI can be a reason for
   generating a new Identity signature, also the converse is true.  An
   Identity signature can be invalidated because other signed
   information (e.g., IP addresses and ports in SDP) has changed, and
   because a domain can sign only when its own domain name is in the
   From URI, it must change the From URI before signing.

2.4.  Using a received From: URI

   Even when authenticated, a received From URI can only indicate a
   domain through which a request has passed, not necessarily the domain
   in which it has originated.  This can be an issue if the UAS expects
   it to indicate a particular originating domain but in fact it
   indicates the domain of an intermediate service provider.  For

      Suppose the UAS has a white list of particular URIs or domains
      from which it accepts communications.  The domain at which a
      request originated might be in the white list, but if the From URI
      indicates another domain through which the request passed, the
      check against the white list might fail.

      Requests from the same originating domain but all routed through
      different intermediate domains might all arrive with different
      From URIs.  Attempts to correlate these requests will probably

      Any attempt by a UAS to correlate a received URI with that of a
      known communication partner and as a result provide relevant
      information to the user will fail if URIs are compared but the
      domain part of the received URI is different from that expected.

      If a user expects a particular communication to be to/from a
      particular domain (e.g., the user's bank), yet the authenticated
      From URI in an received request indicates a service provider's
      domain, the user might not be prepared to proceed with that
      communication, or might proceed but withhold information of a

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      sensitive nature.

   Some of these issues can be resolved if the domain part is ignored
   and only the E.164 number is used for comparison.  However, the last
   of these issues is a far more serious problem:  the user expects a
   communication partner to be from a particular domain (the E.164
   number is not necessarily an important factor).  Seeing that domain
   in the From:  URI, coupled with authentication by means of the
   Identity header field, would satisfy the user's expectation.  Seeing
   a different domain, that of an intermediate service provider, which
   may or may not be known to the user, would not satisfy the user's
   expectation.  The user might not be prepared to accept the unexpected
   URI and might decide not to proceed with the communication.

   This last point is particularly important when the media are to be
   secured using SRTP.  As a basis for this security, the communication
   partner with which encrypted and integrity-protected RTP packets are
   exchanged must be authenticated as the expected or an acceptable
   communication partner.  If this involves knowing the domain of the
   communication partner, then it is important that the From URI
   indicates the domain of the partner and not that of some intermediate
   service provider.  If the Identity signature also covers the
   fingerprint of the certificate used by the partner for establishing
   SRTP keys, then this binds the secure media stream to the From URI.
   If the From URI is not acceptable, the media stream cannot be
   regarded as secure.

3.  Summary of problem

   A SIP URI containing an E.164 number received in a From header field
   is not a reliable source domain of a request, even when authenticated
   by means of the Identity header field.  Only the E.164 number itself
   can be considered reliable, and only the E.164 itself is suitable for
   comparing with known identities at the destination (e.g., in a white
   list, in an address book).  The inability to depend on the domain
   part of an E.164 SIP URI is a serious deficiency in some situations.

4.  Requirements

   A solution addressing the problem must satisfy the following

   REQ1:  When a UAS receives a SIP request originated by a user
      identified by an E.164 number, the UAS must receive a SIP or SIPS
      URI containing that E.164 number and containing the originator's
      domain in the domain part.

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   REQ2:  When a UAS receives a SIP request that includes a SIP or SIPS
      URI identifying the originating user, if that URI contains an
      E.164 number that number alone, when placed in a TEL URI, must be
      suitable for routing a request back to the originating user.

   REQ3:  When a UAS receives a SIP request that includes a SIP or SIPS
      URI identifying the originating user, if that URI contains an
      E.164 number and the originating domain it must be possible to
      include in the request cryptographic evidence from that
      originating domain that binds secure media to that SIP URI.

   REQ1 is in principle met by the From URI, but not if it is modified
   by intermediate domains between the originating domain and the UAS.
   Obviously elimination of such practices would in theory be sufficient
   to satisfy REQ1, but this might not be achievable.  Therefore it
   might require new techniques.

   REQ2 requires new techniques.

   REQ3 can in theory be met by the Identity header field, but this is
   true only if intermediate domains do not modify the From URI or other
   signed information, such as IP addresses and ports in SDP (these are
   often changed by Session Border Controllers, SBCs) and contact URIs
   (these too are often changed by SBCs and other B2BUAs).  Therefore to
   solve this in a way that will work in most practical situations
   requires new techniques.

5.  IANA Considerations

   None; this document is informational.

6.  Security Considerations

   [[This section will be completed in a later version of this

7.  Acknowledgements

   The author thanks Dan Wing for encouraging the writing of this

8.  Informative References

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for

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              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

              International Telecommunications Union, "The International
              Public Telecommunication Numbering Plan", ITU-
              T Recommendation E.164, 1991.

Author's Address

   John Elwell
   Siemens Enterprise Communications GmbH & Co KG
   Hofmannstrasse 51
   D-81379 Munich

   Phone:  +44 115 943 4989
   Email:  john.elwell@siemens.com

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