





Network Working Group                                          M. Watson
Request for Comments: 3324                               Nortel Networks
Category: Informational                                    November 2002


         Short Term Requirements for Network Asserted Identity

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   A Network Asserted Identity is an identity initially derived by a
   Session Initiation Protocol (SIP) network intermediary as a result of
   an authentication process.  This document describes short term
   requirements for the exchange of Network Asserted Identities within
   networks of securely interconnected trusted nodes and to User Agents
   securely connected to such networks.

   There is no requirement for identities asserted by a UA in a SIP
   message to be anything other than the user's desired alias.























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.1 Identity . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.2 Network Asserted Identity  . . . . . . . . . . . . . . . . . .  3
   2.3 Trust Domains  . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.4 Spec(T)  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Generation of Networks Asserted Identity . . . . . . . . . . .  7
   4.  Transport of Network Asserted Identity . . . . . . . . . . . .  7
   4.1 Sending of Networks Asserted Identity within a Trust Domain  .  7
   4.2 Receiving of Network Asserted Identity within a Trust Domain .  7
   4.3 Sending of Network Asserted Identity to entities outside a
       Trust Domain . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.4 Receiving of Network Asserted Identity by a node outside the
       Trust Domain . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Parties with Network Asserted Identities . . . . . . . . . . .  8
   6.  Types of Network Asserted Identity . . . . . . . . . . . . . .  8
   7.  Privacy of Network Asserted Identity . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
       Normative References . . . . . . . . . . . . . . . . . . . . . 10
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11

1. Introduction

   SIP [1] allows users to assert their identity in a number of ways
   e.g., using the From: header.  However, there is no requirement for
   these identities to be anything other than the users desired alias.

   An authenticated identity of a user can be obtained using SIP Digest
   Authentication (or by other means).  However, UAs do not always have
   the necessary key information to authenticate another UA.

   A Network Asserted Identity is an identity initially derived by a SIP
   network intermediary as a result of an authentication process.  This
   may or may not be based on SIP Digest authentication.  This document
   describes short term requirements for the exchange of Network
   Asserted Identities within networks of securely interconnected
   trusted nodes and also to User Agents with secure connections to such
   networks.








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   Such a network is described in this document as a Trust Domain and we
   present a strict definition of trust and Trust Domain for the
   purposes of this document.  These short-term requirements provide
   only for the exchange of Network Asserted Identity within a Trust
   Domain and to an entity directly connected to the trust domain.

   General requirements for transport of Network Asserted Identities on
   the Internet are out of scope of this document.

2. Definitions

2.1 Identity

   An Identity, for the purposes of this document, is a sip:, sips: or
   tel:  URI, and optionally a Display Name.

   The URI MUST be meaningful to the domain identified in the URI (in
   the case of sip: or sips: URIs) or the owner of the E.164 number (in
   the case of tel: URIs), in the sense that when used as a SIP
   Request-URI in a request sent to that domain/number range owner, it
   would cause the request to be routed to the user/line that is
   associated with the identity, or to be processed by service logic
   running on that user's behalf.

   If the URI is a sip: or sips: URI, then depending on the local policy
   of the domain identified in the URI, the URI MAY identify some
   specific entity, such as a person.

   If the URI is a tel: URI, then depending on the local policy of the
   owner of the number range within which the telephone number lies, the
   number MAY identify some specific entity, such as a telephone line.
   However, it should be noted that identifying the owner of the number
   range is a less straightforward process than identifying the domain
   which owns a sip: or sips: URI.

2.2 Network Asserted Identity

   A Network Asserted Identity is an identity derived by a SIP network
   entity as a result of an authentication process, which identifies the
   authenticated entity in the sense defined in Section 2.1.

   In the case of a sip: or sips: URI, the domain included in the URI
   MUST be within the Trust Domain.

   In the case of a tel: URI, the owner of the E.164 number in the URI
   MUST be within the Trust Domain.





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   The authentication process used, or at least it's reliability/
   strength, is a known feature of the Trust Domain using the Network
   Asserted Identity mechanism i.e., in the language of 2.3 below, it is
   defined in Spec(T).

2.3 Trust Domains

   A Trust Domain for the purposes of Network Asserted Identity is a set
   of SIP nodes (UAC, UAS, proxies or other network intermediaries) that
   are trusted to exchange Network  Asserted Identity information in the
   sense described below.

   A node can be a member of a Trust Domain, T, only if the node is know
   to be compliant to a certain set of specifications, Spec(T), which
   characterize the handling of Network Asserted Identity within the
   Trust Domain, T.

   Trust Domains are constructed by human beings who know the properties
   of the equipment they are using/deploying.  In the simplest case, a
   Trust Domain is a set of devices with a single owner/operator who can
   accurately know the behaviour of those devices.

   Such simple Trust Domains may be joined into larger Trust Domains by
   bi-lateral agreements between the owners/operators of the devices.

   We say a node is 'trusted' (with respect to a given Trust Domain) if
   and only if it is a member of that domain.

   We say that a node, A, in the domain is 'trusted by' a node, B, (or
   'B trusts A') if and only if:

      1.  there is a secure connection between the nodes, AND

      2.  B has configuration information indicating that A is a member
          of the Trust Domain.

   Note that B may or may not be a member of the Trust Domain.  For
   example, B may be a UA which trusts a given network intermediary, A
   (e.g., its home proxy).

   A 'secure connection' in this context means that messages cannot be
   read by third parties, cannot be modified by third parties without
   detection and that B can be sure that the message really did come
   from A.  The level of security required is a feature of the Trust
   Domain i.e., it is defined in Spec(T).






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   Within this context, SIP signaling information received by one node
   FROM a node that it trusts is known to have been generated and passed
   through the network according to the procedures of the particular
   specification set Spec(T), and therefore can be known to be valid, or
   at least as valid as specified in the specifications Spec(T).

   Equally, a node can be sure that signaling information passed TO a
   node that it trusts will be handled according to the procedures of
   Spec(T).

   For these capabilities to be useful, Spec(T) must contain
   requirements as to how the Network Asserted Identity is generated,
   how its privacy is protected and how its integrity is maintained as
   it is passed around the network.  A reader of Spec(T) can then make
   an informed judgement about the authenticity and reliability of
   Network Asserted Information received from the Trust Domain T.

   The term 'trusted' (with respect to a given Trust Domain) can be
   applied to a given node in an absolute sense - it is just equivalent
   to saying the node is a member of the Trust Domain.  However, the
   node itself does not know whether another arbitrary node is
   'trusted', even within the Trust Domain.  It does know about certain
   nodes with which it has secure connections as described above.

   With the definition above, statements such as 'A trusted node SHALL
   ...' are just shorthand for 'A node compliant to this specification
   SHALL...'.

   Statements such as 'When a node receives information from a trusted
   node...' are NOT valid, because one node does not have complete
   knowledge about all the other nodes in the trust domain.

   Statements such as 'When a node receives information from another
   node that it trusts...' ARE valid, and should be interpreted
   according to the criteria (1) and (2) above.
















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   The above relationships are illustrated in the following figure:

                                        +------+
                                        |      |
                                        |  F   |
                                        |      |
                                        +------+
                                            x
              ..............................x.........
              .                             x        .
              .    +------+             +------+     .    +------+
              .    |      |             |      |     .    |      |
              .    |  A   |             |  B   |-----.----|  E   |
              .    |      |             |      |     .    |      |
              .    +------+             +------+     .    +------+
              .       \\                   /         .
              .         \\    +------+   //          .
              .           \\  |      | //            .
              .             \ |  C   |/              .
              .               |      |               .
              .               +------+               .
              .                   |      Trust Domain.
              ........................................
                                  |
                                  |
                              +------+
                              |      |
                              |  D   |
                              |      |
                              +------+

          xxxxxx   Insecure connection
          ------   Secure connection

          ......
          .    .All boxes within the dotted line
          ......are part of the same Trust Domain


   o  A, B and C are part of the same trust domain

   o  A trusts C, but A does not trust B

   o  since E knows that B is inside of the trust domain, E

   o  trusts B, but B does not trust E

   o  B does not trust F, F does not trust B



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2.4 Spec(T)

   An aspect of the definition of a trust domain is that all the
   elements in that domain are compliant to a set of configurations and
   specifications generally referred to as Spec(T).  Spec(T) is not a
   specification in the sense of a written document; rather, its an
   agreed upon set of information that all elements are aware of.
   Proper processing of the asserted identities requires that the
   elements know what is actually being asserted, how it was determined,
   and what the privacy policies are.  All of that information is
   characterized by Spec(T).

3. Generation of Networks Asserted Identity

   A Network Asserted Identity is generated by a network intermediary
   following an Authentication process which authenticates the entity
   (UA) to be identified.

   The Authentication process(es) used are a characteristic feature of
   the Trust Domain, and MUST be specified in Spec(T).

   It shall be possible for a UA to provide a preferred identity to the
   network intermediary, which MAY be used to inform the generation of
   the Network Asserted Identity according to the policies of the Trust
   Domain.

4. Transport of Network Asserted Identity

4.1 Sending of Networks Asserted Identity within a Trust Domain

   It shall be possible for one node within a Trust Domain to securely
   send a Network Asserted Identity to another node that it trusts.

4.2 Receiving of Network Asserted Identity within a Trust Domain

   It shall be possible for one node within a Trust Domain to receive a
   Network Asserted identity from another node that it trusts.

4.3 Sending of Network Asserted Identity to entities outside a Trust
    Domain

   If a node, A, within the Trust Domain, is trusted by a node, B,
   outside the Trust Domain, then it shall be possible for A to securely
   send a Network Asserted Identity to B, if allowed by the privacy
   policies of the user that has been identified, and the trust domain.

   This is most often used to pass a Network Asserted Identity directly
   to a UA.



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4.4 Receiving of Network Asserted Identity by a node outside the Trust
    Domain

   It shall be possible for a node outside the Trust Domain to receive a
   Network Asserted Identity from a node that it trusts.

   Network Asserted Identity received in this way may be considered
   valid, and used for display to the user, input data for services etc.

   Network Asserted Identity information received by one node from a
   node which it does not trust carries no guarantee of authenticity or
   integrity because it is not known that the procedures of Spec(T) were
   followed to generate and transport the information.  Such information
   MUST NOT be used.  (i.e., it shall not be displayed to the user,
   passed to other nodes, used as input data for services, etc.)

5. Parties with Network Asserted Identities

   A Network Asserted Identity identifies the originator of the message
   in which it was received.

   For example,

      a Network Asserted Identity received in an initial INVITE (outside
      the context of any existing dialog) identifies the calling party.

      a Network Asserted Identity received in a 180 Ringing response to
      such an INVITE identifies the party who is ringing.

      a Network Asserted Identity received in a 200 response to such an
      INVITE identifies the party who has answered.

6. Types of Network Asserted Identity

   It shall be possible to assert multiple identities associated with a
   given party (in a given message), provided that these are of distinct
   types.

   The types of identity supported shall be sip:, sips: and tel: URIs,
   all of which identify the user as described in Section 2.1.  It is
   not required to transport both a sip: and sips: URI.

   It shall be possible for the capability to transport additional types
   of identity associated with a single party to be introduced in
   future.






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7. Privacy of Network Asserted Identity

   The means by which any privacy requirements in respect of the Network
   Asserted Identity are determined are outside the scope of this
   document.

   It shall be possible to indicate within a message containing a
   Network Asserted Identity that this Network Asserted Identity is
   subject to a privacy requirement which prevents it being passed to
   other users.  This indication should not carry any semantics as to
   the reason for this privacy requirement.

   It shall be possible to indicate that the user has requested that the
   Network Asserted Identity be not passed to other users.  This is
   distinct from the above indication, in that it implies specific user
   intent with respect to the Network Asserted Identity.

   The mechanism shall support Trust Domain policies where the above two
   indications are equivalent (i.e., the only possible reason for a
   privacy requirement is a request from the user), and policies where
   they are not.

   In this case, the Network Asserted Identity specification shall
   require that the mechanism of Section 4.3 SHALL NOT be used i.e., a
   trusted node shall not pass the identity to a node it does not trust.
   However, the mechanism of Section 4.3 MAY be used to transfer the
   identity within the trusted network.

   Note that 'anonymity' requests from users or subscribers may well
   require functionality in addition to the above handling of Network
   Asserted Identities.  Such additional functionality is out of the
   scope of this document.

8. Security Considerations

   The requirements in this document are NOT intended to result in a
   mechanism with general applicability between arbitrary hosts on the
   Internet.

   Rather, the intention is to state requirements for a mechanism to be
   used within a community of devices which are known to obey the
   specification of the mechanism (Spec(T)) and between which there are
   secure connections.  Such a community is known here as a Trust
   Domain.

   The requirements on the mechanisms used for security and to initially
   derive the Network Asserted Identity must be part of the
   specification Spec(T).



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   The requirements also support the transfer of information from a node
   within the Trust Domain, via a secure connection to a node outside
   the Trust Domain.

   Use of this mechanism in any other context has serious security
   shortcomings, namely that there is absolutely no guarantee that the
   information has not been modified, or was even correct in the first
   place.

9. IANA Considerations

   This document does not have any implications for IANA.

10. Acknowledgments

   Thanks are due to Jon Peterson, Cullen Jennings, Allison Mankin and
   Jonathan Rosenberg for comments on this document.

Normative References

   [1]  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.

Author's Address

   Mark Watson
   Nortel Networks
   Maidenhead Office Park
   Westacott Way
   Maidenhead, BERKS  SL6 3QH
   UK

   EMail: mwatson@nortelnetworks.com

















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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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