ftp.cc.uoc.gr
rfc3325
This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 3744, EID 3894
Network Working Group                                        C. Jennings
Request for Comments: 3325                                 Cisco Systems
Category: Informational                                      J. Peterson
                                                           NeuStar, Inc.
                                                               M. Watson
                                                         Nortel Networks
                                                           November 2002


    Private Extensions to the Session Initiation Protocol (SIP) for
               Asserted Identity within Trusted Networks

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

   This document describes private extensions to the Session Initiation
   Protocol (SIP) that enable a network of trusted SIP servers to assert
   the identity of authenticated users, and the application of existing
   privacy mechanisms to the identity problem.  The use of these
   extensions is only applicable inside an administrative domain with
   previously agreed-upon policies for generation, transport and usage
   of such information.  This document does NOT offer a general privacy
   or identity model suitable for use between different trust domains,
   or use in the Internet at large.

Table of Contents

   1.   Applicability Statement  . . . . . . . . . . . . . . . . . .   2
   2.   Conventions  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.   Overview . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.   Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . .   5
   6.   Hints for Multiple Identities  . . . . . . . . . . . . . . .   6
   7.   Requesting Privacy . . . . . . . . . . . . . . . . . . . . .   6
   8.   User Agent Server Behavior . . . . . . . . . . . . . . . . .   7
   9.   Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . .   7
        9.1  The P-Asserted-Identity Header  . . . . . . . . . . . .   8
        9.2  The P-Preferred-Identity Header . . . . . . . . . . . .   8
        9.3  The "id" Privacy Type . . . . . . . . . . . . . . . . .   9

   10.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .   9
        10.1 Network Asserted Identity passed to trusted gateway . .   9
        10.2 Network Asserted Identity Withheld  . . . . . . . . . .  11
   11.  Example of Spec(T) . . . . . . . . . . . . . . . . . . . . .  13
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  14
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  14
        13.1 Registration of new SIP header fields . . . . . . . . .  14
        13.2 Registration of "id" privacy type for SIP Privacy header 15
   14.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  15
        Normative References . . . . . . . . . . . . . . . . . . . .  15
        Informational References . . . . . . . . . . . . . . . . . .  16
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  17
        Full Copyright Statement . . . . . . . . . . . . . . . . . .  18

1. Applicability Statement

   This document describes private extensions to SIP [1] that enable a
   network of trusted SIP servers to assert the identity of end users or
   end systems, and to convey indications of end-user requested privacy.
   The use of these extensions is only applicable inside a 'Trust
   Domain' as defined in Short term requirements for Network Asserted
   Identity [5].  Nodes in such a Trust Domain are explicitly trusted by
   its users and end-systems to publicly assert the identity of each
   party, and to be responsible for withholding that identity outside of
   the Trust Domain when privacy is requested.  The means by which the
   network determines the identity to assert is outside the scope of
   this document (though it commonly entails some form of
   authentication).

   A key requirement of [5] is that the behavior of all nodes within a
   given Trust Domain 'T' is known to comply to a certain set of
   specifications known as 'Spec(T)'.  Spec(T) MUST specify behavior for
   the following:

   1. The manner in which users are authenticated

   2. The mechanisms used to secure the communication among nodes within
      the Trust Domain

   3. The mechanisms used to secure the communication between UAs and
      nodes within the Trust Domain

   4. The manner used to determine which hosts are part of the Trust
      Domain

   5. The default privacy handling when no Privacy header field is
      present

   6. That nodes in the Trust Domain are compliant to SIP [1]

   7. That nodes in the Trust Domain are compliant to this document

   8. Privacy handling for identity as described in Section 7.

   An example of a suitable Spec(T) is shown in Section 11.

   This document does NOT offer a general privacy or identity model
   suitable for inter-domain use or use in the Internet at large.  Its
   assumptions about the trust relationship between the user and the
   network may not apply in many applications.  For example, these
   extensions do not accommodate a model whereby end users can
   independently assert their identity by use of the extensions defined
   here.  Furthermore, since the asserted identities are not
   cryptographically certified, they are subject to forgery, replay, and
   falsification in any architecture that does not meet the requirements
   of [5].

   The asserted identities also lack an indication of who specifically
   is asserting the identity, and so it must be assumed that the Trust
   Domain is asserting the identity.  Therefore, the information is only
   meaningful when securely received from a node known to be a member of
   the Trust Domain.

   Despite these limitations, there are sufficiently useful specialized
   deployments that meet the assumptions described above, and can accept
   the limitations that result, to warrant informational publication of
   this mechanism.  An example deployment would be a closed network
   which emulates a traditional circuit switched telephone network.

2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [3].

   Throughout this document requirements for or references to proxy
   servers or proxy behavior apply similarly to other intermediaries
   within a Trust Domain (ex: B2BUAs).

   The terms Identity, Network Asserted Identity and Trust Domain in
   this document have meanings as defined in [5].

3. Introduction

   Various providers offering a telephony service over IP networks have
   selected SIP as a call establishment protocol.  Their environments
   require a way for trusted network elements operated by the service
   providers (for example SIP proxy servers) to communicate the identity
   of the subscribers to such a service, yet also need to withhold this
   information from entities that are not trusted when necessary.  Such
   networks typically assume some level of transitive trust amongst
   providers and the devices they operate.

   These networks need to support certain traditional telephony services
   and meet basic regulatory and public safety requirements.  These
   include Calling Identity Delivery services, Calling Identity Delivery
   Blocking, and the ability to trace the originator of a call.  While
   baseline SIP can support each of these services independently,
   certain combinations cannot be supported without the extensions
   described in this document.  For example, a caller that wants to
   maintain privacy and consequently provides limited information in the
   SIP From header field will not be identifiable by recipients of the
   call unless they rely on some other means to discover the identity of
   the caller.  Masking identity information at the originating user
   agent will prevent certain services, e.g., call trace, from working
   in the Public Switched Telephone Network (PSTN) or being performed at
   intermediaries not privy to the authenticated identity of the user.

   This document attempts to provide a network asserted identity service
   using a very limited, simple mechanism, based on requirements in [5].
   This work is derived from a previous attempt, [6], to solve several
   problems related to privacy and identity in Trust Domains.  A more
   comprehensive mechanism, [7] which uses cryptography to address this
   problem is the subject of current study by the SIP working group.

   Providing privacy in a SIP network is more complicated than in the
   PSTN.  In SIP networks, the participants in a session are typically
   able to exchange IP traffic directly without involving any SIP
   service provider.  The IP addresses used for these sessions may
   themselves reveal private information.  A general purpose mechanism
   for providing privacy in a SIP environment is discussed in [2].  This
   document applies that privacy mechanism to the problem of network
   asserted identity.

4. Overview

   The mechanism proposed in this document relies on a new header field
   called 'P-Asserted-Identity' that contains a URI (commonly a SIP URI)
   and an optional display-name, for example:

      P-Asserted-Identity: "Cullen Jennings" <sip:fluffy@cisco.com>

   A proxy server which handles a message can, after authenticating the
   originating user in some way (for example: Digest authentication),
   insert such a P-Asserted-Identity header field into the message and
   forward it to other trusted proxies.  A proxy that is about to
   forward a message to a proxy server or UA that it does not trust MUST
   remove all the P-Asserted-Identity header field values if the user
   requested that this information be kept private.  Users can request
   this type of privacy as described in Section 7.

   The formal syntax for the P-Asserted-Identity header is presented in
   Section 9.

5. Proxy Behavior

   A proxy in a Trust Domain can receive a message from a node that it
   trusts, or a node that it does not trust.  When a proxy receives a
   message from a node it does not trust and it wishes to add a P-
   Asserted-Identity header field, the proxy MUST authenticate the
   originator of the message, and use the identity which results from
   this authentication to insert a P-Asserted-Identity header field into
   the message.

   If the proxy receives a message (request or response) from a node
   that it trusts, it can use the information in the P-Asserted-Identity
   header field, if any, as if it had authenticated the user itself.

   If there is no P-Asserted-Identity header field present, a proxy MAY
   add one containing at most one SIP or SIPS URI, and at most one tel
   URL.  If the proxy received the message from an element that it does
   not trust and there is a P-Asserted-Identity header present which
   contains a SIP or SIPS URI, the proxy MUST replace that SIP or SIPS
   URI with a single SIP or SIPS URI or remove this header field.
   Similarly, if the proxy received the message from an element that it
   does not trust and there is a P-Asserted-Identity header present
   which contains a tel URI, the proxy MUST replace that tel URI with a
   single tel URI or remove the header field.

   When a proxy forwards a message to another node, it must first
   determine if it trusts that node or not.  If it trusts the node, the
   proxy does not remove any P-Asserted-Identity header fields that it

   generated itself, or that it received from a trusted source.  If it
   does not trust the element, then the proxy MUST examine the Privacy
   header field (if present) to determine if the user requested that
   asserted identity information be kept private.

6. Hints for Multiple Identities

   If a P-Preferred-Identity header field is present in the message that
   a proxy receives from an entity that it does not trust, the proxy MAY
   use this information as a hint suggesting which of multiple valid
   identities for the authenticated user should be asserted.  If such a
   hint does not correspond to any valid identity known to the proxy for
   that user, the proxy can add a P-Asserted-Identity header of its own
   construction, or it can reject the request (for example, with a 403
   Forbidden).  The proxy MUST remove the user-provided P-Preferred-
   Identity header from any message it forwards.

   A user agent only sends a P-Preferred-Identity header field to proxy
   servers in a Trust Domain; user agents MUST NOT populate the P-
   Preferred-Identity header field in a message that is not sent
   directly to a proxy that is trusted by the user agent.  Were a user
   agent to send a message containing a P-Preferred-Identity header
   field to a node outside a Trust Domain, then the hinted identity
   might not be managed appropriately by the network, which could have
   negative ramifications for privacy.

7. Requesting Privacy

   Parties who wish to request the removal of P-Asserted-Identity header
   fields before they are transmitted to an element that is not trusted
   may add the "id" privacy token defined in this document to the
   Privacy header field.  The Privacy header field is defined in [6].
   If this token is present, proxies MUST remove all the P-Asserted-
   Identity header fields before forwarding messages to elements that
   are not trusted.  If the Privacy header field value is set to "none"
   then the proxy MUST NOT remove the P-Asserted-Identity header fields.

   When a proxy is forwarding the request to an element that is not
   trusted and there is no Privacy header field, the proxy MAY include
   the P-Asserted-Identity header field or it MAY remove it.  This
   decision is a policy matter of the Trust Domain and MUST be specified
   in Spec(T).  It is RECOMMENDED that the P-Asserted-Identity header
   fields SHOULD NOT be removed unless local privacy policies prevent
   it, because removal may cause services based on Asserted Identity to
   fail.

   However, it should be noted that unless all users of the Trust Domain
   have access to appropriate privacy services, forwarding of the P-
   Asserted-Identity may result in disclosure of information which the
   user has not requested and cannot prevent.  It is therefore STRONGLY
   RECOMMENDED that all users have access to privacy services as
   described in this document.

   Formal specification of the "id" Privacy header priv-value is
   described in Section 9.3.  Some general guidelines for when users
   require privacy are given in [2].

   If multiple P-Asserted-Identity header field values are present in a
   message, and privacy of the P-Asserted-Identity header field is
   requested, then all instances of the header field values MUST be
   removed before forwarding the request to an entity that is not
   trusted.

8. User Agent Server Behavior

   Typically, a user agent renders the value of a P-Asserted-Identity
   header field that it receives to its user.  It may consider the
   identity provided by a Trust Domain to be privileged, or
   intrinsically more trustworthy than the From header field of a
   request.  However, any specific behavior is specific to
   implementations or services.  This document also does not mandate any
   user agent handling for multiple P-Asserted-Identity header field
   values that happen to appear in a message (such as a SIP URI
   alongside a tel URL).

   However, if a User Agent Server receives a message from a previous
   element that it does not trust, it MUST NOT use the P-Asserted-
   Identity header field in any way.

   If a UA is part of the Trust Domain from which it received a message
   containing a P-Asserted-Identity header field, then it can use the
   value freely but it MUST ensure that it does not forward the
   information to any element that is not part of the Trust Domain, if
   the user has requested that asserted identity information be kept
   private.

   If a UA is not part of the Trust Domain from which it received a
   message containing a P-Asserted-Identity header field, then it can
   assume this information does not need to be kept private.

9. Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [4].

9.1 The P-Asserted-Identity Header

   The P-Asserted-Identity header field is used among trusted SIP
   entities (typically intermediaries) to carry the identity of the user
   sending a SIP message as it was verified by authentication.

      PAssertedID = "P-Asserted-Identity" HCOLON PAssertedID-value
                      *(COMMA PAssertedID-value)
      PAssertedID-value = name-addr / addr-spec

   A P-Asserted-Identity header field value MUST consist of exactly one 
name-addr or addr-spec.  If the URI contains a comma, the URI MUST
be enclosed in angle brackets (< and >).  There may be one or two P-Asserted-Identity
EID 3744 (Verified) is as follows:

Section: 9.1

Original Text:

A P-Asserted-Identity header field value MUST consist of exactly one
name-addr or addr-spec.

Corrected Text:

A P-Asserted-Identity header field value MUST consist of exactly one
name-addr or addr-spec.  If the URI contains a comma, the URI MUST
be enclosed in angle brackets (< and >).
Notes:
While the P-Asserted-Identity and P-Preferred-Identity header fields have an ambiguity only for "," (not for ";" and "?"), we note that usage of ";" and "?" also must be enclosed in angle brackets to preserve consistency with the RFC 3261 section 20 bracket rule.
values. If there is one value, it MUST be a sip, sips, or tel URI. If there are two values, one value MUST be a sip or sips URI and the other MUST be a tel URI. It is worth noting that proxies can (and will) add and remove this header field. This document adds the following entry to Table 2 of [1]: Header field where proxy ACK BYE CAN INV OPT REG ------------ ----- ----- --- --- --- --- --- --- P-Asserted-Identity adr - o - o o - SUB NOT REF INF UPD PRA --- --- --- --- --- --- o o o - - - 9.2 The P-Preferred-Identity Header The P-Preferred-Identity header field is used from a user agent to a trusted proxy to carry the identity the user sending the SIP message wishes to be used for the P-Asserted-Header field value that the trusted element will insert. PPreferredID = "P-Preferred-Identity" HCOLON PPreferredID-value *(COMMA PPreferredID-value) PPreferredID-value = name-addr / addr-spec A P-Preferred-Identity header field value MUST consist of exactly one name-addr or addr-spec. If the URI contains a comma, the URI MUST be enclosed in angle brackets (< and >). There may be one or two P-Preferred-Identity
EID 3894 (Verified) is as follows:

Section: 9.2

Original Text:

A P-Preferred-Identity header field value MUST consist of exactly one
name-addr or addr-spec.

Corrected Text:

A P-Preferred-Identity header field value MUST consist of exactly one
name-addr or addr-spec.  If the URI contains a comma, the URI MUST
be enclosed in angle brackets (< and >).
Notes:
While the P-Asserted-Identity and P-Preferred-Identity header fields have an ambiguity only for "," (not for ";" and "?"), we note that usage of ";" and "?" also must be enclosed in angle brackets to preserve consistency with the RFC 3261 section 20 bracket rule.
values. If there is one value, it MUST be a sip, sips, or tel URI. If there are two values, one value MUST be a sip or sips URI and the other MUST be a tel URI. It is worth noting that proxies can (and will) remove this header field. This document adds the following entry to Table 2 of [1]: Header field where proxy ACK BYE CAN INV OPT REG ------------ ----- ----- --- --- --- --- --- --- P-Preferred-Identity adr - o - o o - SUB NOT REF INF UPD PRA --- --- --- --- --- --- o o o - - - 9.3 The "id" Privacy Type This specification adds a new privacy type ("priv-value") to the Privacy header, defined in [2]. The presence of this privacy type in a Privacy header field indicates that the user would like the Network Asserted Identity to be kept private with respect to SIP entities outside the Trust Domain with which the user authenticated. Note that a user requesting multiple types of privacy MUST include all of the requested privacy types in its Privacy header field value. priv-value = "id" Example: Privacy: id 10. Examples 10.1 Network Asserted Identity passed to trusted gateway In this example, proxy.cisco.com creates a P-Asserted-Identity header field from an identity it discovered from SIP Digest authentication. It forwards this information to a trusted proxy which forwards it to a trusted gateway. Note that these examples consist of partial SIP messages that illustrate only those headers relevant to the authenticated identity problem. * F1 useragent.cisco.com -> proxy.cisco.com INVITE sip:+14085551212@cisco.com SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-123 To: <sip:+14085551212@cisco.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 1 INVITE Max-Forwards: 70 Privacy: id * F2 proxy.cisco.com -> useragent.cisco.com SIP/2.0 407 Proxy Authorization Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-123 To: <sip:+14085551212@cisco.com>;tag=123456 From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 1 INVITE Proxy-Authenticate: .... realm="sip.cisco.com" * F3 useragent.cisco.com -> proxy.cisco.com INVITE sip:+14085551212@cisco.com SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-124 To: <sip:+14085551212@cisco.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 70 Privacy: id Proxy-Authorization: .... realm="sip.cisco.com" user="fluffy" * F4 proxy.cisco.com -> proxy.pstn.net (trusted) INVITE sip:+14085551212@proxy.pstn.net SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-124 Via: SIP/2.0/TCP proxy.cisco.com;branch=z9hG4bK-abc To: <sip:+14085551212@cisco.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 69 P-Asserted-Identity: "Cullen Jennings" <sip:fluffy@cisco.com> P-Asserted-Identity: tel:+14085264000 Privacy: id * F5 proxy.pstn.net -> gw.pstn.net (trusted) INVITE sip:+14085551212@gw.pstn.net SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-124 Via: SIP/2.0/TCP proxy.cisco.com;branch=z9hG4bK-abc Via: SIP/2.0/TCP proxy.pstn.net;branch=z9hG4bK-a1b2 To: <sip:+14085551212@cisco.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 68 P-Asserted-Identity: "Cullen Jennings" <sip:fluffy@cisco.com> P-Asserted-Identity: tel:+14085264000 Privacy: id 10.2 Network Asserted Identity Withheld In this example, the User Agent sends an INVITE that indicates it would prefer the identity sip:fluffy@cisco.com to the first proxy, which authenticates this with SIP Digest. The first proxy creates a P-Asserted-Identity header field and forwards it to a trusted proxy (outbound.cisco.com). The next proxy removes the P-Asserted-Identity header field and the request for Privacy before forwarding this request onward to the biloxi.com proxy server which it does not trust. * F1 useragent.cisco.com -> proxy.cisco.com INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-a111 To: <sip:bob@biloxi.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 1 INVITE Max-Forwards: 70 Privacy: id P-Preferred-Identity: "Cullen Jennings" <sip:fluffy@cisco.com> * F2 proxy.cisco.com -> useragent.cisco.com SIP/2.0 407 Proxy Authorization Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-a111 To: <sip:bob@biloxi.com>;tag=123456 From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 1 INVITE Proxy-Authenticate: .... realm="cisco.com" * F3 useragent.cisco.com -> proxy.cisco.com INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-a123 To: <sip:bob@biloxi.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 70 Privacy: id P-Preferred-Identity: "Cullen Jennings" <sip:fluffy@cisco.com> Proxy-Authorization: .... realm="cisco.com" user="fluffy" * F4 proxy.cisco.com -> outbound.cisco.com (trusted) INVITE sip:bob@biloxi SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-a123 Via: SIP/2.0/TCP proxy.cisco.com;branch=z9hG4bK-b234 To: <sip:bob@biloxi.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 69 P-Asserted-Identity: "Cullen Jennings" <sip:fluffy@vovida.org> Privacy: id * F5 outbound.cisco.com -> proxy.biloxi.com (not trusted) INVITE sip:bob@biloxi SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-a123 Via: SIP/2.0/TCP proxy.cisco.com;branch=z9hG4bK-b234 Via: SIP/2.0/TCP outbound.cisco.com;branch=z9hG4bK-c345 To: <sip:bob@biloxi.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 68 Privacy: id * F6 proxy.biloxi.com -> bobster.biloxi.com INVITE sip:bob@bobster.biloxi.com SIP/2.0 Via: SIP/2.0/TCP useragent.cisco.com;branch=z9hG4bK-a123 Via: SIP/2.0/TCP proxy.cisco.com;branch=z9hG4bK-b234 Via: SIP/2.0/TCP outbound.cisco.com;branch=z9hG4bK-c345 Via: SIP/2.0/TCP proxy.biloxi.com;branch=z9hG4bK-d456 To: <sip:bob@biloxi.com> From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748 Call-ID: 245780247857024504 CSeq: 2 INVITE Max-Forwards: 67 Privacy: id 11. Example of Spec(T) The integrity of the mechanism described in this document relies on one node knowing (through configuration) that all of the nodes in a Trust Domain will behave in a predetermined way. This requires the predetermined behavior to be clearly defined and for all nodes in the Trust Domain to be compliant. The specification set that all nodes in a Trust Domain T must comply with is termed 'Spec(T)'. The remainder of this section presents an example Spec(T), which is not normative in any way. 1. Protocol requirements The following specifications MUST be supported: 1. RFC 3261 2. RFC 3325 2. Authentication requirements Users MUST be authenticated using SIP Digest Authentication. 3. Security requirements Connections between nodes within the Trust Domain and between UAs and nodes in the Trust Domain MUST use TLS using a cipher suite of RSA_WITH_AES_128_CBC_SHA1. Mutual authentication between nodes in the trust domain MUST be performed and confidentiality MUST be negotiated. 4. Scope of Trust Domain The Trust Domain specified in this agreement consists of hosts which posses a valid certificate which is a) signed by examplerootca.org; b) whose subjectAltName ends with one of the following domain names: trusted.div1.carrier-a.net, trusted.div2.carrier-a.net, sip.carrier-b.com; and c) whose domain name corresponds to the hostname in the subjectAltName in the certificate. 5. Implicit handling when no Privacy header is present The elements in the trust domain must support the 'id' privacy service therefore absence of a Privacy header can be assumed to indicate that the user is not requesting any privacy. If no Privacy header field is present in a request, elements in this Trust Domain MUST act as if no privacy is requested. 12. Security Considerations The mechanism provided in this document is a partial consideration of the problem of identity and privacy in SIP. For example, these mechanisms provide no means by which end users can securely share identity information end-to-end without a trusted service provider. Identity information that the user designates as 'private' can be inspected by any intermediaries participating in the Trust Domain. This information is secured by transitive trust, which is only as reliable as the weakest link in the chain of trust. When a trusted entity sends a message to any destination with that party's identity in a P-Asserted-Identity header field, the entity MUST take precautions to protect the identity information from eavesdropping and interception to protect the confidentiality and integrity of that identity information. The use of transport or network layer hop-by-hop security mechanisms, such as TLS or IPSec with appropriate cipher suites, can satisfy this requirement. 13. IANA Considerations 13.1 Registration of new SIP header fields This document defines two new private SIP header fields, "P- Asserted-Identity" and "P-Preferred-Identity". As recommended by the policy of the Transport Area, these headers have been registered by the IANA in the SIP header registry, using the RFC number of this document as its reference. Name of Header: P-Asserted-Identity Short form: none Registrant: Cullen Jennings fluffy@cisco.com Normative description: Section 9.1 of this document Name of Header: P-Preferred-Identity Short form: none Registrant: Cullen Jennings fluffy@cisco.com Normative description: Section 9.2 of this document 13.2 Registration of "id" privacy type for SIP Privacy header Name of privacy type: id Short Description: Privacy requested for Third-Party Asserted Identity Registrant: Cullen Jennings fluffy@cisco.com Normative description: Section 9.3 of this document 14. Acknowledgements Thanks to Bill Marshall and Flemming Andreason [6], Mark Watson [5], and Jon Peterson [7] for authoring drafts which represent the bulk of the text making up this document. Thanks to many people for useful comments including Jonathan Rosenberg, Rohan Mahy and Paul Kyzivat. 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. [2] Peterson, J., "A Privacy Mechanism for the Session Initiation Protocol (SIP)", RFC 3323, November 2002. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. Informational References [5] Watson, M., "Short Term Requirements for Network Asserted Identity", RFC 3324, November 2002. [6] Andreasen, F., "SIP Extensions for Network-Asserted Caller Identity and Privacy within Trusted Networks", Work in Progress. [7] Peterson, J., "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", Work in Progress. Authors' Addresses Cullen Jennings Cisco Systems 170 West Tasman Drive MS: SJC-21/3 San Jose, CA 95134 USA Phone: +1 408 527-9132 EMail: fluffy@cisco.com Jon Peterson NeuStar, Inc. 1800 Sutter Street, Suite 570 Concord, CA 94520 USA Phone: +1 925/363-8720 EMail: Jon.Peterson@NeuStar.biz Mark Watson Nortel Networks Maidenhead Office Park (Bray House) Westacott Way Maidenhead, Berkshire England Phone: +44 (0)1628-434456 EMail: mwatson@nortelnetworks.com Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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