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Versions: (draft-tschofenig-sip-saml) 00 01 02 03 04 05 06 07 08

SIP                                                        H. Tschofenig
Internet-Draft                                    Nokia Siemens Networks
Updates: 4474 (if approved)                                    J. Hodges
Intended status: Standards Track                             J. Peterson
Expires: January 15, 2009                                  NeuStar, Inc.
                                                                 J. Polk
                                                                   Cisco
                                                               D. Sicker
                                                              CU Boulder
                                                           July 14, 2008


                      SIP SAML Profile and Binding
                       draft-ietf-sip-saml-04.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 15, 2009.












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Abstract

   This document specifies a Session Initiation Protocol (SIP) profile
   of Security Assertion Markup Language (SAML) as well as a SAML SIP
   binding.  The defined SIP SAML Profile composes with the mechanisms
   defined in the SIP Identity specification and satisfy requirements
   presented in "Trait-based Authorization Requirements for the Session
   Initiation Protocol (SIP)".











































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  SAML Introduction  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  SAML Assertions  . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Abstract Request/Response Protocol . . . . . . . . . . . .  9
   4.  Specification Scope  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Employing SAML in SIP  . . . . . . . . . . . . . . . . . . . . 12
   6.  SIP SAML Profiles  . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  AS-driven SIP SAML URI-based Attribute  Assertion
           Fetch Profile  . . . . . . . . . . . . . . . . . . . . . . 14
       6.1.1.  Required Information . . . . . . . . . . . . . . . . . 14
       6.1.2.  Profile Overview . . . . . . . . . . . . . . . . . . . 14
       6.1.3.  Profile Description  . . . . . . . . . . . . . . . . . 18
       6.1.4.  Assertion Profile Description  . . . . . . . . . . . . 21
       6.1.5.  Assertion Verification . . . . . . . . . . . . . . . . 23
     6.2.  The TBD "call-by-value"  Profile . . . . . . . . . . . . . 25
   7.  SAML SIP Binding . . . . . . . . . . . . . . . . . . . . . . . 26
     7.1.  SAML HTTP-URI-based SIP Binding  . . . . . . . . . . . . . 26
   8.  The 'saml-shusb' Option Tag  . . . . . . . . . . . . . . . . . 27
   9.  Example SAML Assertions  . . . . . . . . . . . . . . . . . . . 28
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 33
     10.1. Man-in-the-Middle Attacks and Stolen Assertions  . . . . . 33
     10.2. Forged Assertion . . . . . . . . . . . . . . . . . . . . . 33
     10.3. Replay Attack  . . . . . . . . . . . . . . . . . . . . . . 34
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 35
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 36
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
     13.1. IANA Registration for New SIP Option Tag . . . . . . . . . 37
     13.2. 477 'Use Identity Header with SAML Assertion' Response
           Code . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     13.3. 478 'Unknown SAML Assertion Content' Response Code . . . . 37
     13.4. 479 'Invalid SAML Assertion' Response Code . . . . . . . . 37
   14. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 39
   15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     15.1. -03 to -04 . . . . . . . . . . . . . . . . . . . . . . . . 40
     15.2. -02 to -03 . . . . . . . . . . . . . . . . . . . . . . . . 40
     15.3. -00 to -02 . . . . . . . . . . . . . . . . . . . . . . . . 40
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     16.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44
   Intellectual Property and Copyright Statements . . . . . . . . . . 45







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1.  Introduction

   This document specifies composition of the Security Assertion Markup
   Language (SAML) V2.0 with SIP [RFC3261] in order to accommodate
   richer authorization mechanisms and enable "trait-based
   authorization."  Trait-based authorization is where one is authorized
   to make use of some resource based on roles or traits rather than
   ones identifier(s).  Motivations for trait-based authorization, along
   with use-case scenarios, are presented in [RFC4484].

   Security Assertion Markup Language (SAML) v2.0, "SAMLv2", is an XML-
   based framework for creating and exchanging security information.
   [OASIS.sstc-saml-exec-overview-2.0-cd-01] and
   [OASIS.sstc-saml-tech-overview-2.0-draft-08] provide non-normative
   overviews of SAMLv2.  The SAMLv2 specification set is normatively
   defined by [OASIS.saml-conformance-2.0-os].

   Various means of providing trait-based authorization exist:
   authorization certificates [RFC3281], SPKI [RFC2693], or extensions
   to the authenticated identity body [RFC3893].  The authors selected
   SAML due to its increasing use in environments such as the Liberty
   Alliance, and the Internet2 project, areas where the applicability to
   SIP is widely desired.




























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2.  Terminology

   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 [RFC2119].

   The SIP network element "Authentication Service" is introduced in
   [RFC4474].  We reuse this term to refer to a network element that
   authenticates and authorizes a user and creates a "SIP identity
   assertion".  This system entity is the logical equivalent of a "SAML
   Authority" in the SAML terminology.

   For overall SIP terminology, see [RFC3261].

   In this specification, the term, or term component, "SAML" refers to
   SAML V2.0 in all cases.  For example, the term "SAML assertion"
   implicitly means "SAMLv2 assertion".  For overall SAML terminology,
   see [OASIS.saml-glossary-2.0-os].

   The below list maps other various SIP terms to their SAML
   (rough-)equivalents:



      Element, Network Element:

         System Entity, Entity


      Authentication Service:

         SAML Authority


      Invitee, Invited User, Called Party, Callee:

         Relying Party


      Server, User Agent Server (UAS):

         SAML Responder


      User Agent Client (UAC), client:

         SAML Requester




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   Additional terms defined in the context of this specification:



      profile attribute(s):

         one or more attributes of a "user profile".

      user profile, subject profile:

         the set of various attributes accompanying (i.e., mapped to) a
         user account in many environments.







































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3.  SAML Introduction

   SAML [OASIS.sstc-saml-exec-overview-2.0-cd-01]
   [OASIS.sstc-saml-tech-overview-2.0-draft-08] defines an XML-based
   framework for exchanging "security assertions" between entities.  In
   the course of making, or relying upon such assertions, SAML system
   entities may use SAML protocols, or other protocols, to communicate
   an assertion itself, or the subject of an assertion.

   Thus one can employ SAML to make and encode statements such as "Alice
   has these profile attributes and her domain's certificate is
   available over there, and I'm making this statement, and here's who I
   am."  Then one can cause such an assertion to be conveyed to some
   party who can then rely on it in some fashion for some purpose, for
   example input it into some local policy evaluation for access to some
   resource.  This is done in a particular "context of use".  Such a
   context of use could be, for example, deciding whether to accept and
   act upon a SIP-based invitation to initiate a communication session.

   The specification of how SAML is employed in a particular context of
   use is known as a "SAML profile".  The specification of how SAML
   assertions and/or protocol messages are conveyed in, or over, another
   protocol is known as a "SAML Binding".  Typically, a SAML profile
   specifies the SAML bindings that may be used in its context.  Both
   SAML profiles and SAML bindings reference other SAML specifications,
   especially the SAML Assertions and Protocols, aka "SAML Core",
   specification [OASIS.saml-core-2.0-os].

   There is an additional subtle aspect of SAML profiles that is worth
   highlighting -- the notion of a "SAML assertion profile".  A SAML
   assertion profile is the specification of the assertion contents in
   the context of a particular SAML profile.  It is possibly further
   qualified by a particular implementation and/or deployment context.
   Condensed examples of SAML assertion profiles are:

   o  The SAML assertion must contain at least one authentication
      statement and no other statements.  The relying party must be
      represented in the <AudienceRestriction> element.  The
      SubjectConfirmation Method must be Foo. etc.

   o  The SAML assertion must contain at least one attribute statement
      and may contain more than one.  The values for the subject's
      profile attributes named "Foo" and "Bar" must be present.  An
      authentication statement may be present. etc.

   The primary facets of SAML itself are:





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   o  Assertions

   o  Abstract Request/Response protocol

   We describe each in turn below:

3.1.  SAML Assertions

   A SAML assertion is a package of information including issuer and
   subject, conditions and advice, and/or attribute statements, and/or
   authentication statements and/or other statements.  Statements may or
   may not be present.  The SAML assertion "container" itself contains
   the following information:

   Issuing information:

      Who issued the assertion, when was it issued and the assertion
      identifier.


   Subject information:

      The name of the subject, the security domain and optional subject
      information, like public key.


   Conditions under which the  assertion is valid:

      Special kind of conditions like assertion validity period,
      audience restriction and target restriction.


   Additional advice:

      Explaining how the assertion was made, for example.

   In terms of SAML assertions containing SAML attribute statements or
   SAML authentication statements, here are explanatory examples:

      With a SAML assertion containing a SAML attribute statement, an
      issuing authority is asserting that the subject is associated with
      certain attributes with certain subject profile attribute values.
      For example, user jon@cs.example.com is associated with the
      attribute "Department", which has the value "Computer Science".

      With a SAML assertion containing a SAML authentication statement,
      an issuing authority is asserting that the subject was
      authenticated by certain means at a certain time.



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      With a SAML assertion containing both a SAML attribute statement
      and a SAML authentication statement, an issuing authority is
      asserting the union of the above.

3.2.  Abstract Request/Response Protocol

   SAML defines an abstract request/response protocol for obtaining
   assertions.  See Section 3 "SAML Protocols" of
   [OASIS.saml-core-2.0-os].  A request asks for an assertion.  A
   response returns the requested assertion or an error.  This abstract
   protocol may then be cast into particular contexts of use by binding
   it to specific underlying protocols, e.g., HTTP or SIP, and
   "profiling" it for the specific use case at hand.  The SAML HTTP-
   based web single sign-on profile is one such example (see Section 4.1
   Web Browser SSO Profile of [OASIS.saml-profiles-2.0-os]).  Trait-
   based SIP communication session establishment, the topic of this
   specification, is another.


































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4.  Specification Scope

   The scope of this specification is:

   o  Specify a SIP profile of SAML -- aka a "SIP SAML profile" -- such
      that a subject's profile attributes, and their domain's
      certificate, can be conveyed to a relying party using SAML.  In
      doing so, satisfy the requirements outlined in [RFC4484], and
      compose with [RFC4474].

   The following are outside the scope of this specification:

   o  Defining a means for configuring the runtime behavior, or
      deployment characteristics, of the Authentication Service.

      Discussion:

      For example, a SIP Authentication Service could be implemented
      such that its SAML-based features are employed, or not, on a
      subject-by-subject basis, and/or on a domain-by-domain basis.

   o  The definition of specific conveyed subject profile attributes
      (aka traits).

      Discussion:

      This specification defines a facility enabling "trait-based
      authorization" as discussed in [RFC4484].

      The attributes of interest in trait-based authorization will be
      ones akin to, for example: roles, organizational membership,
      access rights, or authentication event context.  Definition of
      such attributes is application- and/or deployment-context-
      dependent and are not defined in this specification.  However, The
      SAMLv2 specification defines several "SAML Attribute Profiles" for
      encoding attributes from various application domains, e.g., LDAP,
      UUID/GUID, DCE PAC, and XACML, in SAML assertions
      [OASIS.saml-profiles-2.0-os].

      In order for any trait-based system to be practical, participating
      entities must agree on attributes and traits that will be conveyed
      and subsequently relied upon.  Without such agreements, a trait-
      based system cannot be usefully deployed.  This specification does
      not discuss the manner in which participating entites might
      discover one another or agree on the syntax and semantics of
      attributes and traits.

      Note that SAMLv2 specifies a "metadata" facility that may be



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      useful in addressing this need.


















































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5.  Employing SAML in SIP

   Employing SAML in SIP necessitates devising a new SAML profile(s) and
   binding(s) because the those already specified in the SAMLv2
   specification set are specific to other use contexts, e.g., HTTP-
   based web browsing.  Although SIP bears some similarity to HTTP, it
   is a seperately distinct protocol, thus requiring specification of
   SIP-specific SAML profile(s) and binding(s).  This is technically
   straightforward as both SAML and SIP are explicitly extensible.

   The "Authenticated Identity Management in SIP" specification
   [RFC4474] (aka "SIP Identity") facilitates the composition of SAML
   and SIP in that it defines a "mediated authentication architecture"
   where verifying endpoints verify SIP identity assertions -- i.e., the
   "Identity" header value -- signed by an Authentication Service (AS).
   The semantic being that the AS is vouching that it did indeed
   authenticate the calling party.

   Such an Authentication Service, which likely has access to various
   pieces of information concerning the calling party, could also act as
   a SAML Authority, and make such information available to the callee
   via SAML.

   Since [RFC4474] stipulates that the AS must make its certificate
   available for retrieval and convey the availability and access
   mechanism via a URI, in the Identity-Info header, we have an
   opportunity to compose SIP Identity and SAML.

   Such composition can be accomplished by having the resource referred
   to by the URI in the Identity-Info be a SAML assertion conveying both
   the AS's certificate and user profile attributes.  This is the
   approach defined in this specification.  Figure 1 illustrates this
   approach in a high-level summary fashion.  Figure 2, further below,
   illustrates additional details.

















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     +--------+           +--------------+          +--------+
     |Alice@  |           |Authentication|          | Bob@   |
     |example |           |Service       |          |example2|
     |.com    |           |@example.com  |          |.com    |
     |        |           |              |          |        |
     +---+----+           +------+-------+          +---+----+
         |                       |                      |
         |      INVITE           |                      |
         |---------------------->|                      |
         | From:alice@foo.com    |                      |
         |                       |                      |
         |  407 Proxy auth. req. |                      |
         |<----------------------|                      |
         |     Challenge         |                      |
         |                       |                      |
         |       ACK             |                      |
         |---------------------->|                      |
         |                       |                      |
         | INVITE w/authn creds  |                      |
         |---------------------->|                      |
         |                       | INVITE               |
         |                       | w/Identity header    |
         |                       |--------------------->|
         |                       | and Identity-Info    |
         |                       |                      |
         |                       | HTTP GET SAML assn   |
         |                       |<==================== |
         |                       | and domain cert      |
         |                       |                      |
         |                       | HTTP 200 OK + assn   |
         |                       |=====================>|
         |                       | and domain cert      |
         |            200 OK     |                      |
         |<----------------------+----------------------|
         |                       |                      |


            Figure 1: SIP-SAML-based Network Asserted Identity

   Since the AS already being trusted to create and add the Identity
   header containing the SIP Identity Assertion, and to supply a pointer
   to its domain certificate, having it point instead to a SAML
   assertion conveying the domain certificate and possibly some user
   profile attributes, does not significantly alter the first-order
   security considerations examined in [RFC4474].  This specification
   provides some additional security considerations analysis below in
   Section 10.




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6.  SIP SAML Profiles

   This section defines two "SIP SAML profiles":

   o  The "AS-driven SIP SAML URI-based Attribute Assertion Fetch
      Profile"

   o  The to-be-determined (TBD) "call-by-value" profile

6.1.  AS-driven SIP SAML URI-based Attribute  Assertion Fetch Profile

6.1.1.  Required Information

   The information given in this section is similar to the info provided
   when registering something, a MIME Media Type, say, with IANA.  In
   this case, it is for registering this profile with the OASIS SSTC.
   See Section 2 "Specification of Additional Profiles" in
   [OASIS.saml-profiles-2.0-os].

   Identification:

      urn:ietf:params:sip:sip-saml-profile:as:uri:attr:1.0

      @@ NOTE:  This URN must be agreed upon, and then registered with
         IANA per [RFC3553].

   Contact Information:

      @@ someone's or something's contact info goes here

   SAML Confirmation Method Identifiers:

      The SAML V2.0 "{bearer,hok,?}" confirmation method identifier is
      used in this profile.

   Description:

      Given below.

   Updates:

      None.

6.1.2.  Profile Overview

   Figure 2 illustrates this profile's overall protocol flow.  The
   following steps correspond to the labeled interactions in the figure.
   Within an individual step, there may be one or more actual message



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   exchanges depending upon the protocol binding employed for that
   particular step and other implementation-dependent behavior.

   Although this profile is overview is cast in terms of a SIP INVITE
   transaction, the reader should note that the mechanism specified
   herein, and in [RFC4474], may be applied to any SIP request message.

   Figure 2 begins on the next page.











































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     +------------------+    +------------------+   +-----------------+
     |     Caller       |    |Authn Service (AS)|   |     Callee      |
     |Alice@example.com |    |  @example.com    |   | Bob@example2.com|
     +--------+---------+    +--------+---------+   +--------+--------+
   -    -     |                       |                      | (steps)
   ^    ^     |      INVITE           |                      |
   |    |     |---------------------->|                      |   (1a)
   |          | From:alice@foo.com    |                      |
   |    C     | To:sip:bob@example.com|                      |
   |    S     |                       |                      |
   |    e     |  407 Proxy auth. req. |                      |
   |    q     |<----------------------|                      |   (1b)
   |    =     |  Challenge            |                      |
   |    N     |                       |                      |
   |          |      ACK              |                      |
   |    |     |---------------------->|                      |   (1c)
   |    V     |                       |                      |
   |    -     |                       |                      |
        ^     | INVITE + authorization|                      |
   D    |     | header w/ creds       |                      |
        |     |---------------------->|                      |   (2)
   I    |     | From:alice@foo.com    |                      |
        |     | To:sip:bob@example.com|                      |
   A          | Proxy-Authorization:..|                      |
        C     |                       | INVITE               |
   L    S     |                       |--------------------->|   (3)
        e     |                       | From:alice@foo.com   |
   O    q     |                       | To:sip:bob@example2.com
              |                       | Identity: .....      |
   G    =     |                       | Identity-Info:       |
              |                       |   https://example.com|
   |    N     |                       |     /assns/?ID=abcde |
   |          |                       |                      |
   |    +     |                       |URI resolution (eg. HTTP)
   |          |                       |<=====================|   (4)
   |    1     |                       | GET /assns/?ID=abcde |
   |          |                       |                      |
   |    |     |                       | HTTP/1.1 200 OK      |
   |    |     |                       |=====================>|   (5)
   |    |     |                       |  <saml:Assertion>    |
   |    |     |                       |   <saml:Subject>     |
   |    |     |                       |    <saml:NameID>     |
   |    |     |                       |      Alice@example.com
   |    |     |                       |     <saml:SubjConf>
   |    |     |                       |      <saml:SubjConfData>
   |    |     |                       |       <ds:KeyInfo>...
   |    |     |                       |   <saml:AttrStatement>
   |    |     |                       |     foo=bar          |



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   |    |     |            200 OK     |                      |
   |    V     |<----------------------+----------------------|   (6)
   .    -     |                       |                      |
   V


   Figure 2: AS-driven SIP SAML Attribute Fetch Profile: Example INVITE
                                Transaction

   Step 1.  Initial SIP Transaction between Caller and AS

            This optional initial step is comprised of substeps 1a, 1b,
            and 1c in Figure 2.  In this step, the caller, Alice, sends
            a SIP request message, illustrated as an INVITE, indicating
            Bob as the callee (1a), is subsequently challenged by the AS
            (1b), and sends an ACK in response to the challenge (1c).
            The latter message signals the completion of this SIP
            transaction (which is an optional substep of this profile).

   Step 2.  Caller sends SIP Request Message with Authorization
            Credentials to the AS

            Alice then sends an INVITE message in response to the
            challenge, or uses cached credentials for the domain if step
            1 was skipped, as specified in [RFC4474] and [RFC3261].
            Depending on the chosen SIP security mechanism for client
            authentication either digest authentication, client side
            authentication of Transport Layer Security, or a combination
            of both is used to provide the AS with a strong assurance
            about the identity of Alice.

   Step 3.  AS Authorizes the SIP Request and Forwards it to Callee

            First, the AS authorizes the received INVITE message as
            specified in [RFC4474] and [RFC3261].  If the authorization
            is successful, the AS will form the "identity signature" for
            the message and add Identity and Identity-Info header fields
            to the message.  The AS also at this time constructs and
            caches a SAML assertion asserting Alice's profile attributes
            required by Bob's domain (example2.com), and also containing
            a the domain's (example.com) public key certificate, or a
            reference to it.  This certificate MUST contain the public
            key corresponding to the private key used to construct the
            signature whose value was placed in the Identity header.
            The AS constructs a HTTP-based SAML URI Reference
            incorporating the assertion's Assertion ID (see section
            2.3.3 of [OASIS.saml-core-2.0-os]).  The AS uses this URI as
            the value for the Identity-Info header it adds to the INVITE



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            message.

            The AS determines which profile attributes (if any) to
            assert in the <AttributeStatement> via local configuration
            and/or obtaining example2.com's metadata
            [OASIS.saml-metadata-2.0-os].  The AS then sends the updated
            INVITE message to Bob.

   Step 4.  Callee Dereferences HTTP-based SAML URI Reference

            Bob's UAC or SIP Proxy receives the message and begins
            verifying it per the "Verifier Behavior" specified in
            [RFC4474].  In order to accomplish this task, it needs to
            obtain Alice's domain certificate.  It obtains the HTTP-
            based SAML URI Reference from the message's Identity-Info
            header and dereferences it per Section 7.1.  Note that this
            is not a SIP message, but an HTTP message [RFC2616].

   Step 5.  AS Returns SAML Assertion

            Upon receipt of the above HTTP request, which contains an
            embedded reference to Alice's SAML Assertion, Alice's AS
            returns her assertion in an HTTP response message.

            Upon receipt of Alice's SAML Assertion, the AS continues its
            verification of the INVITE message.  If successful, it
            returns a 200 OK message directly to Alice.  Otherwise it
            returns an appropriate SIP error response.

   Step 6.  Callee Returns SIP 200 OK to Caller

            If Bob determines, based upon Alice's identity as asserted
            by the AS, and as further substantiated by the information
            in the SAML assertion, to accept the INVITE, he returns a
            SIP 200 OK message directly to Alice.

6.1.3.  Profile Description

   The following sections provide detailed definitions of the individual
   profile steps.  The relevant illustration is Figure 3, below.  Note
   that this profile is agnostic to the specific SIP request, and also
   that the Sender and Authentication Service (AS) may be seperate or
   co-located in actuality.








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     +------------------+    +------------------+   +------------------+
     |     Sender       |    |Authn Service (AS)|   |    Verifier      |
     |      (UAC)       |    |    (Sender's)    |   |(UAS or Proxy Svr)|
     +--------+---------+    +--------+---------+   +--------+---------+
              |                       |                      | (steps)
              |    SIP Request        |                      |
              |---------------------->|                      |   (1a)
              |                       |                      |
              |  407 Proxy auth. req. |                      |
              |<----------------------|                      |   (1b)
              |  Challenge            |                      |
              |                       |                      |
              |      ACK              |                      |
              |---------------------->|                      |   (1c)
              |                       |                      |
              |                       |                      |
              |SIP Req + authorization|                      |
              | header w/ creds       |                      |
              |---------------------->|                      |   (2)
              |                       |                      |
              |                       |                      |
              |                       | SIP Req + Ident &    |
              |                       | authz headers        |
              |                       |--------------------->|   (3)
              |                       |                      |
              |                       | URI resolution       |
              |                       |<=====================|   (4)
              |                       | (via HTTP)           |
              |                       |                      |
              |                       | HTTP/1.1 200 OK      |
              |                       |=====================>|   (5)
              |                       |                      |
              |                       |                      |
              |                       |                  ?   |   (6)
              |                       |                      |


    Figure 3: AS-driven SIP SAML Attribute Fetch Profile: Message Flow

6.1.3.1.  Initial SIP Transaction between  Sender and AS

   This OPTIONAL step maps to Steps 1 and 2 of Section 5 "Authentication
   Service Behavior" of [RFC4474].  If the SIP request sent by the
   caller in substep 1a is deemed insufficiently authenticated by the AS
   per the rules stipulated by [RFC4474] Steps 1 and 2, then the AS MUST
   authenticate the sender of the message.  The particulars of how this
   is accomplished depend upon implementation and/or deployment
   instantiation as discussed in [RFC4474].  Substeps 1b and 1c as shown



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   in Figure 3 are non-normative and illustrative only.

6.1.3.2.  Sender sends SIP Request Message with  Authorization
          Credentials to the AS

   This step maps to Steps 1 and 2 of Section 5 "Authentication Service
   Behavior" of [RFC4474].  This request is presumed to be made in a
   context such that the AS will not challenge it -- i.e., the AS will
   consider the sender of the message to be authenticated.  If this is
   not true, then this procedure reverts back to Step 1, above.

   Otherwise, the AS carries out all other processing of the message as
   stipulated in [RFC4474] Steps 1 and 2, and if successful, this
   procedure procedes to the next step below.

6.1.3.3.  AS Authorizes the SIP Request and Forwards it to Verifier

   This first portion of this step maps to Steps 3 and 4 of Section 5
   "Authentication Service Behavior" of [RFC4474], which the AS MUST
   perform, although with the following additional substeps:

      The AS MUST construct a SAML assertion according to the "Assertion
      Profile Description" specified in Section 6.1.4 of this
      specification.

      The AS SHOULD construct an HTTPS, and MAY construct an HTTP, URI
      per Section "3.7.5.1 URI Syntax" of [OASIS.saml-bindings-2.0-os].

      The AS MUST use the URI constructed in the immediately preceding
      substep as the value of the Identity-Info header that is added to
      the SIP request message per Step 4 of Section 5 of [RFC4474].

   Upon successful completion of all of the above, the AS forwards the
   request message.

   At this point in this step, after perhaps traversing some number of
   intermediaries, the SIP request message arrives at a SIP network
   entity performing the "verifier" role.  This role and its behavior
   are specified in Section 6 "Verifier Behavior" of [RFC4474].  The
   verifier MUST perform the steps enumerated in the aforementioned
   section, with the following modifications:

      Step 1 of [RFC4474] Section 6 maps to and is updated by, the
      following two steps in this procedure.

      Steps 2, 3, and 4 of [RFC4474] Section 6 may be mapped across this
      latter portion of this step, and/or the following two steps, as
      appropriate.



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6.1.3.4.  Verifier Dereferences HTTP-based SAML  URI Reference

   The verifier SHOULD ascertain whether it has a current cached copy of
   the SIP message sender's SAML assertion and domain certificate.  If
   not, or if the verifier chooses to (e.g., due to local policy), it
   MUST dereference the the HTTP-based SAML URI Reference found in the
   SIP message's Identity-Info header.  To do so, the verifier MUST
   employ the "SAML HTTP-URI-based SIP Binding" specified in
   Section 7.1.

6.1.3.5.  AS Returns SAML Assertion

   This step also employs Section 7.1 "SAML HTTP-URI-based SIP Binding".

   If the prior step returns an HTTP error (e.g., 4xx series), then this
   procedure terminates and the verifier returns (upstream) a SIP 436
   'Bad Identity-Info' Response code.

   Otherwise, the HTTP response message will contain a SAML assertion
   and be denoted as such via the MIME media type of "application/
   samlassertion+xml" [IANA.application.samlassertion-xml].  The
   verifier MUST perform the verification steps specified in
   Section 6.1.5 "Assertion Verification", below.  If successful, then
   this procedure continues with the next step.

6.1.3.6.  Verifier performs Next Step

   The SIP request was successfully processed.  The verifier now
   performs its next step, which depends at least in part on the type of
   SIP request it received.

6.1.4.  Assertion Profile Description

   This section defines the particulars of how the sender, i.e., the
   SAML Authority, MUST construct certain portions of the SAML
   assertions it issues.  The schema for SAML assertions themselves is
   defined in Section 2.3 of [OASIS.saml-core-2.0-os].

   An example SAML assertion, formulated according to this profile is
   given in Section 9.

   Overall SAML assertion profile requirements:

      The SAML assertion MUST be signed by the same key as used to sign
      the contents of the Identity header field.  Signing of SAML
      assertions is defined in Section 5.4 of [OASIS.saml-core-2.0-os].

   In the following subsections, the SAML assertion profile is specified



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   element-by-element, in a top-down, depth-first manner, beginning with
   the outermost element, "<Assertion>".  Where applicable, the
   requirements for an element's XML attributes are also stated, as a
   part of the element's description.  Requirements for any given
   element or XML attribute are only stated when, in the context of use
   of this profile, they are not already sufficiently defined by
   [OASIS.saml-core-2.0-os].

6.1.4.1.  Element: <Assertion>

   Attribute: ID

      The value for the ID XML attribute SHOULD be allocated randomly
      such that the value meets the randomness requirments specified in
      Section 1.3.4 of [OASIS.saml-core-2.0-os].

   Attribute: IssueInstant

      The value for the IssueInstant XML attribute SHOULD be set at the
      time the SAML assertion is created (and cached for subsequent
      retrieval).  This time instant value MAY be temporally the same as
      that encoded in the SIP message's Date header, and MUST be at
      least temporally later, although it is RECOMMENDED that it not be
      10 minutes or more later.

6.1.4.1.1.  Element: <Issuer>

   The value for the Issuer XML element MUST be a value that matches
   either the Issuer or the Issuer Alternative Name fields [RFC3280] in
   the certificate conveyed by the SAML assertion in the ds:
   X509Certificate element located on this path within the SAML
   assertion:

                <Assertion
                  <ds:Signature
                    <ds:KeyInfo
                      <ds:X509Data
                        <ds:X509Certificate

6.1.4.1.2.  Element: <Subject>

   The <Subject> element SHOULD contain both a <NameID> element and a
   <SubjectConfirmation> element.

   The value of the <NameID> element MUST be the same as the Address of
   Record (AoR) value used in computing the "signed-identity-digest"
   which forms the value of the Identity header.  See Section 9 of
   [RFC4474].



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   The <SubjectConfirmation> element attribute Method SHOULD be set to
   the value:

      urn:oasis:names:tc:SAML:2.0:cm:sender-vouches

   Although it MAY be set to some other implementation- and/or
   deployment-specific value.  The <SubjectConfirmation> element itself
   SHOULD be empty.

6.1.4.1.3.  Element: <Conditions>

   The <Conditions> element SHOULD contain an <AudienceRestriction>
   element, which itself SHOULD contain an <Audience> element.  The
   value of the <Audience> element SHOULD be the same as the addr-spec
   of the SIP request's To header field.

   The following XML attributes of the <Conditions> element MUST be set
   as follows:

   Attribute: NotBefore

      The value of the NotBefore XML attribute MUST be set to a time
      instant the same as the value for the IssueInstant XML attribute
      discussed above, or to a later time.

   Attribute: NotOnOrAfter

      The value of the NotOnOrAfter XML attribute MUST be set to a time
      instant later than the value for NotBefore.

6.1.4.1.4.  Element: <AttributeStatement>

   The SAML assertion MAY contain an <AttributeStatement> element.  If
   so, the <AttributeStatement> element will contain attribute-value
   pairs, e.g., of a user profile nature, encoded according to either
   one of the "SAML Attribute Profiles" as specified in
   [OASIS.saml-profiles-2.0-os], or encoded in some implementation-
   and/or deployment-specific attribute profile.

   The attribute-value pairs SHOULD in fact pertain to the entity
   identified in the SIP From header field, since a SAML assertion
   formulated per this overall section is stating that they do.

6.1.5.  Assertion Verification

   This section specifies the steps that a verifier participating in
   this profile MUST perform in addition to the "Verifier Behavior"
   specified in Section 6 of [RFC4474].



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   The steps are:

   1.   Before Step 1 in Section 6 of [RFC4474], the verifier MUST
        extract the AS's domain certificate from the <ds:
        X509Certificate> XML element at the end of the element path
        given in Section 6.1.4.1.1.

   2.   Perform Step 1 in Section 6 of [RFC4474].

   3.   After Step 1 in Section 6 of [RFC4474], but before Step 2 of
        that section, the verifier MUST verify the SAML assertion's
        signature via the procedures specified in Section 5.4 of
        [OASIS.saml-core-2.0-os] as well as [W3C.xmldsig-core].

        @@ TODO: do we need to define a new SIP error response code for
        when a SAML assn signature is bad? e.g., '4xx Invalid SAML
        asssertion'.

   4.   Perform Step 2 in Section 6 of [RFC4474].

   5.   Verify that the signer of the SIP message's Identity header
        field is the same as the signer of the SAML assertion.

   6.   Perform Steps 3 and 4 in Section 6 of [RFC4474].

   7.   Verify that the SAML assertion's <Issuer> element value matches
        the Issuer or the Issuer Alternative Name fields [RFC3280] in
        the AS's domain certificate.

   8.   Verify that the SAML assertion's <NameID> element value is the
        same as the Address of Record (AoR) value in the "signed-
        identity-digest".  See Section 9 of [RFC4474].

   9.   Verify that the SAML assertion's <SubjectConfirmation> element
        value is set to whichever value was configured at
        implementation- or deployment-time.  The default value is:

           urn:oasis:names:tc:SAML:2.0:cm:sender-vouches

   10.  Verify that the SAML assertion contains an <Audience> element,
        and that its value matches the value of the addr-spec of the SIP
        To header field.

   11.  Verify that the validity period denoted by the NotBefore and
        NotOnOrAfter attributes of the <Conditions> element meets the
        requirements given in Section 6.1.4.1.3.





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6.2.  The TBD "call-by-value"  Profile

   To-Be-Determined (TBD)
















































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7.  SAML SIP Binding

   This section specifies one SAML SIP Binding at this time.  Additional
   bindings may be specified in future revisions of this specification.

7.1.  SAML HTTP-URI-based SIP Binding

   This section specifies the "SAML HTTP-URI-based SIP Binding",
   (SHUSB).

   The SHUSB is a profile of the "SAML URI Binding" specified in Section
   3.7 of [OASIS.saml-bindings-2.0-os].  The SAML URI Binding specifies
   a means by which SAML assertions can be referenced by URIs and thus
   be obtained through resolution of such URIs.

   This profile of the SAML URI Binding is congruent with the SAML URI
   Binding -- including support for HTTP-based URIs being mandatory to
   implement -- except for the following further restrictions which are
   specified in the interest of interoperability (section numbers refer
   to [OASIS.saml-bindings-2.0-os]):

   Section 3.7.5.3 Security Considerations:

      Support for TLS 1.0 or SSL 3.0 is mandatory to implement.

   Section 3.7.5.4 Error Reporting:

      All SHOULDs in this section are to be interpreted as MUSTs.























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8.  The 'saml-shusb' Option Tag

   This document creates and IANA registers one new option tag: "saml-
   shusb".  This option tag is to be used, as defined in RFC 3261, in
   the Require, Supported and Unsupported headers.  It is used to
   indicate support for this SIP extension, this option tag is included
   in a Supported header of the SIP request.












































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9.  Example SAML Assertions

   This section presents two examples of a SAML assertion, one unsigned
   (for clarity), the other signed (for accuracy).

   In the first example, Figure 4, the assertion is attesting with
   respect to the subject (lines 7-15) "Alice@example.com" (line 11).
   The validity conditions are expressed in lines 16-23, via both a
   validity period expressed as temporal endpoints, and an "audience
   restriction" stating that this assertion's semantics are valid for
   only the relying party named "example2.com".  Also, the assertion's
   issuer is noted in lines 4-5.

   The above items correspond to some aspects of this specification's
   SAML assertion profile, as noted below in Security Considerations
   dicussions, see: Section 10.1 and Section 10.2.

   In lines 24-36, Alice's telephone number is conveyed, in a "typed"
   fashion, using LDAP/X.500 schema as the typing means.
































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    1 <Assertion ID="_a75adf55-01d7-40cc-929f-dbd8372ebdfc"
    2    IssueInstant="2003-04-17T00:46:02Z" Version="2.0"
    3    xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
    4    <Issuer>
    5       example.com
    6    </Issuer>
    7    <Subject>
    8       <NameID
    9         Format=
   10         "urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
   11         Alice@example.com
   12       </NameID>
   13       <SubjectConfirmation
   14         Method="urn:oasis:names:tc:SAML:2.0:cm:sender-vouches"/>
   15    </Subject>
   16    <Conditions NotBefore="2003-04-17T00:46:02Z"
   17                NotOnOrAfter="2003-04-17T00:51:02Z">
   18       <AudienceRestriction>
   19          <Audience>
   20             example2.com
   21          </Audience>
   22       </AudienceRestriction>
   23    </Conditions>
   24    <AttributeStatement>
   25       <saml:Attribute
   26    xmlns:x500=
   27      "urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
   28    NameFormat=
   29      "urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
   30    Name="urn:oid:2.5.4.20"
   31    FriendlyName="telephoneNumber">
   32          <saml:AttributeValue xsi:type="xs:string">
   33                +1-888-555-1212
   34          </saml:AttributeValue>
   35       </saml:Attribute>
   36    </AttributeStatement>
   37 </Assertion>


       Figure 4: Unsigned SAML Assertion  Illustrating Conveyance of
                             Subject Attribute

   In the second example, Figure 5, the information described above is
   the same, the addition is that this version of the assertion is
   signed.  All the signature information is conveyed in the <ds:
   signature> element, lines 7-47.  Thus this assertion's origin and its
   integrity are assured.  Since this assertion is the same as the one
   in the first example above, other than having a signature added, the



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   second example below addresses the same Security Considerations
   aspects, plus those requiring a Signature.

















































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    1 <Assertion ID="_a75adf55-01d7-40cc-929f-dbd8372ebdfc"
    2    IssueInstant="2003-04-17T00:46:02Z" Version="2.0"
    3    xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
    4    <Issuer>
    5       example.com
    6    </Issuer>
    7    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    8       <ds:SignedInfo>
    9          <ds:CanonicalizationMethod
   10             Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
   11          <ds:SignatureMethod
   12           Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
   13          <ds:Reference
   14           URI="#_a75adf55-01d7-40cc-929f-dbd8372ebdfc">
   15             <ds:Transforms>
   16                <ds:Transform
   17                    Algorithm=
   18          "http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
   19                <ds:Transform
   20                   Algorithm=
   21                      "http://www.w3.org/2001/10/xml-exc-c14n#">
   22                   <InclusiveNamespaces
   23                      PrefixList="#default saml ds xs xsi"
   24                      xmlns=
   25                       "http://www.w3.org/2001/10/xml-exc-c14n#"/>
   26                </ds:Transform>
   27             </ds:Transforms>
   28             <ds:DigestMethod
   29              Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
   30             <ds:DigestValue>
   31                Kclet6XcaOgOWXM4gty6/UNdviI=
   32             </ds:DigestValue>
   33          </ds:Reference>
   34       </ds:SignedInfo>
   35       <ds:SignatureValue>
   36         hq4zk+ZknjggCQgZm7ea8fI7...Hr7wHxvCCRwubfZ6RqVL+wNmeWI4=
   37       </ds:SignatureValue>
   38       <ds:KeyInfo>
   39          <ds:X509Data>
   40              <ds:X509Certificate>
   41     MIICyjCCAjOgAwIBAgICAnUwDQYJKoZIhvcNAQEEBQAwgakxNVBAYTAlVT
   42     MRIwEAYDVQQIEwlXaXNjb .....  dnP6Hr7wHxvCCRwubnZAv2FU78pLX
   43     8I3bsbmRAUg4UP9hH6ABVq4KQKMknxu1xQxLhpR1ylGPdioG8cCx3w/w==
   44              </ds:X509Certificate>
   45          </ds:X509Data>
   46       </ds:KeyInfo>
   47    </ds:Signature>
   48    <Subject>



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   49       <NameID
   50         Format=
   51       "urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
   52         Alice@example.com
   53       </NameID>
   54       <SubjectConfirmation
   55        Method="urn:oasis:names:tc:SAML:2.0:cm:sender-vouches"/>
   56    </Subject>
   57    <Conditions NotBefore="2003-04-17T00:46:02Z"
   58                NotOnOrAfter="2003-04-17T00:51:02Z">
   59       <AudienceRestriction>
   60          <Audience>
   61             example2.com
   62          </Audience>
   63       </AudienceRestriction>
   64    </Conditions>
   65    <AttributeStatement>
   66       <saml:Attribute
   67    xmlns:x500=
   68      "urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
   69    NameFormat=
   70      "urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
   71    Name="urn:oid:2.5.4.20"
   72    FriendlyName="telephoneNumber">
   73          <saml:AttributeValue xsi:type="xs:string">
   74                +1-888-555-1212
   75          </saml:AttributeValue>
   76       </saml:Attribute>
   77    </AttributeStatement>
   78 </Assertion>


   Figure 5: Signed SAML Assertion  Illustrating Conveyance of  Subject
                                 Attribute

















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10.  Security Considerations

   This section discusses security considerations when using SAML with
   SIP.

10.1.  Man-in-the-Middle Attacks and Stolen Assertions

   Threat:

      By making SAML assertions available via HTTP-based requests by a
      potentially unbounded set of requesters, it is conceivably
      possible that anyone would be able to simply request one and
      obtain it.  By SIP intermediaries on the signaling path for
      example.  Or, an HTTP intermediary/proxy could intercept the
      assertion as it is being returned to a requester.

      The attacker could then conceivably attempt to impersonate the
      subject (the putative caller) to some SIP-based target entity.

   Countermeasures:

      Such an attack is implausible for several reasons.  The primary
      reason is that a message constructed by an imposter using a stolen
      assertion that conveys the public key certificate of some domain
      will not verify per [RFC4474] because the imposter will not have
      the corresponding private key with which to generate the signed
      Identity header value.

      Also, the SIP SAML assertion profile specified herein that the
      subject's SAML assertion must adhere to causes it to be not useful
      to arbitrary parties.  The subject's assertion:

      *  should be signed, thus causing any alterations to break its
         integrity and make such alterations detectable.

      *  relying party is represented in the SAML assertion's Audience
         Restriction.

      *  Issuer is represented in the SAML assertion.

      *  validity period for assertion is restricted.

10.2.  Forged Assertion








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   Threat:

      A malicious user could forge or alter a SAML assertion in order to
      communicate with the SIP entities.

   Countermeasures:

      To avoid this kind of attack, the entities must assure that proper
      mechanisms for protecting the SAML assertion are employed, e.g.,
      signing the SAML assertion itself.  Section 5.1 of
      [OASIS.saml-core-2.0-os] specifies the signing of SAML assertions.

      Additionally, the assertion content dictated by the SAML assertion
      profile herein ensures ample evidence for a relying party to
      verify the assertion and its relationship with the received SIP
      request.

10.3.  Replay Attack

   Threat:

      Theft of SIP message protected by the mechanisms described herein
      and replay of it at a later time.

   Countermeasures:

      There are various provisions within [RFC4474] that prevent a
      replay attack.























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11.  Contributors

   The authors would like to thank Marcus Tegnander and Henning
   Schulzrinne for his contributions to earlier versions of this
   document.














































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12.  Acknowledgments

   We would like to thank RL 'Bob' Morgan, Stefan Goeman, Shida
   Schubert, Jason Fischl, Sebastian Felis, Nie Pin, Marcos Dytz, Erkki
   Koivusalo, Richard Barnes, Marc Willekens, Marc Willekens, Steffen
   Fries and Vijay Gurbani for their comments to this draft.  The "AS-
   driven SIP SAML URI-based Attribute Assertion Fetch Profile" is based
   on an idea by Jon Peterson.











































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13.  IANA Considerations

13.1.  IANA Registration for New SIP Option Tag

   The SIP option tag "saml-shusb" is created by this document, with the
   definition and rule in Section 4.4 of this document, to be added to
   sip-parameters within IANA.

13.2.  477 'Use Identity Header with SAML Assertion' Response Code

   This document registers a new SIP response code.  It is sent when a
   verifier receives a SIP request that lacks an Identity header with a
   SAML assertion in order to indicate that the request should be re-
   sent with an Identity header pointing to a SAML assertion.  This
   response code is defined by the following information, which has been
   added to the method and response-code sub-registry under
   http://www.iana.org/assignments/sip-parameters.

   Response Code Number: 477

   Default Reason Phrase: Use Identity Header with SAML Assertion

13.3.  478 'Unknown SAML Assertion Content' Response Code

   This document registers a new SIP response code.  It is used when the
   verifier is unable to parse the content of the SAML assertion
   referenced by the URI of the Identity-Info header, because, for
   example, the assertion contains only unknown elements in in the SAML
   assertion, or the SAML assertion XML document is garbled.  This
   response code is defined by the following information, which has been
   added to the method and response-code sub-registry under
   http://www.iana.org/assignments/sip-parameters.

   Response Code Number: 478

   Default Reason Phrase: Unknown SAML Assertion Content

13.4.  479 'Invalid SAML Assertion' Response Code

   This document registers a new SIP response code.  It is used when the
   verifier is unable to process the SAML assertion referenced by the
   URI of the Identity-Info header, because, for example, the assertion
   is self-signed, or signed by a root certificate authority for whom
   the verifier does not possess a root certificate.  This response code
   is defined by the following information, which has been added to the
   method and response-code sub-registry under
   http://www.iana.org/assignments/sip-parameters.




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   Response Code Number: 479

   Default Reason Phrase: Invalid SAML Assertion
















































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14.  Open Issues

   A list of open issues can be found at:
   http://www.tschofenig.priv.at:8080/saml-sip/















































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15.  Change Log

   RFC Editor - Please remove this section before publication.

15.1.  -03 to -04

   Updated IANA consideration section.

   Added option tag

   Updated acknowledgments section

   Minor editorial changes to the security considerations section

15.2.  -02 to -03

   Denoted that this I-D is intended to update RFC4474 per SIP working
   group consensus at IETF-69.  This is the tact adopted in order to
   address the impedance mismatch between the nature of the URIs
   specified as to be placed in the Identity-Info header field, and what
   is specified in RFC4474 as the allowable value of that header field.

   Added placeholder "TBD" section for a to-be-determined "call-by-
   value" profile, per SIP working group consensus at IETF-69.

   Removed use-case appendicies (per recollection of JHodges during
   IETF-69 discussion as being WG consensus, but such is not noted in
   the minutes).

15.3.  -00 to -02

   Will detail in -04 rev.



















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16.  References

16.1.  Normative References

   [OASIS.saml-bindings-2.0-os]
              Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E.
              Maler, "Bindings for the OASIS Security Assertion Markup
              Language (SAML) V2.0", OASIS
              Standard saml-bindings-2.0-os, March 2005.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

   [OASIS.saml-metadata-2.0-os]
              Cantor, S., Moreh, J., Philpott, R., and E. Maler,
              "Metadata for the Security Assertion Markup Language
              (SAML) V2.0", OASIS Standard saml-metadata-2.0-os,
              March 2005.

   [OASIS.saml-profiles-2.0-os]
              Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
              P., Philpott, R., and E. Maler, "Profiles for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard OASIS.saml-profiles-2.0-os, March 2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, August 1998.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [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.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.




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   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, June 2003.

   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)
              Authenticated Identity Body (AIB) Format", RFC 3893,
              September 2004.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4484]  Peterson, J., Polk, J., Sicker, D., and H. Tschofenig,
              "Trait-Based Authorization Requirements for the Session
              Initiation Protocol (SIP)", RFC 4484, August 2006.

   [W3C.xmldsig-core]
              Eastlake, D., Reagle , J., and D. Solo, "XML-Signature
              Syntax and Processing", W3C Recommendation xmldsig-core,
              October 2000, <http://www.w3.org/TR/xmldsig-core/>.

16.2.  Informative References

   [IANA.application.samlassertion-xml]
              OASIS Security Services Technical Committee (SSTC),
              "application/samlassertion+xml MIME Media Type
              Registration", IANA MIME Media Types Registry application/
              samlassertion+xml, December 2004.

   [OASIS.saml-conformance-2.0-os]
              Mishra, P., Philpott, R., and E. Maler, "Conformance
              Requirements for the Security Assertion Markup Language
              (SAML) V2.0", OASIS Standard saml-conformance-2.0-os,
              March 2005.

   [OASIS.saml-glossary-2.0-os]
              Hodges, J., Philpott, R., and E. Maler, "Glossary for the
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard saml-glossary-2.0-os, March 2005.

   [OASIS.saml-sec-consider-2.0-os]
              Hirsch, F., Philpott, R., and E. Maler, "Security and
              Privacy Considerations for the OASIS Security Markup
              Language (SAML) V2.0", OASIS Standard saml-sec-consider-
              2.0-os, March 2005.



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   [OASIS.sstc-saml-exec-overview-2.0-cd-01]
              Madsen, P. and E. Maler, "SAML V2.0 Executive Overview",
              OASIS SSTC Committee
              Draft sstc-saml-exec-overview-2.0-cd-01, April 2005.

   [OASIS.sstc-saml-protocol-ext-thirdparty-cd-01]
              Cantor, S., "SAML Protocol Extension for Third-Party
              Requests", OASIS SSTC Committee Draft sstc-saml-protocol-
              ext-thirdparty-cd-01, March 2006.

   [OASIS.sstc-saml-tech-overview-2.0-draft-08]
              Hughes, J. and E. Maler, "Security Assertion Markup
              Language (SAML) V2.0 Technical Overview", OASIS SSTC
              Working Draft sstc-saml-tech-overview-2.0-draft-08,
              September 2005.

   [RFC2543]  Handley, M., Schulzrinne, H., Schooler, E., and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC2693]  Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
              B., and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
              September 1999.

   [RFC3281]  Farrell, S. and R. Housley, "An Internet Attribute
              Certificate Profile for Authorization", RFC 3281,
              April 2002.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.





















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Authors' Addresses

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Jeff Hodges
   NeuStar, Inc.
   2000 Broadway Street
   Redwood City, CA  94063
   US

   Email: Jeff.Hodges@neustar.biz


   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520
   US

   Email: jon.peterson@neustar.biz


   James Polk
   Cisco
   2200 East President George Bush Turnpike
   Richardson, Texas  75082
   US

   Email: jmpolk@cisco.com


   Douglas C. Sicker
   University of Colorado at Boulder
   ECOT 430
   Boulder, CO  80309
   US

   Email: douglas.sicker@colorado.edu




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