SIP WG                                                       J. Peterson
Internet-Draft                                                   NeuStar
Expires: August 2, 2003                                    February 2003

   Enhancements for Authenticated Identity Management in the Session
                       Initiation Protocol (SIP)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   The existing mechanisms for expressing identity in the Session
   Initiation Protocol oftentimes do not permit an administrative domain
   to verify securely the identity of the originator of a request.  This
   document recommends practices and conventions for authenticating end
   users, and proposes a way to distribute cryptographically secure
   authenticated identities within SIP messages.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Using an Authentication Service  . . . . . . . . . . . . . . .  5
   4.  How to Share Verified Identities . . . . . . . . . . . . . . .  5
   4.1 Body Added by Client . . . . . . . . . . . . . . . . . . . . .  7
   4.2 Body Added by Authentication Service . . . . . . . . . . . . .  8
   4.3 Using Content Indirection  . . . . . . . . . . . . . . . . . .  8
   5.  Identity in Responses  . . . . . . . . . . . . . . . . . . . .  9
   6.  Receiving an Authentication Token  . . . . . . . . . . . . . . 10
   6.1 Authentication Service Handling of Authentication Tokens . . . 10
   7.  Selective Sharing of Identity  . . . . . . . . . . . . . . . . 10
   7.1 Requesting Privacy . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
       Normative References . . . . . . . . . . . . . . . . . . . . . 13
       Informative References . . . . . . . . . . . . . . . . . . . . 13
   B.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16

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1. Introduction

   This document provides enhancements to the existing mechanisms for
   authenticated identity management in the Session Initiation Protocol
   (SIP [1]).  An identity, for the purposes of this document, is
   defined as a canonical SIP URI employed to reach a user (such as

   RFC3261 enumerates a number of places within a SIP request that a
   user can express an identity for themselves, notably the From header
   field.  However, the recipient of a SIP request has no way to verify
   that the From header field has been populated appropriately without
   some sort of cryptographic authentication mechanism.

   Today, RFC3261 specifies a number of security mechanisms that can be
   used by SIP UAs, including Digest, TLS and S/MIME (and
   implementations may support other security schemes as well).
   However, few SIP user agents today can support the end-user
   certificates necessary to authenticate themselves via TLS or S/MIME,
   and Digest authentication is limited by the fact that the originator
   and destination must share a secret.  It is desirable for SIP user
   agents to be able to send requests to destinations with they have no
   previous association - just as in the telephone network today, one
   can receive a call from someone with whom one has no previous
   association, and still have a reasonable assurance that their
   displayed Caller-ID is accurate.

   Many SIP user agents today support a means of authenticating
   themselves to a SIP registrar - commonly with a shared secret (Digest
   authentication, which MUST be supported by SIP user agents, is
   typically used for this purpose).  Registration allows a user agent
   to express that it is the proper entity to which requests should be
   sent for a particular address-of-record SIP URI.

   Coincidentally, the address-of-record URI of a SIP user is also the
   URI with which a SIP UA populates the From header of requests from
   that user - in other words, the address-of-record is an identity.  So
   in this context users already have a means of providing their
   identity, which makes good sense: since the contents of a From header
   field are essentially a 'return address' for SIP requests, being able
   to prove that you are eligible to receive requests for that 'return
   address' should be identical to proving that you are authorized to
   assert this identity.

   However, the credentials with which a user agent proves to a
   registrar that they are, for example, an authorized recipient of
   requests for '' will not be accepted by a server
   in another domain - these credentials are currently only useful for

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   local registration.  What other domains really want to know about
   your identity is that you are capable of authenticating yourself in
   your own domain.

   Ideally, then, there should be some way of proving to remote domains
   that your local domain has authenticated you.  In the absence of end-
   user certificates in user agents, it is possible to implement a
   mediated authentication architecture for SIP in which requests are
   sent to a server in the user's local domain which authenticates them
   (using the same practices by which the domain would authenticate
   REGISTER requests).  Once a request has been authenticated, the local
   domain then needs some way to communicate to remote domains that it
   has sanctioned the request.  This draft addresses how that identity
   can could be securely shared.

   RFC3261 already describes an architecture very similar to this in
   Section, in which a user agent authenticates itself to a
   local proxy server which in turn authenticates itself to a remote
   proxy server via mutual TLS, creating a two-link chain of transitive
   authentication between the originator and the remote domain.  While
   this works well in some architectures, there are a few respects in
   which this is impractical.  For one, it is possible for SIP requests
   to cross multiple intermediaries in separate administrative domains,
   in which case transitive trust becomes far less compelling.  It also
   requires intermediaries to act as proxies, rather than redirecting
   requests to their destinations (redirection lightens loads on SIP
   intermediaries).  Both of these limitations result from the fact that
   authentication takes place outside the application, at the transport
   layer, rather than within SIP itself.

   One solution to this problem is to use 'trusted' SIP intermediaries
   that assert an identity for users in the form of a privileged SIP
   header.  A mechanism for doing so (with the P-Asserted-Identity
   header) is given in [6].  However, this solution allows only hop-by-
   hop trust between intermediaries, not end-to-end cryptographic
   authentication, and it assumes a managed network of nodes with strict
   mutual trust relationships, an assumption that is incompatible with
   widespread Internet deployment.

   The desired mediated authentication architecture has quite a bit in
   common with the problem space of Kerberos [5].  Ideally, there should
   be a way for a user to authenticate themselves to the local domain,
   and receive some kind of token that they can share with recipients of
   requests that lets them know that the user has been authenticated by
   the local domain.  However, Kerberos support in SIP user agents is
   not widespread, and moreover SIP uses other means (such as Digest) to
   perform key authentication functions already.  An ideal solution
   would adapt existing SIP security mechanisms to address this problem.

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   Therefore, this document defines a new logical role for SIP network
   intermediaries called an 'authentication service'.  Once an
   authentication service has verified the identity of the originator of
   a request, as described above, it creates a cryptographic token that
   contains the authenticated identity of the user, and which has some
   reference integrity with the request itself.  This token can then be
   added to a SIP request and inspected by recipients of the request who
   need a cryptographic guarantee of the identity of the user.

   One possible format for such tokens is the Authenticated Identity
   Body (AIB) described in [4].  Other token formats are a matter for
   further investigation.  Throughout this document, the use of AIB
   format for the token is considered exclusively.  Only tokens that are
   suitable to be carried in a MIME body are considered in this

2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in RFC2119 [2] and indicate requirement levels for
   compliant SIP implementations.

3. Using an Authentication Service

   A SIP user agents sends requests to an authentication service in
   order to receive an authentication token for the request.  How
   exactly the association with an authentication service is learned or
   configured is an implementation-specific matter for the user agent -
   it might be implemented with a pre-loaded Route header.  The
   guidelines given in RFC3261 Sections and should be
   used when connecting to an authentication service; ideally, an
   authentication service should be one hop away from a user agent, it
   should use a lower-layer security protocol such as TLS or IPSec to
   authenticate the authentication service before providing credentials
   (especially shared secrets).

   This document places no requirements on how an authentication
   services authenticates requests.  Since Digest authentication MUST be
   supported by all SIP entities, the use of Digest for this purpose is
   RECOMMENDED for compatibility with the maximum set of user agents.

4. How to Share Verified Identities

   When an authentication service has authenticated the user, it must
   construct an identity URI for that user that will be contained in the
   token.  It is RECOMMENDED that these identities take the form of SIP

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   address-of-record URI (as opposed to contact addresses), as they are
   defined in Section 10 of RFC3261; in other words, URIs of the form

   This identity must be expressed in the authentication token that will
   be signed by the authentication service.  For example, if the
   Authenticated Identity Body (AIB) format described in [4] is used,
   then for an INVITE this identity would be stored in the From header
   field within a 'message/sip' or 'message/sipfrag' [7] body that will
   be signed by the authentication service.

   Once the token has been created, the server MUST sign the token.  The
   subject of the certificate SHOULD be assigned in one of the two
   following ways:

      An authentication service MAY use a common certificate, such as a
      site certificate, for its administrative domain.  The
      subjectAltName of this certificate MUST correspond with the host
      portion of the From header field of the identity in the
      authentication token (if the identity were
      '', the subjectAltName of the certificate
      would be ''); this should be the same certificate that
      the authentication service provides when proving its own identity
      (via TLS or some similar protocol).

      An authentication service MAY hold a certificate corresponding to
      each user in its administrative domain (in other words, a
      certificate whose subjectAltName contains a URI equivalent to the
      address-of-record URI of the user).  In this case, the appropriate
      certificate for the authenticated user will be used to sign the
      authentication token.  Maintaining individual certificates for
      each user is RECOMMENDED, since the name subordination rules
      involved with the use of a common certificate for the domain can
      become complicated.

   After the authentication token has been signed, the authentication
   token MUST somehow be integrated with any existing MIME bodies in the
   request, if necessary by transitioning the outermost MIME body to a
   'multipart/mixed' format, before the request can be forwarded.  Three
   options are considered for ways that an authentication token could be
   added to a SIP message: one in which the authentication service
   pushes the token back to the client for resubmission, one in which
   the authentication service adds the token itself, and one in which
   the client anticipates a URI at which the authentication service will
   make the token available.  Authentication services MUST support the
   mechanism in Section 4.1 and MAY support the mechanism in Section
   4.2; the mechanism in Section 4.3 is included to illustrate a future

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4.1 Body Added by Client

   In this case, the authentication service returns the authentication
   token to the originating user agent, prompting the user agent to
   retry the request with the authentication token attached.  No
   existing SIP mechanism can perform this function.  Therefore, this
   document defines a 428 "Use Authentication Token" response code.

   After a user has been authenticated (in the Digest example, with the
   407 response) an authentication service sends a 428 with a MIME body
   in order to request that a user agent add the enclosed MIME body to
   their request and retry the request.  A 428 MUST have at most a
   single MIME body.  This MIME body MUST be signed by the
   authentication service.

   The use of 428 without any MIME body is also defined in this
   document.  It can be sent by any server to reject a request because
   the request does not contain an authentication token.  A user agent
   receiving this rejection SHOULD retry their request through the same
   server after acquiring a token from an authentication service.

   In order to signal to the authentication services and other
   intermediaries that the originating user agent supports the receipt
   of the 428 response code, a new option-tag has been defined, the
   'auth-id' option-tag.  User agents SHOULD supply the 'auth-id'
   option-tag in a Supported header whenever they provide credentials to
   a server (for example, in Digest authentication, whenever a Proxy-
   Authorization header is added to a request).

   Using the 428 response code may introduce extra round-trip times for
   messages, delaying the setup of requests.  However, there are some
   circumstances under which extra RTTs may not impede performance.  If
   the originating user agent possesses a non-stale nonce (assuming
   Digest authentication) from the authentication service, it can pre-
   emptively include a Proxy-Authorization header, eliminating one RTT
   (the one resulting from a 407).  With regard to the second RTT, note
   that the request needn't necessarily go through the authentication
   service again once the authentication token has been added - it could
   go directly to its destination, which reduce the impact of the second

   There are two good reasons to think that the originating user agent
   should be the party responsible for adding the authentication token
   to the request.  Firstly, because this gives the client the
   opportunity to inspect the body itself (perhaps only to see whether
   or not it is encrypted; see [4]) in order to verify that the
   authenticated identity corresponds with the provided credentials and
   the user's preferences.  Secondly, the client can provide a signature

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   over the entire body of the message (either with S/MIME or some
   header-based mechanism) so that the final recipient of messages can
   be assured that all information in the body is there at the
   originator's behest.

4.2 Body Added by Authentication Service

   Another possibility is that the authentication service could add the
   body to the request itself before forwarding the request.  However,
   the authentication service role is usually played by entities that
   act as proxy servers for most requests, and proxy servers cannot
   modify message bodies (see RFC3261 Section 16.6).  In order to add an
   authentication token, the authentication service needs to act as a
   transparent back-to-back user agent, effectively terminating the
   request and re-originating it with a new body appended to any
   existing MIME bodies (again, transposing to various MIME multipart
   forms as necessary).

   This mechanism has some potential advantages over pushing the
   authentication token back to the originating user agent.  For one, it
   saves one additional round-trip time that would be used by the 428
   response.  It also requires no new SIP mechanisms, whereas the 428
   response necessitates option-tag support.

   However, there are proposed SIP integrity mechanisms that place a
   signature over the entire message body in a SIP message header.  Were
   a server to modify the body of a message that was protected by such
   signature, that would be perceived as an integrity violation by
   downstream recipients of the message.  Presumably, a back-to-back
   user agent function would have to sacrifice this end-to-end
   integrity.  The notion of a transparent back-to-back user agent is
   also ill-defined, and it is questionable if any SIP intermediaries
   should interfere with SIP message bodies.

4.3 Using Content Indirection

   Work is currently underway in the SIP WG to define a content
   indirection [8] mechanism for SIP, a mechanism by which a MIME body
   in a SIP request can refer, with a URL, to a document that it hosted
   somewhere in the network.  This raises another interesting
   possibility for authentication token transport in SIP.

   A SIP user agent could create a content indirection MIME body (using
   the RFC2017 [9] URL MIME External-Body Access-Type) that contains a
   URL that identities a resource controlled by the authentication
   service, anticipating that the authentication service will make the
   authentication token available at that URL.  This URL could be pushed
   by the authentication service to the UAC when the authentication

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   service challenges the UAC (as a new header in the 407 response).
   Once an authentication service has validated the request, it simply
   makes the authentication token available at the anticipated URL;
   recipients of the message would then dereference the URL in order to
   inspect the token.

   This approach could allow user agents to have full control over the
   integrity of SIP requests, while still requiring the extra RTT caused
   by the use of the 428 response code.  It also has numerous advantages
   over other ways of handling authentication tokens issued for SIP
   response messages (see Section 5).

5. Identity in Responses

   Many of the practices described in the preceding sections can be
   applied to responses as well as requests, with some important
   differences.  Primarily, the distinction is that a response cannot be
   challenged or resubmitted in the same manner as a request, and
   therefore the mechanism in Section 4.1 is not usable.  However, when
   a user agent registers under a particular identity, and thereby
   becomes eligible to receive requests and send responses associated
   with that identity, it provides credentials that prove its identity,
   and thus if the registrar is co-located with the proxy that receives
   requests for the user's administrative domain, is in a reasonable
   position to act as an authentication service for responses.

   Note that the identity in an authentication token in a response
   almost certainly will not correspond with the identity asserted in
   the From header field of the response (which is copied from the
   request); the identity in the authentication token represents a
   different entity.  For many requests, the identity in the
   authentication token of a response will correspond to the To header
   field of the request, but there are numerous legitimate ways that
   requests can be retargeted in which this will not be the case.

   An authentication service that also acts as a registrar and inbound
   proxy can add to a response an authentication token that corresponds
   to the identity of the originator of that response in roughly the
   same manner described in Section 4.2 - the authentication service
   adds the authentication token to a response before it forwards the
   response towards the originator of the request.  There is no way for
   an authentication service to perform a function for responses
   comparable to the mechanism described in Section 4.1; however,
   content indirection (see Section 4.3 could provide an alternative
   that would allow the client to retain end-to-end integrity properties
   on responses.

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6. Receiving an Authentication Token

   The manner in which an authentication token is handled is dependent
   on the nature of the token itself; rules for handling the
   Authenticated Identity Body (AIB) format are given [4].

6.1 Authentication Service Handling of Authentication Tokens

   SIP intermediaries generally should not attempt to inspect MIME
   bodies; following the rules of RFC3261 Section 16.6, MIME bodies may
   be encrypted end-to-end or have other properties that make them
   unsuitable for consumption by intermediaries.  However,
   intermediaries that implement the authentication service logical role
   MAY inspect MIME bodies in order to find one with a Content-
   Disposition of 'auth-id'.

   For the most part, the actual value of an authenticated identity is
   not likely to be of interest to a proxy server, though it MAY refuse
   to process a request that does not contain a valid authentication
   token (using the 428 request, as described in Section 4.1).  However,
   authentication services MAY additionally maintain lists of known
   problem users that are banned from making requests to their
   administrative domain, for example, and subsequently reject some
   requests after comparing their authenticated identities to such
   access control lists.

7. Selective Sharing of Identity

   Most of the time, there is no need to restrict the propagation of
   verified identities in the network.  User agents and intermediaries
   benefit from receiving verified identities.  However, in some cases
   intermediaries might want to restrict the distribution of identity
   information, for example if

   o  the authenticated identity body contains an identity that is only
      meaningful as an internal identifier within a particular service
      provider's network, or,

   o  the originating user agent has requested privacy, and the
      unrestricted distribution of the authenticated identity body would
      violate that request.

   If it is not appropriate to share an authenticated identity because a
   user has requested privacy, an authenticated identity body SHOULD NOT
   be created and distributed.  However, in some cases there may be
   other entities in the administrative domain of the authentication
   service that are consumers of the authenticated identity.  If, for
   example, each of these servers needed to challenge the user

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   individually for identity, it might significantly delay the
   processing of the request.  For that reason, it may be appropriate to
   circulate authenticated identity bodies among a controlled set of
   entities.  For that purpose, an encryption mechanism for
   authenticated identities is required.

7.1 Requesting Privacy

   When users authenticate themselves to an authentication service, they
   MAY explicitly notify the service that they do not wish their
   authenticated identity to be circulated.  Usually, the user in
   question would also be taking other steps to preserve their privacy
   (perhaps by including an anonymous From header in the SIP request,
   and following other standard privacy practices).

   Authentication services MUST support the privacy mechanism described
   in RFC3323 [3].  Users requesting privacy should also support the
   mechanisms described in that document.

   In particular, this document uses an identity-specific priv-value
   that can appear in the Privacy header, a value of 'id', which was
   registered by RFC3325 [6].  This Privacy value requests that the
   results of authentication should not be shared by the authenticating
   intermediary.  An authentication service SHOULD NOT create an
   authentication token for a request when 'id' privacy has been
   requested.  If such a token is created, it MUST be encrypted or
   rendered confidential in the manner most appropriate to the token.
   Guidelines for encrypting AIBs are given in [4], and these mechanisms
   MUST be supported by any authentication service that uses AIBs.

8. Security Considerations

   Users SHOULD NOT provide credentials to an authentication service to
   which they cannot initiate a direct connection, preferably one
   secured by a network or transport layer security protocol such as
   TLS.  If a user does not receive a certificate from the
   authentication service over this lower-layer protocol that
   corresponds to the expected domain (especially when they receive a
   challenge via a mechanism such as Digest), then it is possible that a
   rogue server is attempting to pose as a authentication service for a
   domain that it does not control, possibly in an attempt to collect
   shared secrets for that domain.  If a user cannot connect directly to
   the desired authentication service, the user SHOULD at least use a
   SIPS URI to ensure that mutual TLS authentication will be used to
   reach the remote server.

   Relying on an authentication token generated by a remote
   administrative domain assumes that the domain uses some trustworthy

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   practice to authenticate its users.  However, it is possible that
   some domains will implement policies that effectively make users
   unaccountable (such as accepting unauthenticated registrations from
   arbitrary users).  Therefore, it is RECOMMENDED that authentication
   tokens contain some indication of the specific practice (for example,
   Digest) that was used to authenticate this request as a rough
   indicator of credential strength.  No manner of describing
   authentication practices is specified in this document.

   If a common certificate is used by an authentication service (rather
   than individual certificates for each identity), certain problems can
   arise with name subordination.  For example, if an authentication
   service holds a common certificate for the hostname
   '', can it legitimately sign a token containing an
   identity of ''? It is difficult for the
   recipient of a request to ascertain whether or not ''
   is authoritative for the '' domain unless the recipient
   has some foreknowledge of the administration of ''.
   Therefore, it is RECOMMEND that user agent recipients of
   authentication tokens notify end users if there is ANY discrepancy
   between the subjectAltName of the signers certificate and the
   identity within the authentication token.

   Authentication tokens MUST have some form of replay protection.  The
   best protection is to copy the SIP request in its entirety (via the
   'message/sip' MIME type) into the authentication token - in that way,
   it will be clear that this token has been issued for this request,
   since collectively the headers of a SIP request provide a unique
   identifier.  However, SIP requests can be large, and it is reasonable
   to include only a subset of the SIP headers in a request (using the
   'message/sipfrag' MIME type) as long as certain critical headers are
   provided.  For further discussion of this issue, including guidelines
   for including particular headers in a sipfrag, see [4].

   Because the common certificates that can be used by authentication
   services need to assert only the hostname of the authentication
   service, existing certificate authorities can provide adequate
   certificates for this mechanism.  However, not all proxy servers and
   user agents will be able support the root certificates of all
   certificate authorities, and moreover there are some significant
   differences in the policies by which certificate authorities issue
   their certificates.  This document makes no recommendations for the
   usage of particular certificate authorities, nor does it describe any
   particular policies that certificate authorities should follow, but
   it is anticipated that operational experience will create de facto
   standards for the purposes of authentication services.  Some
   federations of service providers, for example, might only trust
   certificates that have been provided by a certificate authority

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   operated by the federation.

9. IANA Considerations

   This document defines a new SIP status code, '428 Use Authentication
   Token'.  The use of this status code is further described in Section

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]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

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

   [4]  Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
        draft-ietf-sip-authid-body-01 (work in progress), October 2002.

Informative References

   [5]  Kohl, J. and C. Neumann, "The Kerberos Network Authentication
        Service (V5)", RFC 1510, September 1993.

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

   [7]  Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
        November 2002.

   [8]  Olson, S., "A Mechanism for Content Indirection in SIP
        Messages", draft-ietf-sip-content-indirect-mech-01 (work in
        progress), August 2002.

   [9]  Freed, N., "Definition of the URL MIME External-Body Access-
        Type", RFC 2017, November 1996.

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Author's Address

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

   Phone: +1 925/363-8720

Appendix A. Acknowledgments

   The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
   Sparks, Jonathan Rosenberg, Mark Watson and Patrik Faltstrom for
   their comments.  Cullen Jennings assisted greatly in the development
   of the content indirection mechanism considered in Section 4.3.

Appendix B. Changelog

   Changes from draft-peterson-sip-identity-01:

      - Split off child draft-ietf-sip-authid-body-00 for defining of
      the AIB

      - Clarified scope in introduction

      - Removed a lot of text that was redundant with RFC3261
      (especially about authentication practices)

      - Added mention of content indirection mechanism for adding token
      to requests and responses

      - Improved Security Considerations (added piece about credential

   Changes from draft-peterson-sip-identity-00:

      - Added a section on authenticated identities in responses

      - Removed hostname convention for authentication services

      - Added text about using 'message/sip' or 'message/sipfrag' in
      authenticated identity bodies, also RECOMMENDED a few more headers
      in sipfrags to increase reference integrity

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      - Various other editorial corrections

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Internet-Draft                SIP Identity                 February 2003

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