SIP WG                                                       J. Peterson
Internet-Draft                                                   NeuStar
Expires: April 28, 2003                                 October 28, 2002

   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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   This Internet-Draft will expire on April 28, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  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 Authentication Service . . . . . . . . . . . . .  6
   4.2 Body Added by Client . . . . . . . . . . . . . . . . . . . . .  7
   4.3 Using Content Indirection  . . . . . . . . . . . . . . . . . .  8
   5.  Identity in Responses  . . . . . . . . . . . . . . . . . . . .  9
   6.  Receiving an Authentication Token  . . . . . . . . . . . . . .  9
   6.1 Authentication Service Handling of Authentication Tokens . . .  9
   7.  Selective Sharing of Identity  . . . . . . . . . . . . . . . . 10
   7.1 Requesting Privacy . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
       Normative References . . . . . . . . . . . . . . . . . . . . . 12
       Informative References . . . . . . . . . . . . . . . . . . . . 13
   B.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15

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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, 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 place to which requests should be sent for a
   particular address-of-record SIP URI.  However, the credentials with
   which a user agent proves to a registrar that they are, for example,
   an authorized recipient of requests for ''
   cannot be shared with a server in another domain - these credentials
   are currently only useful for local registration.

   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
   it turns out that users already have a means of providing their
   identity, if only to a registrar; in fact, 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' is identical to proving that you
   are authorized to assert this identity.  In other words, the best way
   for a SIP user to prove that they can legitimately claim an identity

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   is to provide the same credentials they would need to provide in
   order to register to receive requests for that identity; the two have
   the same security properties.  Since the operator of the registrar
   controls the namespace of local SIP users (assigning the user portion
   of SIP URIs in the domain), it is the ideal arbiter for identity in

   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

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   would adapt existing SIP security mechanisms to address this problem.

   Therefore, this document defines a new logical role for SIP network
   intermediaries (typically proxy servers) called an 'authentication
   service'.  Once an authentication service has verified the identity
   of the originator of a request, as describe 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 would like 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 plausible token formats are a
   matter for further investigation.  Throughout this document, the use
   of the AIB format as a token is considered exclusively.

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 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

4. How to Share Verified Identities

   When an authentication service has authenticated the user, it must
   construct an identity for that user that will be contained in the

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   token.  It is RECOMMENDED that these identities take the form of
   addresses-of-record, 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
   Authentication Identity Body (AIB) format described in [4] is used,
   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.

   In some cases, an authentication service will 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).  The appropriate
   certificate for the authenticated user will be used to sign the
   authentication token.  However, an authentication service MAY instead
   use a common 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).  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 be added to any existing MIME bodies in the request, if
   necessary by transitioning the outermost MIME body to a 'multipart/
   mixed' format.  Two options are considered for ways that an
   authentication token could be added to a SIP message: one in which
   the authentication service adds the token itself, and one in which it
   pushes the token back to the client, essentially asking the client to
   include the token in a retry of the request.  Another possibility,
   using content indirection, is mentioned as a direction for future
   work.  Authentication services MUST support the mechanism in Section
   4.2 and MAY support the mechanism in Section 4.1.

4.1 Body Added by Authentication Service

   The first 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

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   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 push the
   authentication token back to the originating user agent.  For one, it
   saves on additional round-trip times for signaling that result from
   passing the body back to the user agent.  It also requires no new SIP
   mechanisms, whereas any method of asking a user agent to include a
   body in a resubmission to the current request would introduce new
   protocol requirements.

   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

4.2 Body Added by Client

   Alternatively, the authentication service could in some fashion
   return 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 performs this function.
   Therefore, this document defines a 428 "Use Authentication Token"
   response code.

   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 an
   authentication service.

   In order to signal to the authentication services 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

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   Using the 428 response code may introduce extra round-trip times for
   messages, delaying the setup of requests (one RTT for the 407,
   another for the 428).  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.  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 RTT.

   There are two reasons why 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 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.

4.3 Using Content Indirection

   Work [8] is currently underway in the SIP WG to define a content
   indirection 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 management.

   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.  Authentication services
   could define a standard way to anticipate URIs for a particular
   request (for example, an HTTP URL could be structured with a hostname
   corresponding to the authentication service, and a path corresponding
   to a unique identifier selected by the user agent for this request:  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 not requiring the extra RTT
   caused by the use of the 428 response code.  It also has numerous

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   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.  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 the registrar 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 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.  In many network implementations, the identity in the
   authentication token of a response will correspond to the To header
   field of the request, but there are numerous legitimate architectures
   (which contain redirections) in which this will not be the case.

   An authentication service that acts as a registrar 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 4.1 - 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 4.2; 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.

6. Receiving an Authentication Token

   The manner in which an authentication token is handled is dependent
   on the nature of the token itself; for rules for handling the
   Authentication Identity Body (AIB) format, see [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

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   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 4.2).  However,
   proxy servers 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, 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 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 use SIP to 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).

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   Authentication services MUST support the privacy mechanism described
   in [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 [6].  This Privacy value requests that the results of
   authentication should not be shared by the authenticating server.  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
   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.

   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 ''

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   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
   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.

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   [3]  Peterson, J., "A Privacy Mechanism for the Session Initiation
        Protocol (SIP)", draft-ietf-sip-privacy-general-02 (work in
        progress), June 2002.

   [4]  Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
        draft-ietf-sip-authid-body-00 (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", draft-ietf-sip-asserted-identity-02
        (work in progress), June 2002.

   [7]  Sparks, R., "Internet Media Type message/sipfrag", draft-ietf-
        sip-sipfrag-00 (work in progress), September 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.

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.

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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

      - Various other editorial corrections

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Internet-Draft                SIP Identity                  October 2002

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