[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: 00 01 02 RFC 4169

Network Working Group                                        V. Torvinen
Internet-Draft                                                  J. Arkko
Expires: April 28, 2005                                       M. Naslund
                                                                Ericsson
                                                        October 28, 2004



     Hypertext Transfer Protocol (HTTP) Digest Authentication Using
            Authentication and Key Agreement (AKA) Version-2
                  draft-torvinen-http-digest-aka-v2-02.txt


Status of this Memo


   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  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 become aware will be disclosed, in accordance with
   RFC 3668.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."


   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.


   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


   This Internet-Draft will expire on April 28, 2005.


Copyright Notice


   Copyright (C) The Internet Society (2004).


Abstract


   HTTP Digest as specified in [4] is known to be vulnerable to
   man-in-the-middle attacks if the client fails to authenticate the
   server in TLS, or if the same passwords are used for authentication
   in some other context without TLS.  This is a general problem that
   exist not just with HTTP Digest but also with other IETF protocols




Torvinen, et al.         Expires April 28, 2005                 [Page 1]

Internet-Draft             HTTP Digest AKAv2                October 2004



   that use tunneled authentication.  This document specifies version 2
   of the HTTP Digest AKA algorithm [6].  This algorithm can be
   implemented in a way that it is resistant to the man-in-the-middle
   attack.


Table of Contents


   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  HTTP Digest AKAv2  . . . . . . . . . . . . . . . . . . . . . .  6
     3.1   Password generation  . . . . . . . . . . . . . . . . . . .  6
     3.2   Session keys . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Example Digest AKAv2 Operation . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     5.1   Multiple Authentication Schemes and Algorithms . . . . . .  8
     5.2   Session Protection . . . . . . . . . . . . . . . . . . . .  8
     5.3   Man-in-the-middle attacks  . . . . . . . . . . . . . . . .  8
     5.4   Entropy  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
     6.1   Registration Information . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.1   Normative References . . . . . . . . . . . . . . . . . . . . 11
   7.2   Informative References . . . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 14


























Torvinen, et al.         Expires April 28, 2005                 [Page 2]

Internet-Draft             HTTP Digest AKAv2                October 2004



1.  Requirements notation


   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 [3].


2.  Introduction


   The Hypertext Transfer Protocol (HTTP) Digest Authentication,
   described in [4], has been extended in [6] to support Authentication
   and Key Agreement (AKA) mechanism [8].  AKA mechanism performs
   authentication and session key distribution in Universal Mobile
   Telecommunications System (UMTS) networks.  HTTP Digest AKA enables
   the usage of AKA as a one-time password generation mechanism for
   Digest authentication.


   HTTP Digest is known to be vulnerable to man-in-the-middle attacks,
   even when run inside TLS, if the same HTTP Digest authentication
   credentials are used in some other context without TLS.  The attacker
   may initiate a TLS session with a server, and when the server
   challenges the attacker with HTTP Digest, the attacker masquerades
   the server to the victim.  If the victim responds to the challenge,
   the attacker is able to use this response towards the server in HTTP
   Digest.  Note that this attack is an instance of general attack that
   affects a number of IETF protocols such as PIC.  The general problem
   is discussed in [9] and [10].


   Because of the previous vulnerability, the use of HTTP Digest "AKAv1"
   should be limited to the situations where the client is able to
   demonstrate that in addition to the AKA response, it possess the AKA
   session keys.  This is possible, for example, if the underlying
   security protocol uses the AKA generated session keys to protect the
   authentication response.  This is the case for example in the 3GPP IP
   Multimedia Core Network Subsystem (IMS) where HTTP Digest "AKAv1" is
   currently applied.  However, HTTP Digest "AKAv1" should not be used
   with tunnelled security protocols that do not utilize the AKA session
   keys.  For example, the use of HTTP Digest "AKAv1" is not necessarily
   secure with TLS if the server side is authenticated using
   certificates and the client side is authenticated using HTTP Digest
   AKA.


   There are at least four potential solutions to the problem:


   1.  The use of the authentication credentials is limited to one
       application only.  In general, this approach is good and can be
       recommended from security point of view.  However, this will
       increase the total number of authentication credentials for an
       end-user, and may cause scalability problems in the server side.




Torvinen, et al.         Expires April 28, 2005                 [Page 3]

Internet-Draft             HTTP Digest AKAv2                October 2004



   2.  The keys used in the underlying security protocols are somehow
       bind to the keys used in the tunneled authentication protocol.
       However, this would cause problems with the current
       implementations of underlying security protocols.  For example,
       it is not possible to use the session keys from TLS at
       application layer.  Furthermore, this solution would only solve
       the problem when HTTP Digest is used over one hop, and leave the
       problem of using HTTP Digest via multiple hops, e.g.  via proxy
       servers, unsolved.


   3.  Authentication credentials are used in cryptographically
       different way for each media and/or access network.  However, it
       may be difficult to know which underlying media is used below the
       application.


   4.  Authentication credentials are used in cryptographically
       different way for each application.


   This document specifies a new algorithm version for HTTP Digest AKA,
   i.e.  "AKAv2".  "AKAv2" specifies a cryptographically different way
   to use AKA credentials in use cases that are based either on HTTP
   Digest authentication or UMTS authentication (cf.  approach 4 above).
   The only difference to "AKAv1" is that in addition to AKA response
   RES the AKA related session keys, IK and CK, are also used as the
   password for HTTP Digest.  AKAv2 is immune to man-in-the-middle
   attack described above.  However, if AKAv2 is used in some
   environment both with and without some underlying security, such as
   TLS, the problem still exists.


   New HTTP Digest AKA algorithm versions can be registered in IANA
   based on Expert Review.  Documentation of new algorithm versions is
   not mandated as RFCs.  However, "AKAv2" is documented as an RFC
   because the use of different AKA algorithm versions includes security
   implications that the implementators should be aware of.  The
   extension version and security implications are presented in this
   document.


2.1  Terminology


   This chapter explains the terminology used in this document.


   AKA


      Authentication and Key Agreement.


      AKA is a challenge-response based mechanism that uses symmetric
      cryptography.  AKA can be run in a UMTS IM Services Identity
      Module (ISIM) or in UMTS Subscriber Identity Module (USIM), which




Torvinen, et al.         Expires April 28, 2005                 [Page 4]

Internet-Draft             HTTP Digest AKAv2                October 2004



      reside on a smart card like device that also provides tamper
      resistant storage of shared secrets.


   CK


      Cipher Key.  An AKA session key for encryption.


   CK'


      Cipher Key.  HTTP Digest AKAv2 session key for encryption.  CK' is
      derived from CK using a pseudo-random function.


   IK


      Integrity Key.  An AKA session key for integrity check.


   IK'


      Integrity Key.  HTTP Digest AKAv2 session key for integrity check.
      IK' is derived from IK using a pseudo-random function.


   ISIM


      IP Multimedia Services Identity Module.  Sometimes ISIM is
      implemented using USIM.


   RES


      Authentication Response.  Generated by the ISIM.


   PRF


      Pseudo-random function that is used to construct the AKAv2
      password and related session keys IK' and CK'.  In this document,
      PRF is presented in format KD(secret, data) denoting a keyed
      digest algorithm (KD) performed to the data "data" with the secret
      "secret".


   SIM


      Subscriber Identity Module.  GSM counter part for ISIM and USIM.


   UMTS


      Universal Mobile Telecommunications System.


   USIM





Torvinen, et al.         Expires April 28, 2005                 [Page 5]

Internet-Draft             HTTP Digest AKAv2                October 2004



      UMTS Subscriber Identity Module.  UMTS counter part for ISIM and
      SIM.


   XRES


      Expected Authentication Response.  In a successful authentication
      this is equal to RES.



3.  HTTP Digest AKAv2


   In general, the Digest AKAv2 operation is identical to the Digest
   AKAv1 operation described in  [6].  This chapter specifies the parts
   in which Digest AKAv2 is different from Digest AKAv1 operation.  The
   notation used in the Augmented BNF definitions for the new and
   modified syntax elements in this section is as used in SIP [5], and
   any elements not defined in this section are as defined in [6].


   In order to direct the client into using AKAv2 for authentication
   instead of other AKA versions or other HTTP Digest algorithms, the
   AKA version directive of [6] shall have the following new value:



             aka-version         =  "AKAv2"


   The AKA version directive is used as a part of the algorithm field as
   defined in [6].



             Example:  algorithm=AKAv2-MD5



3.1  Password generation


   The client shall use base64 encoded [1] parameters PRF(RES||IK||CK,
   "http-digest-akav2-password") as a "password" when calculating the
   HTTP Digest response directive for AKAv2.


   The server shall use base64 encoded [1] parameters PRF(XRES||IK||CK,
   "http-digest-akav2-password") as a "password" when checking the HTTP
   Digest response or when calculating the "response-auth" of the
   "Authentication-Info" header.


   The pseudo-random function (PRF) used to construct the HTTP Digest
   password is equal to HMAC [2] using the hash algorithm that is used
   in producing the digest and the checksum.  For example, if the
   algorithm is AKAv2-MD5, then the PRF is HMAC_MD5.





Torvinen, et al.         Expires April 28, 2005                 [Page 6]

Internet-Draft             HTTP Digest AKAv2                October 2004



   The string "http-digest-akav2-password" included in the key
   derivation is case sensitive.


3.2  Session keys


   Even though HTTP Digest AKA framework does not specify the use of the
   session keys IK and CK for confidentiality and integrity protection,
   the keys may be used for creating additional security within HTTP
   authentication or some other security mechanism.  However, the
   original session keys IK and CK MUST NOT be directly re-used for such
   additional security in "AKAv2".  Instead, session keys IK' and CK'
   are derived from the original keys IK and CK in the following way:


      IK' = PRF(IK, "http-digest-akav2-integritykey")


      CK' = PRF(CK, "http-digest-akav2-cipherkey")


   Any application using HTTP authentication framework is allowed to use
   these masked session keys.  The unmaksed session keys MAY also be
   re-used in some other context if some other application specific
   strings than "http-digest-akav2-integritykey" or
   "http-digest-akav2-cipherkey" are used to mask the original session
   keys.


   The pseudo-random function (PRF) used to construct the HTTP Digest
   session keys is equal to HMAC [2] using the hash algorithm that is
   used in producing the digest and the checksum.  For example, if the
   algorithm is AKAv2-MD5, then the PRF is HMAC_MD5.  The algorithm MUST
   be used in HMAC format as defined in [2].


   The strings "http-digest-akav2-integritykey" and
   "http-digest-akav2-cipherkey" included in the key derivation are case
   sensitive.


4.  Example Digest AKAv2 Operation


   This document does not introduce any changes to the operations of
   HTTP Digest or HTTP Digest AKA.  Examples defined in [6] applies
   directly to AKAv2 with the following two exceptions:


   1.  The algorithm directive has a prefix "AKAv2" instead of "AKAv1".


   2.  The HTTP Digest password is derived from base64 encoded
       PRF(RES||IK||CK, "http-digest-akav2-password") or
       PRF(XRES||IK||CK, "http-digest-akav2-password") instead of (RES)
       or (XRES) respectively.


   3.  The optional session keys are derived from PRF(IK,




Torvinen, et al.         Expires April 28, 2005                 [Page 7]

Internet-Draft             HTTP Digest AKAv2                October 2004



       "http-digest-akav2-integritykey") and PRF(CK,
       "http-digest-akav2-cipherkey") instead of IK and CK respectively.


   Note that the password in "AKAv1" is in binary format.  "AKAv2"
   password is base64 encoded [1].


5.  Security Considerations


5.1  Multiple Authentication Schemes and Algorithms


   The rules for an user agent for choosing among multiple
   authentication schemes and algorithms are as defined in [6] except
   that the user agent MUST choose "AKAv2" if both "AKAv1" and "AKAv2"
   are present.


   Since HTTP Digest is known to be vulnerable for bidding-down attack
   in environments where multiple authentication schemes and/or
   algorithms are used, the system implementators should pay special
   attention for scenarios where both "AKAv1" and "AKAv2" are used.
   Especially if the AKA generated sessions keys or some other
   additional security measures to authenticate the clients, such as
   client certificates, are not used, the use of both AKA algorithm
   versions should be avoided.


5.2  Session Protection


   Even though "AKAv2" uses the additional integrity (IK) and
   confidentiality (CK) keys as a part of HTTP Digest AKA password,
   these session keys may still be used for creating additional security
   within HTTP authentication or some other security mechanism.  This
   recommendation is based on the assumption that algorithms used in
   HTTP Digest, such as MD5, are sufficiently strong one-way functions,
   and consequently HTTP Digest responses leak no or very little
   computational information about IK and CK.  Furthermore, the session
   keys are masked into IK' and CK' before they can be used for session
   protection.


5.3  Man-in-the-middle attacks


   [9] describe a "man-in-the-middle" attack related to tunnelled
   authentication protocols.  The attack can occur e.g.  in EAP context
   or any similar contexts where tunnelled authentication is used and
   where the same authentication credentials are used without protection
   in some other context or the client fails to authenticate the server.


   For example, the use of TLS with HTTP Digest authentication (i.e.
   TLS for server authentication, and subsequent use of HTTP Digest for
   client authentication) is an instance of such scenario.  HTTP




Torvinen, et al.         Expires April 28, 2005                 [Page 8]

Internet-Draft             HTTP Digest AKAv2                October 2004



   challenges and responses can be fetched from and to different TLS
   tunnels without noticing where they originally came from.
   Especially, the attack is easy to perform if the client fails to
   authenticate the server.  If the same HTTP credentials are used with
   unsecured connection, the attack is also easy to perform.


   This is how the "man-in-the-middle" attack works with HTTP Digest and
   TLS if the victim (i.e.  the client) fails to authenticate the
   server:


   1.  The victim contacts the attacker using TLS.  If the attacker has
       a valid server certificate, the client may continue talking to
       the attacker and use some HTTP authentication compatible
       protocol, such as Session Initiation Protocol (SIP).


   2.  The attacker contacts some real proxy/server also using TLS and
       some HTTP authentication compatible protocol.  The proxy/server
       responds to the attacker with HTTP Authentication challenge.


   3.  The attacker forwards the HTTP Authentication challenge from the
       proxy/server to the victim.  If the victim is not careful, and
       check that the identity in the server certificate in TLS matches
       the realm in the HTTP authentication challenge, it may send a new
       request which carries a valid response to the HTTP Authentication
       challenge.


   4.  The attacker may use the response with the victims HTTP Digest
       username and password to authenticate itself to the proxy/server.


   The man-in-the-middle attack is not possible if the client compares
   the identities in the TLS server certificate and the HTTP Digest
   authentication challenge.  Note that with HTTP Basic, the client
   would send the password to the attacker.


   Another variant of the "man-in-the-middle" attack is the so-called
   "interleaving attack".  This attack is possible if the HTTP Digest
   authentication credentials are used in several contexts, and in one
   of them without protection.


   This is how the attack could proceed:


   1.  The attacker establishes a TLS tunnel to the proxy/server using
       one-way server authentication.  The attacker sends a request to
       the proxy/server.


   2.  The proxy/server challenges the attacker with HTTP Digest
       challenge.





Torvinen, et al.         Expires April 28, 2005                 [Page 9]

Internet-Draft             HTTP Digest AKAv2                October 2004



   3.  The attacker challenges the victim in some other context using
       the challenge carried in the HTTP Digest challenge.  The HTTP
       Digest challenge need to be modified to the format used in the
       protocol of this other context.


   4.  The victim responds with a response.


   5.  The attacker uses the response from the other context for
       authentication in HTTP Digest.


   6.  The proxy/server accepts the response, and delivers the service
       to the attacker.


   In some circumstances, HTTP Digest AKAv1 may be vulnerable for the
   interleaving attack.  In particular, if ISIM is implemented using
   USIM the HTTP Digest AKAv1 should not be used with tunneled security
   protocols unless the AKA related session keys, IK and CK, are somehow
   used with the solution.


   HTTP Digest AKAv2 is not vulnerable for this interleaving attack, and
   it can be used with tunneled security protocols without using the
   related AKA session keys.


5.4  Entropy


   AKAv1 passwords should only be used as one-time passwords if the
   entropy of the used RES value is limited (e.g., only 32 bits).  For
   this reason, the reuse of the same RES value in authenticating
   subsequent requests and responses is not recommended.  Furthermore,
   algorithms such as "MD5-sess", which limit the amount of material
   hashed with a single key, by producing a session key for
   authentication, should not be used with AKAv1.


   Passwords generated using AKAv2 can more securely be used for
   authenticating subsequent requests and responses because the
   concatenation of AKA credentials (i.e.  RES||IK||CK) makes the
   passwords significantly longer, and the pseudo-random function
   heuristically provides an entropy equal to the length of this string,
   or, the length of the PRF output, whichever is the shortest.  The
   user agent does not need to assume that AKAv2 passwords are limited
   to one-time use only, and it may try to re-use the AKAv2 passwords
   with the server.  However, note that AKAv2 passwords can not be
   re-used with HTTP Digest AKAv2 algorithm because such authentication
   challenge will automatically generate a fresh password.  AKAv2
   passwords can be used with other HTTP Digest algorithms, such as
   "MD5".


   The underlying AKA protocol (e.g.  UMTS AKA) has been designed to




Torvinen, et al.         Expires April 28, 2005                [Page 10]

Internet-Draft             HTTP Digest AKAv2                October 2004



   keep CK and IK confidential but will typically send RES in the clear.
   We note that even if (by some unfortunate missuse of AKA) RES values
   were revealed, the inclusion of RES in PRF(RES||IK||CK) is still
   benefitial as it makes pre-calculated dictionaries of IK||CK values
   rather useless (though such dictionaries are anyway infeasible for
   typical sizes of IK and CK).


6.  IANA Considerations


   This document specifies a new aka-version, "AKAv2", to the
   aka-version namespace maintained by IANA.  The procedure for
   allocation of new aka-versions is defined in [6].


6.1  Registration Information


   To: ietf-digest-aka@iana.org


   Subject: Registration of a new AKA version


   Version identifier: "AKAv2"


   Contacts for further information: vesa.torvinen@ericsson.com,
   jari.arkko@ericsson.com or mats.naslund@ericsson.com


7.  References


7.1  Normative References


   [1]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.


   [2]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
        for Message Authentication", RFC 2104, February 1997.


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


   [4]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
        Basic and Digest Access Authentication", RFC 2617, June 1999.


   [5]  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.


   [6]  Niemi, A., Arkko, J. and V. Torvinen, "Hypertext Transfer
        Protocol (HTTP) Digest Authentication Using Authentication and




Torvinen, et al.         Expires April 28, 2005                [Page 11]

Internet-Draft             HTTP Digest AKAv2                October 2004



        Key Agreement (AKA)", RFC 3310, September 2002.


7.2  Informative References


   [7]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October
         1998.


   [8]   3rd Generation Partnership Project, "Security Architecture
         (Release 4)", TS 33.102, December 2001.


   [9]   Asokan, N., Niemi, V. and K. Nyberg, "Man-in-the-Middle in
         Tunnelled Authentication Protocols", Cryptology ePrint Archive,
         http://eprint.iacr.org Report 2002/163, October 2002.


   [10]  Puthenkulam, J., Lortz, V., Palekar, A. and D. Simon, "The
         Compound Authentication Binding Problem", IETF, Work in
         progress draft-puthenkulam-eap-binding-02.txt, March 2003.



Authors' Addresses


   Vesa Torvinen
   Ericsson
   Joukahaisenkatu 1
   Turku  FIN 20520
   Finland


   Phone: +358 40 7230822
   EMail: vesa.torvinen@ericsson.com



   Jari Arkko
   Ericsson
   Hirsalantie 1
   Jorvas  FIN 02420
   Finland


   Phone: +358 40 5079256
   EMail: jari.arkko@ericsson.com












Torvinen, et al.         Expires April 28, 2005                [Page 12]

Internet-Draft             HTTP Digest AKAv2                October 2004



   Mats N„slund
   Ericsson
   Torshamnsgatan 23
   Stockholm  SE 16480
   Sweden


   Phone: +46 8 58533739
   EMail: mats.naslund@ericsson.com












































Torvinen, et al.         Expires April 28, 2005                [Page 13]

Internet-Draft             HTTP Digest AKAv2                October 2004



Intellectual Property Statement


   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.


   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.


   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.



Disclaimer of Validity


   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



Copyright Statement


   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.



Acknowledgment


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




Torvinen, et al.         Expires April 28, 2005                [Page 14]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/