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Network Working Group                                         B. Sterman
Internet-Draft                                           Kayote Networks
Expires: November 30, 2004                                 D. Sadolevsky
                                                          SecureOL, Inc.
                                                             D. Schwartz
                                                         Kayote Networks
                                                             D. Williams
                                                           Cisco Systems
                                                                 W. Beck
                                                     Deutsche Telekom AG
                                                               June 2004


               RADIUS Extension for Digest Authentication
                      draft-sterman-aaa-sip-04.txt

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 30, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   Several protocols borrow the authentication mechanisms from the
   Hypertext Transfer Protocol, HTTP.  This document specifies an



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   extension to RADIUS that allows a RADIUS client in an HTTP-style
   server, upon reception of a request, retrieve and compute Digest
   authentication information from a RADIUS server.  Additionally, a
   scenario describing  the authentication of a user emitting an
   HTTP-style request is provided.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2   Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3   Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.3.1   Scenario 1, RADIUS client chooses nonces . . . . . . .  6
       1.3.2   Scenario 2, RADIUS server chooses nonces . . . . . . .  7
   2.  New RADIUS attributes  . . . . . . . . . . . . . . . . . . . .  9
     2.1   Digest-Response attribute  . . . . . . . . . . . . . . . .  9
     2.2   Digest-Realm attribute . . . . . . . . . . . . . . . . . .  9
     2.3   Digest-Nonce attribute . . . . . . . . . . . . . . . . . . 10
     2.4   Digest-Response-Auth attribute . . . . . . . . . . . . . . 10
     2.5   Digest-Nextnonce attribute . . . . . . . . . . . . . . . . 10
     2.6   Digest-Method attribute  . . . . . . . . . . . . . . . . . 11
     2.7   Digest-URI attribute . . . . . . . . . . . . . . . . . . . 11
     2.8   Digest-QoP attribute . . . . . . . . . . . . . . . . . . . 11
     2.9   Digest-Algorithm attribute . . . . . . . . . . . . . . . . 12
     2.10  Digest-Entity-Body-Hash attribute  . . . . . . . . . . . . 12
     2.11  Digest-CNonce attribute  . . . . . . . . . . . . . . . . . 13
     2.12  Digest-Nonce-Count attribute . . . . . . . . . . . . . . . 13
     2.13  Digest-Username attribute  . . . . . . . . . . . . . . . . 13
     2.14  Digest-Opaque attribute  . . . . . . . . . . . . . . . . . 14
     2.15  Digest-Auth-Param attribute  . . . . . . . . . . . . . . . 14
     2.16  Digest-AKA-Auts attribute  . . . . . . . . . . . . . . . . 14
     2.17  Digest-Domain attribute  . . . . . . . . . . . . . . . . . 15
     2.18  Digest-Stale attribute . . . . . . . . . . . . . . . . . . 15
     2.19  Digest-HA1 attribute . . . . . . . . . . . . . . . . . . . 15
   3.  Detailed Description . . . . . . . . . . . . . . . . . . . . . 17
     3.1   RADIUS Client Behaviour  . . . . . . . . . . . . . . . . . 17
     3.2   RADIUS Server Behaviour  . . . . . . . . . . . . . . . . . 19
   4.  Migration Path to Diameter . . . . . . . . . . . . . . . . . . 21
     4.1   Basic operation  . . . . . . . . . . . . . . . . . . . . . 22
     4.2   Limitations  . . . . . . . . . . . . . . . . . . . . . . . 22
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   7.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Changes from draft-sterman-aaa-sip-03  . . . . . . . . . . . . 28
   9.  Changes from draft-sterman-aaa-sip-02  . . . . . . . . . . . . 29
   10.   Changes from draft-sterman-aaa-sip-01  . . . . . . . . . . . 30
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 31
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 31



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   11.2  Informative References . . . . . . . . . . . . . . . . . . . 31
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
       Intellectual Property and Copyright Statements . . . . . . . . 35















































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1.  Introduction

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2  Motivation

   Digest authentication is a simple authentication mechanism for HTTP
   and SIP.  While it was not too successful in HTTP environments, it is
   the only SIP authentication mechanism that has been widely adopted.
   Due to the limitations and weaknesses of Digest authentication (see
   [RFC2617], section 4 />), additional PKI-based authentication and
   encryption mechanisms have been introduced into SIP: TLS [RFC2246]
   and S/MIME [RFC2633].  The majority of today's SIP clients only
   supports HTTP digest.

   Current RADIUS-based AAA infrastructures have been built and debugged
   over years.  Some deficiencies of RADIUS have been mitigated with
   proprietary (ie costly) extensions.  Operators are therefore
   reluctant to replace their RADIUS infrastructure in order to enable a
   single new authentication mechanism.

   Given the complexity of the alternatives, simple clients will
   continue to support HTTP digest authentication only.  Its
   interoperability with a back-end authentication protocol such as
   RADIUS is needed.

   Operators that are about to replace their RADIUS-based AAA
   infrastructure are strongly recommended to use Diameter.

1.3  Overview

   Figure 1 depicts the basic scenario that is relevant for this
   document.  'HTTP-style Client' and 'RADIUS Client' are entities using
   a protocol with support for HTTP Digest Authentication, like SIP or
   HTTP.












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                     HTTP/SIP            RADIUS
         +------------+          +--------+         +--------+
         | HTTP-style |          | RADIUS |         | RADIUS |
         | Client     |<========>| Client |<------->| Server |
         |            |          |        |         |        |
         +------------+          +--------+         +--------+




                    Figure 1: Overview of operation

   The approach taken here is to extend RADIUS to support Digest
   authentication by mimicking its native support for CHAP
   authentication.  According to [RFC2865], the RADIUS server
   distinguishes between different authentication schemes by looking at
   the presence of an attribute specific for that scheme.  For the three
   natively supported authentication schemes, these attributes are:
   User-Password for PAP (or any other clear-text password scheme),
   CHAP-Password for CHAP, and State + User- Password for
   challenge-response scheme.  This document adds another attribute to
   be used in this role: Digest-Response.  Also according to [RFC2865],
   "An Access-Request packet MUST contain either a User-Password or a
   CHAP-Password or a State.  It MUST NOT contain both a User-Password
   and a CHAP-Password.  If future extensions allow other kinds of
   authentication information to be conveyed, the attribute for that can
   be used instead of User-Password or CHAP-Password." The
   Digest-Response introduced here therefore can be used instead of
   User-Password or CHAP-Password.

   The HTTP Authentication parameters found in the Proxy-Authorization
   or Authorization request header are mapped into newly defined RADIUS
   attributes.  These new RADIUS attributes are defined in the document
   together with some other information required for calculating the
   correct digest response on the RADIUS server with exception of the
   password, which the RADIUS server is assumed to be able to retrieve
   from a data store given the username.

   The nonces required by the digest algorithm are either generated by
   the RADIUS client or by the RADIUS server.  If at least one
   HTTP-style client requires AKA authentication [RFC3310], the RADIUS
   server MUST support nonce generation and its RADIUS clients MUST NOT
   generate nonces locally.








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1.3.1  Scenario 1, RADIUS client chooses nonces





                     HTTP/SIP           RADIUS

            +-----+    (1)    +-----+           +-----+
            |     |==========>|     |           |     |
            |     |    (2)    |     |           |     |
            |     |<==========|     |           |     |
            |     |    (3)    |     |           |     |
            |     |==========>|     |           |     |
            |  A  |           |  B  |    (4)    |  C  |
            |     |           |     |---------->|     |
            |     |           |     |    (5)    |     |
            |     |           |     |<----------|     |
            |     |    (6)    |     |           |     |
            |     |<==========|     |           |     |
            +-----+           +-----+           +-----+

            ====> HTTP/SIP
            ----> RADIUS



                 Figure 2: RADIUS client chooses nonces

   The roles played by the entities in this scenario are as follows:

   A: HTTP client / SIP UA

   B:  {HTTP  server / HTTP proxy server / SIP proxy server / SIP UAS}
   acting also as a RADIUS NAS

   C: RADIUS server

   The relevant order of messages sent in this scenario is as follows:

   A sends B an HTTP/SIP request without authorization header (step 1).
   B challenges A sending an HTTP/SIP  "(Proxy)  Authorization required"
   response containing a locally generated nonce (step 2).  A sends B an
   HTTP/SIP request with authorization header (step 3).  B sends C a
   RADIUS Access-Request with attributes described in this document
   (step 4).  C responds to B with a RADIUS Access-Accept/Access-Reject
   response (step 5).  If credentials were accepted B receives an
   Access-Accept response and the message sent from A is considered



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   authentic.  If B receives an Access-Reject response, however, B then
   responds to  A with a "(Proxy) Authorization required" response (step
   6).

1.3.2  Scenario 2, RADIUS server chooses nonces

   In most cases, the operation outlined in Section 1.3.1 is sufficient.
   It reduces the load on the RADIUS server to a minimum.  However, when
   using AKA [RFC3310] the nonce is partially derived from a precomputed
   authentication vector.  These authentication vectors are often stored
   centrally.

   Figure 3 depicts a scenario, where the RADIUS server chooses nonces.
   It shows a generic case where entities A and B communicate in the
   front-end using protocols such as HTTP/SIP, while entities B and C
   communicate in the back-end using RADIUS.





                     HTTP/SIP           RADIUS

            +-----+    (1)    +-----+           +-----+
            |     |==========>|     |    (2)    |     |
            |     |           |     |---------->|     |
            |     |           |     |    (3)    |     |
            |     |    (4)    |     |<----------|     |
            |     |<==========|     |           |     |
            |     |    (5)    |     |           |     |
            |     |==========>|     |           |     |
            |  A  |           |  B  |    (6)    |  C  |
            |     |           |     |---------->|     |
            |     |           |     |    (7)    |     |
            |     |           |     |<----------|     |
            |     |    (8)    |     |           |     |
            |     |<==========|     |           |     |
            +-----+           +-----+           +-----+

            ====> HTTP/SIP
            ----> RADIUS



                 Figure 3: RADIUS server chooses nonces

   The roles played by the entities in this scenario are as follows:




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   A: HTTP client / SIP UA

   B:  {HTTP  server / HTTP proxy server / SIP proxy server / SIP UAS}
   acting also as a RADIUS NAS

   C: RADIUS server

   The relevant order of messages sent in this scenario is as follows:

   A sends B an HTTP/SIP request without authorization header (step 1).
   B sends an Access-Request message with the newly defined
   Digest-Method and Digest-URI attributes but without a Digest-Nonce
   attribute to the RADIUS server, C (step 2).  C chooses a nonce and
   responds with an Access-Challenge (step 3).  This Access-Challenge
   contains Digest attributes, from which B takes values to construct an
   HTTP/SIP "(Proxy) Authorization required" response.  The remaining
   steps are identical with scenario 1 (Section 1.3.1): B sends this
   response to A (step 4).  A resends its request with its credentials
   (step 5).  B sends an Access-Request to C (step 6).  C checks the
   credentials and replies with Access-Accept or Access-Reject (step 7).
   Dependent on the C's result, B processes A's request or rejects it
   with a "(Proxy) Authorization required" response (step 8).





























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2.  New RADIUS attributes

   DIG-RES, DIG-REALM, DIG-NONCE, DIG-RSPAUTH, DIG-NEXTNONCE,
   DIG-METHOD, DIG-URI, DIG-QOP, DIG-ALG, DIG-BODY, DIG-CNONCE, DIG-NC,
   DIG-USER, DIG-OPAQUE, DIG-AUTHP, DIG-AUTS, DIG-DOMAIN, DIG-STALE and
   DIG-HA1 are placeholders for values that are taken from the RADIUS
   attribute type number space (see Section 5).

   The term 'HTTP-style' denotes any protocol that uses HTTP-like
   headers and uses HTTP digest authentication as described in
   [RFC2617].  Examples are HTTP and SIP.

   If not stated otherwise, the attributes have the following format:




   0                   1                   2
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length       | String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




2.1  Digest-Response attribute

   If this attribute is present, the RADIUS server SHOULD view the
   Access-Request as a Digest one.  When a RADIUS client receives a
   (Proxy-)Authorization header, it puts the request-digest value into a
   Digest-Response attribute.

   Type
         DIG-RES for Digest-Response.
   Length
         34
   String
         This attribute is only used in Access-Requests.  This string
         proves the user knows a password.  The String field is 32
         octets long and contains hexadecimal representation of 16 octet
         digest value as it was calculated by the authenticated client.
         The String field SHOULD be copied from request-digest of
         digest-response ([RFC2617]).

2.2  Digest-Realm attribute

   This string attribute describes a protection space of the RADIUS



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   server.  See [RFC2617] 1.2 for details.

   Type
         DIG-REALM
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         realm directive (realm-value) from the HTTP-style request it
         wants to authenticate.  In Access-Challenge messages, the
         RADIUS server puts the expected realm value into this
         attribute.

2.3  Digest-Nonce attribute

   This attribute holds a random nonce to be used in the HTTP Digest
   calculation.

   Type
         DIG-NONCE
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         nonce directive (nonce-value) from the HTTP-style request it
         wants to authenticate.  If the Access-Request had a
         Digest-Method and a Digest-URI but no Digest-Nonce attribute
         and the RADIUS server is configured to choose nonces, it MUST
         put a Digest-Nonce attribute into its Access-Challenge message.

2.4  Digest-Response-Auth attribute

   Type
         DIG-RSPAUTH for Digest-Response-Auth.
   Length
         34
   String
         This attribute is only used in Access-Accept messages if the
         RADIUS server is configured to choose nonces.  This string
         proves the RADIUS server knows the password.  The RADIUS server
         calculates a digest according to section 3.2.3 of [RFC2617] and
         copies the result into this string.  The RADIUS client puts the
         string into the rspauth directive of the Authentication-Info
         header.

2.5  Digest-Nextnonce attribute

   This attribute holds a random nonce to be used in the HTTP Digest



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   calculation.

   Type
         DIG-NEXTNONCE
   Length
         >=3
   String
         If the RADIUS server is configured to choose nonces and to use
         Authentication-Info, it puts a Digest-Nextnonce attribute into
         its Access-Accept message.  It contains the nonce value that
         SHOULD be used by the client in the next Access-Request
         message.  The RADIUS client MUST put the contents of this
         attribute into the nextnonce directive of its HTTP-style
         response.

2.6  Digest-Method attribute

   This attribute holds the method string to be used in the HTTP Digest
   calculation.

   Type
         DIG-METHOD
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         request method from the HTTP-style request it wants to
         authenticate.  This attribute MUST only be used in
         Access-Request messages.

2.7  Digest-URI attribute

   This attribute holds the URI string to be used in the HTTP Digest
   calculation.

   Type
         DIG-URI
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         request URI (digest-uri-value) from the HTTP-style request it
         wants to authenticate.  The attribute MUST only be used in
         Access-Request messages.

2.8  Digest-QoP attribute

   This attribute holds the Quality of Protection parameter that



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   influences the HTTP Digest calculation.

   Type
         DIG-QOP
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         qop directive (qop-value) from the HTTP-style request it wants
         to authenticate.  In Access-Challenge messages, the RADIUS
         server SHOULD put the desired qop-value into this attribute.

2.9  Digest-Algorithm attribute

   This attribute holds the algorithm parameter that influences the HTTP
   Digest calculation.

   Type
         DIG-ALG
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         algorithm directive from the HTTP-style request it wants to
         authenticate.  In Access-Accept messages, the RADIUS server MAY
         put the desired algorithm into this attribute.

2.10  Digest-Entity-Body-Hash attribute

   When using the qop level 'auth-int', the contents of the message body
   are required for digest calculation.  Instead of sending the complete
   body of the message, only its hash value is sent.  This hash value
   can be used directly in the digest calculation.

   Type
         DIG-BODY
   Length
         34
   String
         String, hexadecimal representation of a digest calculated over
         entity-body of HTTP/SIP request ([RFC2616], [RFC3261]).
         Computed by entity B in figure Figure 2.  This attribute is not
         part of the HTTP Digest response.  See [RFC2617] section
         3.2.2.3.  This attribute MUST only be sent in Access-Request
         packets.






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2.11  Digest-CNonce attribute

   This attribute holds the client nonce parameter that is used in the
   HTTP Digest calculation.

   Type
         DIG-CNONCE
   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         cnonce directive (cnonce-value) from the HTTP-style request it
         wants to authenticate.  The attribute is never used in
         Access-Accept, Access-Challenge or Access-Reject messages.

2.12  Digest-Nonce-Count attribute

   This attribute holds the nonce count parameter that is used to detect
   replay attacks.

   Type
         DIG-NC
   Length
         9
   String
         In Access-Requests, the RADIUS client takes the value of the nc
         directive (nc-value) from the HTTP-style request it wants to
         authenticate.  The attribute MUST only be used in
         Access-Request messages.

2.13  Digest-Username attribute

   This attribute holds the user name parameter that is used in the HTTP
   digest calculation.

   Type
         DIG-USER
   Length
         >= 3
   String
         In Access-Requests, the RADIUS client takes the value of the
         username directive (username-value) from the HTTP-style request
         it wants to authenticate.  The RADIUS server SHOULD NOT use
         this value for password finding, but only for digest
         calculation purpose.  In order to find the user record
         containing the password, the RADIUS server SHOULD use the value
         of the ([RFC2865] -)User-Name attribute.  This attribute MUST
         only be sent in Access-Request packets.



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2.14  Digest-Opaque attribute

   This attribute holds the opaque parameter that is passed to the
   HTTP-style client.  Th HTTP-style client passes this value back to
   the server (ie the RADIUS client) without modification.

   Type
         DIG-OPAQUE
   Length
         >=3
   String
         This attribute is only used when the RADIUS server chooses
         nonces.  In Access-Requests, the RADIUS client takes the value
         of the opaque directive from the HTTP-style request it wants to
         authenticate and puts it into this attribute.  In
         Access-Challenge messages, the RADIUS server MAY include this
         attribute.

2.15  Digest-Auth-Param attribute

   This attribute is a placeholder for future extensions.

   Type
         DIG-AUTHP
   Length
         >=3
   String
         This attribute is for future extensions.  Any extension
         parameter in the digest-response can be put into a
         Digest-Auth-Param attribute.  The string consists of the whole
         parameter, including its name and the equal ('=') sign.  RADIUS
         servers that do not implement a parameter contained in a
         Digest-Auth-Param attribute MUST respond with an Access-Reject
         message.  RADIUS clients that do not implement a parameter
         contained in a Digest-Auth-Param attribute MUST reject the
         original HTTP-style request.  This attribute MAY be used in
         Access-Request and Access-Accept messages.

2.16  Digest-AKA-Auts attribute

   This attribute holds the auts parameter that is used in the AKA
   Digest ([RFC3310]) calculation.

   Type
         DIG-AUTS






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   Length
         >=3
   String
         In Access-Requests, the RADIUS client takes the value of the
         auts directive from the HTTP-style request it wants to
         authenticate.  It is only used if the algorithm of the
         digest-response denotes a version of AKA digest [RFC3310].
         RADIUS servers that do not implement AKA digest MUST respond
         with an Access-Reject message.

2.17  Digest-Domain attribute

   When a RADIUS client has asked for a nonce, the RADIUS server MAY add
   one or more Digest-Domain attributes to its Access-Challenge message.
   The RADIUS client puts them into the quoted, space-separated list of
   URIs of the 'domain' directive of a WWW-Authenticate header.  The
   URIs in the list define the protection space (see [RFC2617], section
   3.2.1).

   Type
         DIG-DOMAIN
   Length
         3
   String
         The string consists of a single URI, that defines a protection
         space.  RADIUS servers MAY send attributes of this type in
         Access-Challenge messages.  RADIUS clients MUST NOT put
         attributes of this type in Access-Request messages.

2.18  Digest-Stale attribute

   If this attribute is present, the RADIUS server did not accept the
   nonce value.

   Type
         DIG-STALE
   Length
         3
   String
         The string consists of a single character.  If the nonce
         presented by the RADIUS client was stale, the character is '1'
         and is '0' otherwise.  The attribute MUST be used in
         Access-Accept messages if the RADIUS server chooses nonces.

2.19  Digest-HA1 attribute

   If this attribute is present, the RADIUS server did not accept the
   nonce value.



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   Type
         DIG-HA1
   Length
         34
   String
         This string contains the hexadecimal representation of H(A1) as
         described in [RFC2617], section 3.2.1 and 3.2.2.2.  This
         attribute is only used in Access-Accept messages.  It is used
         by the RADIUS client to calculate the 'rspauth' directive in an
         Authentication-Info header when the quality of protection
         ('qop') is 'auth-int'.  Digest-HA1 SHOULD only be sent if the
         'algorithm' directive's value is 'MD5-sess' or
         'AKAv1-MD5-sess'.  This attribute MUST NOT be sent if the qop
         parameter was not specified or has value of 'auth'.  If the
         'algorithm' directive's value is 'MD5' or 'AKAv1-MD5', the
         Digest-HA1 attribute MUST NOT be sent by the RADIUS server or
         processed by the RADIUS client, unless the authenticity and
         integrity of the Access-Accept message was secured by
         cryptographic or equivalently secure means.
































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3.  Detailed Description

3.1  RADIUS Client Behaviour

   A RADIUS client without an encrypted or otherwise secured connection
   (see Section 6) to its RADIUS server only accepts unsecured
   connections from its HTTP-style clients (or else the clients would
   have a false sense of security).

   The RADIUS client examines the (Proxy-)Authorization header of an
   incoming HTTP-style request message.  If this header is present and
   contains HTTP digest information, the RADIUS client checks the
   'nonce' parameter.  If the 'nonce' value has not been sent by the
   RADIUS client, it responds with a 401 (Unauthorized) or 407 (Proxy
   Authentication Required) to the HTTP-style client.  In this error
   response, the RADIUS client sends a new nonce.

   If the RADIUS client recognizes the nonce, it takes the header
   parameters and puts them into a RADIUS Access-Request message.  It
   puts the 'response' parameter into a Digest-Response attribute and
   the realm / nonce / qop / algorithm / cnonce / nc / username into the
   respective Digest-Realm / Digest-Nonce / Digest-QoP /
   Digest-Algorithm / Digest-CNonce / Digest-Nonce-Count /
   Digest-Username attributes.  The request URI and the request method
   are put into the Digest-URI and Digest-Method attributes.  Now, the
   RADIUS client sends the Access-Request message to the RADIUS server.

   The RADIUS server processes the message and responds with an
   Access-Accept or an Access-Reject message.

   The RADIUS clients constructs an Authentication-Info header:
   o  If the Access-Accept message contains a Digest-Response-Auth
      attribute, the RADIUS client checks the Digest-QoP attribute:
      *  If the Digest-Qop attribute's value is 'auth' or not specified,
         the RADIUS client puts the Digest-Response-Auth attribute's
         content into the 'rspauth' directive of the HTTP-style
         response.
      *  If the Digest-Qop attribute's value is 'auth-int', the RADIUS
         client ignores the Access-Accept message and behaves like it
         had received an Access-Reject message.
   o  If the Access-Accept message contains a Digest-HA1 attribute, the
      RADIUS client checks the Digest-QoP and Digest-Algorithm
      attributes:
      *  If the Digest-Qop attribute is missing or its value is 'auth',
         the RADIUS client ignores the Digest-HA1 attribute.  It does
         not include an Authentication-Info header into its HTTP-style
         response.




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      *  If the Digest-Qop attribute's value is 'auth-int' and the
         Digest-Algorithm attribute's value is 'MD5-sess' or
         'AKAv1-MD5-sess', the RADIUS client calculates the contents of
         the 'rspauth' directive.  It creates the HTTP-style response
         message and calculates the hash of this message's body.  It
         uses the result and the Digest-URI attribute's value of the
         corresponding Access-Request message to perform the H(A2)
         calculation.  It takes the Digest-Nonce, Digest-Nonce-Count,
         Digest-CNonce and Digest-QoP values of the corresponding
         Access-Request and the Digest-HA1 attribute's value to finish
         the computation of the 'rspauth' value.
      *  If the Digest-Qop attribute's value is 'auth-int' and the
         Digest-Algorithm attribute's value is 'MD5' or 'AKAv1-MD5', the
         RADIUS client MUST NOT use the Digest-HA1 attribute, unless it
         knows for sure that the Access-Accept message was encrypted or
         otherwise protected against eavesdropping.

   The RADIUS server MAY have added a Digest-Nextnonce attribute.  If
   the RADIUS client discovers this, it puts the contents of this
   attribute into a 'nextnonce' directive.  Now it can send an
   HTTP-style response.

   If the RADIUS client did not receive a (Proxy-)Authorization header
   from its HTTP-style client, it obtains a new nonce and sends an error
   response (401 or 407) containing a (Proxy-)Authenticate header.

   If the RADIUS client receives an Access-Reject or no response from
   the RADIUS server, it sends an error response to the HTTP-style
   request it has received.

   The RADIUS client has three ways to obtain nonces: it generates them
   locally, it has received one in a Digest-Nonce attribute of a
   previously received Access-Accept message, or it asks the RADIUS
   server for one.  To do the latter, it sends an Access-Request
   containing a Digest-Method and a Digest-URI attribute but without a
   Digest-Nonce attribute.  The RADIUS server chooses a nonce and
   responds with an Access-Challenge containing a Digest-Nonce
   attribute.

   If the RADIUS server responds with an Access-Reject, the RADIUS
   client MAY generate a nonce locally.  If the RADIUS client does not
   generate nonces locally, the authentication has failed.  The RADIUS
   server can send Digest-QoP, Digest-Algorithm, Digest-Realm,
   Digest-Domain and Digest-Opaque attributes in the Access-Challenge
   carrying the nonce.  If these attributes are present, the client MUST
   use them.

   If the Digest-Stale attribute is present in the Access-Accept message



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   following an Access-Challenge, the RADIUS client sends an error (401
   or 407) response containing WWW-/Proxy-Authenticate header with the
   directives 'stale' and 'nextnonce'.

3.2  RADIUS Server Behaviour

   If the RADIUS server receives an Access-Request message with a
   Digest-Method and a Digest-URI attribute but without a Digest-Nonce
   attribute, it chooses a nonce.  It puts the nonce into a Digest-Nonce
   attribute and sends it in an Access-Challenge message to the RADIUS
   client.  The RADIUS server MUST add Digest-Algorithm, Digest-Realm,
   SHOULD add Digest-QoP and MAY add Digest-Domain, Digest-Opaque
   attributes to the Access-Challenge message.  If the server cannot
   choose a nonce, it replies with an Access-Reject message.

   If the RADIUS server receives an Access-Request message containing a
   Digest-Response attribute, it looks for the following attributes:
   Digest-Realm, Digest-Nonce, Digest-Method, Digest-URI, Digest-QoP,
   Digest-Algorithm, Digest-Username.  Depending on the content of
   Digest-Algorithm and Digest-QoP, it looks for
   Digest-Entity-Body-Hash, Digest-CNonce and Digest-AKA-Auts, too.  See
   [RFC2617] and [RFC3310] for details.  If it has issued a
   Digest-Opaque attribute along with the nonce, the Access-Request MUST
   have a matching Digest-Opaque attribute.

   If it does not find these attributes, it responds with an
   Access-Reject message.  If the attributes are present, the RADIUS
   server calculates the digest response as described in [RFC2617].  To
   look up the password, the RADIUS server uses the RADIUS User-Name
   attribute.  All other values are taken from the Digest attributes
   described in this document.  If the calculated digest response equals
   the string received in the Digest-Response attribute, the
   authentication was successful.  If not, the RADIUS server responds
   with an Access-Reject.

   If the authentication was successful, the RADIUS server adds an
   attribute to the Access-Accept message which can be used by the
   RADIUS client to construct an Authentication-Info header:
   o  If the Digest-QoP attribute's value is 'auth' or unspecified, the
      RADIUS server puts a Digest-Response-Auth attribute into the
      Access-Accept message
   o  If the Digest-QoP attribute's value is 'auth-int' and the
      Digest-Algorithm attribute's value is 'MD5-sess' or
      'AKAv1-MD5-sess', the RADIUS server puts a Digest-HA1 attribute
      into the Access-Accept message.
   o  If the Digest-QoP attribute's value is 'auth-int' and the
      Digest-Algorithm attribute's value is 'MD5' or 'AKAv1-MD5', the
      RADIUS server MUST NOT send a Digest-HA1 attribute unless the



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      connection between RADIUS server and client is encrypted or
      otherwise protected against eavesdropping.

   RADIUS servers issuing nonces MAY construct a Digest-Nextnonce
   attribute.  This is useful to limit the lifetime of a nonce and to
   save a round-trip in future requests (see nextnonce discussion in
   [RFC2617], section 3.2.3).  The Digest-Response attribute and the
   optional Digest-Nextnonce attribute are send to the RADIUS client in
   an Access-Accept message.










































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4.  Migration Path to Diameter

   The following table gives an overview of the mapping between RADIUS
   attributes defined here and the corresponding Diameter AVPs described
   in [I-D.ietf-aaa-diameter-sip-app]:

 +-------------------------+-----------------------------------------+
 | RADIUS                  | Diameter                                |
 +-------------------------+-----------------------------------------+
 | Digest-Realm            | Digest-Realm                            |
 |                         |                                         |
 | Digest-Nonce            | Digest-Nonce                            |
 |                         |                                         |
 | Digest-URI              | Digest-URI                              |
 |                         |                                         |
 | Digest-Domain           | Digest-Domain                           |
 |                         |                                         |
 | Digest-QoP              | Digest-Qop                              |
 |                         |                                         |
 | Digest-Algorithm        | Digest-Algorithm                        |
 |                         |                                         |
 | Digest-CNonce           | Digest-Cnonce                           |
 |                         |                                         |
 | Digest-Nonce-Count      | Digest-Nonce-Count                      |
 |                         |                                         |
 | Digest-Method           | SIP-Method AVP                          |
 |                         |                                         |
 | Digest-Username         | Digest-Username AVP                     |
 |                         |                                         |
 | Digest-Entity-Body-Hash | SIP-Entity-Body-Hash AVP                |
 |                         |                                         |
 | Digest-Response         | SIP-Authorization Digest-Response       |
 |                         |                                         |
 | Digest-Response-Auth    | SIP-Authentication-Info Digest-Response |
 |                         |                                         |
 | Digest-Opaque           | Digest-Opaque AVP                       |
 |                         |                                         |
 | Digest-Auth-Param       | Digest-Auth-Param                       |
 |                         |                                         |
 | Digest-AKA-Auts         | Digest-AKA-Auts                         |
 |                         |                                         |
 | Digest-Stale            | Digest-Stale AVP                        |
 +-------------------------+-----------------------------------------+

                                Table 1






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4.1  Basic operation

   If an Access-Request message contains a Digest-Method and a
   Digest-URI attribute but no Digest-Nonce attribute, the gateway maps
   the RADIUS attributes to Diameter according to Table 2.  The gateway
   construct a MAR message and sends it to the Diameter server.

                     +---------------+------------+
                     | RADIUS        | Diameter   |
                     +---------------+------------+
                     | Digest-URI    | SIP-AOR    |
                     |               |            |
                     | Digest-Method | SIP-Method |
                     +---------------+------------+

                                Table 2

   The Diameter Server responds with a MAA message.  This message
   contains a Result-Code AVP set to the value DIAMETER_MULTI_ROUND_AUTH
   and challenge parameters.  The gateway translates the AVPs and puts
   the resulting RADIUS attributes into an Access-Challenge message.  It
   sends the Access-Challenge message to the RADIUS client.

   The gateway maps an Access-Request message containing a
   Digest-Response attribute to a MAR message with a Diameter
   SIP-Authorization AVP.  All RADIUS attributes of the Access-Request
   message are mapped to the corresponding Diameter AVPs.  The gateway
   sends the MAR message to the Diameter server.

   If the authentication was successful, the Diameter server replies
   with a MAA containing a SIP-Authentication-Info and a Digest-Response
   AVP.  The gateway converts these to the corresponding RADIUS
   attributes and constructs a RADIUS message.  If the Result-Code AVP
   is Diameter_SUCCESS or a Digest-Stale AVP is present, an
   Access-Accept is sent.  In all other cases, an Access-Reject is sent.

4.2  Limitations

   This document covers not all functionality found in
   [I-D.ietf-aaa-diameter-sip-app].
   o  There is no equivalent to Diameter's UAR/UAA, SAR/SAA, LIR/LIA,
      RTR/RTA and PPR/PPA messages
   o  The operational mode where the Diameter server sends the expected
      digest response to the client is not possible.

   The operational mode where the RADIUS client chooses nonces is not
   possible with [I-D.ietf-aaa-diameter-sip-app].




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5.  IANA Considerations

   This document serves as IANA registration request for a number of
   values from the RADIUS attribute type number space:

               +---------------+------------------------+
               | placeholder   | value assigned by IANA |
               +---------------+------------------------+
               | DIG-RES       | TBD                    |
               |               |                        |
               | DIG-REALM     | TBD                    |
               |               |                        |
               | DIG-NONCE     | TBD                    |
               |               |                        |
               | DIG-NEXTNONCE | TBD                    |
               |               |                        |
               | DIG-RSPAUTH   | TBD                    |
               |               |                        |
               | DIG-METHOD    | TBD                    |
               |               |                        |
               | DIG-URI       | TBD                    |
               |               |                        |
               | DIG-QOP       | TBD                    |
               |               |                        |
               | DIG-ALG       | TBD                    |
               |               |                        |
               | DIG-BODY      | TBD                    |
               |               |                        |
               | DIG-CNONCE    | TBD                    |
               |               |                        |
               | DIG-NC        | TBD                    |
               |               |                        |
               | DIG-USER      | TBD                    |
               |               |                        |
               | DIG-OPAQUE    | TBD                    |
               |               |                        |
               | DIG-AUTHP     | TBD                    |
               |               |                        |
               | DIG-AUTS      | TBD                    |
               |               |                        |
               | DIG-DOMAIN    | TBD                    |
               |               |                        |
               | DIG-STALE     | TBD                    |
               |               |                        |
               | DIG-HA1       | TBD                    |
               +---------------+------------------------+

                                Table 3



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6.  Security Considerations

   The RADIUS extensions described in this document make RADIUS a
   transport protocol for the data that is required to perform a digest
   calculation.  It adds the vulnerabilities of HTTP Digest (see
   [RFC2617], section 4) to those of RADIUS (see [RFC2865], section 8 or
   <http://www.untruth.org/~josh/security/radius/radius-auth.html>)).

   If an attacker gets access to a RADIUS client or RADIUS proxy, it can
   perform man-in-the-middle attacks even if the connections between A,
   B and B, C (Figure 2) have been secured with TLS or IPSec.

   SIP or HTTP requests occur much more frequently than dial-in
   requests.  RADIUS servers implementing this specification must meet
   that additional performance requirements.  An attacker could try to
   overload the RADIUS infrastructure by excessively sending SIP or HTTP
   requests.  This kind of attack was more difficult when RADIUS was
   just used for dial-in authentication: the attacker could be
   identified by the DSL / Cable interface or with some help of the PSTN
   provider.

   To make simple denial of service attacks more difficult, RADIUS
   clients MUST check if nonces received from a client have been issued
   by them.  This SHOULD be done statelessly.  For example, a nonce
   could consist of a cryptographically random part and some kind of
   signature of the RADIUS client, as described in [RFC2617], section
   3.2.1.

   RADIUS servers MAY include Digest-QoP and Digest-Algorithm attributes
   in Access-Accept messages.  A man in the middle can modify or remove
   those attributes in a bidding down attack.  In this case, the RADIUS
   client would use a weaker authentication scheme than intended.
   Informational RfC 3579 [RFC3579], section 3.2 describes a
   Message-Authenticator attribute which MAY be used to protect the
   integrity of RADIUS messages.

   The Digest-HA1 attribute contains no random components if the
   algorithm is 'MD5' or 'AKAv1-MD5'.  This makes offline dictionary
   attacks easier and can be used for replay attacks.

   HTTP-style clients can use TLS with server side certificates together
   with HTTP-Digest authentication.  Instead of TLS, IPSec can be used,
   too.  TLS or IPSec secure the connection while Digest Authentication
   authenticates the user.  If a RADIUS client accepts such connections,
   it MUST have an equally secure connection to the RADIUS server.






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7.  Example

   This is an example sniffed from the traffic between a softphone (A),
   a Proxy Server (B) and examplecom RADIUS server (C).  The
   communication between the Proxy Server and a SIP PSTN gateway is
   omitted for brevity.  The SIP messages are not shown completely.



   A->B

      INVITE sip:97226491335@10.0.69.38 SIP/2.0


   B->A

      SIP/2.0 100 Trying


   B->A

      SIP/2.0 407 Proxy Authentication Required
      Proxy-Authenticate: Digest realm="examplecom"
           ,nonce="3bada1a0", algorithm="md5"
      Content-Length: 0


   A->B

      ACK sip:97226491335@10.0.69.38 SIP/2.0


   A->B

      INVITE sip:97226491335@10.0.69.38 SIP/2.0
      Proxy-Authorization: Digest algorithm="md5",nonce="3bada1a0"
           ,opaque="",realm="examplecom"
           ,response="2ae133421cda65d67dc50d13ba0eb9bc"
           ,uri="sip:97226491335@10.0.69.38",username="12345678"


   B->C

      Code = 1 (Access-Request)
      Attributes:
      NAS-IP-Address = a 0 45 26 (10.0.69.38)
      NAS-Port-Type = 5 (Virtual)
      User-Name = "12345678"



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      Digest-Response (DIG-RES) = "2ae133421cda65d67dc50d13ba0eb9bc"
      Digest-Realm (DIG-REALM) = "examplecom"
      Digest-Nonce (DIG-NONCE) = "3bada1a0"
      Digest-Method (DIG-METHOD) = "INVITE"
      Digest-URI (DIG-URI) = "sip:97226491335@10.0.69.38"
      Digest-Algorithm (DIG-ALG) = "md5"
      Digest-Username (DIG-USER) =  "12345678"


   C->B

      Code = 2 (Access-Accept)
      Attributes:
      Digest-Response-Auth (DIG-RSPAUTH) =
                   "6303c41b0e2c3e524e413cafe8cce954"


   B->A

      SIP/2.0 180 Ringing


   B->A

      SIP/2.0 200 OK


   A->B

      ACK sip:97226491335@10.0.69.38:5060 SIP/2.0



   A second example shows the traffic between a web browser (A), web
   server (B) and a RADIUS server (C).



   A->B

      GET /index.html HTTP/1.1


   B->A

      HTTP/1.1 407 Authentication Required
      WWW-Authenticate: Digest realm="examplecom", domain="/index.html",
           ,nonce="a3086ac8", algorithm="md5"



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      Content-Length: 0


   A->B

      GET /index.html HTTP/1.1
      Authorization: Digest algorithm="md5",nonce="a3086ac8"
           ,opaque="",realm="examplecom"
           ,response="369b593b9a79e001256a2b40afe49f4c"
           ,uri="/index.html",username="12345678"


   B->C

      Code = 1 (Access-Request)
      Attributes:
      NAS-IP-Address = a 0 45 26 (10.0.69.38)
      NAS-Port-Type = 5 (Virtual)
      User-Name = "12345678"
      Digest-Response (DIG-RES) = "369b593b9a79e001256a2b40afe49f4c"
      Digest-Realm (DIG-REALM) = "examplecom"
      Digest-Nonce (DIG-NONCE) = "a3086ac8"
      Digest-Method (DIG-METHOD) = "GET"
      Digest-URI (DIG-URI) = "/index.html""
      Digest-Algorithm (DIG-ALG) = "md5"
      Digest-Username (DIG-USER) =  "12345678"


   C->B

      Code = 2 (Access-Accept)
      Attributes:
      Digest-Response-Auth (DIG-RSPAUTH) =
          "e644aa513effbfe1caff67103ff6433c"


   B->A

      HTTP/1.1 200 OK
      ...

      <html>
      ...








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8.  Changes from draft-sterman-aaa-sip-03

   o  addressed 'auth-int' issue
   o  New Digest-Nextnonce attribute
   o  revised abstract, motivational section and examples
   o  Access-Challenge instead of 'Access-Accept carrying a Digest-Nonce
      attribute'
   o  shortened SIP messages in example, removed real-world addresses
      and product names










































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9.  Changes from draft-sterman-aaa-sip-02

   o  Relaxed restrictions for DIG-DOMAIN, DIG-REALM, DIG-OPAQUE,
      DIG-QOP and DIG-ALG
   o  Additional security considerations for DIG-DOMAIN, DIG-QOP and
      DIG-ALG usage in Access-Accept messages













































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10.  Changes from draft-sterman-aaa-sip-01

   o  Replaced Sub-attributes with flat attributes
   o  aligned naming with [I-D.ietf-aaa-diameter-sip-app]
   o  Added how a server must treat unknown attributes.
   o  Added a section 'Migration path to Diameter'
   o  Added an optional attribute for support of the digest scheme
      described in informational [RFC3310].
   o  Added a mode of operation where the RADIUS server chooses the
      nonce.  This was required for AKA [RFC3310], but can be useful for
      ordinary Digest authentication when the qop directive is not used.
      This required the addition of several attributes.







































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11.  References

11.1  Normative References

   [I-D.ietf-aaa-diameter-sip-app]
              Garcia-Martin, M., Belinchon, M., Pallares-Lopez, M.,
              Canales-Valenzuela, C. and K. Tammi, "Diameter Session
              Initiation Protocol (SIP) Application",
              draft-ietf-aaa-diameter-sip-app-03 (work in progress),
              July 2004.

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

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

   [RFC2617]  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.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A. and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2865, June 2000.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M. and E. Schooler,
              "SIP: Session Initiation Protocol", RFC 3261, June 2002.

11.2  Informative References

   [RFC1750]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC2633]  Ramsdell, B., "S/MIME Version 3 Message Specification",
              RFC 2633, June 1999.

   [RFC3310]  Niemi, A., Arkko, J. and V. Torvinen, "Hypertext Transfer
              Protocol (HTTP) Digest Authentication Using Authentication
              and Key Agreement (AKA)", RFC 3310, September 2002.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible



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              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.


Authors' Addresses

   Baruch Sterman
   Kayote Networks
   P.O. Box 1373
   Efrat  90435
   Israel

   EMail: baruch@kayote.com


   Daniel Sadolevsky
   SecureOL, Inc.
   Jerusalem Technology Park
   P.O. Box 16120
   Jerusalem  91160
   Israel

   EMail: dscreat@dscreat.com


   David  Schwartz
   Kayote Networks
   P.O. Box 1373
   Efrat  90435
   Israel

   EMail: david@kayote.com


   David Williams
   Cisco Systems
   7025 Kit Creek Road
   P.O. Box 14987
   Research Triangle Park  NC 27709
   USA

   EMail: dwilli@cisco.com







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   Wolfgang Beck
   Deutsche Telekom AG
   Am Kavalleriesand 3
   Darmstadt  64295
   Germany

   EMail: beckw@t-systems.com












































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Appendix A.  Acknowledgements

   We would like to acknowledge Kevin Mcdermott (Cisco Systems) /or
   providing comments and experimental implementation.















































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