Network Working Group                                   Murtaza S. Chiba
INTERNET-DRAFT                                             Gopal Dommety
Obsoletes: 3576                                              Mark Eklund
Category: Informational                              Cisco Systems, Inc.
<draft-ietf-radext-rfc3576bis-01.txt>
<draft-ietf-radext-rfc3576bis-02.txt>                       David Mitton
21
24 March 2007                                         RSA Security, Inc.
                                                           Bernard Aboba
                                                   Microsoft Corporation

Dynamic Authorization Extensions to Remote Authentication Dial In User
                            Service (RADIUS)

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 25, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).  All Rights Reserved.

Abstract

   This document describes a currently deployed extension to the Remote
   Authentication Dial In User Service (RADIUS) protocol, allowing
   dynamic changes to a user session, as implemented by network access
   server products.  This includes support for disconnecting users and
   changing authorizations applicable to a user session.

Table of Contents

1.     Introduction ..........................................    3
   1.1       Applicability ...................................    3
   1.2       Requirements Language ...........................    4
   1.3       Terminology .....................................    4
2.     Overview  .............................................    5
   2.1       Disconnect Messages (DM) ........................    5
   2.2       Change-of-Authorization Messages (CoA) ..........    5
   2.3       Packet Format ...................................    6
3.     Attributes ............................................   10
   3.1       State ...........................................   12
   3.2       Message-Authenticator ...........................   12
   3.3       Error-Cause .....................................   13
   3.4       Table of Attributes .............................   16
4.     Diameter Considerations ...............................   20
5.     IANA Considerations ...................................   22
6.     Security Considerations ...............................   22
   6.1       Authorization Issues ............................   22
   6.2       Impersonation ...................................   23
   6.3       IPsec Usage Guidelines ..........................   24
   6.4       Replay Protection ...............................   27
7.     Example Traces ........................................   27
8.     References ............................................   28
   8.1       Normative References ............................   28
   8.2       Informative References ..........................   29
ACKNOWLEDGMENTS ..............................................   30
AUTHORS' ADDRESSES ...........................................   30
Appendix A - Changes from RFC 3576 ...........................   31   32
Full Copyright Statement .....................................   32   33
Intellectual Property ........................................   32   33

1.  Introduction

   The RADIUS protocol, defined in [RFC2865], does not support
   unsolicited messages sent from the RADIUS server to the Network
   Access Server (NAS).

   However, there are many instances in which it is desirable for
   changes to be made to session characteristics, without requiring the
   NAS to initiate the exchange.  For example, it may be desirable for
   administrators to be able to terminate a user session in progress.
   Alternatively, if the user changes authorization level, this may
   require that authorization attributes be added/deleted from a user
   session.

   To overcome these limitations, several vendors have implemented
   additional RADIUS commands in order to be able to support unsolicited
   messages sent from the RADIUS server to the NAS.  These extended
   commands provide support for Disconnect and Change-of-Authorization
   (CoA) packets.  Disconnect packets cause a user session to be
   terminated immediately, whereas CoA packets modify session
   authorization attributes such as data filters.

1.1.  Applicability

   This protocol is being recommended for publication as an
   Informational RFC rather than as a standards-track RFC because of
   problems that cannot be fixed without creating incompatibilities with
   deployed implementations.  This includes security vulnerabilities, as
   well as semantic ambiguities resulting from the design of the Change-
   of-Authorization (CoA) commands.  While fixes are recommended, they
   cannot be made mandatory since this would be incompatible with
   existing implementations.

   Existing implementations of this protocol do not support
   authorization checks, so that an ISP sharing a NAS with another ISP
   could disconnect or change authorizations for another ISP's users.
   In order to remedy this problem, a "Reverse Path Forwarding" check is
   recommended.  See Section 6.1. for details.

   Existing implementations utilize per-packet authentication and
   integrity protection algorithms with known weaknesses [MD5Attack].
   To provide stronger per-packet authentication and integrity
   protection, the use of IPsec is recommended.  See Section 6.3 for
   details.

   Existing implementations lack replay protection.  In order to support
   replay detection, it is recommended that an Event-Timestamp Attribute
   be added to all packets in situations where IPsec replay protection
   is not employed.  See Section 6.4 for details.

   The approach taken with CoA commands in existing implementations
   results in a semantic ambiguity.  Existing implementations of the
   CoA-Request identify the affected session, as well as supply the
   authorization changes.  Since RADIUS Attributes included within
   existing implementations of the CoA-Request can be used for session
   identification or authorization change, it may not be clear which
   function a given attribute is serving.

   The problem does not exist within the Diameter protocol [RFC3588], in
   which server-initiated authorization change is initiated using a Re-
   Auth-Request (RAR) command identifying the session via User-Name and
   Session-Id AVPs and containing a Re-Auth-Request-Type AVP with value
   "AUTHORIZE_ONLY".  This results in initiation of a standard
   Request/Response sequence where authorization changes are supplied.
   As a result, in no command can Diameter AVPs have multiple potential
   meanings.

1.2.  Requirements Language

   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].

1.3.  Terminology

   This document frequently uses the following terms:

Network Access Server (NAS)
     The device providing access to the network.

service
     The NAS provides a service to the user, such as IEEE 802 or PPP.

session
     Each service provided by the NAS to a user constitutes a session,
     with the beginning of the session defined as the point where
     service is first provided and the end of the session defined as the
     point where service is ended.  A user may have multiple sessions in
     parallel or series if the NAS supports that.

silently discard
     This means the implementation discards the packet without further
     processing.  The implementation SHOULD provide the capability of
     logging the error, including the contents of the silently discarded
     packet, and SHOULD record the event in a statistics counter.

2.  Overview

   This section describes the most commonly implemented features of
   Disconnect and Change-of-Authorization packets.

2.1.  Disconnect Messages (DM)

   A Disconnect-Request packet is sent by the RADIUS server in order to
   terminate a user session on a NAS and discard all associated session
   context.  The Disconnect-Request packet is sent to UDP port 3799, and
   identifies the NAS as well as the user session to be terminated by
   inclusion of the identification attributes described in Section 3.

   +----------+   Disconnect-Request     +----------+
   |          |   <--------------------  |          |
   |    NAS   |                          |  RADIUS  |
   |          |   Disconnect-Response    |  Server  |
   |          |   ---------------------> |          |
   +----------+                          +----------+

   The NAS responds to a Disconnect-Request packet sent by a RADIUS
   server with a Disconnect-ACK if all associated session context is
   discarded and the user session is no longer connected, or a
   Disconnect-NAK, if the NAS was unable to disconnect the session and
   discard all associated session context.  A Disconnect-ACK MAY contain
   the Attribute Acct-Terminate-Cause (49) [RFC2866] with the value set
   to 6 for Admin-Reset.

2.2.  Change-of-Authorization Messages (CoA)

   CoA-Request packets contain information for dynamically changing
   session authorizations.  Typically this is used to change data
   filters.  The data filters can be of either the ingress or egress
   kind, and are sent in addition to the identification attributes as
   described in section 3.  The port used, and packet format (described
   in Section 2.3), are the same as that for Disconnect-Request packets.

   The following attributes MAY be sent in a CoA-Request:

   Filter-ID (11) -        Indicates the name of a data filter list
                           to be applied for the session that the
                           identification attributes map to.

   NAS-Filter-Rule (TBD) - Provides a filter list to be applied
                           for the session that the identification
                           attributes map to [RFCFilter].

   +----------+      CoA-Request         +----------+
   |          |  <--------------------   |          |
   |   NAS    |                          |  RADIUS  |
   |          |     CoA-Response         |  Server  |
   |          |   ---------------------> |          |
   +----------+                          +----------+

   The NAS responds to a CoA-Request sent by a RADIUS server with a CoA-
   ACK if the NAS is able to successfully change the authorizations for
   the user session, or a CoA-NAK if the Request is unsuccessful.  A NAS
   MUST respond to a CoA-Request including a Service-Type Attribute with
   value "Authorize Only" with a CoA-NAK; a CoA-ACK MUST NOT be sent.  A
   NAS MUST respond to a CoA-Request including a Service-Type Attribute
   with an unsupported value with a CoA-NAK; an Error-Cause Attribute
   with value "Unsupported Service" MAY be included.

2.3.  Packet Format

   For either Disconnect-Request or CoA-Request packets UDP port 3799 is
   used as the destination port.  For responses, the source and
   destination ports are reversed.  Exactly one RADIUS packet is
   encapsulated in the UDP Data field.

   A summary of the data format is shown below. The fields are
   transmitted from left to right.

   The packet format consists of the fields: Code, Identifier, Length,
   Authenticator, and Attributes in Type:Length:Value (TLV) format.  All
   fields hold the same meaning as those described in RADIUS [RFC2865].
   The Authenticator field MUST be calculated in the same way as is
   specified for an Accounting-Request in [RFC2866].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                         Authenticator                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      The Code field is one octet, and identifies the type of RADIUS
      packet.  Packets received with an invalid Code field MUST be
      silently discarded.  RADIUS codes (decimal) for this extension are
      assigned as follows:

      40 - Disconnect-Request [RFC3575]
      41 - Disconnect-ACK [RFC3575]
      42 - Disconnect-NAK [RFC3575]
      43 - CoA-Request [RFC3575]
      44 - CoA-ACK [RFC3575]
      45 - CoA-NAK [RFC3575]

   Identifier

      The Identifier field is one octet, and aids in matching requests
      and replies.  RADIUS clients implementing this specification MUST
      be capable of detecting a duplicate request if it has the same
      server source IP address, source UDP port and Identifier within a
      short span of time.

      Unlike RADIUS as defined in [RFC2865], the responsibility for
      retransmission of Disconnect-Request and CoA-Request packets lies
      with the RADIUS server.  If after sending these packets, the
      RADIUS server does not receive a response, it will retransmit.

      The Identifier field MUST be changed whenever the content of the
      Attributes field changes, or whenever a valid reply has been
      received for a previous request.  For retransmissions where the
      contents are identical, the Identifier MUST remain unchanged.

      If the RADIUS server is retransmitting a Disconnect-Request or
      CoA-Request to the same client as before, and the Attributes
      haven't changed, the same Request Authenticator, Identifier and
      source port MUST be used.  If any Attributes have changed, a new
      Authenticator and Identifier MUST be used.

      If the Request to a primary proxy fails, a secondary proxy must be
      queried, if available.  Issues relating to failover algorithms are
      described in [RFC3539].  Since this represents a new request, a
      new Request Authenticator and Identifier MUST be used.  However,
      where the RADIUS server is sending directly to the client,
      failover typically does not make sense, since Disconnect or CoA
      packets need to be delivered to the NAS where the session resides.

   Length

      The Length field is two octets.  It indicates the length of the
      packet including the Code, Identifier, Length, Authenticator and
      Attribute fields.  Octets outside the range of the Length field
      MUST be treated as padding and ignored on reception.  If the
      packet is shorter than the Length field indicates, it MUST be
      silently discarded.  The minimum length is 20 and maximum length
      is 4096.

   Authenticator

      The Authenticator field is sixteen (16) octets.  The most
      significant octet is transmitted first.  This value is used to
      authenticate packets between the RADIUS server and client.

      Request Authenticator

         In  Request packets, the Authenticator value is a 16 octet MD5
         [RFC1321] checksum, called the Request Authenticator.  The
         Request Authenticator is calculated the same way as for an
         Accounting-Request, specified in [RFC2866].

         Note that the Request Authenticator of a Disconnect or CoA-
         Request cannot be computed the same way as the Request
         Authenticator of a RADIUS Access-Request, because there is no
         User-Password Attribute in a Disconnect-Request or CoA-Request.

      Response Authenticator

         The Authenticator field in a Response packet (e.g. Disconnect-
         ACK, Disconnect-NAK, CoA-ACK, or CoA-NAK) is called the
         Response Authenticator, and contains a one-way MD5 hash
         calculated over a stream of octets consisting of the Code,
         Identifier, Length, the Request Authenticator field from the
         packet being replied to, and the response Attributes if any,
         followed by the shared secret.  The resulting 16 octet MD5 hash
         value is stored in the Authenticator field of the Response
         packet.

      Administrative note: As noted in [RFC2865] Section 3, the secret
      (password shared between the client and the RADIUS server) SHOULD
      be at least as large and unguessable as a well-chosen password.
      RADIUS clients MUST use the source IP address of the RADIUS UDP
      packet to decide which shared secret to use, so that requests can
      be proxied.

   Attributes

      In Disconnect and CoA-Request packets, all Attributes are treated
      as mandatory.  A NAS MUST respond to a CoA-Request containing one
      or more unsupported Attributes or Attribute values with a CoA-NAK;
      a Disconnect-Request containing one or more unsupported Attributes
      or Attribute values MUST be answered with a Disconnect-NAK.  State
      changes resulting from a CoA-Request MUST be atomic: if the
      Request is successful, a CoA-ACK is sent, and all requested
      authorization changes MUST be made.  If the CoA-Request is
      unsuccessful, a CoA-NAK MUST be sent, and the requested
      authorization changes MUST NOT be made.  Similarly, a state change
      MUST NOT occur as a result of an unsuccessful Disconnect-Request;
      here a Disconnect-NAK MUST be sent.

      Since within this specification attributes may be used for
      identification, authorization or other purposes, even if a NAS
      implements an attribute for use with RADIUS authentication and
      accounting, it may not support inclusion of that attribute within
      Disconnect-Request or CoA-Request packets, given the difference in
      attribute semantics.  This is true even for attributes specified
      within [RFC2865], [RFC2868], [RFC2869], [RFC3162] or [RFC3579]
      as allowable within Access-Accept packets. packets (such as within
      [RFC2865],[RFC2868],[RFC2869],[RFC3162],[RFC3579],[RFC4675],
      [RFCFilter][RFCDelegated]).  As a result, if unsupported
      attributes beyond those specified in Section 3.5 are included within Disconnect-Request or CoA-Request
      packets, the RADIUS server may receive a Disconnect-NAK/CoA-NAK in
      response, possibly containing an Error-Cause attribute with value
      Unsupported Attribute (401).

      If there are any Proxy-State Attributes in a Disconnect-Request or
      CoA-Request received from the server, the forwarding proxy or NAS
      MUST include those Proxy-State Attributes in its response to the
      server.

      A forwarding proxy or NAS MUST NOT modify existing Proxy-State,
      State, or Class Attributes present in the packet.  The forwarding
      proxy or NAS MUST treat any Proxy-State attributes already in the
      packet as opaque data.  Its operation MUST NOT depend on the
      content of Proxy-State attributes added by previous proxies.  The
      forwarding proxy MUST NOT modify any other Proxy-State Attributes
      that were in the packet; it may choose not to forward them, but it
      MUST NOT change their contents.  If the forwarding proxy omits the
      Proxy-State Attributes in the request, it MUST attach them to the
      response before sending it.

      When the proxy forwards a Disconnect or CoA-Request, it MAY add a
      Proxy-State Attribute, but it MUST NOT add more than one.  If a
      Proxy-State Attribute is added to a packet when forwarding the
      packet, the Proxy-State Attribute MUST be added after any existing
      Proxy-State attributes.  The forwarding proxy MUST NOT change the
      order of any attributes of the same type, including Proxy-State.
      Other Attributes can be placed before, after or even between the
      Proxy-State Attributes.

      When the proxy receives a response to a CoA-Request or Disconnect-
      Request, it MUST remove its own Proxy-State (the last Proxy- State
      in the packet) before forwarding the response.  Since Disconnect
      and CoA responses are authenticated on the entire packet contents,
      the stripping of the Proxy-State Attribute invalidates the
      integrity check - so the proxy needs to recompute it.

3.  Attributes

   In Disconnect-Request and CoA-Request packets, certain attributes are
   used to uniquely identify the NAS as well as a user session on the
   NAS.  All NAS identification attributes included in a Request packet
   MUST match in order for a Disconnect-Request or CoA-Request to be
   successful; otherwise a Disconnect-NAK or CoA-NAK SHOULD be sent.
   For session identification attributes, the User-Name and Acct-
   Session-Id Attributes, if included, MUST match in order for a
   Disconnect-Request or CoA-Request to be successful; other session
   identification attributes SHOULD match.  Where a mismatch of session
   identification attributes is detected, a Disconnect-NAK or CoA-NAK
   SHOULD  be sent.

   The ability to use NAS or session identification attributes to map to
   unique/multiple sessions is beyond the scope of this document.
   Identification attributes include NAS and session identification
   attributes, as described below.

     NAS identification attributes

     Attribute             #    Reference  Description
     ---------            ---   ---------  -----------
     NAS-IP-Address        4    [RFC2865]  The IPv4 address of the NAS.
     NAS-Identifier       32    [RFC2865]  String identifying the NAS.
     NAS-IPv6-Address     95    [RFC3162]  The IPv6 address of the NAS.

     Session identification attributes

     Attribute              #   Reference  Description
     ---------             ---  ---------  -----------
     User-Name              1   [RFC2865]  The name of the user
                                           associated with the session.
     NAS-Port               5   [RFC2865]  The port on which the
                                           session is terminated.
     Framed-IP-Address      8   [RFC2865]  The IPv4 address associated
                                           with the session.
     Called-Station-Id     30   [RFC2865]  The link address to which
                                           the session is connected.
     Calling-Station-Id    31   [RFC2865]  The link address from which
                                           the session is connected.

     Attribute              #   Reference  Description
     ---------             ---  ---------  -----------
     Acct-Session-Id       44   [RFC2866]  The identifier uniquely
                                           identifying the session
                                           on the NAS.
     Acct-Multi-Session-Id 50   [RFC2866]  The identifier uniquely
                                           identifying related sessions.
     NAS-Port-Type         61   [RFC2865]  The type of port used.
     NAS-Port-Id           87   [RFC2869]  String identifying the port
                                           where the session is.
     Originating-Line-Info 94   [RFC4005]  Provides information on the
                                           characteristics of the line
                                           from which a session
                                           originated.
     Framed-Interface-Id   96   [RFC3162]  The IPv6 Interface Identifier
                                           associated with the session;
                                           always sent with
                                           Framed-IPv6-Prefix.
     Framed-IPv6-Prefix    97   [RFC3162]  The IPv6 prefix associated
                                           with the session, always sent
                                           with Framed-Interface-Id.

   To address security concerns described in Section 6.1, and to enable
   Diameter/RADIUS translation, the User-Name Attribute SHOULD be
   present in Disconnect-Request or CoA-Request packets; one or more
   additional session identification attributes MAY also be present.
   For example, where a Diameter client utilizes the same Session-Id for
   both authorization and accounting, inclusion of an Acct-Session-Id
   Attribute in a Disconnect-Request or CoA-Request can assist with
   Diameter/RADIUS translation, since Diameter RAR and ASR commands
   include a Session-Id AVP.

   Where a NAS offers multiple services, confusion may result with
   respect to interpretation of a CoA-Request or Disconnect-Request.  In
   order to prevent confusion a RADIUS Server SHOULD identify the
   session as specifically as possible.  For example, an Acct-Session-Id
   attribute SHOULD be included in Disconnect-Request and CoA-Request
   packets, rather than just the User-Name attribute.

   To address security concerns described in Section 6.2, one or more of
   the NAS-IP-Address or NAS-IPv6-Address Attributes SHOULD be present
   in Disconnect-Request or CoA-Request packets; the NAS-Identifier
   Attribute MAY be present in addition.

   If one or more authorization changes specified in a CoA-Request
   cannot be carried out, or if one or more attributes or attribute-
   values is unsupported, a CoA-NAK MUST be sent.  Similarly, if there
   are one or more unsupported attributes or attribute values in a
   Disconnect-Request, a Disconnect-NAK MUST be sent.

   A CoA-Request containing a Service-Type Attribute with value
   "Authorize Only" MUST contain only NAS or session identification
   attributes, as well as Service-Type and State attributes.  If other
   attributes are included in such a CoA-Request, implementations MUST
   send a CoA-NAK; an Error-Cause Attribute with value "Unsupported
   Attribute" MAY be included.

   A Disconnect-Request MUST contain only NAS and session identification
   attributes (see Section 3).  If other attributes are included in a
   Disconnect-Request, implementations MUST send a Disconnect-NAK; an
   Error-Cause Attribute with value "Unsupported Attribute" MAY be
   included.

3.1.  State

   [RFC2865] Section 5.44 states:

      An Access-Request MUST contain either a User-Password or a CHAP-
      Password or State.  An Access-Request 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 in an Access-Request instead of
      User-Password or CHAP-Password.

   In order to satisfy the requirements of [RFC2865] Section 5.44, an
   Access-Request with Service-Type="Authorize-Only" MUST contain a
   State attribute.

   In order to provide a State attribute to the NAS, a server sending a
   CoA-Request with a Service-Type value of "Authorize-Only" MUST
   include a State Attribute, and the NAS MUST include the State
   Attribute unchanged in the Access-Request.  A NAS receiving a CoA-
   Request containing a Service-Type value of "Authorize-Only" but
   lacking a State attribute MUST send a CoA-NAK and SHOULD include an
   Error-Cause attribute with value 402 (Missing Attribute).

3.2.  Message-Authenticator

   The Message-Authenticator Attribute MAY be used to authenticate and
   integrity-protect CoA-Request, CoA-ACK, CoA-NAK, Disconnect-Request,
   Disconnect-ACK and Disconnect-NAK packets order to prevent spoofing.

   A RADIUS client receiving a CoA-Request or Disconnect-Request with a
   Message-Authenticator Attribute present MUST calculate the correct
   value of the Message-Authenticator and silently discard the packet if
   it does not match the value sent.  A RADIUS server receiving a
   CoA/Disconnect-ACK or CoA/Disconnect-NAK with a Message-Authenticator
   Attribute present MUST calculate the correct value of the Message-
   Authenticator and silently discard the packet if it does not match
   the value sent.

   When a Message-Authenticator Attribute is included within a CoA-
   Request or Disconnect-Request, it is calculated as follows:

      Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
      Request Authenticator, Attributes)

      When the HMAC-MD5 message integrity check is calculated the
      Request Authenticator field and Message-Authenticator Attribute
      should be considered to be sixteen octets of zero.  The Message-
      Authenticator Attribute is calculated and inserted in the packet
      before the Request Authenticator is calculated.

      When a Message-Authenticator Attribute is included within a CoA-
      ACK, CoA-NAK, Disconnect-ACK or Disconnect-NAK, it is calculated
      as follows:

         Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
         Request Authenticator, Attributes)

      When the HMAC-MD5 message integrity check is calculated the
      Message-Authenticator Attribute should be considered to be sixteen
      octets of zero.  The Request Authenticator is taken from the
      corresponding CoA/Disconnect-Request.  The Message-Authenticator
      is calculated and inserted in the packet before the Response
      Authenticator is calculated.

3.3.  Error-Cause

   Description

      It is possible that the NAS cannot honor Disconnect-Request or
      CoA-Request packets for some reason.  The Error-Cause Attribute
      provides more detail on the cause of the problem.  It MAY be
      included within Disconnect-ACK, Disconnect-NAK and CoA-NAK
      packets.

      A summary of the Error-Cause Attribute format is shown below.  The
      fields are transmitted from left to right.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |             Value
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Value (cont)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      101 for Error-Cause

   Length

      6

   Value

      The Value field is four octets, containing an integer specifying
      the cause of the error. Values 0-199 and 300-399 are reserved.
      Values 200-299 represent successful completion, so that these
      values may only be sent within Disconnect-ACK or CoA-ACK packets
      and MUST NOT be sent within a Disconnect-NAK or CoA-NAK.  Values
      400-499 represent fatal errors committed by the RADIUS server, so
      that they MAY be sent within CoA-NAK or Disconnect-NAK packets,
      and MUST NOT be sent within CoA-ACK or Disconnect-ACK packets.
      Values 500-599 represent fatal errors occurring on a NAS or RADIUS
      proxy, so that they MAY be sent within CoA-NAK and Disconnect-NAK
      packets, and MUST NOT be sent within CoA-ACK or Disconnect-ACK
      packets.  Error-Cause values SHOULD be logged by the RADIUS
      server.  Error-Code values (expressed in decimal) include:

       #     Value
      ---    -----
      201    Residual Session Context Removed
      202    Invalid EAP Packet (Ignored)
      401    Unsupported Attribute
      402    Missing Attribute
      403    NAS Identification Mismatch
      404    Invalid Request
      405    Unsupported Service
      406    Unsupported Extension
      501    Administratively Prohibited
      502    Request Not Routable (Proxy)
      503    Session Context Not Found
      504    Session Context Not Removable
      505    Other Proxy Processing Error
      506    Resources Unavailable
      507    Request Initiated

      "Residual Session Context Removed" is sent in response to a
      Disconnect-Request if the user session is no longer active, but
      residual session context was found and successfully removed.  This
      value is only sent within a Disconnect-ACK and MUST NOT be sent
      within a CoA-ACK, Disconnect-NAK or CoA-NAK.

      "Invalid EAP Packet (Ignored)" is a non-fatal error that MUST NOT
      be sent by implementations of this specification.

      "Unsupported Attribute" is a fatal error sent if a Request
      contains an attribute (such as a Vendor-Specific or EAP-Message
      Attribute) that is not supported.

      "Missing Attribute" is a fatal error sent if critical attributes
      (such as NAS or session identification attributes) are missing
      from a Request.

      "NAS Identification Mismatch" is a fatal error sent if one or more
      NAS identification attributes (see Section 3) do not match the
      identity of the NAS receiving the Request.

      "Invalid Request" is a fatal error sent if some other aspect of
      the Request is invalid, such as if one or more attributes (such as
      EAP- Message Attribute(s)) are not formatted properly.

      "Unsupported Service" is a fatal error sent if a Service-Type
      Attribute included with the Request is sent with an invalid or
      unsupported value.  This error cannot be sent in response to a
      Disconnect-Request.

      "Unsupported Extension" is a fatal error sent due to lack of
      support for an extension such as Disconnect and/or CoA packets.
      This will typically be sent by a proxy receiving an ICMP port
      unreachable message after attempting to forward a Request to the
      NAS.

      "Administratively Prohibited" is a fatal error sent if the NAS is
      configured to prohibit honoring of Request packets for the
      specified session.

      "Request Not Routable" is a fatal error which MAY be sent by a
      RADIUS proxy and MUST NOT be sent by a NAS.  It indicates that the
      RADIUS proxy was unable to determine how to route the Request to
      the NAS.  For example, this can occur if the required entries are
      not present in the proxy's realm routing table.

      "Session Context Not Found" is a fatal error sent if the session
      context identified in the Request does not exist on the NAS.

      "Session Context Not Removable" is a fatal error sent in response
      to a Disconnect-Request if the NAS was able to locate the session
      context, but could not remove it for some reason.  It MUST NOT be
      sent within a CoA-ACK, CoA-NAK or Disconnect-ACK, only within a
      Disconnect-NAK.

      "Other Proxy Processing Error" is a fatal error sent in response
      to a Request that could not be processed by a proxy, for reasons
      other than routing.

      "Resources Unavailable" is a fatal error sent when a Request could
      not be honored due to lack of available NAS resources (memory,
      non- volatile storage, etc.).

      "Request Initiated" is a fatal error sent in response to a CoA-
      Request including a Service-Type Attribute with a value of
      "Authorize Only".  It indicates that the CoA-Request has not been
      honored, but that a RADIUS Access-Request including a Service-Type
      Attribute with value "Authorize Only" is being sent to the RADIUS
      server.

3.4.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in which packets, and in what quantity.

   Change-of-Authorization Messages

   Request   ACK      NAK   #   Attribute
   0-1       0        0     1   User-Name [Note 1]
   0-1       0        0     4   NAS-IP-Address [Note 1]
   0-1       0        0     5   NAS-Port [Note 1]
   0-1       0        0-1   6   Service-Type [Note 6]
   0-1       0        0     7   Framed-Protocol [Note 3]
   0-1       0        0     8   Framed-IP-Address [Note 1] 3]
   0-1       0        0     9   Framed-IP-Netmask [Note 3]
   0-1       0        0    10   Framed-Routing [Note 3]
   0+        0        0    11   Filter-ID [Note 3]
   0-1       0        0    12   Framed-MTU [Note 3]
   0+        0        0    13   Framed-Compression [Note 3]
   0+        0        0    14   Login-IP-Host [Note 3]
   0-1       0        0    15   Login-Service [Note 3]
   0-1       0        0    16   Login-TCP-Port [Note 3]
   0+        0        0    18   Reply-Message [Note 2]
   Request   ACK      NAK   #   Attribute
   Request   ACK      NAK   #   Attribute
   0-1       0        0    19   Callback-Number [Note 3]
   0-1       0        0    20   Callback-Id [Note 3]
   0+        0        0    22   Framed-Route [Note 3]
   0-1       0        0    23   Framed-IPX-Network [Note 3]
   0-1       0-1      0-1  24   State [Note 7]
   0+        0        0    25   Class [Note 3]
   0+        0        0    26   Vendor-Specific [Note 3]
   0-1       0        0    27   Session-Timeout [Note 3]
   0-1       0        0    28   Idle-Timeout [Note 3]
   0-1       0        0    29   Termination-Action [Note 3]
   0-1       0        0    30   Called-Station-Id [Note 1]
   0-1       0        0    31   Calling-Station-Id [Note 1]
   0-1       0        0    32   NAS-Identifier [Note 1]
   0+        0+       0+   33   Proxy-State
   0-1       0        0    34   Login-LAT-Service [Note 3]
   0-1       0        0    35   Login-LAT-Node [Note 3]
   0-1       0        0    36   Login-LAT-Group [Note 3]
   0-1       0        0    37   Framed-AppleTalk-Link [Note 3]
   0+        0        0    38   Framed-AppleTalk-Network [Note 3]
   0-1       0        0    39   Framed-AppleTalk-Zone [Note 3]
   0-1       0        0    44   Acct-Session-Id [Note 1]
   0-1       0        0    50   Acct-Multi-Session-Id [Note 1]
   0-1       0-1      0-1  55   Event-Timestamp
   0+        0        0    56   Egress-VLANID [Note 3]
   0-1       0        0    57   Ingress-Filters [Note 3]
   0+        0        0    58   Egress-VLAN-Name [Note 3]
   0-1       0        0    59   User-Priority-Table [Note 3]
   0-1       0        0    61   NAS-Port-Type [Note 1]
   0-1       0        0    62   Port-Limit [Note 3]
   0-1       0        0    63   Login-LAT-Port [Note 3]
   0+        0        0    64   Tunnel-Type [Note 5]
   0+        0        0    65   Tunnel-Medium-Type [Note 5]
   0+        0        0    66   Tunnel-Client-Endpoint [Note 5]
   0+        0        0    67   Tunnel-Server-Endpoint [Note 5]
   0+        0        0    69   Tunnel-Password [Note 5]
   0-1       0        0    71   ARAP-Features [Note 3]
   0-1       0        0    72   ARAP-Zone-Access [Note 3]
   0+        0        0    78   Configuration-Token [Note 3]
   0+        0-1      0    79   EAP-Message [Note 2]
   0-1       0-1      0-1  80   Message-Authenticator
   0+        0        0    81   Tunnel-Private-Group-ID [Note 5]
   0+        0        0    82   Tunnel-Assignment-ID [Note 5]
   0+        0        0    83   Tunnel-Preference [Note 5]
   0-1       0        0    85   Acct-Interim-Interval [Note 3]
   0-1       0        0    87   NAS-Port-Id [Note 1]
   0-1       0        0    88   Framed-Pool [Note 3]
   Request   ACK      NAK   #   Attribute
   Request   ACK      NAK   #   Attribute
   0+        0        0    90   Tunnel-Client-Auth-ID [Note 5]
   0+        0        0    91   Tunnel-Server-Auth-ID [Note 5]
   0-1       0        0    94   Originating-Line-Info [Note 1]
   0-1       0        0    95   NAS-IPv6-Address [Note 1]
   Request   ACK      NAK   #   Attribute
   Request   ACK      NAK   #   Attribute
   0-1       0        0    96   Framed-Interface-Id [Note 1] 3]
   0+        0        0    97   Framed-IPv6-Prefix [Note 1] 8]
   0+        0        0    98   Login-IPv6-Host [Note 3]
   0+        0        0    99   Framed-IPv6-Route [Note 3]
   0-1       0        0   100   Framed-IPv6-Pool [Note 3] 8]
   0         0        0+  101   Error-Cause
   0-1       0        0   TBD   NAS-Filter-Rule [Note 3]
   0+        0        0   TBD   Delegated-IPv6-Prefix [Note 8]
   Request   ACK      NAK   #   Attribute

   Disconnect Messages

   Request   ACK      NAK   #   Attribute
   0-1       0        0     1   User-Name [Note 1]
   0-1       0        0     4   NAS-IP-Address [Note 1]
   0-1       0        0     5   NAS-Port [Note 1]
   0         0        0     6   Service-Type
   0-1       0        0     8   Framed-IP-Address [Note 1]
   0+        0        0    18   Reply-Message [Note 2]
   0         0        0    24   State
   0+        0        0    25   Class [Note 4]
   0+        0        0    26   Vendor-Specific
   0-1       0        0    30   Called-Station-Id [Note 1]
   0-1       0        0    31   Calling-Station-Id [Note 1]
   0-1       0        0    32   NAS-Identifier [Note 1]
   0+        0+       0+   33   Proxy-State
   0-1       0        0    44   Acct-Session-Id [Note 1]
   0-1       0-1      0    49   Acct-Terminate-Cause
   0-1       0        0    50   Acct-Multi-Session-Id [Note 1]
   0-1       0-1      0-1  55   Event-Timestamp
   0-1       0        0    61   NAS-Port-Type [Note 1]
   0+        0-1      0    79   EAP-Message [Note 2]
   0-1       0-1      0-1  80   Message-Authenticator
   0-1       0        0    87   NAS-Port-Id [Note 1]
   0-1       0        0    94   Orginating-Line-Info [Note 1]
   0-1       0        0    95   NAS-IPv6-Address [Note 1]
   0-1       0        0    96   Framed-Interface-Id [Note 1]
   0+        0        0    97   Framed-IPv6-Prefix [Note 1]
   0         0+       0+  101   Error-Cause
   Request   ACK      NAK   #   Attribute

   The following table defines the meaning of the above table entries.

0     This attribute MUST NOT be present in packet.
0+    Zero or more instances of this attribute MAY be present in packet.
0-1   Zero or one instance of this attribute MAY be present in packet.

1     Exactly one instance of this attribute MUST be present in packet.

   [Note 1] Where NAS or session identification attributes are included
   in Disconnect-Request or CoA-Request packets, they are used for
   identification purposes only.  These attributes MUST NOT be used for
   purposes other than identification (e.g. within CoA-Request packets
   to request authorization changes).

   [Note 2] The Reply-Message Attribute is used to present a displayable
   message to the user.  The message is only displayed as a result of a
   successful Disconnect-Request or CoA-Request (where a Disconnect-ACK
   or CoA-ACK is subsequently sent).  Where EAP is used for
   authentication, an EAP-Message/Notification-Request Attribute is sent
   instead, and Disconnect-ACK or CoA-ACK packets contain an EAP-
   Message/Notification-Response Attribute.

   [Note 3] When included within a CoA-Request, these attributes
   represent an authorization change request.  When one of these
   attributes is omitted from a CoA-Request, the NAS assumes that the
   attribute value is to remain unchanged.  Attributes included in a
   CoA-Request replace all existing value(s) of the same attribute(s).

   [Note 4] When included within a successful Disconnect-Request (where
   a Disconnect-ACK is subsequently sent), the Class Attribute SHOULD be
   sent unmodified by the client to the accounting server in the
   Accounting Stop packet.  If the Disconnect-Request is unsuccessful,
   then the Class Attribute is not processed.

   [Note 5] When included within a CoA-Request, these attributes
   represent an authorization change request.  Where tunnel attribute(s)
   are included within a successful CoA-Request, all existing tunnel
   attributes are removed and replaced by the new attribute(s).

   [Note 6] Support for the Service-Type of "Authorize Only" is OPTIONAL
   on the NAS and RADIUS server.  A NAS supporting the "Authorize Only"
   Service-Type value within a CoA-Request packet MUST respond with a
   CoA-NAK containing a Service-Type Attribute with value "Authorize
   Only", and an Error-Cause Attribute with value "Request Initiated".
   The NAS then sends an Access-Request to the RADIUS server with a
   Service-Type Attribute with value "Authorize Only".  This Access-
   Request SHOULD contain the NAS attributes from the CoA-Request, as
   well as the session attributes from the CoA-Request legal for
   inclusion in an Access-Request as specified in [RFC2865], [RFC2868],
   [RFC2869] and [RFC3162].  As noted in [RFC2869] Section 5.19, a
   Message-Authenticator attribute SHOULD be included in an Access-
   Request that does not contain a User-Password, CHAP-Password, ARAP-
   Password or EAP-Message Attribute.  The RADIUS server should send
   back an Access-Accept to (re-)authorize the session or an Access-
   Reject to refuse to (re-)authorize it.

   A NAS that does not support the Service-Type Attribute with the value
   "Authorize Only" within a CoA-Request MUST respond with a CoA-NAK
   including no Service-Type Attribute; an Error-Cause Attribute with
   value "Unsupported Service" MAY be included.

   [Note 7] The State Attribute is available to be sent by the RADIUS
   server to the NAS in a CoA-Request packet and MUST be sent unmodified
   from the NAS to the RADIUS server in a subsequent ACK or NAK packet.
   If a Service-Type Attribute with value "Authorize Only" is included
   in a CoA-Request then a State Attribute MUST be present, and MUST be
   sent unmodified from the NAS to the RADIUS server in the resulting
   Access-Request sent to the RADIUS server, if any.  The State
   Attribute is also available to be sent by the RADIUS server to the
   NAS in a CoA-Request that also includes a Termination-Action
   Attribute with the value of RADIUS-Request.  If the client performs
   the Termination-Action by sending a new Access-Request upon
   termination of the current session, it MUST include the State
   Attribute unchanged in that Access-Request.  In either usage, the
   client MUST NOT interpret the Attribute locally.  A CoA-Request
   packet must have only zero or one State Attribute.  Usage of the
   State Attribute is implementation dependent.

   [Note 8] These attributes are typically included in a CoA-Request for
   the purposes of renumbering.

4.  Diameter Considerations

   Due to differences in handling change-of-authorization requests in
   RADIUS and Diameter, it may be difficult or impossible for a
   Diameter/RADIUS gateway to successfully translate a Diameter Re-Auth-
   Request (RAR) to a CoA-Request and vice versa.  For example, since a
   CoA-Request only initiates an authorization change but does not
   initiate re-authentication, a RAR command containing a Re-Auth-
   Request-Type AVP with value "AUTHORIZE_AUTHENTICATE" cannot be
   directly translated to a CoA-Request.  A Diameter/RADIUS gateway
   receiving a CoA-Request containing authorization changes will need to
   translate this into two Diameter exchange.  First, the
   Diameter/RADIUS gateway will issue a RAR command including a Session-
   Id AVP and a Re-Auth-Request-Type AVP with value "AUTHORIZE ONLY".
   Then the Diameter/RADIUS gateway will respond to the ensuing access
   request with a response including the authorization attributes
   gleaned from the CoA-Request.  For the translation to be possible,
   the CoA-Request MUST include a Acct-Session-Id Attribute.  If the
   Diameter client uses the same Session-Id for both authorization and
   accounting, then the Diameter/RADIUS gateway can copy the contents of
   the Acct-Session-Id Attribute into the Session-Id AVP;  otherwise, it
   will need to map the Acct-Session-Id value to an equivalent Session-
   Id for use within a RAR command.

   To simplify translation between RADIUS and Diameter, a server
   compliant with this specification MAY include a Service-Type
   Attribute with value "Authorize Only" within a CoA-Request.  Such a
   CoA-Request MUST contain a State Attribute.  A NAS supporting the
   "Authorize Only" Service-Type within a CoA-Request responds with a
   CoA-NAK containing a Service-Type Attribute with value "Authorize
   Only", and an Error-Cause Attribute with value "Request Initiated".
   The NAS will then send an Access-Request containing a Service-Type
   Attribute with a value of "Authorize Only", along with a State
   Attribute.  A Diameter/RADIUS gateway receiving a CoA-Request
   containing a Service-Type with value "Authorize Only" translates this
   to a RAR with Re-Auth-Request-Type AVP with value "AUTHORIZE ONLY".
   The received RAA is then translated to a CoA-NAK with a Service-Type
   value of "Authorize Only".   If the Result-Code AVP in the RAA has a
   value in the success category, then an Error-Cause Attribute with
   value "Request Initiated" is included in the CoA-NAK.   If the
   Result-Code AVP in the RAA has a value indicating a Protocol Error or
   a Transient or Permanent Failure, then an alternate Error-Cause
   Attribute is returned as suggested below.

   Within Diameter, a server can request that a session be aborted by
   sending an Abort-Session-Request (ASR), identifying the session to be
   terminated using Session-ID and User-Name AVPs.  The ASR command is
   translated to a Disconnect-Request containing an Acct-Session-Id and
   User-Name attribute.  If the Diameter client utilizes the same
   Session-Id in both authorization and accounting, then the value of
   the Session-ID AVP may be placed in the Acct-Session-Id attribute;
   otherwise the value of the Session-ID AVP will need to be mapped to
   an appropriate Acct-Session-Id value.   For a Disconnect-Request to
   be translatable to an ASR, an Acct-Session-Id attribute MUST be
   present.  If the Diameter client utilizes the same Session-Id in both
   authorization and accounting, then the value of the Acct-Session-Id
   may be placed into the Session-ID AVP within the ASR;  otherwise the
   value of the Acct-Session-Id will need to be mapped to an appropriate
   Session-ID value.

   An Abort-Session-Answer (ASA) command is sent in response to an ASR
   in order to indicate the disposition of the request.  A
   Diameter/RADIUS gateway receiving a Disconnect-ACK translates this to
   an ASA command with a Result-Code AVP of "DIAMETER_SUCCESS".  A
   Disconnect-NAK received from the server is translated to an ASA
   command with a Result-Code AVP which depends on the value of the
   Error-Cause Attribute.  Suggested translations between Error-Cause
   Attribute values and Result-Code AVP values are included below:

    #    Error-Cause Attribute Value   Result-Code AVP
   ---   ---------------------------  ------------------------
   201   Residual Session Context     DIAMETER_SUCCESS
         Removed
   202   Invalid EAP Packet           DIAMETER_LIMITED_SUCCESS
         (Ignored)
   401   Unsupported Attribute        DIAMETER_AVP_UNSUPPORTED
   402   Missing Attribute            DIAMETER_MISSING_AVP
   403   NAS Identification           DIAMETER_REALM_NOT_SERVED
         Mismatch
   404   Invalid Request              DIAMETER_UNABLE_TO_COMPLY
   405   Unsupported Service          DIAMETER_COMMAND_UNSUPPORTED
   406   Unsupported Extension        DIAMETER_APPLICATION_UNSUPPORTED
   501   Administratively             DIAMETER_AUTHORIZATION_REJECTED
         Prohibited
   502   Request Not Routable (Proxy) DIAMETER_UNABLE_TO_DELIVER
   503   Session Context Not Found    DIAMETER_UNKNOWN_SESSION_ID
   504   Session Context Not          DIAMETER_AUTHORIZATION_REJECTED
         Removable
   505   Other Proxy Processing       DIAMETER_UNABLE_TO_COMPLY
         Error
   506   Resources Unavailable        DIAMETER_RESOURCES_EXCEEDED
   507   Request Initiated            DIAMETER_SUCCESS

   Since both the ASR/ASA and Disconnect-Request/Disconnect-
   NAK/Disconnect-ACK exchanges involve just a request and response,
   inclusion of an "Authorize Only" Service-Type within a Disconnect-
   Request is not needed to assist in Diameter/RADIUS translation, and
   may make translation more difficult.  As a result, the Service-Type
   Attribute MUST NOT be used within a Disconnect-Request.

5.  IANA Considerations

   This specification contains no actions for IANA.  All protocol
   parameters required for this document were previously approved as
   part of the publication of [RFC3576].

6.  Security Considerations

6.1.  Authorization Issues

   Where a NAS is shared by multiple providers, it is undesirable for
   one provider to be able to send Disconnect-Request or CoA-Requests
   affecting the sessions of another provider.

   A NAS or RADIUS proxy MUST silently discard Disconnect-Request or
   CoA-Request packets from untrusted sources.  By default, a RADIUS
   proxy SHOULD perform a "reverse path forwarding" (RPF) check to
   verify that a Disconnect-Request or CoA-Request originates from an
   authorized RADIUS server.  In addition, it SHOULD be possible to
   explicitly authorize additional sources of Disconnect-Request or CoA-
   Request packets relating to certain classes of sessions.  For
   example, a particular source can be explicitly authorized to send
   CoA-Request packets relating to users within a set of realms.

   To perform the RPF check, the proxy uses the session identification
   attributes included in Disconnect-Request or CoA-Request packets, in
   order to determine the RADIUS server(s) to which an equivalent
   Access-Request could be routed.  If the source address of the
   Disconnect-Request or CoA-Request is within this set, then the
   Request is forwarded; otherwise it MUST be silently discarded.

   Typically the proxy will extract the realm from the Network Access
   Identifier [RFC4282] included within the User-Name Attribute, and
   determine the corresponding RADIUS servers in the proxy routing
   tables.  The RADIUS servers for that realm are then compared against
   the source address of the packet.  Where no RADIUS proxy is present,
   the RPF check will need to be performed by the NAS itself.

   Since authorization to send a Disconnect-Request or CoA-Request is
   determined based on the source address and the corresponding shared
   secret, the NASes or proxies SHOULD configure a different shared
   secret for each RADIUS server.

6.2.  Impersonation

   [RFC2865] Section 3 states:

      A RADIUS server MUST use the source IP address of the RADIUS
      UDP packet to decide which shared secret to use, so that
      RADIUS requests can be proxied.

   When RADIUS requests are forwarded by a proxy, the NAS-IP-Address or
   NAS-IPv6-Address Attributes will typically not match the source
   address observed by the RADIUS server.  Since the NAS-Identifier
   Attribute need not contain an FQDN, this attribute may not be
   resolvable to the source address observed by the RADIUS server, even
   when no proxy is present.

   As a result, the authenticity check performed by a RADIUS server or
   proxy does not verify the correctness of NAS identification
   attributes.  This makes it possible for a rogue NAS to forge NAS-IP-
   Address, NAS-IPv6-Address or NAS-Identifier Attributes within a
   RADIUS Access-Request in order to impersonate another NAS.  It is
   also possible for a rogue NAS to forge session identification
   attributes such as the Called-Station-Id, Calling-Station-Id, or
   Originating-Line-Info [RFC4005].  This could fool the RADIUS server
   into sending Disconnect-Request or CoA-Request packets containing
   forged session identification attributes to a NAS targeted by an
   attacker.

   To address these vulnerabilities RADIUS proxies SHOULD check whether
   NAS identification attributes (see Section 3) match the source
   address of packets originating from the NAS.  Where one or more
   attributes do not match, Disconnect-Request or CoA-Request packets
   SHOULD be silently discarded.

   Such a check may not always be possible.  Since the NAS-Identifier
   Attribute need not correspond to an FQDN, it may not be resolvable to
   an IP address to be matched against the source address.  Also, where
   a NAT exists between the RADIUS client and proxy, checking the NAS-
   IP-Address or NAS-IPv6-Address Attributes may not be feasible.

6.3.  IPsec Usage Guidelines

   In addition to security vulnerabilities unique to Disconnect or CoA
   packets, the protocol exchanges described in this document are
   susceptible to the same vulnerabilities as RADIUS [RFC2865].  It is
   RECOMMENDED that IPsec be employed to afford better security.

   Implementations of this specification SHOULD support IPsec [RFC4301]
   along with IKEv1 [RFC2409] for key management.  IPsec ESP [RFC4303]
   with non-null transform SHOULD be supported, and IPsec ESP with a
   non-null encryption transform and authentication support SHOULD be
   used to provide per-packet confidentiality, authentication, integrity
   and replay protection.  IKE SHOULD be used for key management.

   Within RADIUS [RFC2865], a shared secret is used for hiding of
   Attributes such as User-Password, as well as in computation of the
   Response Authenticator.  In RADIUS accounting [RFC2866], the shared
   secret is used in computation of both the Request Authenticator and
   the Response Authenticator.

   Since in RADIUS a shared secret is used to provide confidentiality as
   well as integrity protection and authentication, only use of IPsec
   ESP with a non-null transform can provide security services
   sufficient to substitute for RADIUS application-layer security.
   Therefore, where IPsec AH or ESP null is used, it will typically
   still be necessary to configure a RADIUS shared secret.

   Where RADIUS is run over IPsec ESP with a non-null transform, the
   secret shared between the NAS and the RADIUS server MAY NOT be
   configured.  In this case, a shared secret of zero length MUST be
   assumed.  However, a RADIUS server that cannot know whether incoming
   traffic is IPsec-protected MUST be configured with a non-null RADIUS
   shared secret.

   When IPsec ESP is used with RADIUS, per-packet authentication,
   integrity and replay protection MUST be used.  3DES-CBC MUST be
   supported as an encryption transform and AES-CBC SHOULD be supported.
   AES-CBC SHOULD be offered as a preferred encryption transform if
   supported.  HMAC-SHA1-96 MUST be supported as an authentication
   transform.  DES-CBC SHOULD NOT be used as the encryption transform.

   A typical IPsec policy for an IPsec-capable RADIUS client is
   "Initiate IPsec, from me to any destination port UDP 1812".  This
   IPsec policy causes an IPsec SA to be set up by the RADIUS client
   prior to sending RADIUS traffic.  If some RADIUS servers contacted by
   the client do not support IPsec, then a more granular policy will be
   required: "Initiate IPsec, from me to IPsec-Capable-RADIUS-Server,
   destination port UDP 1812."

   For a client implementing this specification the policy would be
   "Accept IPsec, from any to me, destination port UDP 3799".  This
   causes the RADIUS client to accept (but not require) use of IPsec.
   It may not be appropriate to require IPsec for all RADIUS servers
   connecting to an IPsec-enabled RADIUS client, since some RADIUS
   servers may not support IPsec.

   For an IPsec-capable RADIUS server, a typical IPsec policy is "Accept
   IPsec, from any to me, destination port 1812".  This causes the
   RADIUS server to accept (but not require) use of IPsec.  It may not
   be appropriate to require IPsec for all RADIUS clients connecting to
   an IPsec-enabled RADIUS server, since some RADIUS clients may not
   support IPsec.

   For servers implementing this specification, the policy would be
   "Initiate IPsec, from me to any, destination port UDP 3799".  This
   causes the RADIUS server to initiate IPsec when sending RADIUS
   extension traffic to any RADIUS client.  If some RADIUS clients
   contacted by the server do not support IPsec, then a more granular
   policy will be required, such as "Initiate IPsec, from me to IPsec-
   capable-RADIUS-client, destination port UDP 3799".

   Where IPsec is used for security, and no RADIUS shared secret is
   configured, it is important that the RADIUS client and server perform
   an authorization check.  Before enabling a host to act as a RADIUS
   client, the RADIUS server SHOULD check whether the host is authorized
   to provide network access.  Similarly, before enabling a host to act
   as a RADIUS server, the RADIUS client SHOULD check whether the host
   is authorized for that role.

   RADIUS servers can be configured with the IP addresses (for IKE
   Aggressive Mode with pre-shared keys) or FQDNs (for certificate
   authentication) of RADIUS clients.  Alternatively, if a separate
   Certification Authority (CA) exists for RADIUS clients, then the
   RADIUS server can configure this CA as a trust anchor [RFC3280] for
   use with IPsec.

   Similarly, RADIUS clients can be configured with the IP addresses
   (for IKE Aggressive Mode with pre-shared keys) or FQDNs (for
   certificate authentication) of RADIUS servers.  Alternatively, if a
   separate CA exists for RADIUS servers, then the RADIUS client can
   configure this CA as a trust anchor for use with IPsec.

   Since unlike SSL/TLS, IKE does not permit certificate policies to be
   set on a per-port basis, certificate policies need to apply to all
   uses of IPsec on RADIUS clients and servers.  In IPsec deployment
   supporting only certificate authentication, a management station
   initiating an IPsec-protected telnet session to the RADIUS server
   would need to obtain a certificate chaining to the RADIUS client CA.
   Issuing such a certificate migh not be appropriate if the management
   station was not authorized as a RADIUS client.

   Where RADIUS clients may obtain their IP address dynamically (such as
   an Access Point supporting DHCP), Main Mode with pre-shared keys
   [RFC2409] SHOULD NOT be used, since this requires use of a group pre-
   shared key; instead, Aggressive Mode SHOULD be used. Where RADIUS
   client addresses are statically assigned either Aggressive Mode or
   Main Mode MAY be used.  With certificate authentication, Main Mode
   SHOULD be used.

   Care needs to be taken with IKE Phase 1 Identity Payload selection in
   order to enable mapping of identities to pre-shared keys even with
   Aggressive Mode.  Where the ID_IPV4_ADDR or ID_IPV6_ADDR Identity
   Payloads are used and addresses are dynamically assigned, mapping of
   identities to keys is not possible, so that group pre-shared keys are
   still a practical necessity.  As a result, the ID_FQDN identity
   payload SHOULD be employed in situations where Aggressive mode is
   utilized along with pre-shared keys and IP addresses are dynamically
   assigned.  This approach also has other advantages, since it allows
   the RADIUS server and client to configure themselves based on the
   fully qualified domain name of their peers.

   Note that with IPsec, security services are negotiated at the
   granularity of an IPsec SA, so that RADIUS exchanges requiring a set
   of security services different from those negotiated with existing
   IPsec SAs will need to negotiate a new IPsec SA.  Separate IPsec SAs
   are also advisable where quality of service considerations dictate
   different handling RADIUS conversations.  Attempting to apply
   different quality of service to connections handled by the same IPsec
   SA can result in reordering, and falling outside the replay window.
   For a discussion of the issues, see [RFC2983].

6.4.  Replay Protection

   Where IPsec replay protection is not used, an Event-Timestamp (55)
   [RFC2869] Attribute SHOULD be included within CoA-Request and
   Disconnect-Request packets, and MAY be included within CoA-ACK, CoA-
   NAK, Disconnect-ACK and Disconnect-NAK packets.

   When the Event-Timestamp attribute is present, both the NAS and the
   RADIUS server MUST check that the Event-Timestamp Attribute is
   current within an acceptable time window. If the Event-Timestamp
   Attribute is not current, then the packet MUST be silently discarded.
   This implies the need for loose time synchronization within the
   network, which can be achieved by a variety of means, including SNTP,
   as described in [RFC4330].  Implementations SHOULD be configurable to
   discard CoA-Request or Disconnect-Request packets not containing an
   Event-Timestamp attribute.

   If the Event-Timestamp Attribute is included, it represents the time
   at which the original packet was sent, and therefore it SHOULD NOT be
   updated when the packet is retransmitted.  If the Event-Timestamp
   attribute is not updated, this implies that the Identifier is not
   changed in retransmitted packets.  As a result, the ability to detect
   replay within the time window is dependent on support for duplicate
   detection within that same window.  As noted in Section 2.3,
   duplicate detection is REQUIRED for RADIUS clients implementing this
   specification.

   The time window used for duplicate detection MUST be the same as the
   window used to detect stale Event-Timestamp Attributes.  Since the
   RADIUS Identifier cannot be repeated within the selected time window,
   no more than 256 Requests can be accepted within the time window.  As
   a result, the chosen time window will depend on the expected maximum
   volume of CoA/Disconnect-Requests, so that unnecessary discards can
   be avoided.  A default time window of 300 seconds should be adequate
   in many circumstances.

7.  Example Traces

   Disconnect Request with User-Name:

      0: xxxx xxxx xxxx xxxx xxxx 2801 001c 1b23    .B.....$.-(....#
     16: 624c 3543 ceba 55f1 be55 a714 ca5e 0108    bL5C..U..U...^..
     32: 6d63 6869 6261
   Disconnect Request with Acct-Session-ID:

      0: xxxx xxxx xxxx xxxx xxxx 2801 001e ad0d    .B..... ~.(.....
     16: 8e53 55b6 bd02 a0cb ace6 4e38 77bd 2c0a    .SU.......N8w.,.
     32: 3930 3233 3435 3637                        90234567

   Disconnect Request with Framed-IP-Address:

      0: xxxx xxxx xxxx xxxx xxxx 2801 001a 0bda    .B....."2.(.....
     16: 33fe 765b 05f0 fd9c c32a 2f6b 5182 0806    3.v[.....*/kQ...
     32: 0a00 0203

8.  References

8.1.  Normative References

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
          April 1992.

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

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
          RFC 2409, November 1998.

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

[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RFC2869] Rigney, C., Willats W. and P. Calhoun, "RADIUS Extensions",
          RFC 2869, June 2000.

[RFC3162] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC
          3162, August 2001.

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

[RFC3575] Aboba, B., "IANA Considerations for RADIUS", RFC 3575, July
          2003.

[RFC3579] Aboba, B. and P. Calhoun, "RADIUS Support for Extensible
          Authentication Protocol (EAP)", RFC 3579, September 2003.

[RFC4282] Aboba, B., Beadles, M., Arkko, J. and P. Eronen,  "The Network
          Access Identifier", RFC 4282, December 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet
          Protocol", RFC 4301, December 2005.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
          December 2005.

8.2.  Informative References

[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.
          and I. Goyret, "RADIUS Attributes for Tunnel Protocol
          Support", RFC 2868, June 2000.

[RFC2983] Black, D. "Differentiated Services and Tunnels", RFC 2983,
          October 2000.

[RFC3539] Aboba,  B. and J. Wood, "Authentication, Authorization and
          Accounting Transport Profile", RFC 3539, June 2003.

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

[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B. Aboba,
          "Dynamic Authorization Extensions to Remote Authentication
          Dial In User Service (RADIUS)", RFC 3576, July 2003.

[RFC4005] Calhoun, P., Zorn, G., Spence, D. and D. Mitton, "Diameter
          Network Access Server Application", RFC 4005, August 2005.

[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
          IPv4, IPv6 and OSI", RFC 4330, January 2006.

[RFC4675] Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Attributes for
          Virtual LAN and Priority Support", RFC 4675, September 2006.

[MD5Attack]
          Dobbertin, H., "The Status of MD5 After a Recent Attack",
          CryptoBytes Vol.2 No.2, Summer 1996.

[RFCDelegated]
          Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
          Attribute", draft-ietf-radext-delegated-prefix-05.txt,
          Internet draft (work in progress), October 2006.

[RFCFilter]
          Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Filter Rule
          Attribute", draft-ietf-radext-filter-08.txt, Internet draft
          (work in progress), January 2007.

Acknowledgments

   This protocol was first developed and distributed by Ascend
   Communications.  Example code was distributed in their free server
   kit.

   The authors would like to acknowledge the valuable suggestions and
   feedback from the following people:

      Avi Lior <avi@bridgewatersystems.com>,
      Randy Bush <randy@psg.net>,
      Steve Bellovin <smb@research.att.com>
      Glen Zorn <gwz@cisco.com>,
      Mark Jones <mjones@bridgewatersystems.com>,
      Claudio Lapidus <clapidus@hotmail.com>,
      Anurag Batta <Anurag_Batta@3com.com>,
      Kuntal Chowdhury <chowdury@nortelnetworks.com>
      Tim Moore <timmoore@microsoft.com>
      Russ Housley <housley@vigilsec.com>
      Joe Salowey <jsalowey@cisco.com>

Authors' Addresses

   Murtaza Chiba
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose CA, 95134

   EMail: mchiba@cisco.com
   Phone: +1 408 525 7198

   Gopal Dommety
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA 95134

   EMail: gdommety@cisco.com
   Phone: +1 408 525 1404

   Mark Eklund
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA 95134

   EMail: meklund@cisco.com
   Phone: +1 865 671 6255

   David Mitton
   RSA Security, Inc.
   174 Middlesex Turnpike
   Bedford, MA 01730

   EMail: dmitton@circularnetworks.com

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernarda@microsoft.com
   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329

Appendix A - Changes from RFC 3576

   This Appendix lists the major changes between [RFC3576] and this
   document.  Minor changes, including style, grammar, spelling, and
   editorial changes are not mentioned here.

   o Added details relating to handling of the Proxy-State Attribute.
   Added requirement for duplicate detection on the RADIUS client
   (Section 2.3).

   o Removed Framed-IP-Address, Framed-Interface-Id and Framed-
   IPv6-Prefix from the list of session identification attributes
   (Section 3).

   o Added requirements for inclusion of the State Attribute in CoA-
   Request packets with a Service-Type of "Authorize Only" (Section
   3.1).

   o Added clarification on the calculation of the Message-Authenticator
   Attribute (Section 3.2).

   o Added statement that support for "Authorize Only" Service-Type is
   optional
   optional.  Updated CoA-Request Attribute Table to include Filter-
   Rule, Delegated-IPv6-Prefix, Egress-VLANID, Ingress-Filters, Egress-
   VLAN-Name and User-Priority-Table Attributes (Section 3.5). 3.4).

   o Use of a Service-Type Attribute within a Disconnect-Request is
   prohibited (Section 3.4, 4).

   o Clarified use of Framed-IPv6-Prefix, Framed-IPv6-Pool and
   Delegated-IPv6-Prefix Attributes in renumbering (Section 3.4).

   o Added Diameter Considerations (Section 5).

   o Clarified that the Event-Timestamp Attribute should not be
   recalculated on retransmission.  The implications for replay and
   duplicate detection are discussed (Section 6.4).

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Open issues

   Open issues relating to this specification are tracked on the
   following web site:

   http://www.drizzle.com/~aboba/RADEXT/