





Network Working Group                                           M. Chiba
Request for Comments: 3576                                    G. Dommety
Category: Informational                                        M. Eklund
                                                     Cisco Systems, Inc.
                                                               D. Mitton
                                                  Circular Logic, UnLtd.
                                                                B. Aboba
                                                   Microsoft Corporation
                                                               July 2003


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

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  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.




















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Applicability. . . . . . . . . . . . . . . . . . . . . .  3
       1.2.  Requirements Language  . . . . . . . . . . . . . . . . .  5
       1.3.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.  Disconnect Messages (DM) . . . . . . . . . . . . . . . .  5
       2.2.  Change-of-Authorization Messages (CoA) . . . . . . . . .  6
       2.3.  Packet Format. . . . . . . . . . . . . . . . . . . . . .  7
   3.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.1.  Error-Cause. . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.  Table of Attributes. . . . . . . . . . . . . . . . . . . 16
   4.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 20
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 21
       5.1.  Authorization Issues . . . . . . . . . . . . . . . . . . 21
       5.2.  Impersonation. . . . . . . . . . . . . . . . . . . . . . 22
       5.3.  IPsec Usage Guidelines . . . . . . . . . . . . . . . . . 22
       5.4.  Replay Protection. . . . . . . . . . . . . . . . . . . . 25
   6.  Example Traces . . . . . . . . . . . . . . . . . . . . . . . . 26
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
       7.1.  Normative References . . . . . . . . . . . . . . . . . . 26
       7.2.  Informative References . . . . . . . . . . . . . . . . . 27
   8.  Intellectual Property Statement. . . . . . . . . . . . . . . . 28
   9.  Acknowledgements.  . . . . . . . . . . . . . . . . . . . . . . 28
   10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29
   11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 30
























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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) messages.  Disconnect messages cause a user session to be
   terminated immediately, whereas CoA messages 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 5.1. 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 5.3. for
   details.







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   Existing implementations lack replay protection.  In order to support
   replay detection, it is recommended that the Event-Timestamp
   Attribute be added to all messages in situations where IPsec replay
   protection is not employed.  Implementations should be configurable
   to silently discard messages lacking the Event-Timestamp Attribute.
   See Section 5.4. 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 [Diameter], in which authorization
   change is requested by a command using Attribute Value Pairs (AVPs)
   solely for identification, resulting 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.

   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 existing
   implementations of this specification to equivalent messages in
   Diameter.  For example, a Diameter command changing any attribute
   used for identification within existing CoA-Request implementations
   cannot be translated, since such an authorization change is
   impossible to carry out in existing implementations.  Similarly,
   translation between existing implementations of Disconnect-Request or
   CoA-Request messages and Diameter is tricky because a Disconnect-
   Request or CoA-Request message will need to be translated to multiple
   Diameter commands.

   To simplify translation between RADIUS and Diameter, a Service-Type
   Attribute with value "Authorize Only" can (optionally) be included
   within a Disconnect-Request or CoA-Request.  Such a Request contains
   only identification attributes.  A NAS supporting the "Authorize
   Only" Service-Type within a Disconnect-Request or CoA-Request
   responds with a 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".  This usage
   sequence is akin to what occurs in Diameter and so is more easily
   translated by a Diameter/RADIUS gateway.





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1.2.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  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 messages.

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.



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   +----------+   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 NAS MUST respond to a
   Disconnect-Request including a Service-Type Attribute with value
   "Authorize Only" with a Disconnect-NAK; a Disconnect-ACK MUST NOT be
   sent.  A NAS MUST respond to a Disconnect-Request including a
   Service-Type Attribute with an unsupported value with a Disconnect-
   NAK; an Error-Cause Attribute with value "Unsupported Service" MAY be
   included.  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.  This is typically 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
   Messages.

   The following attribute 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.

   +----------+      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



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   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 messages 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 [RFC2882]
      41 - Disconnect-ACK [RFC2882]
      42 - Disconnect-NAK [RFC2882]
      43 - CoA-Request [RFC2882]
      44 - CoA-ACK [RFC2882]
      45 - CoA-NAK [RFC2882]




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   Identifier

      The Identifier field is one octet, and aids in matching requests
      and replies.  The RADIUS client can detect a duplicate request if
      it has the same server source IP address and 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 messages lies
      with the RADIUS server.  If after sending these messages, 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 have
      not 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.

      Note that if the Event-Timestamp Attribute is included, it will be
      updated when the packet is retransmitted, changing the content of
      the Attributes field and requiring a new Identifier and Request
      Authenticator.

      If the Request to a primary proxy fails, a secondary proxy must be
      queried, if available.  Issues relating to failover algorithms are
      described in [AAATransport].  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
      messages 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 the maximum
      length is 4096.





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   Authenticator

      The Authenticator field is sixteen (16) octets.  The most
      significant octet is transmitted first.  This value is used to
      authenticate the messages 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 done 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 messages, 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





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      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 messages, given the difference
      in attribute semantics.  This is true even for attributes
      specified within [RFC2865], [RFC2868], [RFC2869] or [RFC3162] as
      allowable within Access-Accept messages.

      As a result, attributes beyond those specified in Section 3.2.
      SHOULD NOT be included within Disconnect or CoA messages since
      this could produce unpredictable results.

      When using a forwarding proxy, the proxy must be able to alter the
      packet as it passes through in each direction.  When the proxy
      forwards a Disconnect or CoA-Request, it MAY add a Proxy-State
      Attribute, and when the proxy forwards a response, it MUST remove
      its Proxy-State Attribute if it added one.  Proxy-State is always
      added or removed after any other Proxy-States, but no other
      assumptions regarding its location within the list of Attributes
      can be made.  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.  A forwarding proxy MUST NOT modify existing Proxy-
      State, State, or Class Attributes present in the packet.

      If there are any Proxy-State Attributes in a Disconnect-Request or
      CoA-Request received from the server, the forwarding proxy MUST
      include those Proxy-State Attributes in its response to the
      server.  The forwarding proxy MAY include the Proxy-State
      Attributes in the Disconnect-Request or CoA-Request when it
      forwards the request, or it MAY omit them in the forwarded
      request.  If the forwarding proxy omits the Proxy-State Attributes
      in the request, it MUST attach them to the response before sending
      it to the server.












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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 message
   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.



























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   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.
   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    [NASREQ]   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 5.1., 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.  To address security concerns described in Section
   5.2., 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.




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   Where a Service-Type Attribute with value "Authorize Only" is
   included within a CoA-Request or Disconnect-Request, attributes
   representing an authorization change MUST NOT be included; only
   identification attributes are permitted.  If attributes other than
   NAS or session identification 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.
   Similarly, if attributes other than NAS or session identification
   attributes are included in such a Disconnect-Request, implementations
   MUST send a Disconnect-NAK; an Error-Cause Attribute with value
   "Unsupported Attribute" MAY be included.

3.1.  Error-Cause

   Description

      It is possible that the NAS cannot honor Disconnect-Request or
      CoA-Request messages 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
      messages.

      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 message
      and MUST NOT be sent within a Disconnect-NAK or CoA-NAK.  Values



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      400-499 represent fatal errors committed by the RADIUS server, so
      that they MAY be sent within CoA-NAK or Disconnect-NAK messages,
      and MUST NOT be sent within CoA-ACK or Disconnect-ACK messages.
      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
      messages, and MUST NOT be sent within CoA-ACK or Disconnect-ACK
      messages.  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.




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   "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.

   "Unsupported Extension" is a fatal error sent due to lack of support
   for an extension such as Disconnect and/or CoA messages.  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 messages 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 Request
   including a Service-Type Attribute with a value of "Authorize Only".
   It indicates that the Disconnect-Request or 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.





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3.2.  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]
   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]
   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
   0-1       0        0    61   NAS-Port-Type [Note 1]
   Request   ACK      NAK   #   Attribute



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   Request   ACK      NAK   #   Attribute
   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]
   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]
   0-1       0        0    96   Framed-Interface-Id [Note 1]
   0+        0        0    97   Framed-IPv6-Prefix [Note 1]
   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]
   0         0        0+  101   Error-Cause
   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]
   0-1       0        0-1   6   Service-Type [Note 6]
   0-1       0        0     8   Framed-IP-Address [Note 1]
   0+        0        0    18   Reply-Message [Note 2]
   0-1       0-1      0-1  24   State [Note 7]
   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
   Request   ACK      NAK   #   Attribute



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   Request   ACK      NAK   #   Attribute
   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   Originating-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

   [Note 1] Where NAS or session identification attributes are included
   in Disconnect-Request or CoA-Request messages, they are used for
   identification purposes only.  These attributes MUST NOT be used for
   purposes other than identification (e.g. within CoA-Request messages
   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 messages 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 sent within a successful CoA-Request, all existing tunnel
   attributes are removed and replaced by the new attribute(s).





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   [Note 6] When included within a Disconnect-Request or CoA-Request, a
   Service-Type Attribute with value "Authorize Only" indicates that the
   Request only contains NAS and session identification attributes, and
   that the NAS should attempt reauthorization by sending an Access-
   Request with a Service-Type Attribute with value "Authorize Only".
   This enables a usage model akin to that supported in Diameter, thus
   easing translation between the two protocols.  Support for the
   Service-Type Attribute is optional within CoA-Request and
   Disconnect-Request messages; where it is not included, the Request
   message may contain both identification and authorization attributes.
   A NAS that does not support the Service-Type Attribute with the value
   "Authorize Only" within a Disconnect-Request MUST respond with a
   Disconnect-NAK including no Service-Type Attribute; an Error-Cause
   Attribute with value "Unsupported Service" MAY be included.  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.

   A NAS supporting the "Authorize Only" Service-Type value within
   Disconnect-Request or CoA-Request messages MUST respond with a
   Disconnect-NAK or CoA-NAK respectively, 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 Disconnect or CoA-Request, as well as the session
   attributes from the 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.

   [Note 7] The State Attribute is available to be sent by the RADIUS
   server to the NAS in a Disconnect-Request or CoA-Request message and
   MUST be sent unmodified from the NAS to the RADIUS server in a
   subsequent ACK or NAK message.  If a Service-Type Attribute with
   value "Authorize Only" is included in a Disconnect-Request or CoA-
   Request along with a State Attribute, then the State Attribute 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



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   Attribute unchanged in that Access-Request.  In either usage, the
   client MUST NOT interpret the Attribute locally.  A Disconnect-
   Request or CoA-Request packet must have only zero or one State
   Attribute.  Usage of the State Attribute is implementation dependent.
   If the RADIUS server does not recognize the State Attribute in the
   Access-Request, then it MUST send an Access-Reject.

   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.

4.  IANA Considerations

   This document uses the RADIUS [RFC2865] namespace, see
   <http://www.iana.org/assignments/radius-types>.  There are six
   updates for the section: RADIUS Packet Type Codes.  These Packet
   Types are allocated in [RADIANA]:

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

   Allocation of a new Service-Type value for "Authorize Only" is
   requested.  This document also uses the UDP [RFC768] namespace, see
   <http://www.iana.org/assignments/port-numbers>.  The authors request
   a port assignment from the Registered ports range.  Finally, this
   specification allocates the Error-Cause Attribute (101) with the
   following decimal values:

    #     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)



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   503    Session Context Not Found
   504    Session Context Not Removable
   505    Other Proxy Processing Error
   506    Resources Unavailable
   507    Request Initiated

5.  Security Considerations

5.1.  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 messages 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 messages 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 messages, 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 [RFC2486] 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.









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5.2.  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 [NASREQ].  This could fool the RADIUS server
   into sending Disconnect-Request or CoA-Request messages 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 messages
   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.

5.3.  IPsec Usage Guidelines

   In addition to security vulnerabilities unique to Disconnect or CoA
   messages, 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.






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   Implementations of this specification SHOULD support IPsec [RFC2401]
   along with IKE [RFC2409] for key management.  IPsec ESP [RFC2406]
   with a 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
   Attributes such as User-Password, as well as used 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.



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   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 might 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



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   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].

5.4.  Replay Protection

   Where IPsec replay protection is not used, the Event-Timestamp (55)
   Attribute [RFC2869] SHOULD be included within all messages.  When
   this 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 message MUST be silently discarded.  This implies
   the need for time synchronization within the network, which can be
   achieved by a variety of means, including secure NTP, as described in
   [NTPAUTH].

   Both the NAS and the RADIUS server SHOULD be configurable to silently
   discard messages lacking an Event-Timestamp Attribute.  A default
   time window of 300 seconds is recommended.









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6.  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

7.  References

7.1.  Normative References

   [RFC1305]      Mills, D., "Network Time Protocol (version 3)
                  Specification, Implementation and Analysis", RFC 1305,
                  March 1992.

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

   [RFC2104]      Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                  Keyed-Hashing for Message Authentication", RFC 2104,
                  February 1997.

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

   [RFC2401]      Kent, S. and R. Atkinson, "Security Architecture for
                  the Internet Protocol", RFC 2401, November 1998.

   [RFC2406]      Kent, S. and R. Atkinson, "IP Encapsulating Security
                  Payload (ESP)", RFC 2406, November 1998.

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





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   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26, RFC
                  2434, October 1998.

   [RFC2486]      Aboba, B. and M. Beadles, "The Network Access
                  Identifier", RFC 2486, January 1999.

   [RFC2865]      Rigney, C., Willens, S., Rubens, A. and W. Simpson,
                  "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.

   [RADIANA]      Aboba, B., "IANA Considerations for RADIUS (Remote
                  Authentication Dial In User Service)", RFC 3575, July
                  2003.

7.2.  Informative References

   [RFC2882]      Mitton, D., "Network Access Server Requirements:
                  Extended RADIUS Practices", RFC 2882, July 2000.

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

   [AAATransport] Aboba,  B. and J. Wood, "Authentication, Authorization
                  and Accounting (AAA) Transport Profile", RFC 3539,
                  June 2003.

   [Diameter]     Calhoun, P., et al., "Diameter Base Protocol", Work in
                  Progress.

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

   [NASREQ]       Calhoun, P., et al., "Diameter Network Access Server
                  Application", Work in Progress.



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   [NTPAUTH]      Mills, D., "Public Key Cryptography for the Network
                  Time Protocol", Work in Progress.

8.  Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards- related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementers or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

9.  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>, and
      Tim Moore <timmoore@microsoft.com>.
      Russ Housley <housley@vigilsec.com>







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10.  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
   Circular Logic UnLtd.
   733 Turnpike Street #154
   North Andover, MA 01845

   EMail: david@mitton.com
   Phone: +1 978 683 1814

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

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









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11.  Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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



















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