Network Working Group                                        DeKok, Alan
INTERNET-DRAFT                                                FreeRADIUS
Updates: 5176                                                J. Korhonen
Category: Standards Track
30 July 2018

                   Dynamic Authorization Proxying in
      Remote Authorization Dial-In User Service Protocol (RADIUS)


   RFC 5176 defines Change of Authorization (CoA) and Disconnect Message
   (DM) behavior for RADIUS.  Section 3.1 of that document suggests that
   proxying these messages is possible, but gives no guidance as to how
   that is done.  This specification corrects that omission for
   scenarios where networks use Realm-based proxying as defined in

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 30, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

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Table of Contents

1.  Introduction .............................................    4
   1.1.  Terminology .........................................    4
   1.2.  Requirements Language ...............................    5
2.  Problem Statement ........................................    6
   2.1.  Typical RADIUS Proxying .............................    6
   2.2.  CoA Processing ......................................    6
   2.3.  Failure of CoA Proxying .............................    7
3.  How to Perform CoA Proxying ..............................    7
   3.1.  Changes to Access-Request and Accounting-Request pack    8
   3.2.  Proxying of CoA-Request and Disconnect-Request packet    8
   3.3.  Operator-NAS-Identifier .............................    9
4.  Requirements .............................................   11
   4.1.  Requirements on Home Servers ........................   11
   4.2.  Requirements on Visited Networks ....................   11
   4.3.  Requirements on Proxies .............................   12
      4.3.1.  Security Requirements on Proxies ...............   12
      4.3.2.  Filtering Requirements on Proxies ..............   13
5.  Functionality ............................................   14
   5.1.  User Login ..........................................   14
   5.2.  CoA Proxying ........................................   14
6.  Security Considerations ..................................   15
7.  IANA Considerations ......................................   15
8.  References ...............................................   15
   8.1.  Normative References ................................   15
   8.2.  Informative References ..............................   16

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1.  Introduction

   RFC 5176 [RFC5176] defines Change of Authorization (CoA) and
   Disconnect Message (DM) behavior for RADIUS.  Section 3.1 of that
   document suggests that proxying these messages is possible, but gives
   no guidance as to how that is done.  This omission means that in
   practice, proxying of CoA packets is impossible.

   We partially correct that ommission here by explaining how proxying
   of these packets can be done by leveraging an existing RADIUS
   attribute, Operator-Name (Section 4.1 of [RFC5580]).  We then explain
   how this attribute can be used by CoA proxies to route packets
   "backwards" through a RADIUS proxy chain to the Visited Network.  We
   introduce a new attribute; Operator-NAS-Identifier, which permits
   packets to be routed from the RADIUS server at the Visited Network to
   the NAS.  We then explain how use of this attribute can increase
   privacy of the internal implementation of the visited network.

   This correction is limited to the use-case of CoA proxying to Realm-
   based proxying as defined in [RFC7542].  Other forms of CoA proxying
   are possible, but are not specified here.

   We conclude with a discussion of the security implications of the
   design, and show how they are acceptable.

1.1.  Terminology

   This document frequently uses the following terms:


      Change of Authorization, e.g. CoA-Request, or CoA-ACK, or CoA-NAK,
      as defined in [RFC5176].  That specification also defines
      Disconnect-Request, Disconnect-ACK, and Disconnect-NAK.  For
      simplicity here, where we use "CoA", we mean a generic "CoA-
      Request or Disconnect-Request" packet.  We use "CoA-Request" or
      "Disconnect-Request" to refer to the specific packet types.

   Network Access Identifier

      The Network Access Identifier (NAI) [RFC7542] is the user identity
      submitted by the client during network access authentication.  The
      purpose of the NAI is to identify the user as well as to assist in
      the routing of the authentication request.  Please note that the
      NAI may not necessarily be the same as the user's email address or
      the user identity submitted in an application layer

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   Network Access Server

      The Network Access Server (NAS) is the device that clients connect
      to in order to get access to the network.  In PPTP terminology,
      this is referred to as the PPTP Access Concentrator (PAC), and in
      L2TP terminology, it is referred to as the L2TP Access
      Concentrator (LAC).  In IEEE 802.11, it is referred to as an
      Access Point.

   Home Network

      The network which holds the authentication credentials for a user.

   Visited Network

      A network other than the home network, where the user attempts to
      gain network access.  The Visited Network typically has a
      relationship with the Home Network, possibly through one or more
      intermediary proxies.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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2.  Problem Statement

   This section describes how RADIUS proxying works, how CoA packets
   work, and why CoA proxying as discussed in RFC 5176 is insufficient
   in practice.

2.1.  Typical RADIUS Proxying

   When a RADIUS server proxies an Access-Request packet, it typically
   does so based on the contents of the User-Name attribute, which
   contains a Network Access Identifier [RFC7542].  Other methods are
   possible, but we restrict ourselves to this usage, as it is the most
   common one.

   The proxy server looks up the "Realm" portion of the NAI in a logical
   AAA routing table, as described in Section 3 of [RFC7542].  The entry
   in that table is the "next hop" to which the packet is sent.  This
   "next hop" may be another proxy, or it may be the home server for
   that realm.

   If the "next hop" is a proxy, it will perform the same Realm lookup,
   and then proxy the packet.  Alternatively, if the "next hop" is the
   Home Server for that realm, it will try to authenticate the user, and
   respond with an Access-Accept, Access-Reject, or Access-Challenge.

   The RADIUS client will match the response packet to an outstanding
   request.  If the client is part of a proxy, it will then proxy that
   response packet in turn to the system that originated the Access-
   Request.  This process occurs until the response packet arrives at
   the NAS.

   The proxies are typically stateful with respect to ongoing request /
   response packets, but stateless with respect to user sessions.  Once
   a response has been received by the proxy, it can discard all
   information about the request packet.

   The same proxy method is used for Accounting-Request packets.  The
   combination of the two methods allows proxies to connect Visited
   Networks to Home Networks for all AAA purposes.

2.2.  CoA Processing

   [RFC5176] describes how CoA clients send packets to CoA servers.  We
   note that system comprising the CoA client is typically co-located
   with, or is the same as, the RADIUS server.  Similarly, the CoA
   server is a system that is either co-located with, or is the same as,
   the RADIUS client.

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   In the case of packets sent inside of one network, the source and
   destination of CoA packets is locally determined.  There is thus no
   need for standardization of that process, as networks are free to
   send CoA packets whenever they want, for whatever reason they want.

2.3.  Failure of CoA Proxying

   The situation is more complicated when multiple networks are
   involved.  [RFC5176] suggests that CoA proxying is permitted, but
   makes no suggestions for how it should be done.

   If proxies tracked user sessions, it might be possible for a proxy to
   match an incoming CoA-Request to a user session, and then to proxy
   that packet to the RADIUS client that originated the Access-Request
   for that sessions.

   There are many problems with such a scenario.  The CoA server may, in
   fact, not be co-located with the RADIUS client.  The RADIUS client
   may be down, but there may be a different CoA server which could
   successfully process the packet.  User session tracking can be
   expensive and complicated for a proxy, and many proxies do not record
   user sessions.  Finally, [RFC5176] is silent on the topic of what
   attributes constitute "session identification attributes", which
   makes it impossible for a proxy to determine if a CoA packet matches
   a particular user session.

   The result of all of these issues is that CoA proxying cannot be
   performed when using the behavior defined in [RFC5176].

3.  How to Perform CoA Proxying

   The solution to the above problem is to use the Operator-Name
   attribute defined in [RFC5580], Section 4.1.  We repeat a portion of
   that definition here for clarity:

      This attribute carries the operator namespace identifier and the
      operator name.  The operator name is combined with the namespace
      identifier to uniquely identify the owner of an access network.

   Followed by a description of the REALM namespace:

      REALM ('1' (0x31)):

      The REALM operator namespace can be used to indicate operator
      names based on any registered domain name.  Such names are
      required to be unique, and the rights to use a given realm name
      are obtained coincident with acquiring the rights to use a
      particular Fully Qualified Domain Name (FQDN). ...

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   In short, the Operator-Name attribute contains the an ASCII "1",
   followed by the Realm of the Visited Network.  e.g. for the
   "" realm, the Operator-Name attribute contains the text
   "".  This information is precisely what is needed by
   intermediate nodes, in order to perform CoA proxying.

3.1.  Changes to Access-Request and Accounting-Request packets

   When a Visited Network proxies an Access-Request or Accounting-
   Request packet outside of its network, it SHOULD include an Operator-
   Name attribute in the packet, as discussed in Section 4.1 of
   [RFC5580].  The contents of the Operator-Name should be "1", followed
   by the realm name of the Visited Network.  Where the Visited Network
   has more than one realm name, a "canonical" one should be chosen, and
   used for all packets.

   Visited Networks MUST use a consistent value for Operator-Name for
   one user session.  That is, sending "" in an Access-
   Request packet, and "" in an Accounting-Request packet
   for that same session is forbidden.  Such behavior would make it look
   like a single user session was activee simultaneously in two
   different Visited Networks, which is impossible.

   Proxies that record user session information SHOULD also record
   Operator-Name.  Proxies that do not record user session information
   do not need to record Operator-Name.

   Home Networks SHOULD record Operator-Name along with any other
   information that they record about user sessions.  Home Networks that
   expect to send CoA packets to Visited Networks MUST record Operator-
   Name for each user session that originates from a Visited Network.
   Failure to record the Operator-Name would mean that the Home Network
   would not know where to send any CoA packet.

   Networks that contain both the RADIUS client and RADIUS server do not
   need to create, record or track Operator-Name.  That is, if the
   Visited Network and Home Network are the same, there is no need to
   use the Operator-Name attribute.

3.2.  Proxying of CoA-Request and Disconnect-Request packets

   When a Home Network wishes to send a CoA-Request or Disconnect-
   Request packet to a Visited Network, it MUST include an Operator-Name
   attribute in the packet.  The value of the Operator-Name MUST be the
   value which was recorded earlier for that user session.

   The Home Network MUST lookup the realm from the Operator-Name in a
   logical "realm routing table", as discussed in [RFC7542] Section 3.

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   That logical realm table is defined there as:

      a logical AAA routing table, where the "utf8-realm" portion
      acts as a key, and the values stored in the table are one or more
      "next hop" AAA servers.

   In order to support proxying of CoA packets, this table is extended
   to include a mapping between "utf8-realm" and one or more "next hop"
   CoA servers.

   When proxying CoA-Request and Disconnect-Request packets, the lookups
   will return data from the "CoA server" field, instead of the "AAA
   server" field.

   In practice, this process means that CoA proxying works exactly like
   "normal" RADIUS proxying, except that the proxy decision is made
   using the realm from the Operator-Name attribute, instead of using
   the realm from the User-Name attribute.

   Proxies that receive the CoA packet MUST look up the realm from the
   Operator-Name in a logical "realm routing table", as with Home
   Servers, above.  The packet is then sent to the realm which was found
   in that table.  This process continues with any subsequent proxies
   until the packet reaches the Visited Network.

   The Visited Network can then send the CoA packet to the NAS, and
   return any response packet back up the proxy chain to the Home

   The only missing piece here is how the Visited Network gets the
   packet from its CoA server to the NAS.  The Visited Network could use
   NAS-Identifier, NAS-IP-Address, or NAS-IPv6-Address, but these
   attributes may be incorrect, or may be missing entirely.

   These attributes may be incorrect because proxies that forward
   Access-Request packets often re-write them for internal policy
   reasons.  These attributes may be missing, because the Visited
   Network may not want all upstream proxies and Home Servers to have
   detailed information about the internals of its private network.

   We therefore need a way to identifier a NAS in the Visited Network,
   in a way which is both private, and which does not use any existing

3.3.  Operator-NAS-Identifier

   The Operator-NAS-Identifier attribute contains opaque information
   that identifies a NAS in a Visited Network.  It MAY appear in the

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   following packets: Access-Request, Accounting-Request, CoA-Request,
   or Disconnect-Request.  Operator-NAS-Identifier MUST NOT appear in
   any other packet.

   Operator-NAS-Identifier MAY occur in a packet if the packet also
   contains an Operator-Name attribute.  Operator-NAS-Identifier MUST
   NOT appear in a packet if there is no Operator-Name in the packet.
   Operator-NAS-Identifier MUST NOT occur more than once in a packet.

   An Operator-NAS-Identifer attribute SHOULD be added to an Access-
   Request or Accounting-Request packet by a Visited Network just before
   proxying a packet to an external RADIUS server.  When the Operator-
   NAS-Identifer attribute is added to a packet, the following
   attributes MUST be deleted: NAS-IP-Address, NAS-IPv6-Address, NAS-
   Identifier.  The proxy MUST then add a NAS-Identifier attribute, in
   order satisfy the requirements of Section 4.1 of [RFC2865], and
   Section 4.1 of [RFC2866].  When a server receives a packet that
   already contains an Operator-NAS-Identifer attribute, no such editing
   is performed.

   The Operator-NAS-Identifier attribute parallels the Operator-Name
   attribute that was defined in Section 4.1 of [RFC5580].

   We suggest that the contents of the NAS-Identifier be the Realm name
   of the Visited Network.  That is, for everyone outside of the Visited
   Network, there is only one NAS: the Visited Network itself.  For the
   Visited Network, the identity of the NAS is private information,
   which is opaque to everyone else.

   The new Operator-NAS-Identifier attribute is defined as follows.


      An opaque token describing the NAS a user has logged into.


      TBD.  To be assigned by IANA from the "short extended space".


      4 to 23.

      Implementations supporting this attribute MUST be able to handle
      between one (1) and twenty (20) octets of data.  Implementations
      creating an Operator-NAS-Identifier MUST NOT create attributes
      with more than twenty octets of data.  A twenty octet string is
      more than sufficient to individually address all of the NASes on

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      the planet.

   Data Type

      string.  See [RFC8044] Section 3.6 for a definition.


      The contents of this attribute are an opaque token interpretable
      only by the Visited Network.

      This token MUST allow the Visited Network to direct the packet to
      the NAS for the users session.  In practice, this requirement
      means that for non-trivial use-cases, the Visited Network will
      either track these tokens in a database, or it will create tokens
      that can be decoded in order to reveal the identity of the NAS.

4.  Requirements

4.1.  Requirements on Home Servers

   The Operator-NAS-Identifier attribute MUST be stored by a Home Server
   along with any user session identification attributes.  When sending
   a CoA packet for a user session, the Home Server MUST include any
   Operator-NAS-Identifier it has recorded for that session.

   A Home Server MUST NOT send CoA packets for users of other networks.
   The provisions of the next few sections describe how other
   participants in the RADIUS ecosystem can enforce this requirement.

4.2.  Requirements on Visited Networks

   A Visited Network which receives a CoA packet that will be proxied
   MUST perform all of the operations required for proxies by Section
   4.3.2.  This requirement is because we assume that the Visited
   Network has a proxy in between the NAS and any external (i.e. third-
   party) proxy.  Situations where a NAS sends packets directly to a
   third-party RADIUS server are outside of the scope of this

   Due to the limited number of attributes allowed in CoA packets by
   [RFC5176] Section 2.3, a Visited Network MUST remove the Operator-
   Name and Operator-NAS-Identifier attributes from any CoA-Request or
   Disconnect-Request packet prior to proxying that packet to the final
   CoA server (i.e. NAS).  This requirement is phrase more generically
   below, in Section 4.3.2.

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   A Visited Network may create an Operator-NAS-Identifier via many
   methods.  The value SHOULD be cryptographically strong, and SHOULD be
   verifiable by the Visited Network, without requiring it to track in a
   database every individual value of Operator-NAS-identifier which was

   Exactly how this requirement is implemented is outside of the scope
   of this document.

4.3.  Requirements on Proxies

   There are a number of requirements on proxies, both CoA proxies and
   RADIUS proxies.  For the purpose of this section, we assume that each
   RADIUS proxy shares a common administration with a corresponding CoA
   proxy, and that the two systems can communicate electronically.
   There is no requirement that these systems are co-located.

4.3.1.  Security Requirements on Proxies

   Section 6.1 of [RFC5176] has some security requirements on proxies
   that handle CoA-Request and Disconnect-Request packets:

      ... a proxy MAY perform a "reverse path
      forwarding" (RPF) check to verify that a Disconnect-Request or
      CoA-Request originates from an authorized Dynamic Authorization

   We strengthen that requirement by saying that a proxy MUST perform a
   "reverse path forwarding" (RPF) check to verify that a Disconnect-
   Request or CoA-Request originates from an authorized Dynamic
   Authorization Client.  Without this check, a proxy may forward forged
   packets, and thus contribute to the forgery problem instead of
   preventing it.

   Proxies that record user session information SHOULD verify the
   contents of a received CoA packet against the recorded data for that
   user session.  If the proxy determines that the information in the
   packet does not match the recorded user session, it SHOULD return a
   CoA-NAK or Disconnect-NAK packet, that contains an Error-Cause
   attribute having value 503 ("Session Context Not Found").

   We recognize that because a RADIUS proxy will see Access-Request and
   Accounting-Request packets, it will have sufficient information to
   forge CoA packets.  The RADIUS proxy will thus have the ability to
   subsequently disconnect any user who was authenticated through

   We suggest that the real-world effect of this security problem is

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   minimal.  RADIUS proxies can already return Access-Accept or Access-
   Reject for Access-Request packets, and can change authorization
   attributes contained in an Access-Accept.  Allowing a proxy to change
   (or disconnect) a user session post-authentication is not
   substantially different from changing (or refusing to connect) a user
   session during the initial process of authentiction.

   The largest problem is that there are no provisions in RADIUS for
   "end to end" security.  That is, the Visited Network and Home Network
   cannot communicate privately in the presence of proxies.  This
   limitation originates from the design of RADIUS for Access-Request
   and Accounting-Request packets.  That limitation is then carried over
   to CoA-Request and Disconnect-Request packets.

   We cannot therefore prevent proxies or Home Servers from forging CoA
   packets.  We can only create scenarios where that forgery is hard to
   perform, and/or is likely to be detected.

4.3.2.  Filtering Requirements on Proxies

   Section 2.3 of [RFC5176] makes the following requirement for CoA

         In CoA-Request and Disconnect-Request packets, all attributes
         MUST be treated as mandatory.

   These requirements are too stringent for a CoA proxy.  Instead, we
   say that for a CoA proxy, all attributes MUST NOT be treated as
   mandatory.  Proxies SHOULD perform proxying based on Operator-Name,
   though other schemes are possible, but are not discussed here.
   Proxies SHOULD forward all packets as-is, with minimal changes.  Only
   the final CoA server (i.e NAS) can make a decision on which
   attributes are mandatory and which are not.

   Where Operator-Realm and Operator-NAS-Identifier is received by a
   proxy, the proxy MUST pass those attributes through unchanged.  This
   requirement applies to all proxies, including ones that forward any
   or all of Access-Request, Accounting-Request, CoA-Request, and
   Disconnect-Request packets.

   All attributes added by a RADIUS proxy when sending packets from the
   Visited Network to the Home Network Network MUST be removed by the
   corresponding CoA proxy from packets that travel the reverse path.
   That is, any attribute editing that is done on the "forward" path
   MUST be undone on the "reverse" path.

   The result is that a NAS will only ever receive CoA packets that
   either contain attributes sent by the NAS to it's local RADIUS

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   server, or contain attributes that are sent by the Home Server in
   order to perform a change of authorization.

   We note that the above requirement applies not only to Operator-Name
   and Operator-NAS-Identifier, but also to any future attributes that
   are added by a RADIUS proxy.

5.  Functionality

   This section describes how the two attributes work together to permit
   CoA proxying.

5.1.  User Login

   In this scenario, we follow a roaming user attempting authentication
   in a Visited Network.  The login attempt is done via a NAS in the
   Visited Network.  That NAS will send an Access-Request packet to the
   visited RADIUS server.  The visited RADIUS server will see that the
   user is roaming, and will add an Operator-Name attribute, with value
   "1" followed by it's own realm name.  e.g. "".  The
   visited RADIUS server MAY also add an Operator-NAS-Identifier.

   The visited RADIUS server will then proxy the authentication request
   to an upstream server.  That server may be the Home Server, or it may
   be a proxy.  In the case of a proxy, the proxy will forward the
   packet, until the packet reaches the Home Server.

   The Home Server will record the Operator-Name and Operator-NAS-
   Identifier along with other information about the users session, if
   those attributes are present in a packet.

5.2.  CoA Proxying

   When the Home Server determines that a user should be disconnected,
   it looks up the Operator-Name and Operator-NAS-Identifer, along with
   other user session identifiers as described in [RFC5176].  The Home
   Server then looks up the realm from the Operator-Name attribute in
   the logical AAA routing table, in order to find the "next hop" CoA
   server for that realm (that may be a proxy).  The Disconnect-Request
   is then sent to that CoA server.

   The CoA server receives the request, and if it is a proxy, performs a
   similar lookup as done by the Home Server.  The packet is then
   proxied repeatedly until it reaches the Visited Network.

   If the proxy cannot find a destination for the request, or if no
   Operator-Name attribute exists in the request, the proxy will return
   a CoA-NAK with Error-Cause 502 (Request Not Routable).

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   The Visited Network will receive the CoA-Request packet, and will use
   the Operator-NAS-Identifier (if available) attribute to determine
   which local CoA server (i.e. NAS) the packet should be sent to.  If
   there is no Opertor-NAS-Identifier attribute, the Visited Network may
   use other means to locate the NAS, such as consulting a local
   database that tracks user sessions.

   The Operator-Name and Operator-NAS-Identifer attributes are then
   removed from the packet, and it is then sent to the CoA server.

   If no CoA server can be found, the Visited Network return a CoA-NAK
   with Error-Cause 403 (NAS Identification Mismatch).

   Any response from the CoA server (NAS) is returned to the Home
   Network, via the normal method of returning responses to requests.

6.  Security Considerations

   This specification incorporates by reference the [RFC6929] Section
   11.  In short, RADIUS has many known issues which are discussed in
   detail there, and which do not need to be repeated here.

   This specification adds one new attribute, and defines new behavior
   for RADIUS proxying.  As this behavior mirrors existing RADIUS
   proxying, we do not believe that it introduces any new security

   The Operator-NAS-Identifier SHOULD be created by the Visited Network
   such that its contents are opaque to all other parties.  This ensures
   that anyone observing unencrypted RADIUS traffic gains no information
   about the internals of the Visited Network.

7.  IANA Considerations

   IANA is instructed to allocate one new RADIUS attribute, as per
   Section 3.3, above.

8.  References

8.1.  Normative References

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

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

DeKok, Alan                   Informational                    [Page 15]

INTERNET-DRAFT  Dynamic Authorization Proxying in RADIUS    30 July 2018

     Chiba, M. et al, "Dynamic Authorization Extensions to Remote
     Authentication Dial In User Service (RADIUS)", RFC 5176, January

     Tschofenig H., Ed. "Carrying Location Objects in RADIUS and
     Diameter", RFC 5580, August 2009.

     DeKok A. and Lior, A., "Remote Authentication Dial-In User Service
     (RADIUS) Protocol Extensions", RFC 6929, April 2013.

     DeKok A., "The Network Access Identifier", RFC 7542, May 2015.

     DeKok A., "Data Types in the Remote Authentication Dial-In User
     Service Protocol (RADIUS)", RFC 8044, January 2017.

     Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key
     Words", RFC 8174, May 2017.

8.2.  Informative References

     Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project


   Jouni Korhonen


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