Network Working Group                                           A. DeKok
INTERNET-DRAFT                                                FreeRADIUS
Category: Proposed Standard
Expires: June 16, September 1, 2009
16 December 2008
                            RADIUS Over TCP

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   The Remote Authentication Dial In User Server (RADIUS) Protocol has
   traditionally used the User Datagram Protocol (UDP) as it's
   underlying transport layer.  This document defines RADIUS over the
   Transmission Control Protocol (TCP).

Table of Contents

1.  Introduction .............................................    3    4
   1.1.  Applicability of Reliable Transport .................    3    4
   1.2.  Terminology .........................................    5    6
   1.3.  Requirements Language ...............................    5    6
2.  Changes to RADIUS ........................................    5    6
   2.1.  Packet Format .......................................    6    7
   2.2.  Assigned Ports for RADIUS Over TCP ..................    6    7
   2.3.  Management Information Base (MIB) ...................    6    7
   2.4.  Interaction with RadSec .............................    7    8
   2.5.  RADIUS Proxies ......................................    7    8
   2.6.  TCP Specific Issues .................................    9   10
      2.6.1.  Duplicates and Retransmissions .................    9   10
      2.6.2.  Shared Secrets .................................   10   11
      2.6.3.  Malformed Packets and Unknown Clients ..........   11   12
      2.6.4.  Limitations of the ID Field ....................   11   12
      2.6.5.  EAP Sessions ...................................   12   13
      2.6.6.  TCP Applications are not UDP Applications ......   12   13
3.  Diameter Considerations ..................................   13   14
4.  IANA Considerations ......................................   13   14
5.  Security Considerations ..................................   13   14
6.  References ...............................................   13   14
   6.1.  Normative References ................................   13   14
   6.2.  Informative References ..............................   14   15

1.  Introduction

   The RADIUS Protocol has been defined in [RFC2865] as using the User
   Datagram Protocol (UDP) for the underlying transport layer.  While
   there are a number of benefits to using UDP as outlined in [RFC2865]
   Section 2.4, there are also some limitations:

      * Unreliable transport.  As a result, systems using RADIUS have to
      implement application-layer timers and re-transmissions, as
      described in [RFC5080] Section 2.2.1.

      * Packet fragmentation.  [RFC2865] Section 3 permits RADIUS
      packets up to 4096 octets in length.  These packets are larger
      than the default Internet MTU (576), resulting in fragmentation of
      the packets at the IP layer.  Transport of fragmented UDP packets
      appears to be a poorly tested code path on network devices.  Some
      devices appear to be incapable of transporting fragmented UDP
      packets, making it difficult to deploy RADIUS in a network where
      those devices are deployed.

      * Connectionless transport.  Neither clients nor servers can
      reliably detect when the other is down.  This information has to
      be deduced instead from the absence of a reply to a request.

   As RADIUS is widely deployed, and has been widely deployed for well
   over a decade, these issues are relatively minor.  However, new
   systems may be interested in choosing a different set of trade-offs
   than those outlined in [RFC2865] Section 2.4.  For those systems, we
   define RADIUS over TCP.

1.1.  Applicability of Reliable Transport

   The intent of this document is to address transport issues related to
   RadSec [RADSEC].  The use of "bare" TCP transport is NOT RECOMMENDED,
   as there has been little implementational or operational experience
   with it.  Additionally, [RFC2865] Section 2.4 contains a list of
   reasons why UDP was originally chosen as the transport protocol for
   RADIUS.  UDP SHOULD be used as transport protocol in all cases where
   the rationale given in [RFC2865] Section 2.4 applies.

   There are a number of benefits to using a reliable transport.  For
   example, when RADIUS is used to carry EAP conversions [RFC3579], the
   EAP exchanges may involve 5 round trips at the RADIUS application
   layer.  We may assume a probability P of packet loss in each
   direction (with P having a value of 1% or less).  Any one
   authentication attempt will then have at least one lost packet, with
   a probability of approximately (10 * P).

   These lost packets require the supplicant and/or the NAS to re-
   transmit packets at the application layer.  The difficulty with this
   approach is that retransmission implementations have historically
   been poor.  Some implementations retransmit packets, others do not,
   and others send new packets rather then performing retransmission.
   Some implementations are incapable of detecting EAP retransmissions,
   and will instead treat the retransmitted packet as an error.

   These retransmissions have a high likelihood of causing the entire
   authentication session to fail.  For a system with a million logins a
   day, and having a packet loss probability of P=0.01%, we expect that
   0.1% of connections will experience a lost packet.  That is, 1,000
   user sessions each day will experience authentication failure.

   In addition, transport of fragmented UDP packets is a poorly tested
   code path on network devices.  Some devices appear to be incapable of
   transporting fragmented UDP packets, meaning that the packet loss
   rate for fragmented packets approaches 100 percent.  The net effect
   can be to prevent the deployment of authentication methods such as
   EAP-TLS that require large RADIUS packets.

   Using a reliable transport method such as TCP means that RADIUS
   implementations can remove all application-layer retransmissions, and
   instead rely on the Operating System (OS) kernel's well-tested TCP
   transport to ensure reliable delivery.  In addition, most TCP
   implementations discover Path MTU better than RADIUS application
   implementations, resulting in significantly fewer fragmented packets.
   Modern TCP implementations also implement anti-spoofing provisions,
   which is more difficult to do in UDP applications.

   Transporting RADIUS over TCP means that the RADIUS applications can
   leverage these additional protections offered by TCP.

   However, there are also some drawbacks to using TCP.  RADIUS over TCP
   has some drawbacks, as noted in [RFC2865] Section 2.4.  [RFC3539]
   Section 2 discusses further issues with using TCP as a transport for
   Authentication, Authorization, and/or Accounting (AAA) protocols such
   as RADIUS.

   Specifically, as noted in [RFC3539] Section 2.1, for systems
   originating low numbers of RADIUS request packets, inter-packet
   spacing is often larger than the packet RTT.  In those situations,
   RADIUS over TCP SHOULD NOT be used.

   In general, RADIUS clients generating small amounts of RADIUS traffic
   SHOULD NOT use TCP.  This suggestion will usually apply to most
   NASes, and to most clients that originate CoA-Request and Disconnect-
   Request packets.

   RADIUS over TCP is most applicable to RADIUS proxies that exchange a
   large volume of packets with RADIUS clients and servers (10's to
   1000's of packets per second).  In those situations, RADIUS over TCP
   may be a good fit, and may result in increased network stability and

1.2.  Terminology

   This document uses the following terms:

RADIUS client
     A device that provides an access service for a user to a network.
     Also referred to as a Network Access Server, or NAS.

RADIUS server
     A RADIUS authentication, authorization, and/or accounting (AAA)
     server is an entity that provides one or more AAA services to a

RADIUS proxy
     A RADIUS proxy acts as a RADIUS server to the NAS, and a RADIUS
     client to the RADIUS server.

RADIUS request packet
     A packet originated by a RADIUS client to a RADIUS server.  e.g.
     Access-Request, Accounting-Request, CoA-Request, or Disconnect-

RADIUS response packet
     A packet sent by a RADIUS server to a RADIUS client, in response to
     a RADIUS request packet.  e.g. Access-Accept, Access-Reject,
     Access-Challenge, Accounting-Response, CoA-ACK, etc.

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Changes to RADIUS

   Adding TCP as a RADIUS transport has a number of impacts on the
   protocol, on applications using the protocol, and on networks that
   deploy the protocol.  In short, RADIUS over TCP is little more than
   sending RADIUS formatted messages over a TCP connection.

   As always, there are additional details that need to be discussed.
   This section outlines the various impacts of using RADIUS over TCP,
   and the discusses the proposal in more detail.

2.1.  Packet Format

   The RADIUS packet format is unchanged from [RFC2865], [RFC2866], and
   [RFC5176].  Specifically, all of the following portions of RADIUS
   MUST be unchanged when using RADIUS over TCP:

      * Packet format
      * Permitted codes
      * Request Authenticator calculation
      * Response Authenticator calculation
      * Minimum packet length
      * Maximum packet length
      * Attribute format
      * Vendor-Specific Attribute (VSA) format
      * Permitted data types
      * Calculations of dynamic attributes such as CHAP-Challenge,
        or Message-Authenticator.
      * Calculation of "encrypted" attributes such as Tunnel-Password.

   The changes to RADIUS implementations required to implement this
   specification are largely limited to the portions that send and
   receive packets on the network.

2.2.  Assigned Ports for RADIUS Over TCP

   IANA has already assigned TCP ports for RADIUS transport, as outlined

      * radius          1812/tcp
      * radius-acct     1813/tcp
      * radius-dynauth  3799/tcp

   These ports are unused by existing RADIUS applications.
   Implementations SHOULD use the assigned values as the default ports
   for RADIUS over TCP.

   The early deployment of RADIUS was done using UDP port number 1645,
   which conflicts with the "datametrics" service.  Implementations
   using RADIUS over TCP MUST NOT use TCP ports 1645 or 1646 as the
   default ports for this specification.

2.3.  Management Information Base (MIB)

   The MIB Module definitions in [RFC4668], [RFC4669], [RFC4670],
   [RFC4671], [RFC4672], and [RFC4673] each contain only one reference
   to UDP.  These references are in the DESCRIPTION field of the MIB
   Module definition, and are in the form of "The UDP port" or "the UDP
   destination port".

   Implementations of RADIUS over TCP SHOULD re-use these MIB Modules to
   perform statistics counting for RADIUS over TCP connections.
   However, implementors are warned that there is no way for these MIB
   Modules to distinguish between packets sent over UDP or over TCP
   transport.  Similarly, there is no requirement in RADIUS that the
   RADIUS services offered over UDP on a particular IP address and port
   are identical to the RADIUS services offered over TCP on a particular
   IP address and the same (numerical) port.

   Implementations of RADIUS over TCP SHOULD include the protocol (UDP)
   or (TCP) in the radiusAuthServIdent, radiusAuthClientID,
   radiusAuthClientIdentifier, radiusAccServIdent, radiusAccClientID, or
   radiusAccClientIdentifier fields of the MIB Module.  This information
   can help the administrator distinguish capabilities of systems in the

2.4.  Interaction with RadSec

   IANA has already assigned TCP ports for RadSec (i.e. RADIUS over TLS
   over TCP), as outlined below:

      * radsec          2083/tcp

   This value SHOULD be used as the default port for RADIUS over TLS
   (i.e. RadSec).  The "radius" port (1812/tcp) SHOULD NOT be used for

2.5.  RADIUS Proxies

   As RADIUS is a "hop by hop" protocol, a RADIUS proxy effectively
   shields the client from any information about downstream servers.
   While the client may be able to deduce the operational state of the
   local server (i.e. proxy), it cannot make any determination about the
   operational state of the downstream servers.

   If a request is proxied through intermediate proxies, it is not
   possible to detect which of the later hops is responsible for the
   absence of a reply.  An intermediate proxy also cannot signal that
   the outage lies in a later hop because RADIUS does not have the
   ability to carry such signalling information.  This issue is further
   exacerbated by some proxy implementations that do not reply to a
   client if they do not receive a reply to a proxied request.

   When UDP was used as a transport protocol, the absence of a reply can
   cause a client to deduce (incorrectly) that the proxy is unavailable.

   The client could then fail over to another server, or conclude that
   no "live" servers are available (OKAY state in [RFC3539] Appendix A).
   This situation is made even worse when requests are sent through a
   proxy to multiple destinations.  Failures in one destination may
   result in service outages for other destinations, if the client
   erroneously believes that the proxy is unresponsive.

   For RADIUS over TCP, the continued existence of the TCP connection
   SHOULD be used to deduce that the service on the other end of the
   connection is still responsive.  Further, the application layer
   watchdog defined in [RFC3539] Section 3.4 enables clients to
   determine that the server is "live", even though it may not have
   responded recently to other, non-watchdog requests.

   RADIUS clients using RADIUS over TCP MUST NOT decide that a
   connection is down until the application layer watchdog algorithm has
   marked it DOWN ([RFC3539] Appendix A).  RADIUS clients using RADIUS
   over TCP MUST NOT decide that a RADIUS server is unresponsive until
   all TCP connections to it have been marked DOWN.

   Additional issues with RADIUS proxies involve transport protocol
   changes where the proxy receives packets on one transport protocol,
   and forwards them on a different transport protocol.  There are
   several situations in which the law of "conservation of packets"
   could be violated on an end-to-end basis (e.g. where more packets
   could enter the system than could leave it on a short-term basis):

      * Where TCP is used between proxies, it is possible that the
      bandwidth consumed by incoming UDP packets destined to a given
      upstream server could exceed the sending rate of a single TCP
      connection to that server, based on the window size/RTT estimate.

      * It is possible for the incoming rate of TCP packets destined to
      a given realm to exceed the UDP throughput achievable using the
      transport guidelines established in [RFC5080].  This could happen,
      for example, where the TCP window between proxies has opened, but
      packet loss is being experienced on the UDP leg, so that the
      effective congestion window on the UDP side is 1.

   Intrinsically, proxy systems operate with multiple control loops
   instead of one end-to-end loop, and so are less stable.  This is true
   even for TCP-TCP proxies.  As discussed in [RFC3539], the only way to
   achieve stability equivalent to a single TCP connection is to mimic
   the end-to-end behavior of a single TCP connection.  This typically
   is not achievable with an application-layer RADIUS implementation,
   regardless of transport.

2.6.  TCP Specific Issues

   The guidelines defined in [RFC3539] for implementing an AAA protocol
   operating over a reliable transport MUST be followed by implementors
   of this specification.

   The Application Layer Watchdog defined in [RFC3539] Section 3.4 MUST
   be used.  The Status-Server packet [STATUS] MUST be used as the
   application layer watchdog message.  Implementations MUST reserve one
   RADIUS ID per connection for the application layer watchdog message.
   This restriction is described further below.

   Implementations MUST NOT confuse UDP and TCP transport.  That is,
   RADIUS clients and servers MUST be treated as unique based on a key
   of the three-tuple (IP address, port, transport protocol).
   Implementations MUST be configurable to have different shared secrets
   for UDP and TCP to the same destination IP address and numerical

   This requirement does not forbid the traditional practice of using
   primary and secondary servers in a fail-over relationship.  Instead,
   it requires that two services sharing an IP address and numerical
   port, but differing in transport protocol, MUST be treated as
   independent services for the purpose of fail-over, load-balancing,

   Whenever the underlying operating system permits the use of TCP
   keepalive socket options, their use is RECOMMENDED.

2.6.1.  Duplicates and Retransmissions

   As TCP is a reliable transport, implementors of this specification
   MUST NOT retransmit RADIUS request packets over the same TCP
   connection.  Similarly, if there is no response to a RADIUS packet
   over one TCP connection, implementations MUST NOT retransmit that
   packet over a different TCP connection to the same destination IP
   address and port, while the first connection is in the OKAY state
   ([RFC3539] Appendix A).

   However, if the TCP connection is broken or closed, retransmissions
   over new connections are permissible.  RADIUS request packets that
   have not yet received a response MAY be transmitted by a RADIUS
   client over a new TCP connection.  As this procedure involves using a
   new source port, the ID of the packet MAY change.  If the ID changes,
   any security attributes such as Message-Authenticator MUST be

   If a TCP connection is broken or closed, any cached RADIUS response
   packets ([RFC5080] Section 2.2.2) associated with that connection
   MUST be discarded.  A RADIUS server SHOULD stop processing of any
   requests associated with that TCP connection.  No response to these
   requests can be sent over the TCP connection, so any further
   processing is pointless.  This requirement applies not only to RADIUS
   servers, but also to proxies.  When a client's connection to a proxy
   server is closed, there may be responses from a home server that were
   supposed to be sent by the proxy back over that connection to the
   client.  Since the client connection is closed, those responses from
   the home server to the proxy server SHOULD be silently discarded by
   the proxy.

   Despite the above discussion, RADIUS servers SHOULD still perform
   duplicate detection on received packets, as described in [RFC5080]
   Section 2.2.2.  This detection can prevent duplicate processing of
   packets from non-conformant clients.

   As noted previously, RADIUS packets SHOULD NOT be re-transmitted to
   the same destination IP and numerical port, but over a different
   transport layer.  There is no guarantee in RADIUS that the two ports
   are in any way related.  This requirement does not forbid the
   practice of putting multiple servers into a fail-over or load-balance

   Much of the discussion in this section can be summarized by the
   following requirement.  RADIUS requests MAY be re-transmitted
   verbatim only if the following 5-tuple (Client IP address, Client
   port, Transport Protocol, Server IP address, Server port) remains the
   same.  If any field of that 5-tuple changes, the packet MUST NOT be
   considered to be a re-transmission.  Instead, the packet MUST be
   considered to be a new request, and be treated accordingly.  This
   involves updating header calculations, packet signatures, associated
   timers and counters, etc.

   The above requirement is necessary, but not sufficient in all cases.
   Other specifications give additional situations where the packet is
   to be considered as a new request.  Those recommendations MUST also
   be followed.

2.6.2.  Shared Secrets

   The use of shared secrets in calculating the Response Authenticator,
   and other attributes such as User-Password or Message-Authenticator
   [RFC3579] MUST be unchanged from previous specifications.

   Clients and servers MUST be able to store and manage shared secrets
   based on the key described above, of (IP address, port, transport

2.6.3.  Malformed Packets and Unknown Clients

   The RADIUS specifications ([RFC2865], etc.) say that an
   implementation should "silently discard" a packet in a number of
   circumstances.  This action has no further consequences for UDP
   transport, as the "next" packet is completely independent of the
   previous one.

   When TCP is used as a transport, decoding the "next" packet on a
   connection depends on the proper decoding of the previous packet.  As
   a result, the behavior with respect to discarded packets has to

   Implementations of this specification SHOULD treat the "silently
   discard" texts referenced above as "silently discard and close the
   connection."  That is, the TCP connection MUST be closed if any of
   the following circumstances are seen:

      * Packet from an unknown client
      * Packet where the RADIUS "length" field is less than the minimum
        RADIUS packet length
      * Packet where the RADIUS "length" field is more than the maximum
        RADIUS packet length
      * Packet that has an Attribute "length" field has value of zero
        or one (0 or 1).
      * Packet where the attributes do not exactly fill the packet
      * Packet where the Request Authenticator fails validation
        (where applicable).
      * Packet where the Response Authenticator fails validation
        (where applicable).
      * Packet where the Message-Authenticator attribute fails
        validation (where applicable).

   TCP connections MAY be closed if any of the following circumstances
   are seen.  Alternatively, the TCP connection MAY remain open if any
   of the following circumstances are seen, but the invalid packet MUST
   BE silently discarded.

      * Packet with an invalid code field
      * Response packets that do not match any outstanding request

   These requirements minimize the possibility for a misbehaving client
   or server to wreak havoc on the network.

2.6.4.  Limitations of the ID Field

   The RADIUS ID field is one octet in size.  As a result, any one TCP
   connection can have only 256 "in flight" RADIUS packets at a time.

   If more than 256 simultaneous "in flight" packets are required,
   additional TCP connections will need to be opened.  This limitation
   is also noted in [RFC3539] Section 2.4.

   An additional limit is the requirement to send a Status-Server packet
   over the same TCP connection as is used for normal requests.  As
   noted in [STATUS], the response to a Status-Server packet is either
   an Access-Accept or an Accounting-Response.  If all IDs were
   allocated to normal requests, then there would be no free Id to use
   for the Status-Server packet, and it could not be sent over the

   Implementations SHOULD reserve ID zero on each TCP connection for
   Status-Server packets.  This value was picked arbitrarily, as there
   is no reason to choose any one value over another for this use.

   Implementors may be tempted to extend RADIUS to permit more than 256
   outstanding packets on one connection.  However, doing so will likely
   require fundamental changes to the RADIUS protocol, and as such, is
   outside of the scope of this specification.

2.6.5.  EAP Sessions

   When RADIUS clients send EAP requests using RADIUS over TCP, they
   SHOULD choose the same TCP connection for all packets related to one
   EAP conversation.  A simple method that may work in many situations
   is to hash the contents of the Calling-Station-Id attribute, which
   normally contains the MAC address.  The output of that hash can be
   used to select a particular TCP connection.

   If this practice is used, then the client SHOULD also reserve one
   RADIUS Id per TCP connection for a particular EAP session.

   The retransmission requirements of Section 2.6.1, above, MUST be
   applied to RADIUS encapsulated EAP packets.  That is, EAP
   retransmissions MUST NOT result in retransmissions of RADIUS packets
   over a particular TCP connection.  EAP retransmissions MAY result in
   retransmission of RADIUS packets over a different TCP connection, but
   only when the previous TCP connection is marked DOWN as per the
   algorithm in [RFC3539] Appendix A.

2.6.6.  TCP Applications are not UDP Applications

   Implementors should be aware that programming a robust TCP
   application can be very different from programming a robust UDP
   application.  We RECOMMEND that implementors of this specification
   familiarize themselves with TCP application programming concepts.  We
   RECOMMEND also that existing TCP applications be examined with an eye
   to robustness, performance, scalability, etc.

   Clients and servers SHOULD implement configurable connection limits.
   Clients and servers SHOULD implement configurable rate limiting on
   new connections.  Allowing an unbounded number or rate of TCP
   connections may result in resource exhaustion.

   Further discussion of implementation issues is outside of the scope
   of this document.

3.  Diameter Considerations

   This document defines TCP as a transport layer for RADIUS.  It
   defines no new RADIUS attributes or codes.  The only interaction with
   Diameter is in a RADIUS to Diameter, or in a Diameter to RADIUS
   gateway.  The RADIUS side of such a gateway MAY implement RADIUS over
   TCP, but this change has no effect on Diameter.

4.  IANA Considerations

   This document requires no action by IANA.

5.  Security Considerations

   As the RADIUS packet format, signing, and client verification are
   unchanged from prior specifications, all of the security issues
   outlined in previous specifications for RADIUS over UDP are also
   applicable here.

   As noted above, clients and servers SHOULD support configurable
   connection limits.  Allowing an unlimited number of connections may
   result in resource exhaustion.

   There are no (at this time) other known security issues for RADIUS
   over TCP transport.

6.  References

6.1.  Normative References

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

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

[RFC3539] Aboba, B. et al., "Authentication, Authorization and
          Accounting (AAA) Transport Profile", RFC 3539, June 2003.

6.2.  Informative References

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

[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
          In User Service) Support For Extensible Authentication
          Protocol (EAP)", RFC 3579, September 2003.

[RFC4668] Nelson, D, "RADIUS Authentication Client MIB for IPv6", RFC
          4668, August 2006.

[RFC4669] Nelson, D, "RADIUS Authentication Server MIB for IPv6", RFC
          4669, August 2006.

[RFC4670] Nelson, D, "RADIUS Accounting Client MIB for IPv6", RFC 4670,
          August 2006.

[RFC4671] Nelson, D, "RADIUS Accounting Server MIB for IPv6", RFC 4671,
          August 2006.

[RFC4672] Nelson, D, "RADIUS Dynamic Authorization Client MIB", RFC
          4672, August 2006.

[RFC4673] Nelson, D, "RADIUS Dynamic Authorization Server MIB", RFC
          4673, August 2006.

[RFC5080] Nelson, D. and DeKok, A, "Common Remote Authentication Dial In
          User Service (RADIUS) Implementation Issues and Suggested
          Fixes", RFC 5080, December 2007.

[RFC5176] Chiba, M. et al., "Dynamic Authorization Extensions to Remote
          Authentication Dial In User Service (RADIUS)", RFC 5176,
          January 2008.

[STATUS]  DeKok, A., "Use of Status-Server Packets in the Remote
          Authentication Dial In User Service (RADIUS) Protocol", draft-
          ietf-radext-status-server-02.txt, November 2008
          ietf-radext-status-server-03.txt, March 2009 (work in

[RADSEC]  Winter, S. et. al., "TLS encryption for RADIUS over TCP
          (RadSec)", draft-ietf-radext-radsec-02.txt, October 2008 draft-ietf-radext-radsec-03.txt, Februrary 2009
          (work in progress).

   None at this time.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project


Open issues

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