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RFC 7360

Experimental
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Datagram Transport Layer Security (DTLS) as a Transport Layer for RADIUS

 


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Internet Engineering Task Force (IETF)                          A. DeKok
Request for Comments: 7360                                    FreeRADIUS
Category: Experimental                                    September 2014
ISSN: 2070-1721


                Datagram Transport Layer Security (DTLS)
                    as a Transport Layer for RADIUS

Abstract

   The RADIUS protocol defined in RFC 2865 has limited support for
   authentication and encryption of RADIUS packets.  The protocol
   transports data in the clear, although some parts of the packets can
   have obfuscated content.  Packets may be replayed verbatim by an
   attacker, and client-server authentication is based on fixed shared
   secrets.  This document specifies how the Datagram Transport Layer
   Security (DTLS) protocol may be used as a fix for these problems.  It
   also describes how implementations of this proposal can coexist with
   current RADIUS systems.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7360.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................5
      1.2. Requirements Language ......................................5
      1.3. Document Status ............................................5
   2. Building on Existing Foundations ................................6
      2.1. Changes to RADIUS ..........................................7
      2.2. Similarities with RADIUS/TLS ...............................8
           2.2.1. Changes from RADIUS/TLS to RADIUS/DTLS ..............8
   3. Interaction with RADIUS/UDP .....................................9
      3.1. DTLS Port and Packet Types ................................10
      3.2. Server Behavior ...........................................10
   4. Client Behavior ................................................11
   5. Session Management .............................................12
      5.1. Server Session Management .................................12
           5.1.1. Session Opening and Closing ........................13
      5.2. Client Session Management .................................15
   6. Implementation Guidelines ......................................16
      6.1. Client Implementations ....................................17
      6.2. Server Implementations ....................................18
   7. Diameter Considerations ........................................18
   8. IANA Considerations ............................................18
   9. Implementation Status ..........................................18
      9.1. Radsecproxy ...............................................19
      9.2. jradius ...................................................19
   10. Security Considerations .......................................19
      10.1. Crypto-Agility ...........................................20
      10.2. Legacy RADIUS Security ...................................21
      10.3. Resource Exhaustion ......................................22
      10.4. Client-Server Authentication with DTLS ...................22
      10.5. Network Address Translation ..............................24
      10.6. Wildcard Clients .........................................24
      10.7. Session Closing ..........................................25
      10.8. Client Subsystems ........................................25
   11. References ....................................................26
      11.1. Normative References .....................................26
      11.2. Informative References ...................................27
   Acknowledgments ...................................................27

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1.  Introduction

   The RADIUS protocol as described in [RFC2865], [RFC2866], [RFC5176],
   and others has traditionally used methods based on MD5 [RFC1321] for
   per-packet authentication and integrity checks.  However, the MD5
   algorithm has known weaknesses such as [MD5Attack] and [MD5Break].
   As a result, some specifications, such as [RFC5176], have recommended
   using IPsec to secure RADIUS traffic.

   While RADIUS over IPsec has been widely deployed, there are
   difficulties with this approach.  The simplest point against IPsec is
   that there is no straightforward way for an application to control or
   monitor the network security policies.  That is, the requirement that
   the RADIUS traffic be encrypted and/or authenticated is implicit in
   the network configuration, and it cannot be enforced by the RADIUS
   application.

   This specification takes a different approach.  We define a method
   for using DTLS [RFC6347] as a RADIUS transport protocol.  This
   approach has the benefit that the RADIUS application can directly
   monitor and control the security policies associated with the traffic
   that it processes.

   Another benefit is that RADIUS over DTLS continues to be a UDP-based
   protocol.  The change from RADIUS/UDP is largely to add DTLS support,
   and make any necessary related changes to RADIUS.  This allows
   implementations to remain UDP based, without changing to a TCP
   architecture.

   This specification does not, however, solve all of the problems
   associated with RADIUS/UDP.  The DTLS protocol does not add reliable
   or in-order transport to RADIUS.  DTLS also does not support
   fragmentation of application-layer messages, or of the DTLS messages
   themselves.  This specification therefore shares with traditional
   RADIUS the issues of order, reliability, and fragmentation.  These
   issues are dealt with in RADIUS/TCP [RFC6613] and RADIUS/TLS
   [RFC6614].

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1.1.  Terminology

   This document uses the following terms:

   RADIUS/DTLS
      This term is a shorthand for "RADIUS over DTLS".

   RADIUS/DTLS client
      This term refers both to RADIUS clients as defined in [RFC2865]
      and to Dynamic Authorization clients as defined in [RFC5176] that
      implement RADIUS/DTLS.

   RADIUS/DTLS server
      This term refers both to RADIUS servers as defined in [RFC2865]
      and to Dynamic Authorization servers as defined in [RFC5176] that
      implement RADIUS/DTLS.

   RADIUS/UDP
      RADIUS over UDP, as defined in [RFC2865].

   RADIUS/TLS
      RADIUS over TLS, as defined in [RFC6614].

   silently discard
      This means that the implementation discards the packet without
      further processing.

1.2.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in [RFC2119].

1.3.  Document Status

   This document is an Experimental RFC.

   It contains one of several approaches to address known cryptographic
   weaknesses of the RADIUS protocol, such as described in [RFC6614].
   This specification does not fulfill all recommendations for an
   Authentication, Authorization, and Accounting (AAA) transport profile
   as per [RFC3539]; however, unlike [RFC6614], it is based on UDP and
   therefore does not have head-of-line blocking issues.

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   If this specification is indeed selected for advancement to Standards
   Track, certificate verification options ([RFC6614], Section 2.3,
   point 2) will need to be refined.

   Another experimental characteristic of this specification is the
   question of key management between RADIUS/DTLS peers.  RADIUS/UDP
   only allowed for manual key management, i.e., distribution of a
   shared secret between a client and a server.  RADIUS/DTLS allows
   manual distribution of long-term proofs of peer identity, by using
   TLS-PSK ciphersuites.  RADIUS/DTLS also allows the use of X.509
   certificates in a PKIX infrastructure.  It remains to be seen if one
   of these methods will prevail or if both will find their place in
   real-life deployments.  The authors can imagine pre-shared keys
   (PSKs) to be popular in small-scale deployments (Small Office, Home
   Office (SOHO) or isolated enterprise deployments) where scalability
   is not an issue and the deployment of a Certification Authority (CA)
   is considered too much of a hassle; however, the authors can also
   imagine large roaming consortia to make use of PKIX.  Readers of this
   specification are encouraged to read the discussion of key management
   issues within [RFC6421] as well as [RFC4107].

   It has yet to be decided whether this approach is to be chosen for
   Standards Track.  One key aspect to judge whether the approach is
   usable on a large scale is by observing the uptake, usability, and
   operational behavior of the protocol in large-scale, real-life
   deployments.

2.  Building on Existing Foundations

   Adding DTLS as a RADIUS transport protocol requires a number of
   changes to systems implementing standard RADIUS.  This section
   outlines those changes, and defines new behaviors necessary to
   implement DTLS.

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2.1.  Changes to RADIUS

   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/DTLS:

      * 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 "obfuscated" attributes such as User-Password and
        Tunnel-Password.

   In short, the application creates a RADIUS packet via the usual
   methods, and then instead of sending it over a UDP socket, sends the
   packet to a DTLS layer for encapsulation.  DTLS then acts as a
   transport layer for RADIUS: hence, the names "RADIUS/UDP" and
   "RADIUS/DTLS".

   The requirement that RADIUS remain largely unchanged ensures the
   simplest possible implementation and widest interoperability of this
   specification.

   We note that the DTLS encapsulation of RADIUS means that RADIUS
   packets have an additional overhead due to DTLS.  Implementations
   MUST support sending and receiving encapsulated RADIUS packets of
   4096 octets in length, with a corresponding increase in the maximum
   size of the encapsulated DTLS packets.  This larger packet size may
   cause the packet to be larger than the Path MTU (PMTU), where a
   RADIUS/UDP packet may be smaller.  See Section 5.2, below, for more
   discussion.

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   The only changes made from RADIUS/UDP to RADIUS/DTLS are the
   following two items:

   (1) The Length checks defined in [RFC2865], Section 3, MUST use the
       length of the decrypted DTLS data instead of the UDP packet
       length.  They MUST treat any decrypted DTLS data octets outside
       the range of the Length field as padding and ignore it on
       reception.

   (2) The shared secret used to compute the MD5 integrity checks and
       the attribute encryption MUST be "radius/dtls".

   All other aspects of RADIUS are unchanged.

2.2.  Similarities with RADIUS/TLS

   While this specification can be thought of as RADIUS/TLS over UDP
   instead of the Transmission Control Protocol (TCP), there are some
   differences between the two methods.  The bulk of [RFC6614] applies
   to this specification, so we do not repeat it here.

   This section explains the differences between RADIUS/TLS and
   RADIUS/DTLS, as semantic "patches" to [RFC6614].  The changes are as
   follows:

      * We replace references to "TCP" with "UDP"

      * We replace references to "RADIUS/TLS" with "RADIUS/DTLS"

      * We replace references to "TLS" with "DTLS"

   Those changes are sufficient to cover the majority of the differences
   between the two specifications.  The next section reviews some more
   detailed changes from [RFC6614], giving additional commentary only
   where necessary.

2.2.1.  Changes from RADIUS/TLS to RADIUS/DTLS

   This section describes how particular sections of [RFC6614] apply to
   RADIUS/DTLS.

   Section 2.1 applies to RADIUS/DTLS, with the exception that the
   RADIUS/DTLS port is UDP/2083.

   Section 2.2 applies to RADIUS/DTLS.  Servers and clients need to be
   pre-configured to use RADIUS/DTLS for a given endpoint.

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   Most of Section 2.3 applies also to RADIUS/DTLS.  Item (1) should be
   interpreted as applying to DTLS session initiation, instead of TCP
   connection establishment.  Item (2) applies, except for the
   recommendation that implementations "SHOULD" support
   TLS_RSA_WITH_RC4_128_SHA.  This recommendation is a historical
   artifact of RADIUS/TLS, and it does not apply to RADIUS/DTLS.  Item
   (3) applies to RADIUS/DTLS.  Item (4) applies, except that the fixed
   shared secret is "radius/dtls", as described above.

   Section 2.4 applies to RADIUS/DTLS.  Client identities SHOULD be
   determined from DTLS parameters, instead of relying solely on the
   source IP address of the packet.

   Section 2.5 does not apply to RADIUS/DTLS.  The relationship between
   RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged from
   RADIUS/UDP.

   Sections 3.1, 3.2, and 3.3 apply to RADIUS/DTLS.

   Section 3.4 item (1) does not apply to RADIUS/DTLS.  Each RADIUS
   packet is encapsulated in one DTLS packet, and there is no "stream"
   of RADIUS packets inside of a TLS session.  Implementors MUST enforce
   the requirements of [RFC2865], Section 3, for the RADIUS Length
   field, using the length of the decrypted DTLS data for the checks.
   This check replaces the RADIUS method of using the Length field from
   the UDP packet.

   Section 3.4 items (2), (3), (4), and (5) apply to RADIUS/DTLS.

   Section 4 does not apply to RADIUS/DTLS.  Protocol compatibility
   considerations are defined in this document.

   Section 6 applies to RADIUS/DTLS.

3.  Interaction with RADIUS/UDP

   Transitioning to DTLS is a process that needs to be done carefully.
   A poorly handled transition is complex for administrators and
   potentially subject to security downgrade attacks.  It is not
   sufficient to just disable RADIUS/UDP and enable RADIUS/DTLS.  RADIUS
   has no provisions for protocol negotiation, so simply disabling
   RADIUS/UDP would result in timeouts, lost traffic, and network
   instabilities.

   The end result of this specification is that nearly all RADIUS/UDP
   implementations should transition to using a secure alternative.  In
   some cases, RADIUS/UDP may remain where IPsec is used as a transport,
   or where implementation and/or business reasons preclude a change.

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   However, we do not recommend long-term use of RADIUS/UDP outside of
   isolated and secure networks.

   This section describes how clients and servers should use
   RADIUS/DTLS, and how it interacts with RADIUS/UDP.

3.1.  DTLS Port and Packet Types

   The default destination port number for RADIUS/DTLS is UDP/2083.
   There are no separate ports for authentication, accounting, and
   dynamic authorization changes.  The source port is arbitrary.  The
   text in [RFC6614], Section 3.4, describes issues surrounding the use
   of one port for multiple packet types.  We recognize that
   implementations may allow the use of RADIUS/DTLS over non-standard
   ports.  In that case, the references to UDP/2083 in this document
   should be read as applying to any port used for transport of
   RADIUS/DTLS traffic.

3.2.  Server Behavior

   When a server receives packets on UDP/2083, all packets MUST be
   treated as being DTLS.  RADIUS/UDP packets MUST NOT be accepted on
   this port.

   Servers MUST NOT accept DTLS packets on the old RADIUS/UDP ports.
   Early versions of this specification permitted this behavior.  It is
   forbidden here, as it depended on behavior in DTLS that may change
   without notice.

   Servers MUST authenticate clients.  RADIUS is designed to be used by
   mutually trusted systems.  Allowing anonymous clients would ensure
   privacy for RADIUS/DTLS traffic, but would negate all other security
   aspects of the protocol.

   As RADIUS has no provisions for capability signaling, there is no way
   for a server to indicate to a client that it should transition to
   using DTLS.  This action has to be taken by the administrators of the
   two systems, using a method other than RADIUS.  This method will
   likely be out of band, or manual configuration will need to be used.

   Some servers maintain a list of allowed clients per destination port.
   Others maintain a global list of clients that are permitted to send
   packets to any port.  Where a client can send packets to multiple
   ports, the server MUST maintain a "DTLS Required" flag per client.

   This flag indicates whether or not the client is required to use
   DTLS.  When set, the flag indicates that the only traffic accepted
   from the client is over UDP/2083.  When packets are received from a

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   client on non-DTLS ports, for which DTLS is required, the server MUST
   silently discard these packets, as there is no RADIUS/UDP shared
   secret available.

   This flag will often be set by an administrator.  However, if a
   server receives DTLS traffic from a client, it SHOULD notify the
   administrator that DTLS is available for that client.  It MAY mark
   the client as "DTLS Required".

   It is RECOMMENDED that servers support the following Perfect Forward
   Secrecy (PFS) ciphersuites:

      o  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256

      o  TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   Allowing RADIUS/UDP and RADIUS/DTLS from the same client exposes the
   traffic to downbidding attacks and is NOT RECOMMENDED.

4.  Client Behavior

   When a client sends packets to the assigned RADIUS/DTLS port, all
   packets MUST be DTLS.  RADIUS/UDP packets MUST NOT be sent to this
   port.

   Clients MUST authenticate themselves to servers via credentials that
   are unique to each client.

   It is RECOMMENDED that clients support the following PFS
   ciphersuites:

      o  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256

      o  TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   RADIUS/DTLS clients SHOULD NOT probe servers to see if they support
   DTLS transport.  Instead, clients SHOULD use DTLS as a transport
   layer only when administratively configured.  If a client is
   configured to use DTLS and the server appears to be unresponsive, the
   client MUST NOT fall back to using RADIUS/UDP.  Instead, the client
   should treat the server as being down.

   RADIUS clients often had multiple independent RADIUS implementations
   and/or processes that originate packets.  This practice was simple to
   implement, but the result is that each independent subsystem must
   independently discover network issues or server failures.  It is
   therefore RECOMMENDED that clients with multiple internal RADIUS
   sources use a local proxy as described in Section 6.1, below.

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   Clients may implement "pools" of servers for fail-over or load-
   balancing.  These pools SHOULD NOT mix RADIUS/UDP and RADIUS/DTLS
   servers.

5.  Session Management

   Where [RFC6614] can rely on the TCP state machine to perform session
   tracking, this specification cannot.  As a result, implementations of
   this specification may need to perform session management of the DTLS
   session in the application layer.  This section describes logically
   how this tracking is done.  Implementations may choose to use the
   method described here, or another, equivalent method.

   We note that [RFC5080], Section 2.2.2, already mandates a duplicate
   detection cache.  The session tracking described below can be seen as
   an extension of that cache, where entries contain DTLS sessions
   instead of RADIUS/UDP packets.

   [RFC5080], Section 2.2.2, describes how duplicate RADIUS/UDP requests
   result in the retransmission of a previously cached RADIUS/UDP
   response.  Due to DTLS sequence window requirements, a server MUST
   NOT retransmit a previously sent DTLS packet.  Instead, it should
   cache the RADIUS response packet, and re-process it through DTLS to
   create a new RADIUS/DTLS packet, every time it is necessary to
   retransmit a RADIUS response.

5.1.  Server Session Management

   A RADIUS/DTLS server MUST track ongoing DTLS sessions for each, based
   on the following 4-tuple:

      * source IP address
      * source port
      * destination IP address
      * destination port

   Note that this 4-tuple is independent of IP address version (IPv4 or
   IPv6).

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   Each 4-tuple points to a unique session entry, which usually contains
   the following information:

   DTLS Session
      Any information required to maintain and manage the DTLS session.

   Last Traffic
      A variable containing a timestamp that indicates when this session
      last received valid traffic.  If "Last Traffic" is not used, this
      variable may not exist.

   DTLS Data
      An implementation-specific variable that may contain information
      about the active DTLS session.  This variable may be empty or
      nonexistent.

      This data will typically contain information such as idle
      timeouts, session lifetimes, and other implementation-specific
      data.

5.1.1.  Session Opening and Closing

   Session tracking is subject to Denial-of-Service (DoS) attacks due to
   the ability of an attacker to forge UDP traffic.  RADIUS/DTLS servers
   SHOULD use the stateless cookie tracking technique described in
   [RFC6347], Section 4.2.1.  DTLS sessions SHOULD NOT be tracked until
   a ClientHello packet has been received with an appropriate Cookie
   value.  Server implementation SHOULD have a way of tracking DTLS
   sessions that are partially set up.  Servers MUST limit both the
   number and impact on resources of partial sessions.

   Sessions (both 4-tuple and entry) MUST be deleted when a TLS Closure
   Alert ([RFC5246], Section 7.2.1) or a fatal TLS Error Alert
   ([RFC5246], Section 7.2.2) is received.  When a session is deleted
   due to it failing security requirements, the DTLS session MUST be
   closed, any TLS session resumption parameters for that session MUST
   be discarded, and all tracking information MUST be deleted.

   Sessions MUST also be deleted when a RADIUS packet fails validation
   due to a packet being malformed, or when it has an invalid Message-
   Authenticator or invalid Request Authenticator.  There are other
   cases when the specifications require that a packet received via a
   DTLS session be "silently discarded".  In those cases,
   implementations MAY delete the underlying session as described above.
   There are few reasons to communicate with a Network Access Server
   (NAS) that is not implementing RADIUS.

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   A session MUST be deleted when non-RADIUS traffic is received over
   it.  This specification is for RADIUS, and there is no reason to
   allow non-RADIUS traffic over a RADIUS/DTLS session.  A session MUST
   be deleted when RADIUS traffic fails to pass security checks.  There
   is no reason to permit insecure networks.  A session SHOULD NOT be
   deleted when a well-formed, but "unexpected", RADIUS packet is
   received over it.  Future specifications may extend RADIUS/DTLS, and
   we do not want to forbid those specifications.

   The goal of the above requirements is to ensure security, while
   maintaining flexibility.  Any security-related issue causes the
   connection to be closed.  After the security restrictions have been
   applied, any unexpected traffic may be safely ignored, as it cannot
   cause a security issue.  There is no need to close the session for
   unexpected but valid traffic, and the session can safely remain open.

   Once a DTLS session is established, a RADIUS/DTLS server SHOULD use
   DTLS Heartbeats [RFC6520] to determine connectivity between the two
   servers.  A server SHOULD also use watchdog packets from the client
   to determine that the session is still active.

   As UDP does not guarantee delivery of messages, RADIUS/DTLS servers
   that do not implement an application-layer watchdog MUST also
   maintain a "Last Traffic" timestamp per DTLS session.  The
   granularity of this timestamp is not critical and could be limited to
   one-second intervals.  The timestamp SHOULD be updated on reception
   of a valid RADIUS/DTLS packet, or a DTLS Heartbeat, but no more than
   once per interval.  The timestamp MUST NOT be updated in other
   situations.

   When a session has not received a packet for a period of time, it is
   labeled "idle".  The server SHOULD delete idle DTLS sessions after an
   "idle timeout".  The server MAY cache the TLS session parameters, in
   order to provide for fast session resumption.

   This session "idle timeout" SHOULD be exposed to the administrator as
   a configurable setting.  It SHOULD NOT be set to less than 60 seconds
   and SHOULD NOT be set to more than 600 seconds (10 minutes).  The
   minimum useful value for this timer is determined by the application-
   layer watchdog mechanism defined in the following section.

   RADIUS/DTLS servers SHOULD also monitor the total number of open
   sessions.  They SHOULD have a "maximum sessions" setting exposed to
   administrators as a configurable parameter.  When this maximum is
   reached and a new session is started, the server MUST either drop an
   old session in order to open the new one or not create a new session.

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   RADIUS/DTLS servers SHOULD implement session resumption, preferably
   stateless session resumption as given in [RFC5077].  This practice
   lowers the time and effort required to start a DTLS session with a
   client and increases network responsiveness.

   Since UDP is stateless, the potential exists for the client to
   initiate a new DTLS session using a particular 4-tuple, before the
   server has closed the old session.  For security reasons, the server
   MUST keep the old session active until either it has received secure
   notification from the client that the session is closed or the server
   decides to close the session based on idle timeouts.  Taking any
   other action would permit unauthenticated clients to perform a DoS
   attack, by reusing a 4-tuple and thus causing the server to close an
   active (and authenticated) DTLS session.

   As a result, servers MUST ignore any attempts to reuse an existing
   4-tuple from an active session.  This requirement can likely be
   reached by simply processing the packet through the existing session,
   as with any other packet received via that 4-tuple.  Non-compliant,
   or unexpected packets will be ignored by the DTLS layer.

   The above requirement is mitigated by the suggestion in Section 6.1,
   below, that the client use a local proxy for all RADIUS traffic.
   That proxy can then track the ports that it uses and ensure that
   reuse of 4-tuples is avoided.  The exact process by which this
   tracking is done is outside of the scope of this document.

5.2.  Client Session Management

   Clients SHOULD use PMTU discovery [RFC6520] to determine the PMTU
   between the client and server, prior to sending any RADIUS traffic.
   Once a DTLS session is established, a RADIUS/DTLS client SHOULD use
   DTLS Heartbeats [RFC6520] to determine connectivity between the two
   systems.  RADIUS/DTLS clients SHOULD also use the application-layer
   watchdog algorithm defined in [RFC3539] to determine server
   responsiveness.  The Status-Server packet defined in [RFC5997] SHOULD
   be used as the "watchdog packet" in any application-layer watchdog
   algorithm.

   RADIUS/DTLS clients SHOULD proactively close sessions when they have
   been idle for a period of time.  Clients SHOULD close a session when
   the DTLS Heartbeat algorithm indicates that the session is no longer
   active.  Clients SHOULD close a session when no traffic other than
   watchdog packets and (possibly) watchdog responses has been sent for
   three watchdog timeouts.  This behavior ensures that clients do not
   waste resources on the server by causing it to track idle sessions.

Top      ToC       Page 16 
   When a client fails to implement both DTLS Heartbeats and watchdog
   packets, it has no way of knowing that a DTLS session has been
   closed.  Therefore, there is the possibility that the server closes
   the session without the client knowing.  When that happens, the
   client may later transmit packets in a session, and those packets
   will be ignored by the server.  The client is then forced to time out
   those packets and then the session, leading to delays and network
   instabilities.

   For these reasons, it is RECOMMENDED that all DTLS sessions be
   configured to use DTLS Heartbeats and/or watchdog packets.

   DTLS sessions MUST also be deleted when a RADIUS packet fails
   validation due to a packet being malformed, or when it has an invalid
   Message-Authenticator or invalid Response Authenticator.  There are
   other cases when the specifications require that a packet received
   via a DTLS session be "silently discarded".  In those cases,
   implementations MAY delete the underlying DTLS session.

   RADIUS/DTLS clients should not send both RADIUS/UDP and RADIUS/DTLS
   packets to different servers from the same source socket.  This
   practice causes increased complexity in the client application and
   increases the potential for security breaches due to implementation
   issues.

   RADIUS/DTLS clients SHOULD implement session resumption, preferably
   stateless session resumption as given in [RFC5077].  This practice
   lowers the time and effort required to start a DTLS session with a
   server and increases network responsiveness.

6.  Implementation Guidelines

   The text above describes the protocol.  In this section, we give
   additional implementation guidelines.  These guidelines are not part
   of the protocol, but they may help implementors create simple,
   secure, and interoperable implementations.

   Where a TLS-PSK method is used, implementations MUST support keys of
   at least 16 octets in length.  Implementations SHOULD support key
   lengths of 32 octets and SHOULD allow for longer keys.  The key data
   MUST be capable of being any value (0 through 255, inclusive).
   Implementations MUST NOT limit themselves to using textual keys.  It
   is RECOMMENDED that the administration interface allow for the keys
   to be entered as human-readable strings in hex format.

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   When creating keys for use with PSK ciphersuites, it is RECOMMENDED
   that keys be derived from a Cryptographically Secure Pseudorandom
   Number Generator (CSPRNG) instead of administrators inventing keys on
   their own.  If managing keys is too complicated, a certificate-based
   TLS method SHOULD be used instead.

6.1.  Client Implementations

   RADIUS/DTLS clients should use connected sockets where possible.  Use
   of connected sockets means that the underlying kernel tracks the
   sessions, so that the client subsystem does not need to manage
   multiple sessions on one socket.

   RADIUS/DTLS clients should use a single source (IP + port) when
   sending packets to a particular RADIUS/DTLS server.  Doing so
   minimizes the number of DTLS session setups.  It also ensures that
   information about the home server state is discovered only once.

   In practice, this means that RADIUS/DTLS clients with multiple
   internal RADIUS sources should use a local proxy that arbitrates all
   RADIUS traffic between the client and all servers.  The proxy should
   accept traffic only from the authorized subsystems on the client
   machine and should proxy that traffic to known servers.  Each
   authorized subsystem should include an attribute that uniquely
   identifies that subsystem to the proxy, so that the proxy can apply
   origin-specific proxy rules and security policies.  We suggest using
   NAS-Identifier for this purpose.

   The local proxy should be able to interact with multiple servers at
   the same time.  There is no requirement that each server have its own
   unique proxy on the client, as that would be inefficient.

   The suggestion to use a local proxy means that there is only one
   process that discovers network and/or connectivity issues with a
   server.  If each client subsystem communicated directly with a
   server, issues with that server would have to be discovered
   independently by each subsystem.  The side effect would be increased
   delays in re-routing traffic, error reporting, and network
   instabilities.

   Each client subsystem can include a subsystem-specific NAS-Identifier
   in each request.  The format of this attribute is implementation-
   specific.  The proxy should verify that the request originated from
   the local system, ideally via a loopback address.  The proxy MUST
   then rewrite any subsystem-specific NAS-Identifier to a NAS-
   Identifier that identifies the client as a whole, or, remove the NAS-
   Identifier entirely and replace it with NAS-IP-Address or NAS-
   IPv6-Address.

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   In traditional RADIUS, the cost to set up a new "session" between a
   client and server was minimal.  The client subsystem could simply
   open a port, send a packet, wait for the response, and then close the
   port.  With RADIUS/DTLS, the connection setup is significantly more
   expensive.  In addition, there may be a requirement to use DTLS in
   order to communicate with a server, as RADIUS/UDP may not be
   supported by that server.  The knowledge of what protocol to use is
   best managed by a dedicated RADIUS subsystem, rather than by each
   individual subsystem on the client.

6.2.  Server Implementations

   RADIUS/DTLS servers should not use connected sockets to read DTLS
   packets from a client.  This recommendation exists because a
   connected UDP socket will accept packets only from one source IP
   address and port.  This limitation would prevent the server from
   accepting packets from multiple clients on the same port.

7.  Diameter Considerations

   This specification defines a transport layer for RADIUS.  It makes no
   other changes to the RADIUS protocol.  As a result, there are no
   Diameter considerations.

8.  IANA Considerations

   No new RADIUS attributes or packet codes are defined.  IANA has
   updated the "Service Name and Transport Protocol Port Number
   Registry".  The entries corresponding to port service name "radsec",
   port number "2083", and transport protocol "UDP" have been updated as
   follows:

      o  Assignee: IESG

      o  Contact: IETF Chair

      o  Reference: This document

      o  Assignment Notes: The UDP port 2083 was already previously
         assigned by IANA for "RadSec", an early implementation of
         RADIUS/TLS, prior to issuance of this RFC.

9.  Implementation Status

   This section records the status of known implementations of
   RADIUS/DTLS at the time of writing, and is based on a proposal
   described in [RFC6982].

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   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing Internet-
   Drafts to RFCs.

9.1.  Radsecproxy

   Organization: Radsecproxy

   URL:       https://software.uninett.no/radsecproxy/

   Maturity:  Widely used software based on early versions of this
              document.
              The use of the DTLS functionality is not clear.

   Coverage:  The bulk of this specification is implemented, based on
              earlier versions of this document.  Exact revisions that
              were implemented are unknown.

   Licensing: Freely distributable with acknowledgment.

   Implementation experience: No comments from implementors.

9.2.  jradius

   Organization: Coova

   URL:       http://www.coova.org/JRadius/RadSec

   Maturity:  Production software based on early versions of this
              document.
              The use of the DTLS functionality is not clear.

   Coverage:  The bulk of this specification is implemented, based on
              earlier versions of this document.  Exact revisions that
              were implemented are unknown.

   Licensing: Freely distributable with requirement to redistribute
              source.

   Implementation experience: No comments from implementors.

10.  Security Considerations

   The bulk of this specification is devoted to discussing security
   considerations related to RADIUS.  However, we discuss a few
   additional issues here.

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   This specification relies on the existing DTLS, RADIUS/UDP, and
   RADIUS/TLS specifications.  As a result, all security considerations
   for DTLS apply to the DTLS portion of RADIUS/DTLS.  Similarly, the
   TLS and RADIUS security issues discussed in [RFC6614] also apply to
   this specification.  Most of the security considerations for RADIUS
   apply to the RADIUS portion of the specification.

   However, many security considerations raised in the RADIUS documents
   are related to RADIUS encryption and authorization.  Those issues are
   largely mitigated when DTLS is used as a transport method.  The
   issues that are not mitigated by this specification are related to
   the RADIUS packet format and handling, which is unchanged in this
   specification.

   This specification also suggests that implementations use a session
   tracking table.  This table is an extension of the duplicate
   detection cache mandated in [RFC5080], Section 2.2.2.  The changes
   given here are that DTLS-specific information is tracked for each
   table entry.  Section 5.1.1, above, describes steps to mitigate any
   DoS issues that result from tracking additional information.

   The fixed shared secret given above in Section 2.2.1 is acceptable
   only when DTLS is used with a non-null encryption method.  When a
   DTLS session uses a null encryption method due to misconfiguration or
   implementation error, all of the RADIUS traffic will be readable by
   an observer.  Therefore, implementations MUST NOT use null encryption
   methods for RADIUS/DTLS.

   For systems that perform protocol-based firewalling and/or filtering,
   it is RECOMMENDED that they be configured to permit only DTLS over
   the RADIUS/DTLS port.

10.1.  Crypto-Agility

   Section 4.2 of [RFC6421] makes a number of recommendations about
   security properties of new RADIUS proposals.  All of those
   recommendations are satisfied by using DTLS as the transport layer.

   Section 4.3 of [RFC6421] makes a number of recommendations about
   backwards compatibility with RADIUS.  Section 3, above, addresses
   these concerns in detail.

   Section 4.4 of [RFC6421] recommends that change control be ceded to
   the IETF, and that interoperability is possible.  Both requirements
   are satisfied.

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   Section 4.5 of [RFC6421] requires that the new security methods apply
   to all packet types.  This requirement is satisfied by allowing DTLS
   to be used for all RADIUS traffic.  In addition, Section 3, above,
   addresses concerns about documenting the transition from legacy
   RADIUS to crypto-agile RADIUS.

   Section 4.6 of [RFC6421] requires automated key management.  This
   requirement is satisfied by using DTLS key management.

10.2.  Legacy RADIUS Security

   We reiterate here the poor security of the legacy RADIUS protocol.
   We suggest that RADIUS clients and servers implement either this
   specification or [RFC6614].  New attacks on MD5 have appeared over
   the past few years, and there is a distinct possibility that MD5 may
   be completely broken in the near future.  Such a break would mean
   that RADIUS/UDP was completely insecure.

   The existence of fast and cheap attacks on MD5 could result in a loss
   of all network security that depends on RADIUS.  Attackers could
   obtain user passwords and possibly gain complete network access.  We
   cannot overstate the disastrous consequences of a successful attack
   on RADIUS.

   We also caution implementors (especially client implementors) about
   using RADIUS/DTLS.  It may be tempting to use the shared secret as
   the basis for a TLS-PSK method and to leave the user interface
   otherwise unchanged.  This practice MUST NOT be used.  The
   administrator MUST be given the option to use DTLS.  Any shared
   secret used for RADIUS/UDP MUST NOT be used for DTLS.  Reusing a
   shared secret between RADIUS/UDP and RADIUS/DTLS would negate all of
   the benefits found by using DTLS.

   RADIUS/DTLS client implementors MUST expose a configuration that
   allows the administrator to choose the ciphersuite.  Where
   certificates are used, RADIUS/DTLS client implementors MUST expose a
   configuration that allows an administrator to configure all
   certificates necessary for certificate-based authentication.  These
   certificates include client, server, and root certificates.

   TLS-PSK methods are susceptible to dictionary attacks.  Section 6,
   above, recommends deriving TLS-PSK keys from a Cryptographically
   Secure Pseudorandom Number Generator (CSPRNG), which makes dictionary
   attacks significantly more difficult.  Servers SHOULD track failed
   client connections by TLS-PSK ID and block TLS-PSK IDs that seem to
   be attempting brute-force searches of the keyspace.

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   The historic RADIUS practice of using shared secrets (here, PSKs)
   that are minor variations of words is NOT RECOMMENDED, as it would
   negate all of the security of DTLS.

10.3.  Resource Exhaustion

   The use of DTLS allows DoS attacks and resource-exhaustion attacks
   that were not possible in RADIUS/UDP.  These attacks are similar to
   those described in [RFC6614], Section 6, for TCP.

   Session tracking, as described in Section 5.1, can result in resource
   exhaustion.  Therefore, servers MUST limit the absolute number of
   sessions that they track.  When the total number of sessions tracked
   is going to exceed the configured limit, servers MAY free up
   resources by closing the session that has been idle for the longest
   time.  Doing so may free up idle resources that then allow the server
   to accept a new session.

   Servers MUST limit the number of partially open DTLS sessions.  These
   limits SHOULD be exposed to the administrator as configurable
   settings.

10.4.  Client-Server Authentication with DTLS

   We expect that the initial deployment of DTLS will follow the
   RADIUS/UDP model of statically configured client-server
   relationships.  The specification for dynamic discovery of RADIUS
   servers is under development, so we will not address that here.

   Static configuration of client-server relationships for RADIUS/UDP
   means that a client has a fixed IP address for a server and a shared
   secret used to authenticate traffic sent to that address.  The server
   in turn has a fixed IP address for a client and a shared secret used
   to authenticate traffic from that address.  This model needs to be
   extended for RADIUS/DTLS.

   Instead of a shared secret, TLS credentials MUST be used by each
   party to authenticate the other.  The issue of identity is more
   problematic.  As with RADIUS/UDP, IP addresses may be used as a key
   to determine the authentication credentials that a client will
   present to a server or which credentials a server will accept from a
   client.  This is the fixed IP address model of RADIUS/UDP, with the
   shared secret replaced by TLS credentials.

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   There are, however, additional considerations with RADIUS/DTLS.  When
   a client is configured with a hostname for a server, the server may
   present to the client a certificate containing a hostname.  The
   client MUST then verify that the hostnames match.  Any mismatch is a
   security violation, and the connection MUST be closed.

   A RADIUS/DTLS server MAY be configured with a "wildcard" IP address
   match for clients, instead of a unique fixed IP address for each
   client.  In that case, clients MUST be individually configured with a
   unique certificate.  When the server receives a connection from a
   client, it MUST determine client identity from the client
   certificate, and MUST authenticate (or not) the client based on that
   certificate.  See [RFC6614], Section 2.4, for a discussion of how to
   match a certificate to a client identity.

   However, servers SHOULD use IP address filtering to minimize the
   possibility of attacks.  That is, they SHOULD permit clients only
   from a limited IP address range or ranges.  They SHOULD silently
   discard all traffic from outside of those ranges.

   Since the client-server relationship is static, the authentication
   credentials for that relationship must also be statically configured.
   That is, a client connecting to a DTLS server SHOULD be pre-
   configured with the server's credentials (e.g., PSK or certificate).
   If the server fails to present the correct credentials, the DTLS
   session MUST be closed.  Each server SHOULD be pre-configured with
   sufficient information to authenticate connecting clients.

   The requirement for clients to be individually configured with a
   unique certificate can be met by using a private CA for certificates
   used in RADIUS/DTLS environments.  If a client were configured to use
   a public CA, then it could accept as valid any server that has a
   certificate signed by that CA.  While the traffic would be secure
   from third-party observers, the server would, however, have
   unrestricted access to all of the RADIUS traffic, including all user
   credentials and passwords.

   Therefore, clients SHOULD NOT be pre-configured with a list of known
   public CAs by the vendor or manufacturer.  Instead, the clients
   SHOULD start off with an empty CA list.  The addition of a CA SHOULD
   be done only when manually configured by an administrator.

Top      ToC       Page 24 
   This scenario is the opposite of web browsers, where they are pre-
   configured with many known CAs.  The goal there is security from
   third-party observers, but also the ability to communicate with any
   unknown site that presents a signed certificate.  In contrast, the
   goal of RADIUS/DTLS is both security from third-party observers and
   the ability to communicate with only a small set of well-known
   servers.

   This requirement does not prevent clients from using hostnames
   instead of IP addresses for locating a particular server.  Instead,
   it means that the credentials for that server should be pre-
   configured on the client, and associated with that hostname.  This
   requirement does suggest that in the absence of a specification for
   dynamic discovery, clients SHOULD use only those servers that have
   been manually configured by an administrator.

10.5.  Network Address Translation

   Network Address Translation (NAT) is fundamentally incompatible with
   RADIUS/UDP.  RADIUS/UDP uses the source IP address to determine the
   shared secret for the client, and NAT hides many clients behind one
   source IP address.  As a result, RADIUS/UDP clients cannot be located
   behind a NAT gateway.

   In addition, port reuse on a NAT gateway means that packets from
   different clients may appear to come from the same source port on the
   NAT.  That is, a RADIUS server may receive a RADIUS/DTLS packet from
   one source IP/port combination, followed by the reception of a
   RADIUS/UDP packet from that same source IP/port combination.  If this
   behavior is allowed, then the server would have an inconsistent view
   of the client's security profile, allowing an attacker to choose the
   most insecure method.

   If more than one client is located behind a NAT gateway, then every
   client behind the NAT MUST use a secure transport such as TLS or
   DTLS.  As discussed below, a method for uniquely identifying each
   client MUST be used.

10.6.  Wildcard Clients

   Some RADIUS server implementations allow for "wildcard" clients --
   that is, clients with an IPv4 netmask of other than 32 or an IPv6
   netmask of other than 128.  That practice is not recommended for
   RADIUS/UDP, as it means multiple clients will use the same shared
   secret.

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   The use of RADIUS/DTLS can allow for the safe usage of wildcards.
   When RADIUS/DTLS is used with wildcards, clients MUST be uniquely
   identified using TLS parameters, and any certificate or PSK used MUST
   be unique to each client.

10.7.  Session Closing

   Section 5.1.1, above, requires that DTLS sessions be closed when the
   transported RADIUS packets are malformed or fail the authenticator
   checks.  The reason is that the session is expected to be used for
   transport of RADIUS packets only.

   Any non-RADIUS traffic on that session means the other party is
   misbehaving and is a potential security risk.  Similarly, any RADIUS
   traffic failing authentication vector or Message-Authenticator
   validation means that two parties do not have a common shared secret,
   and the session is therefore unauthenticated and insecure.

   We wish to avoid the situation where a third party can send well-
   formed RADIUS packets that cause a DTLS session to close.  Therefore,
   in other situations, the session SHOULD remain open in the face of
   non-conformant packets.

10.8.  Client Subsystems

   Many traditional clients treat RADIUS as subsystem-specific.  That
   is, each subsystem on the client has its own RADIUS implementation
   and configuration.  These independent implementations work for simple
   systems, but break down for RADIUS when multiple servers, fail-over,
   and load-balancing are required.  They have even worse issues when
   DTLS is enabled.

   As noted in Section 6.1, above, clients SHOULD use a local proxy that
   arbitrates all RADIUS traffic between the client and all servers.
   This proxy will encapsulate all knowledge about servers, including
   security policies, fail-over, and load-balancing.  All client
   subsystems SHOULD communicate with this local proxy, ideally over a
   loopback address.  The requirements on using strong shared secrets
   still apply.

   The benefit of this configuration is that there is one place in the
   client that arbitrates all RADIUS traffic.  Subsystems that do not
   implement DTLS can remain unaware of DTLS.  DTLS sessions opened by
   the proxy can remain open for long periods of time, even when client
   subsystems are restarted.  The proxy can do RADIUS/UDP to some
   servers and RADIUS/DTLS to others.

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   Delegation of responsibilities and separation of tasks are important
   security principles.  By moving all RADIUS/DTLS knowledge to a DTLS-
   aware proxy, security analysis becomes simpler, and enforcement of
   correct security becomes easier.

11.  References

11.1.  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 2000.

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

   [RFC5077]   Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
               "Transport Layer Security (TLS) Session Resumption
               without Server-Side State", RFC 5077, January 2008.

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

   [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5997]   DeKok, A., "Use of Status-Server Packets in the Remote
               Authentication Dial In User Service (RADIUS) Protocol",
               RFC 5997, August 2010.

   [RFC6347]   Rescorla, E. and N. Modadugu, "Datagram Transport Layer
               Security Version 1.2", RFC 6347, January 2012.

   [RFC6520]   Seggelmann, R., Tuexen, M., and M. Williams, "Transport
               Layer Security (TLS) and Datagram Transport Layer
               Security (DTLS) Heartbeat Extension", RFC 6520, February
               2012.

   [RFC6613]   DeKok, A., "RADIUS over TCP", RFC 6613, May 2012.

   [RFC6614]   Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
               "Transport Layer Security (TLS) Encryption for RADIUS",
               RFC 6614, May 2012.

Top      ToC       Page 27 
11.2.  Informative References

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

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

   [RFC4107]   Bellovin, S. and R. Housley, "Guidelines for
               Cryptographic Key Management", BCP 107, RFC 4107, June
               2005.

   [RFC5176]   Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
               Aboba, "Dynamic Authorization Extensions to Remote
               Authentication Dial In User Service (RADIUS)", RFC 5176,
               January 2008.

   [RFC6421]   Nelson, D., Ed., "Crypto-Agility Requirements for Remote
               Authentication Dial-In User Service (RADIUS)", RFC 6421,
               November 2011.

   [RFC6982]   Sheffer, Y. and A. Farrel, "Improving Awareness of
               Running Code: The Implementation Status Section", RFC
               6982, July 2013.

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

   [MD5Break]  Wang, X. and H. Yu, "How to Break MD5 and Other Hash
               Functions", EUROCRYPT '05 Proceedings of the 24th annual
               international conference on Theory and Applications of
               Cryptographic Techniques, pp. 19-35, ISBN 3-540-25910-4,
               2005.

Acknowledgments

   Parts of the text in Section 3 defining the Request and Response
   Authenticators were taken with minor edits from [RFC2865], Section 3.

Author's Address

   Alan DeKok
   The FreeRADIUS Server Project
   URI: http://freeradius.org
   EMail: aland@freeradius.org