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Terminal Access Controller Access-Control System Plus (TACACS+) over TLS 1.3
draft-ietf-opsawg-tacacs-tls13-10

Document Type Active Internet-Draft (opsawg WG)
Authors Thorsten Dahm , John Heasley , dcmgash@cisco.com , Andrej Ota
Last updated 2024-05-22
Replaces draft-dahm-tacacs-tls13
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draft-ietf-opsawg-tacacs-tls13-10
Operations and Management Area Working Group                     T. Dahm
Internet-Draft                                                          
Updates: 8907 (if approved)                                   J. Heasley
Intended status: Standards Track                                     NTT
Expires: 23 November 2024                               D.C. Medway Gash
                                                     Cisco Systems, Inc.
                                                                  A. Ota
                                                             Google Inc.
                                                             22 May 2024

Terminal Access Controller Access-Control System Plus (TACACS+) over TLS
                                  1.3
                   draft-ietf-opsawg-tacacs-tls13-10

Abstract

   The Terminal Access Controller Access-Control System Plus (TACACS+)
   Protocol provides device administration for routers, network access
   servers and other networked computing devices via one or more
   centralized servers.  This document adds Transport Layer Security
   (TLS 1.3) support to TACACS+ and obsoletes former inferior security
   mechanisms.

   This document updates RFC8907.

Requirements Language

   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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 23 November 2024.

Copyright Notice

   Copyright (c) 2024 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Technical Definitions . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Obfuscation . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Non-TLS Connection  . . . . . . . . . . . . . . . . . . .   3
     2.3.  TLS Connection  . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  TACACS+ Server  . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  Peer  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  TACACS+ over TLS  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Separating TLS Connections  . . . . . . . . . . . . . . .   4
     3.2.  TLS Connection  . . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Cipher Suites Requirements  . . . . . . . . . . . . .   5
       3.2.2.  TLS Authentication  . . . . . . . . . . . . . . . . .   6
     3.3.  TLS Identification  . . . . . . . . . . . . . . . . . . .   7
   4.  Obsolescence of TACACS+ Obfuscation . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     5.1.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       5.1.1.  TLS Use . . . . . . . . . . . . . . . . . . . . . . .   9
       5.1.2.  TLS 0-RTT . . . . . . . . . . . . . . . . . . . . . .   9
       5.1.3.  TLS Options . . . . . . . . . . . . . . . . . . . . .   9
       5.1.4.  Unreachable Certification Authority (CA)  . . . . . .   9
       5.1.5.  TLS Server Name Indicator (SNI) . . . . . . . . . . .  10
     5.2.  TACACS+ Configuration . . . . . . . . . . . . . . . . . .  10
     5.3.  Well-Known TCP/IP Port  . . . . . . . . . . . . . . . . .  10
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .  11
     6.1.  Migration . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Maintaining Non-TLS TACACS+ Clients . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  12
   10. Informative References  . . . . . . . . . . . . . . . . . . .  14

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The Terminal Access Controller Access-Control System Plus (TACACS+)
   Protocol [RFC8907] provides Device Administration for routers,
   network access servers and other networked computing devices via one
   or more centralized servers.  The protocol provides authentication,
   authorization and accounting services (AAA) for TACACS+ clients
   within the Device Administration use case.

   While the content of the protocol is highly sensitive, TACACS+ lacks
   effective confidentiality, integrity, and authentication of the
   connection and network traffic between the server and client,
   requiring secure transport to safeguard a deployment.  The existing
   TACACS+ mechanisms are extremely weak as described in Section 10 of
   [RFC8907].

   To address these deficiencies, this document updates the TACACS+
   protocol to use TLS 1.3 [RFC8446] authentication and encryption, and
   obsoletes the use of its former mechanisms (Section 10.5 of
   [RFC8907]).

2.  Technical Definitions

   The terms defined in Section 3 of [RFC8907] are fully applicable here
   and will not be repeated.  The following terms are also used in this
   document.

2.1.  Obfuscation

   TACACS+ was originally intended to incorporate a mechanism for
   securing the body of its packets.  The algorithm is categorized as
   Obfuscation in Section 10.5.2 of [RFC8907].  The term should be
   interpreted to mean "plaintext", it is used to ensure that the
   algorithm is not mistaken for encryption.

2.2.  Non-TLS Connection

   This term refers to the connection defined in [RFC8907].  It is a
   connection without TLS and therefore being plaintext or possibly
   using unsecure TACACS+ authentication and obfuscation.  The use of
   well-known TCP/IP server port number 49 is specified as the default
   for Non-TLS connections.

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2.3.  TLS Connection

   A TLS connection is a TCP/IP connection with TLS authentication and
   encryption used by TACACS+ for transport.  A TLS connection for
   TACACS+ is always between one TACACS+ client and one TACACS+ Server.

2.4.  TACACS+ Server

   A TACACS+ server is an instance of the server as defined in
   Section 3.2 of [RFC8907] that responds to TACACS+ traffic bound to a
   specific port number on a host.  This document distinguishes the
   TACACS+ server instance from the host itself.  A host may have
   multiple TACACS+ servers running, each listening on different port
   numbers.

2.5.  Peer

   The peer of a TACACS+ client (or server) in the context of a TACACS+
   connection, is a server (or client).  Together, the ends of a TACACS+
   connection are referred to as peers.

3.  TACACS+ over TLS

   TACACS+ over TLS takes the protocol defined in [RFC8907], removes the
   option for MD5 obfuscation, and specifies that TLS 1.3 be used for
   transport (Section 3.1 elaborates TLS version support).  A new well-
   known default server port number is used.  The next sections provide
   further details and guidance.

   TLS is introduced into TACACS+ to fulfill the following requirements:

   1.  Confidentiality and Integrity: The MD5 obfuscation specified in
       [RFC8907] has been shown to be insecure [RFC6151].  This prevents
       TACACS+ being used in a [FIPS-140-3] compliant deployment.
       Securing TACACS+ protocol with TLS is intended to provide
       confidentiality and integrity without requiring the provision of
       a secured network.

   2.  Peer authentication: The authentication capabilities of TLS
       replace the pre-shared keys of obfuscation for mutual
       authentication.

3.1.  Separating TLS Connections

   All data exchanged by TACACS+ peers MUST be encrypted, including the
   mutual authentication of the peers.  Therefore, when a TCP connection
   is established for the service, a TLS handshake begins immediately.

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   To ensure separation of TACACS+ traffic that uses TLS from that which
   does not (Section 5.3), they will be deployed on different ports.
   Due to the widespread use of default port number settings in numerous
   existing TACACS+ client configurations, a well-known system TCP/IP
   port number will be requested from IANA.  Client implementations can
   ensure traffic is kept separate.  The designated port number is [TBD]
   (Section 7) with the service name [TBDN] (Section 7).

   Under exceptional circumstances, this document permits any other TCP
   port number to be configured when required by deployment specifics,
   but the implications in Section 5.3 have to be considered by
   operators.

3.2.  TLS Connection

   A TACACS+ client initiates a TLS connection by making a TCP
   connection to a configured server on the TACACS+ TLS port number
   ([TBD]) (Section 3.1).  Once the TCP connection is established, the
   client MUST immediately begin the TLS negotiation before sending any
   TACACS+ protocol data.

   TLS 1.3 [RFC8446] must be used for transport, though it is expected
   that TACACS+ as described in this document will work with future
   versions of TLS.  Earlier versions of TLS MUST NOT be used.

   When resumption is supported, the resumption ticket_lifetime SHOULD
   be configurable, including a zero seconds lifetime.

   Once the TLS connection is established, the exchange of TACACS+ data
   proceeds as defined in [RFC8907], except that it is transmitted over
   TLS as TLS application data and without TACACS+ obfuscation
   (Section 4)

   The connection persists until the server or client closes it, either
   due to an error, or at the conclusion of the TACACS+ session, or, if
   Single Connection Mode (Section 4.3 of [RFC8907]) has been
   negotiated, when an inactivity timeout occurs.  Why it closed has no
   bearing on TLS resumption, unless closed by a TLS error, in which
   case the ticket might be invalidated.

3.2.1.  Cipher Suites Requirements

   Implementations MUST support the TLS 1.3 mandatory cipher suites
   (Section 9.1 of [RFC8446]).  The cipher suites offered or accepted
   SHOULD be configurable so that operators can adapt.

   This document makes no cipher suite recommendations.  Readers should
   refer to [BCP195] for guidance.

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3.2.2.  TLS Authentication

   Certificate based mutual authentication MUST be supported.  Each peer
   MUST validate the certificate path of the remote peer, including
   revocation checking, as described in Section 3.2.2.1.

   If the verification succeeds, the authentication is successful and
   the connection is permitted.  Policy may impose further constraints
   upon the peer, allowing or denying the connection based on
   certificate fields or any other parameters exposed by the
   implementation.

   Unless disabled by configuration, a peer MUST NOT permit connection
   of any peer that presents an invalid TLS Certificate.

   In addition to full certificate-based TLS authentication,
   implementations MAY support:

   *  Raw Public Keys (There is no definitive description of Raw Public
      Keys in TLS 1.3 at time of writing, so [RFC7250] must be used in
      context of [RFC8446].

   *  Pre-Shared Keys (PSKs) [RFC8446], also known as external PSKs in
      TLS 1.3, which are not resumption PSKs.

   The details of these mechanisms are considered out-of-scope for this
   document.  Please refer to the RFCs above for implementation,
   deployment, and security considerations.

3.2.2.1.  TLS Certificate Path Verification

   The implementation of certificate based mutual authentication MUST
   support certificate Path verification as described in Section 6 of
   [RFC5280].

   In some deployments, a peer could be isolated from a remote peer's
   Certification Authority (CA).  Implementations for these deployments
   MUST support certificate chains (a.k.a. bundles or chains of trust),
   where the entire chain of the remote's certificate is stored on the
   local peer.

   TLS Cached Information Extension [RFC7924] SHOULD be implemented.
   This MAY be augmented with Raw Public Keys [RFC7250], though
   revocation must be handled as it is not part of the standard.

   Other approaches may be used for loading the intermediate
   certificates onto the client, but MUST include support for revocation
   checking.  For example, [RFC5280] details the AIA (Authority

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   Information Access) extension to provide information about the issuer
   of the certificate in which the extension appears.  It can be used to
   provide the address of the Online Certificate Status Protocol (OCSP)
   responder from where revocation status of the certificate (which
   includes the extension) can be checked.

3.3.  TLS Identification

   For the client-side validation of presented server identities,
   implementations MUST follow [RFC9525] validation techniques.
   Identifier types DNS-ID, IP-ID or SRV-ID are applicable for use with
   the TLS TACACS+ protocol, selected by operators depending upon the
   deployment design.  TLS TACACS+ does not use URI-IDs for server
   identity verification.  The wildcard character MUST NOT be included
   in the presented server identities.

   For the server-side validation of client identities, Implementations
   must support the ability to configure which fields of a certificate
   are used for client identification, to verify that the client is a
   valid source for the received certificate and that it is permitted
   access to TACACS+. Implementations MUST support either:

   Network address based validation methods as described in Section 5.2
   of [RFC5425].

   or

   Client Identity validation of a shared identity in the certificate
   subjectAltName.  This is applicable in deployments where the client
   securely supports an identity which is shared with the server.  This
   approach allows a client's network location to be reconfigured
   without issuing a new client certificate, in this case, only the
   server mapping needs to be updated.

   Implementations MUST support the TLS Server Name Indication extension
   (SNI) (Section 3 of [RFC6066]), and MUST support the ability to
   configure the server's domain name, so that it may be included in the
   SNI "server_name" extension of the client hello.  Please see
   Section 5.1.5 for securitly related operator considerations.

   Certificate provisioning is out of scope of this document.

4.  Obsolescence of TACACS+ Obfuscation

   [RFC8907] describes the obfuscation mechanism, documented in
   Section 5.2 of [RFC5425].  Such a method is weak.

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   The introduction of TLS PSK, certificate peer authentication, and TLS
   encryption to TACACS+ replaces these former mechanisms and so
   obfuscation is hereby obsoleted.  This section describes how the
   TACACS+ client and servers MUST operate with regards to the
   obfuscation mechanism.

   Peers MUST NOT use obfuscation with TLS.

   A TACACS+ client initiating a TACACS+ TLS connection MUST set the
   TAC_PLUS_UNENCRYPTED_FLAG bit, thereby asserting that obfuscation is
   not used for the session.  All subsequent packets MUST have the
   TAC_PLUS_UNENCRYPTED_FLAG set.

   A TACACS+ server that receives a packet with the
   TAC_PLUS_UNENCRYPTED_FLAG not set (cleared) over a TLS connection,
   MUST return an error of TAC_PLUS_AUTHEN_STATUS_ERROR,
   TAC_PLUS_AUTHOR_STATUS_ERROR, or TAC_PLUS_ACCT_STATUS_ERROR as
   appropriate for the TACACS+ message type, with the
   TAC_PLUS_UNENCRYPTED_FLAG set, and terminate the session.  This
   behavior corresponds to that defined in Section 4.5 of [RFC8907] Data
   Obfuscation for TAC_PLUS_UNENCRYPTED_FLAG or key mismatches.

   A TACACS+ client that receives a packet with the
   TAC_PLUS_UNENCRYPTED_FLAG not set (i.e., cleared), MUST terminate the
   session, and SHOULD log this error.

5.  Security Considerations

5.1.  TLS

   This document improves the confidentiality, integrity, and
   authentication of the connection and network traffic between TACACS+
   peers by adding TLS support.

   Simply adding TLS support to the protocol does not guarantee the
   protection of the server and clients.  It is essential for the
   operators and equipment vendors to adhere to the latest best
   practices for ensuring the integrity of network devices and selecting
   secure TLS key and encryption algorithms.

   RFC9325 offers substantial guidance for implementing protocols that
   use TLS and their deployment.  Those implementing and deploying
   Secure TACACS+ must adhere to the recommendations relevant to TLS 1.3
   outlined in RFC9325, or its subsequent versions.

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   This document outlines additional restrictions permissible under
   RFC9325.  For example, any recommendations referring to TLS 1.2,
   including the mandatory support, are not relevant for Secure TACACS+
   as TLS 1.3 or above is mandated.

5.1.1.  TLS Use

   Non-TLS connections should not be used for new TACACS+ deployments.

   TACACS+ servers that have TLS support MUST NOT allow Non-TLS
   connections, because of the threat of downgrade attacks, as described
   in Section 5.2.  Instead, separate Non-TLS TACACS+ servers can be set
   up to cater for these clients.

   Further, TLS TACACS+ servers and Non-TLS TACACS+ servers SHOULD NOT
   be deployed on the same host.  Non-TLS connections would be better
   served by deploying the required Non-TLS TACACS+ servers on separate
   hosts.

   It is NOT RECOMMENDED to deploy TACACS+ without TLS authentication
   and encryption, unless within test and debug environments.  Also see
   [RFC3365].

5.1.2.  TLS 0-RTT

   TLS 1.3 resumption and PSK techniques make it possible to send Early
   Data, aka. 0-RTT data, data that is sent before the TLS handshake
   completes.  Replay of this data is a risk.  Given the sensitivity of
   TACACS+ data, clients MUST NOT send data until the full TLS handshake
   completes; that is, clients MUST NOT send 0-RTT data and servers MUST
   abruptly disconnect clients that do.

5.1.3.  TLS Options

   Recommendations in [BCP195] MUST be followed, in order to determine
   which TLS versions and algorithms should be supported, deprecated,
   obsoleted, or abandoned.

   Also, Section 9 of [RFC8446] prescribes mandatory supported options.

5.1.4.  Unreachable Certification Authority (CA)

   Operators should be cognizant of the potential of server and/or
   client isolation from their peer's CA by network failures.  Isolation
   from a public key certificate's CA will cause the verification of the
   certificate to fail and thus TLS authentication of the peer to fail.
   The approach mentioned in Section 3.2.2.1 can help address this, and
   should be considered where implemented.

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5.1.5.  TLS Server Name Indicator (SNI)

   Operators should be aware that the TLS SNI extension is part of the
   TLS client hello, and is therefore subject to eavesdropping.  Also
   see Section 11.1 of [RFC6066].

5.2.  TACACS+ Configuration

   Implementors must ensure that the configuration scheme introduced for
   enabling TLS is straightforward and leaves no room for ambiguity
   regarding whether TLS or Non-TLS will be used between the TACACS+
   client and the TACACS+ server.

   This document recommends the use of a separate port number that TLS
   enabled TACACS+ servers will listen to.  Where deployments have not
   overridden the defaults explicitly, TACACS+ client implementations
   MUST use the correct values:

   *  for Non-TLS connection TACACS+: Port 49.

   *  for TLS connection TACACS+: (TBD).

   Implementors may offer a single option for TACACS+ clients and
   servers to disable all Non-TLS TACACS+ operations.  When enabled on a
   TACACS+ server, it will not respond to any requests from Non-TLS
   TACACS+ client connections.  When enabled on a TACACS+ client, it
   will not establish any Non-TLS TACACS+ server connections.

5.3.  Well-Known TCP/IP Port

   A new port is considered appropriate and superior to a "STARTTLS"
   command or other negotiation method because it allows:

   *  ease of blocking the unobfuscated or obfuscated connections by the
      TCP/IP port number,

   *  passive Intrusion Detection Systems (IDSs) monitoring the
      unobfuscated to be unaffected by the introduction of TLS,

   *  avoidance of Man in the Middle (MitM) attacks that can interfere
      with STARTTLS,

   *  and helps prevent the accidental exposure of sensitive information
      due to misconfiguration.

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   However, co-existence of inferior authentication and obfuscated,
   whether an Non-TLS connection or deprecated parts that compose TLS,
   also presents opportunity for down-grade attacks.  Causing failure of
   connections to the TLS-enabled service or the negotiation of shared
   algorithm support are two such down-grade attacks.

   The simplest way to address exposure from Non-TLS connection methods
   is to refuse Non-TLS connections at the server entirely, perhaps
   using separate servers for Non-TLS connections and TLS.

   Another approach is mutual configuration that requires TLS.  Clients
   and servers SHOULD support configuration that requires peers,
   globally and individually, use TLS.  Furthermore, peers SHOULD be
   configurable to limit offered or recognized TLS versions and
   algorithms to those recommended by standards bodies and implementers.

6.  Operational Considerations

   Operational and deployment considerations are spread throughout the
   document.  While avoiding repetition, it is useful for the impatient
   to direct particular attention to Sections 5.2 and 5.1.5.  However,
   it is important that the entire Section 5 is observed.

6.1.  Migration

   In Section 5.2, it is mentioned that for an optimal deployment of TLS
   TACACS+, TLS should be universally applied throughout the deployment.
   However, during the migration process from a Non-TLS TACACS+
   deployment, operators may need to support both TLS and Non-TLS
   TACACS+ servers.  This migration phase allows operators to gradually
   transition their deployments from an insecure state to a more secure
   one, but it is important to note that it is vulnerable to downgrade
   attacks.  Therefore, the migration phase should be considered
   insecure until it is fully completed.  To mitigate this hazard:

   *  the period where any client is configured with both TLS and Non-
      TLS servers should be minimized.

   *  the operator must consider the impact of mixed TLS and Non-TLS on
      security.

6.2.  Maintaining Non-TLS TACACS+ Clients

   Some TACACS+ client devices in a deployment may not implement TLS.
   These devices will require access to Non-TLS TACACS+ servers.
   Operators must follow the recommendation of Section 5.1.1 and deploy
   separate TACACS+ servers for these Non-TLS clients from those used
   for the TLS clients.

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7.  IANA Considerations

   The authors request that, when this draft is accepted by the working
   group, the OPSAWG Chairs submit a request to IANA for an early
   allocation, per [RFC4020] and [RFC6335], of a new well-known system
   TCP/IP port number for the service name "tacacss" (referenced in this
   document also as "TACACS+ TLS well-known port ([TBD])"), described as
   "TACACS+ over TLS".  The service name "tacacss" follows the common
   practice of appending an "s" to the name given to the Non-TLS well-
   known port name.  This allocation is justified in Section 5.3.

   This document requests IANA to add a new entity from the "Service
   name and Transport Protocol Port Number Registry" available at
   https://www.iana.org/assignments/service-name-port-numbers/

   Service Name: tacacss

   Port Number: [TBD]

   Transport Protocol: TCP

   Description: TLS Secure Login Host Protocol (TACACSS)

   Assignee: IESG

   Contact: IETF Chair

   Reference: [TBDN] (This Document)

   RFC EDITOR: this port number should replace "[TBD]" and the service
   name should replace "[TBDN]" within this document.

   Considerations about service discovery are out of scope of this
   document.

8.  Acknowledgments

   The author(s) would like to thank Russ Housley, Steven M.  Bellovin,
   Stephen Farrell, Alan DeKok, Warren Kumari, Tom Petch, Tirumal Reddy,
   Valery Smyslov, and Mohamed Boucadair for their support, insightful
   review, and/or comments.  [RFC5425] was also used as a basis for the
   approach to TLS.

9.  Normative References

   [BCP195]   Best Current Practice 195,
              <https://www.rfc-editor.org/info/bcp195>.
              At the time of writing, this BCP comprises the following:

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              Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5425]  Miao, F., Ed., Ma, Y., Ed., and J. Salowey, Ed.,
              "Transport Layer Security (TLS) Transport Mapping for
              Syslog", RFC 5425, DOI 10.17487/RFC5425, March 2009,
              <https://www.rfc-editor.org/info/rfc5425>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <https://www.rfc-editor.org/info/rfc7924>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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   [RFC8907]  Dahm, T., Ota, A., Medway Gash, D.C., Carrel, D., and L.
              Grant, "The Terminal Access Controller Access-Control
              System Plus (TACACS+) Protocol", RFC 8907,
              DOI 10.17487/RFC8907, September 2020,
              <https://www.rfc-editor.org/info/rfc8907>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/info/rfc9525>.

10.  Informative References

   [FIPS-140-3]
              National Institute of Standards and Technology, U.S.
              Department of Commerce, "NIST Federal Information
              Processing Standards (FIPS) Publication 140-3",
              <https://csrc.nist.gov/pubs/fips/140-3/final>.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, DOI 10.17487/RFC3365, August 2002,
              <https://www.rfc-editor.org/info/rfc3365>.

   [RFC4020]  Kompella, K. and A. Zinin, "Early IANA Allocation of
              Standards Track Code Points", RFC 4020,
              DOI 10.17487/RFC4020, February 2005,
              <https://www.rfc-editor.org/info/rfc4020>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [TLSCSREC] IANA, "Transport Layer Security (TLS) Parameters",
              <https://www.iana.org/assignments/tls-parameters/tls-
              parameters.xhtml#tls-parameters-4>.

Authors' Addresses

   Thorsten Dahm
   Email: thorsten.dahm@gmail.com

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   John Heasley
   NTT
   Email: heas@shrubbery.net

   Douglas C. Medway Gash
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA 95134
   United States of America
   Email: dcmgash@cisco.com

   Andrej Ota
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America
   Email: andrej@ota.si

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