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TACACS+ TLS 1.3
draft-ietf-opsawg-tacacs-tls13-02

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Thorsten Dahm , dcmgash@cisco.com , Andrej Ota , John Heasley
Last updated 2023-03-13
Replaces draft-dahm-tacacs-tls13
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draft-ietf-opsawg-tacacs-tls13-02
Operations and Management Area Working Group                     T. Dahm
Internet-Draft                                                          
Updates: RFC8907 (if approved)                                   D. Gash
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: 14 September 2023                                        A. Ota
                                                                        
                                                              J. Heasley
                                                                     NTT
                                                           13 March 2023

                            TACACS+ TLS 1.3
                   draft-ietf-opsawg-tacacs-tls13-02

Abstract

   The TACACS+ Protocol [RFC8907] provides device administration for
   routers, network access servers and other networked computing devices
   via one or more centralized servers.  This document, a companion to
   the TACACS+ protocol [RFC8907], adds Transport Layer Security
   (currently defined by TLS 1.3 [RFC8446]) support and obsoletes former
   inferior security mechanisms.

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
   [BCP14] 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."

   This Internet-Draft will expire on 14 September 2023.

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Copyright Notice

   Copyright (c) 2023 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.  Unsecure Connection . . . . . . . . . . . . . . . . . . .   3
     2.2.  Peer  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  TLS Connection  . . . . . . . . . . . . . . . . . . . . .   3
     2.4.  Obfuscation . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  TLS for TACACS+ . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Well-Known TCP/IP Port  . . . . . . . . . . . . . . . . .   4
     3.2.  TLS Connection  . . . . . . . . . . . . . . . . . . . . .   4
       3.2.1.  Cipher Requirements . . . . . . . . . . . . . . . . .   5
       3.2.2.  TLS Authentication  . . . . . . . . . . . . . . . . .   5
     3.3.  TLS Identification  . . . . . . . . . . . . . . . . . . .   6
   4.  Obsolescence of TACACS+ Obfuscation . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  TLS Use . . . . . . . . . . . . . . . . . . . . . . .   7
       5.1.2.  TLS 0-RTT . . . . . . . . . . . . . . . . . . . . . .   7
       5.1.3.  TLS PSK . . . . . . . . . . . . . . . . . . . . . . .   8
       5.1.4.  TLS Options . . . . . . . . . . . . . . . . . . . . .   8
       5.1.5.  Unreachable Certificate Authority (CA)  . . . . . . .   8
       5.1.6.  TLS Server Name Indicator (SNI) . . . . . . . . . . .   8
     5.2.  Well-Known TCP/IP Port  . . . . . . . . . . . . . . . . .   8
   6.  Operator Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   10. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   The 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 for TACACS+
   clients.

   While the content of the protocol is highly sensitive, TACACS+ lacks
   modern and/or effective confidentiality, integrity, and
   authentication of the connection and network traffic between the
   server and client.  The existing mechanisms of TACACS+ are extremely
   weak and the Security Considerations section of the TACACS+ Protocol
   [RFC8907] adequately describes this.

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

2.  Technical Definitions

   The Technical Definitions section of the TACACS+ Protocol [RFC8907]
   is fully applicable here and will not be repeated, though might be
   augmented.  The following terms are also used in this document.

2.1.  Unsecure Connection

   This is another term for a Connection as defined in TACACS+ Protocol
   [RFC8907].  It is a Connection without TLS and therefore being
   plaintext or possibly using unsecure TACACS+ authentication and
   obfuscation.

2.2.  Peer

   This refers to a TACACS+ Server or Client.

2.3.  TLS Connection

   A TLS Connection is a TCP/IP connection with TLS authentication and
   encryption used by TACACS+ for transport, similar to a Connection as
   defined in TACACS+ Protocol [RFC8907].

2.4.  Obfuscation

   Inferior form of encryption used in TACACS+, referred to as
   obfuscation in [RFC5425], Section 5.2 to indicate that it is not
   encryption and is utterly insufficient.

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3.  TLS for TACACS+

   TACACS+ connections are TCP/IP connections initiated by the Client to
   the Server.  The well-known TCP/IP port 49 on the Server is used for
   unobfuscated and obfuscated connections as defined in the TACACS+
   Protocol [RFC8907].  A connection might be used for only a single
   Session or the multiplexing of multiple Sessions in TACACS+ Single
   Connection Mode.

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

   1.  Confidentiality and Integrity: The MD5 obfuscation specified in
       the original protocol definition is not fit for purpose,
       requiring that TACACS+ be deployed over a secured network.
       Securing TACACS+ protocol with TLS is intended to provide
       confidentiality and integrity without requiring the provision of
       a secured network.

   2.  Peer authentication: The use of shared keys to add and remove the
       MD5 obfuscation was intended to provide a form of Peer
       authentication for the TACACS+ protocol.  This document obsoletes
       the MD5 obfuscation, and specifies that the authentication
       capabilities of TLS are used to allow the Peers to authenticate
       each other.

3.1.  Well-Known TCP/IP Port

   All data exchanged by TACACS+ Peers MUST be encrypted, including the
   authentication of the Peers.  Therefore, TLS Hello MUST be initiated
   by the client immediately upon the establishment of the TCP/IP
   connection.

   This document favors the predictable use of TLS security for a
   deployment, see (Section 5.2).  TACACS+ TLS will therefore follow
   [RFC7605], where a different well-known system TCP/IP port is
   assigned by IANA, port [TBD] (Section 7) with the service name [TBDN]
   (Section 7), for TLS connections.

   TACACS+ TLS could use any other TCP port by operator configuration,
   though Section 5.2 should still be considered.

3.2.  TLS Connection

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

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   Implementations MUST support TLS 1.3 [RFC8446] and MAY permit TLS 1.3
   session resumption.  If 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 normal, except that it is transmitted over TLS as TLS
   application data and without TACACS+ obfuscation (see Section 4)

   The connection persists until the Server or Client closes it.  It
   might be closed due to an error or at the conclusion of the TACACS+
   Session.  If Single Connection Mode has been negotiated, it might
   remain open after a successful Session, until an error or 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 Requirements

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

   This document makes no cipher suite recommendations, but
   recommendations can be found in the TLS Cipher Suites section of the
   [TLSCSREC].

3.2.2.  TLS Authentication

   Implementations MUST support certificate-based TLS authentication and
   certificate revocation bi-directionally for authentication, identity
   verification and policy purposes.  Certificate path verification as
   described in Section 3.2.2.1 MUST be supported.

   If this 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 disconnect a Peer that
   offers an invalid TLS Certificate.

3.2.2.1.  TLS Certificate Path Verification

   Implementations MUST support certificate Path verification as
   described in [RFC5280].

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   Because a Peer could be isolated from a remote Peer's Certificate
   Authority (CA), implementations MUST support certificate chains (aka.
   bundles or chains of trust), where the entire chain of the remote's
   certificate is stored on the local Peer.

3.3.  TLS Identification

   In addition to authentication of TLS certificates, implementations
   MUST support policy consideration of Peer-identifying certificate
   fields and policy used to verify that the Peer is a valid source for
   the received certificate and that it is permitted access to TACACS+.
   Implementations MUST support either:

   Network location based validation methods as described in [RFC5425],
   Section 5.2.

   or

   Device Identity based validation methods where the peer’s identity is
   used in the certificate subjectName.  This is applicable in
   deployments where the device securely supports an identity which is
   shared with its peer.  This approach allows a peer's network location
   to be reconfigured without issuing a new client certificate.  Only
   the local server mapping needs to be updated.

   Implementations SHOULD support the TLS Server Name Inidication
   extension ([RFC6066], Section 3).  Policy can be applied to this
   attribute and it can be useful for load balancing or multiplexing at
   the server.

4.  Obsolescence of TACACS+ Obfuscation

   The original draft of TACACS+ described the Obfuscation mechanism,
   documented in [RFC5425], Section 5.2.  It is insufficient for modern
   purposes.

   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.

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   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 the behavior defined in RFC8907 Section 4.5.
   Data Obfuscation [RFC8907] for TAC_PLUS_UNENCRYPTED_FLAG or key
   mismatches.

   A TACACS+ client that receives a packet with the
   TAC_PLUS_UNENCRYPTED_FLAG not set (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.  This does not in itself protect the
   server nor clients; the operator and equipment vendors have a role.
   That role is to diligently follow current best practices for
   maintaining the integrity of network devices and selection of TLS key
   and encryption algorithms.

5.1.1.  TLS Use

   TLS encryption SHOULD be used in deployments when both the Clients
   and Servers support it.  Servers that support TLS encryption MAY be
   configured to allow Unsecure Connections when TLS encryption is not
   supported by the Client, but this is NOT RECOMMENDED because of the
   threat of downgrade attacks, as described in Section 5.2.  Unsecure
   Connections would be better served by separate Servers from the TLS
   Servers.

   It is NOT RECOMMENDED to deploy TACACS+ without TLS authentication
   and encryption, including TLS using the NULL algorithm, except for
   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 possible.  Given the sensitivity
   of TACACS+ data, a Client MUST NOT send data until the full TLS
   handshake completes; that is, Clients MUST NOT send 0-RTT data and
   Servers MAY abruptly disconnect Clients that do.

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5.1.3.  TLS PSK

   Implementations MAY support TLS authentication with Pre-Shared Keys
   (PSKs), also known as external PSKs in TLS 1.3, which are not
   resumption PSKs.  PSKs SHOULD NOT be shared among Clients or Servers
   to limit exposure of a compromised key and to ease key rotation.
   Also see [RFC8773] and [I-D.ietf-tls-external-psk-guidance].

   PSKs are otherwise considered out-of-scope for this document.

5.1.4.  TLS Options

   Unfortunately, no single and timely TLS recommendations document
   exists.  Therefore, implementers and operators SHOULD make use of the
   various RFCs to determine which TLS versions and algorithms should be
   supported, deprecated, obsoleted, or abandoned, in the absence of
   updates to this document.  Useful examples are the TLS specifications
   themselves (TLS 1.3 [RFC8446]), which prescribes mandatory support in
   Section 9, and TLS Recommendations [RFC7525].

5.1.5.  Unreachable Certificate 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.
   Operators SHOULD consider loading certificate chains on devices and
   servers to avoid this failure.

   Certificate caching and Raw Public Keys [RFC7250] are other methods
   to help address this, but both are out of scope for this document.
   Certificate fingerprints are another option.

5.1.6.  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 [RFC6066], Section 11.1.

5.2.  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,

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   *  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.

   However, co-existence of inferior authentication and obfuscated,
   whether an Unsecure 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.

   Servers and Clients could maintain a cache of Peers that have engaged
   in TACACS+ TLS connections and demand TLS from that point forward.
   However, this too has potential to be a Denial of Service (DoS)
   vector, whereby an attacker causes a server to believe that a client
   that does not support TLS has successfully connected with TLS.

   The simplest way to address exposure from Unsecure Connection methods
   is to refuse Unsecure Connections at the server entirely, perhaps
   using separate servers for Unsecure 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.  Operator Considerations

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

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 5.2.

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

8.  Acknowledgments

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

9.  Normative References

   [BCP14]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

              Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, May 2017.

              <https://www.rfc-editor.org/bcp/bcp14.txt>

   [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>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7525, DOI 10.17487/RFC7525, May 2015,
              <https://www.rfc-editor.org/info/rfc7525>.

   [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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   [RFC8773]  Housley, R., "TLS 1.3 Extension for Certificate-Based
              Authentication with an External Pre-Shared Key", RFC 8773,
              DOI 10.17487/RFC8773, March 2020,
              <https://www.rfc-editor.org/info/rfc8773>.

   [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>.

10.  Informative References

   [I-D.ietf-tls-external-psk-guidance]
              Housley, R., Hoyland, J., Sethi, M., and C. A. Wood,
              "Guidance for External Pre-Shared Key (PSK) Usage in TLS",
              Work in Progress, Internet-Draft, draft-ietf-tls-external-
              psk-guidance-06, 4 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              external-psk-guidance-06>.

   [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>.

   [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>.

   [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>.

   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <https://www.rfc-editor.org/info/rfc7605>.

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   [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

   Douglas Gash
   Cisco Systems, Inc.
   Email: dcmgash@cisco.com

   Andrej Ota
   Email: andrej@ota.si

   John Heasley
   NTT
   Email: heas@shrubbery.net

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