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Versions: (draft-reddy-opswg-mud-tls) 00 01 02

OPSWG WG                                                        T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 D. Wing
Expires: March 6, 2020                                            Citrix
                                                       September 3, 2019

                  MUD (D)TLS profiles for IoT devices


   This memo extends Manufacturer Usage Description (MUD) to model DTLS
   and TLS usage.  This allows a network element to notice abnormal DTLS
   or TLS usage which has been strong indicator of other software
   running on the endpoint, typically malware.

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 March 6, 2020.

Copyright Notice

   Copyright (c) 2019 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 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of MUD (D)TLS profiles for IoT devices . . . . . . .   4
   4.  (D)TLS 1.3 handshake  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Full (D)TLS 1.3 handshake inspection  . . . . . . . . . .   5
     4.2.  Encrypted SNI . . . . . . . . . . . . . . . . . . . . . .   6
   5.  (D)TLS profile YANG module  . . . . . . . . . . . . . . . . .   7
     5.1.  Tree Structure  . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  MUD File Example  . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Encryption is necessary to protect the privacy of end users using IoT
   devices.  In a network setting, TLS [RFC8446] and DTLS
   [I-D.ietf-tls-dtls13] are the dominant protocols to provide
   encryption for IoT device traffic.  Unfortunately in conjunction with
   IoT applications rise of encryption, malware is also using encryption
   which thwarts network-based analysis such as deep packet inspection
   (DPI).  Other mechanisms are needed to notice malware is running on
   the IoT device.

   Malware frequently uses its own libraries for its activities, and
   those libraries are re-used much like any other software engineering
   project.  Research [malware] indicates there are observable
   differences in how malware uses encryption compared with non-malware
   uses encryption.  There are several interesting findings specific to
   DTLS and TLS which were found common to malware:

   o  Older and weaker cryptographic parameters (e.g.,

   o  TLS SNI and server certificates are composed of subjects with
      characteristics of a domain generation algorithm (DGA) (e.g.,

   o  Higher use of self-signed certificates compared with typical
      legitimate software.

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   o  Discrepancies in the server name indication (SNI) TLS extension in
      the ClientHello message and the DNS names in the
      SubjectAltName(SAN) X.509 extension in the server certificate

   o  Discrepancies in the key exchange algorithm and the client public
      key length in comparison with legitimate flows.  As a reminder,
      Client Key Exchange message has been removed from TLS 1.3.

   o  Lower diversity in TLS client advertised TLS extensions compared
      to legitimate clients.

   If observable (D)TLS profile parameters are used, the following
   functions are possible which have a the favorable impact on network

   o  Permit intended DTLS or TLS use and to block malicious DTLS or TLS
      use.  This is superior to the layer 3 and layer 4 ACLs of
      Manufacturer Usage Description Specification (MUD) [RFC8520] which
      are not suitable for broad communication patterns.

   o  Ensure TLS certificates are valid.  Several TLS deployments have
      been vulnerable to active Man-In-The-Middle (MITM) attacks because
      of lack of certificate validation.  By observing (D)TLS profile
      parameters, a network element can detect when the TLS SNI
      mismatches the SubjectAltName and detect when the server's
      certificate is invalid, and alert those situations.  In TLS 1.2,
      the ClientHello, ServerHello and Certificate messages are all sent
      in clear-text, however in TLS 1.3, the Certificate message is
      encrypted thereby hiding the server identity from any
      intermediary.  In TLS 1.3, the middle-box needs to act as a TLS
      proxy to validate the server certificate and to detect TLS SNI
      mismatch with the server certificate.

   o  Support new communication patterns.  IoT device can learn a new
      capability, and the new capability changes the way the IoT device
      communicates with other devices located in the local network and
      Internet.  In other words, if IP addresses and domain names the
      IoT device connects to rapidly changes based on a new capability
      and MUD rules using ACLs cannot be rapidly updated.  In such a
      case, observable (D)TLS profile parameters can be used to permit
      intended use and to block malicious behaviour of IoT device.

   This document extends MUD [RFC8520] to model observable (D)TLS
   profile parameters.  Using these (D)TLS profile parameters, an active
   MUD-enforcing firewall can identify MUD non-compliant DTLS and TLS
   behavior that can indicate malware is running on the IoT device.
   This detection can prevent malware download, block access to

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   malicious domains, enforce use of strong ciphers, stop data
   exfiltration, etc.  In addition, organizations may have policies
   around acceptable ciphers and certificates on the websites the IoT
   devices connect to.  Examples include no use of old and less secure
   versions of TLS, no use of self-signed certificates, deny-list or
   accept-list of Certificate Authorities, valid certificate expiration
   time, etc.  These policies can be enforced by observing the (D)TLS
   profile parameters.  Enterprise firewall can use the IoT device's
   (D)TLS profile parameters to identify legitimate flows by observation
   of (D)TLS sessions, and can make inferences to permit legitimate
   flows and to block malicious flows.  The proposed technique is also
   suitable in deployments where decryption techniques are not ideal due
   to privacy concerns, non-cooperating end-points and expense.

2.  Terminology

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

   "(D)TLS" is used for statements that apply to both Transport Layer
   Security [RFC8446] and Datagram Transport Layer Security [RFC6347].
   Specific terms are used for any statement that applies to either
   protocol alone.

3.  Overview of MUD (D)TLS profiles for IoT devices

   In Enterprise networks, protection and detection are typically done
   both on end hosts and in the network.  Host agents have deep
   visibility on the devices where they are installed, whereas the
   network has broader visibility.  Installing host agents may not be a
   viable option on IoT devices, and network-based security can only be
   used to protect such IoT devices.  (D)TLS profile parameters of IoT
   device can be used by middle-boxes to detect and block malware
   communication, while at the same time preserving the privacy of
   legitimate uses of encryption.  Middle-boxes need not proxy (D)TLS
   but can passively observe the parameters of (D)TLS handshakes from
   IoT devices and gain good visibility into TLS 1.0 to 1.2 parameters
   and partial visibility into TLS 1.3 parameters.  Malicious agents can
   try to use the (D)TLS profile parameters as legitimate agents to
   evade detection but it becomes a challenge to mimic the behavior of
   various IoT device types and IoT device models from several
   manufacturers.  In other words, malware developers will have to
   develop malicious agents per IoT device type, manufacturer and model
   (which will be several thousands), infect the device with specific
   malware agent and will have keep up with the updates to (D)TLS

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   profile parameters of IoT devices.  Further, the malware command and
   control server certificates needs to be signed by the same certifying
   authorities trusted by the IoT devices.

   The compromised IoT devices are typically used for launching DDoS
   attacks (Section 3 of [RFC8576]) on victims while the owner/
   administrator of the network is not aware about such misbehaviors.
   Some of the DDoS attacks like Slowloris and Transport Layer Security
   (TLS) re-negotiation can be detected by observing the (D)TLS profile
   parameters.  For example, the victim server certificate need not be
   signed by the same certifying authorities trusted by the IoT device.

4.  (D)TLS 1.3 handshake

   In (D)TLS 1.3, full (D)TLS handshake inspection is not possible since
   all (D)TLS handshake messages excluding the ClientHello message are
   encrypted.  (D)TLS 1.3 has introduced new extensions in the handshake
   record layers called Encrypted Extensions.  Using these extensions
   handshake messages will be encrypted and network devices (such as a
   firewall) are incapable deciphering the handshake, thus cannot view
   the server certificate.  However, a few parameters in the ServerHello
   are still visible such as the chosen cipher.

4.1.  Full (D)TLS 1.3 handshake inspection

   To obtain more visibility into negotiated TLS 1.3 parameters, a
   middlebox needs to act as a (D)TLS 1.3 proxy.  The middlebox MUST
   follow the behaviour explained in Section 9.3 of [RFC8446] to act as
   a compliant (D)TLS 1.3 proxy.

   To function as a (D)TLS proxy the middlebox creates a signed
   certificate using itself as a certificate authority.  That
   certificate authority has to be trusted by the (D)TLS client.  The
   following steps explain the mechanism to automatically bootstrap IoT
   devices with the middlebox's CA certificate.

   Bootstrapping Remote Secure Key Infrastructures (BRSKI) discussed in
   [I-D.ietf-anima-bootstrapping-keyinfra] provides a solution for
   secure automated bootstrap of devices.  BRSKI specifies means to
   provision credentials on devices to be used to operationally access
   networks.  In addition, BRSKI provides an automated mechanism for the
   bootstrap distribution of CA certificates from the Enrollment over
   Secure Transport (EST) [RFC7030] server.  The IoT device can use
   BRSKI to automatically bootstrap the IoT device using the IoT
   manufacturer provisioned X.509 certificate, in combination with a
   registrar provided by the local network and IoT device manufacturer's
   authorizing service (MASA).

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   1.  The IoT device authenticates to the local network using the IoT
       manufacturer provisioned X.509 certificate.  The IoT device can
       request and get a voucher from the MASA service via the
       registrar.  The voucher is signed by the MASA service and
       includes the local network's CA public key.

   2.  The IoT device validates the signed voucher using the
       manufacturer installed trust anchor associated with the MASA,
       stores the CA's public key and validates the provisional TLS
       connection to the registrar.

   3.  The IoT device requests the full EST distribution of current CA
       certificates (Section 5.9.1 in
       [I-D.ietf-anima-bootstrapping-keyinfra]) from the registrar
       operating as a BRSKI-EST server.  The IoT device stores the CA
       certificates as Explicit Trust Anchor database entries.  The IoT
       device uses the Explicit Trust Anchor database to validate the
       server certificate.

   4.  The middle-box uses the "supported_versions" TLS extension
       (defined in TLS 1.3 to negotiate the supported TLS versions
       between client and server) to determine the TLS version.  During
       the (D)TLS handshake, If (D)TLS version 1.3 is used, the middle-
       box ((D)TLS proxy) modifies the certificate provided by the
       server and signs it with the private key from the local CA
       certificate.  The middle-box has visibility into further
       exchanges between the IoT device and server which enables it to
       inspect the (D)TLS 1.3 handshake, enforce the MUD (D)TLS profile
       and can inspect subsequent network traffic.

   5.  The IoT device uses the Explicit Trust Anchor database to
       validate the server certificate.

4.2.  Encrypted SNI

   To increase privacy, encrypted SNI (ESNI,
   [I-D.ietf-tls-sni-encryption]) prevents passive observation of the
   TLS Server Name Indication which improves privacy.  To effectively
   provide that privacy protection, SNI encryption needs to be used in
   conjunction with DNS encryption (e.g., DNS-over-(D)TLS or DNS-over-
   HTTPS).  An in-line network device (e.g., firewall) passively
   inspecting an encrypted SNI (D)TLS handshake cannot observe the
   encrypted SNI nor observe the encrypted DNS traffic.  If an IoT
   device is pre-configured to use public DNS-over-(D)TLS or DNS-over-
   HTTPS servers, the middle-box needs to act as a DNS-over-TLS or DNS-
   over-HTTPS proxy and replace the esni_keys in the ESNI record with
   the middle box's esni_keys.  Instead of an unappealing DNS-over-TLS
   or DNS-over-HTTPS proxy, the IoT device can be bootstrapped to

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   discover and authenticate DNS-over-(D)TLS and DNS-over-HTTPS servers
   provided by a local network using
   [I-D.reddy-dprive-bootstrap-dns-server] and [I-D.sah-resinfo-doh].
   The local DNS-over-(D)TLS and DNS-over-HTTPS server replaces the
   sni_keys in the ESNI record with the middle box's esni_keys.

   Note that if an IoT device is pre-configured to use public DNS-
   over-(D)TLS or DNS-over-HTTPS servers, the MUD policy enforcement
   point is moved to that public server, which cannot enforce the MUD
   policy based on domain names (Section 8 of [RFC8520]).  Thus the use
   of a public DNS-over-(D)TLS or DNS-over-HTTPS server is incompatible
   with MUD in general.  A local DNS server is necessary to allow MUD
   policy enforcement on the local network.

5.  (D)TLS profile YANG module

   This document specifies a YANG module for representing (D)TLS
   profile.  The (D)TLS profile YANG module provides a method for
   firewall to observe the (D)TLS profile parameters in the (D)TLS
   handshake to permit intended use and to block malicious behavior.
   This module uses the common YANG types defined in [RFC6991] , rules
   defined in [RFC8519] and cryptographic types defined in

   The (D)TLS profiles and the parameters in each (D)TLS profile include
   the following:

   o  Profile name

   o  (D)TLS version in ClientHello.legacy_version

   o  (D)TLS versions supported by the IoT device.  As a reminder,
      "supported_versions" extension defined in (D)TLS 1.3 is used by
      the client to indicate which versions of (D)TLS it supports and a
      client is considered to be attempting to negotiate (D)TLS 1.3 if
      the ClientHello contains a "supported_versions" extension with
      0x0304 contained in its body.

   o  If GREASE [I-D.ietf-tls-grease] (Generate Random Extensions And
      Sustain Extensibility) values are offered by the client or not.

   o  List of supported symmetric encryption algorithms

   o  List of supported compression methods

   o  List of extension types

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   o  List of trust anchor certificates used by the IoT device.  If the
      server certificate is signed by one of the trust anchors, the
      middle-box continues with the connection as normal.  Otherwise,
      the middle-box will react as if the the server certificate
      validation has failed and takes appropriate action (e.g, block the
      (D)TLS session).  Note that server certificate is encrypted in
      (D)TLS 1.3 and the middle-box without acting as (D)TLS proxy
      cannot validate the server certificate.

   o  List of SPKI pin set pre-configured on the client to validate
      self-signed server certificates or raw public keys.  A SPKI pin
      set is a cryptographic digest to "pin" public key information in a
      manner similar to HTTP Public Key Pinning (HPKP) [RFC7469].  If
      SPKI pin set is present in the (D)TLS profile of a IoT device and
      the server certificate does not pass the PKIX certification path
      validation, the middle-box computes the SPKI Fingerprint for the
      public key found in the server's certificate (or in the raw public
      key, if the server provides that instead).  If a computed
      fingerprint exactly matches one of the SPKI pin sets in the (D)TLS
      profile, the middle-box continues with the connection as normal.
      Otherwise, the middle-box will act on the SPKI validation failure
      and takes appropriate action.

   o  Cryptographic hash algorithm used to generate the SPKI pinsets

   o  List of pre-shared key exchange modes

   o  List of DHE or ECDHE groups supported by the client

   o  List signature algorithms the client can validate in X.509 server

   o  List of client key exchange algorithms and the client public key
      lengths in versions prior to (D)TLS 1.3

   The (D)TLS profile parameters MUST NOT include the GREASE values for
   extension types, supported groups, signature algorithms, (D)TLS
   versions, pre-shared key exchange modes and cipher suites.  Note that
   the GREASE values are random and peers will ignore these values and

   If the (D)TLS profile parameters are not observed in a (D)TLS session
   from the IoT device, the default behaviour is to block the (D)TLS

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5.1.  Tree Structure

   This document augments the "ietf-mud" MUD YANG module defined in
   [RFC8520] for signaling the IoT device (D)TLS profile.  This document
   defines the YANG module "reddy-opsawg-mud-tls-profile", which has the
   following tree structure:

module: reddy-opsawg-mud-tls-profile
  augment /mud:mud/mud:from-device-policy:
    +--rw client-profile
       +--rw tls-profiles* [profile-name]
          +--rw profile-name              string
          +--rw protocol-version?         uint16
          +--rw supported_versions*       uint16
          +--rw grease_extension?         boolean
          +--rw encryption-algorithms*    encryption-algorithm
          +--rw compression-methods*      compression-method
          +--rw extension-types*          extension-type
          +--rw acceptlist-ta-certs*      ct:trust-anchor-cert-cms
          +--rw SPKI-pin-sets*            SPKI-pin-set
          +--rw SPKI-hash-algorithm       ct:hash-algorithm-t
          +--rw psk-key-exchange-modes*   psk-key-exchange-mode
          +--rw supported-groups*         supported-group
          +--rw signature-algorithms*     signature-algorithm
          +--rw client-public-keys
          |  +--rw key-exchange-algorithms*     key-exchange-algorithm
          |  +--rw client-public-key-lengths*   client-public-key-length
          +--rw actions
             +--rw forwarding    identityref

5.2.  YANG Module

module reddy-opsawg-mud-tls-profile {
   yang-version 1.1;
   namespace "urn:ietf:params:xml:ns:yang:reddy-opsawg-mud-tls-profile";
   prefix mud-tls-profile;

   import ietf-crypto-types {
     prefix ct;
     reference "draft-ietf-netconf-crypto-types-01:
                Common YANG Data Types for Cryptography";

   import ietf-inet-types {
     prefix inet;
     reference "Section 4 of RFC 6991";

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   import ietf-mud {
     prefix mud;
     reference "RFC 8520";

   import ietf-access-control-list {
     prefix ietf-acl;
       "RFC 8519: YANG Data Model for Network Access
                  Control Lists (ACLs)";

     "IETF Operations and Management Area Working Group Working Group";
      "Editor:  Konda, Tirumaleswar Reddy

     "This module contains YANG definition for the IoT device
      (D)TLS profile.

      Copyright (c) 2019 IETF Trust and the persons identified as
      authors of the code.  All rights reserved.

      Redistribution and use in source and binary forms, with or
      without modification, is permitted pursuant to, and subject
      to the license terms contained in, the Simplified BSD License
      set forth in Section 4.c of the IETF Trust's Legal Provisions
      Relating to IETF Documents

      This version of this YANG module is part of RFC XXXX; see
      the RFC itself for full legal notices.";

   revision 2019-06-12 {
       "Initial revision.";

   typedef compression-method {
     type uint8;
     description "Compression method.";

   typedef extension-type {
     type uint16;
     description "Extension type.";

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   typedef encryption-algorithm {
     type uint16;
     description "Encryption algorithms.";

   typedef supported-group {
     type uint16;
     description "supported DHE or ECDHE group.";

   typedef SPKI-pin-set {
     type binary;
     description "Subject Public Key Info pin set.";

   typedef signature-algorithm {
     type uint16;
     description "Signature algorithm";

   typedef key-exchange-algorithm {
     type uint8;
     description "key exchange algorithm";

   typedef psk-key-exchange-mode {
     type uint8;
     description "pre-shared key exchange mode";

   typedef client-public-key-length {
     type uint8;
     description "client public key length";

   augment "/mud:mud/mud:from-device-policy" {
     container client-profile {
       list tls-profiles {
         key "profile-name";
          "(D)TLS version profiles supported by the client";
        leaf profile-name {
          type string {
            length "1..64";

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            "The name of (D)TLS profile; space and special
            characters are not allowed.";
         leaf protocol-version {
           type uint16;
           description "Legacy protocol version.";
         leaf-list supported_versions {
           type uint16;
             "TLS versions supported by the client indicated
              in the supported_versions extension in (D)TLS 1.3.";
         leaf Grease_extension {
           type boolean;
            "If set to 'true', Grease extension values are offered by
             the client.";
         leaf-list encryption-algorithms {
           type encryption-algorithm;
           description "Encryption algorithms";
         leaf-list compression-methods {
           type compression-method;
            description "Compression methods";
         leaf-list extension-types {
           type extension-type;
           description "Extension Types";
         leaf-list acceptlist-ta-certs {
           type ct:trust-anchor-cert-cms;
             "A list of trust anchor certificates used by the client.";
         leaf-list SPKI-pin-sets {
            type SPKI-pin-set;
             "A list of SPKI pin sets pre-configured on the client
              to validate self-signed server certificate or
              raw public key.";
         leaf SPKI-hash-algorithm {
           type ct:hash-algorithm-t;
             "cryptographic hash algorithm used to generate the
              SPKI pinset.";

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         leaf-list psk-key-exchange-modes {
           type psk-key-exchange-mode;
             "pre-shared key exchange modes";
         leaf-list supported-groups {
            type supported-group;
             "A list of DHE or ECDHE groups supported by the client.";
         leaf-list signature-algorithms {
            type signature-algorithm;
             "A list signature algorithms the client can validate
              in X.509 certificates.";
         container client-public-keys {
           leaf-list key-exchange-algorithms {
             type key-exchange-algorithm;
             "Key exchange algorithms supported by the client";
           leaf-list client-public-key-lengths {
             type client-public-key-length;
             "client public key lengths";
         container actions {
           "Definitions of action for this profile.";
           leaf forwarding {
             type identityref {
               base ietf-acl:forwarding-action;
             mandatory true;
             "Specifies the forwarding action for the (D)TLS profile.";
             "RFC 8519: YANG Data Model for Network Access
                           Control Lists (ACLs)";

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6.  MUD File Example

   This example below contains (D)TLS profile parameters for a IoT
   device.  JSON encoding of YANG modelled data [RFC7951] is used to
   illustrate the example.

      "ietf-mud:mud": {
        "mud-version": 1,
         "mud-url": "https://example.com/IoTDevice",
         "last-update": "2019-18-06T03:56:40.105+10:00",
         "cache-validity": 100,
         "is-supported": true,
         "systeminfo": "IoT device name",
         "reddy-opsawg-mud-tls-profile:from-device-policy": {
           "client-profile": {
             "tls-version-profile" : [
                    "protocol-version" : 771,
                    "supported_versions_ext" : "FALSE",
                    "encryption-algorithms" : [31354, 4865, 4866, 4867],
                    "extension-types" : [10],
                    "supported-groups" : [29],
                    "actions": {
                      "forwarding": "accept"

7.  Security Considerations

   Security considerations in [RFC8520] need to be taken into

8.  IANA Considerations

   This document requests IANA to register the following URIs in the
   "ns" subregistry within the "IETF XML Registry" [RFC3688]:

         URI: urn:ietf:params:xml:ns:yang:reddy-opsawg-mud-tls-profile
         Registrant Contact: The IESG.
         XML: N/A; the requested URI is an XML namespace.

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9.  Acknowledgments

   Thanks to Shashank Jain and Harsha Joshi for the discussion and

10.  References

10.1.  Normative References

              Watsen, K. and H. Wang, "Common YANG Data Types for
              Cryptography", draft-ietf-netconf-crypto-types-10 (work in
              progress), July 2019.

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-32 (work in progress), July

              Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-04 (work in progress), August 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,

   [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,

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Internet-Draft       MUD TLS profile for IoT devices      September 2019

   [RFC8519]  Jethanandani, M., Agarwal, S., Huang, L., and D. Blair,
              "YANG Data Model for Network Access Control Lists (ACLs)",
              RFC 8519, DOI 10.17487/RFC8519, March 2019,

10.2.  Informative References

              Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-26 (work in progress), August 2019.

              Huitema, C. and E. Rescorla, "Issues and Requirements for
              SNI Encryption in TLS", draft-ietf-tls-sni-encryption-05
              (work in progress), August 2019.

              K, R., Wing, D., Richardson, M., and M. Boucadair, "A
              Bootstrapping Procedure to Discover and Authenticate DNS-
              over-(D)TLS and DNS-over-HTTPS Servers", draft-reddy-
              dprive-bootstrap-dns-server-04 (work in progress), June

              Sood, P., Arends, R., and P. Hoffman, "DNS Resolver
              Information: "doh"", draft-sah-resinfo-doh-00 (work in
              progress), May 2019.

   [malware]  Anderson, B., Paul, S., and D. McGrew, "Deciphering
              Malware's use of TLS (without Decryption)", July 2016,

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <https://www.rfc-editor.org/info/rfc7469>.

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,

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Internet-Draft       MUD TLS profile for IoT devices      September 2019

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,

   [RFC8576]  Garcia-Morchon, O., Kumar, S., and M. Sethi, "Internet of
              Things (IoT) Security: State of the Art and Challenges",
              RFC 8576, DOI 10.17487/RFC8576, April 2019,

Authors' Addresses

   Tirumaleswar Reddy
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071

   Email: kondtir@gmail.com

   Dan Wing
   Citrix Systems, Inc.
   4988 Great America Pkwy
   Santa Clara, CA  95054

   Email: danwing@gmail.com

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