TLS                                                     E. Rescorla, Ed.
Internet-Draft                                                RTFM, Inc.
Updates: 6347 (if approved)                           H. Tschofenig, Ed.
Intended status: Standards Track                              T. Fossati
Expires: May 6, 2021                                         Arm Limited
                                                       November 02, 2020

                  Connection Identifiers for DTLS 1.2


   This document specifies the Connection ID (CID) construct for the
   Datagram Transport Layer Security (DTLS) protocol version 1.2.

   A CID is an identifier carried in the record layer header that gives
   the recipient additional information for selecting the appropriate
   security association.  In "classical" DTLS, selecting a security
   association of an incoming DTLS record is accomplished with the help
   of the 5-tuple.  If the source IP address and/or source port changes
   during the lifetime of an ongoing DTLS session then the receiver will
   be unable to locate the correct security context.

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
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

   Copyright (c) 2020 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

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   ( 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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  The "connection_id" Extension . . . . . . . . . . . . . . . .   3
   4.  Record Layer Extensions . . . . . . . . . . . . . . . . . . .   5
   5.  Record Payload Protection . . . . . . . . . . . . . . . . . .   7
     5.1.  Block Ciphers . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Block Ciphers with Encrypt-then-MAC processing  . . . . .   8
     5.3.  AEAD Ciphers  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Peer Address Update . . . . . . . . . . . . . . . . . . . . .   8
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Working Group Information  . . . . . . . . . . . . .  16
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  16
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1.  Introduction

   The Datagram Transport Layer Security (DTLS) [RFC6347] protocol was
   designed for securing connection-less transports, like UDP.  DTLS,
   like TLS, starts with a handshake, which can be computationally
   demanding (particularly when public key cryptography is used).  After
   a successful handshake, symmetric key cryptography is used to apply
   data origin authentication, integrity and confidentiality protection.
   This two-step approach allows endpoints to amortize the cost of the
   initial handshake across subsequent application data protection.
   Ideally, the second phase where application data is protected lasts
   over a long period of time since the established keys will only need
   to be updated once the key lifetime expires.

   In DTLS as specified in RFC 6347, the IP address and port of the peer
   are used to identify the DTLS association.  Unfortunately, in some
   cases, such as NAT rebinding, these values are insufficient.  This is
   a particular issue in the Internet of Things when devices enter
   extended sleep periods to increase their battery lifetime.  The NAT
   rebinding leads to connection failure, with the resulting cost of a
   new handshake.

   This document defines an extension to DTLS 1.2 to add a CID to the
   DTLS record layer.  The presence of the CID is negotiated via a DTLS

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

   This document assumes familiarity with DTLS 1.2 [RFC6347].

3.  The "connection_id" Extension

   This document defines the "connection_id" extension, which is used in
   ClientHello and ServerHello messages.

   The extension type is specified as follows.

     enum {
        connection_id(TBD1), (65535)
     } ExtensionType;

   The extension_data field of this extension, when included in the
   ClientHello, MUST contain the ConnectionId structure.  This structure

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   contains the CID value the client wishes the server to use when
   sending messages to the client.  A zero-length CID value indicates
   that the client is prepared to send with a CID but does not wish the
   server to use one when sending.  Alternatively, this can be
   interpreted as the client wishes the server to use a zero-length CID;
   the result is the same.

     struct {
         opaque cid<0..2^8-1>;
     } ConnectionId;

   A server willing to use CIDs will respond with a "connection_id"
   extension in the ServerHello, containing the CID it wishes the client
   to use when sending messages towards it.  A zero-length value
   indicates that the server will send with the client's CID but does
   not wish the client to include a CID (or again, alternately, to use a
   zero-length CID).

   Because each party sends the value in the "connection_id" extension
   it wants to receive as a CID in encrypted records, it is possible for
   an endpoint to use a globally constant length for such connection
   identifiers.  This can in turn ease parsing and connection lookup,
   for example by having the length in question be a compile-time
   constant.  Such implementations MUST still be able to send CIDs of
   different length to other parties.  Implementations that want to use
   variable-length CIDs are responsible for constructing the CID in such
   a way that its length can be determined on reception.  Note that
   there is no CID length information included in the record itself.

   In DTLS 1.2, CIDs are exchanged at the beginning of the DTLS session
   only.  There is no dedicated "CID update" message that allows new
   CIDs to be established mid-session, because DTLS 1.2 in general does
   not allow TLS 1.3-style post-handshake messages that do not
   themselves begin other handshakes.  When a DTLS session is resumed or
   renegotiated, the "connection_id" extension is negotiated afresh.

   If DTLS peers have not negotiated the use of CIDs then the RFC
   6347-defined record format and content type MUST be used.

   If DTLS peers have negotiated the use of a CIDs using the ClientHello
   and the ServerHello messages then the peers need to take the
   following steps.

   The DTLS peers determine whether incoming and outgoing messages need
   to use the new record format, i.e., the record format containing the
   CID.  The new record format with the the tls12_cid content type is
   only used once encryption is enabled.  Plaintext payloads never use
   the new record type and the CID content type.

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   For sending, if a zero-length CID has been negotiated then the RFC
   6347-defined record format and content type MUST be used (see
   Section 4.1 of [RFC6347]) else the new record layer format with the
   tls12_cid content type defined in Figure 3 MUST be used.

   When transmitting a datagram with the tls12_cid content type, the new
   MAC computation defined in Section 5 MUST be used.

   For receiving, if the tls12_cid content type is set, then the CID is
   used to look up the connection and the security association.  If the
   tls12_cid content type is not set, then the connection and security
   association is looked up by the 5-tuple and a check MUST be made to
   determine whether the expected CID value is indeed zero length.  If
   the check fails, then the datagram MUST be dropped.

   When receiving a datagram with the tls12_cid content type, the new
   MAC computation defined in Section 5 MUST be used.  When receiving a
   datagram with the RFC 6347-defined record format the MAC calculation
   defined in Section 4.1.2 of [RFC6347] MUST be used.

4.  Record Layer Extensions

   This specification defines the DTLS 1.2 record layer format and
   [I-D.ietf-tls-dtls13] specifies how to carry the CID in DTLS 1.3.

   To allow a receiver to determine whether a record has a CID or not,
   connections which have negotiated this extension use a distinguished
   record type tls12_cid(TBD2).  Use of this content type has the
   following three implications:

   -  The CID field is present and contains one or more bytes.

   -  The MAC calculation follows the process described in Section 5.

   -  The true content type is inside the encryption envelope, as
      described below.

   Plaintext records are not impacted by this extension.  Hence, the
   format of the DTLSPlaintext structure is left unchanged, as shown in
   Figure 1.

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        struct {
            ContentType type;
            ProtocolVersion version;
            uint16 epoch;
            uint48 sequence_number;
            uint16 length;
            opaque fragment[DTLSPlaintext.length];
        } DTLSPlaintext;

               Figure 1: DTLS 1.2 Plaintext Record Payload.

   When CIDs are being used, the content to be sent is first wrapped
   along with its content type and optional padding into a
   DTLSInnerPlaintext structure.  This newly introduced structure is
   shown in Figure 2.  The DTLSInnerPlaintext byte sequence is then
   encrypted.  To create the DTLSCiphertext structure shown in Figure 3
   the CID is added.

        struct {
            opaque content[length];
            ContentType real_type;
            uint8 zeros[length_of_padding];
        } DTLSInnerPlaintext;

            Figure 2: New DTLSInnerPlaintext Payload Structure.

   content  Corresponds to the fragment of a given length.

   real_type  The content type describing the payload.

   zeros  An arbitrary-length run of zero-valued bytes may appear in the
      cleartext after the type field.  This provides an opportunity for
      senders to pad any DTLS record by a chosen amount as long as the
      total stays within record size limits.  See Section 5.4 of
      [RFC8446] for more details.  (Note that the term TLSInnerPlaintext
      in RFC 8446 refers to DTLSInnerPlaintext in this specification.)

        struct {
            ContentType outer_type = tls12_cid;
            ProtocolVersion version;
            uint16 epoch;
            uint48 sequence_number;
            opaque cid[cid_length];               // New field
            uint16 length;
            opaque enc_content[DTLSCiphertext.length];
        } DTLSCiphertext;

            Figure 3: DTLS 1.2 CID-enhanced Ciphertext Record.

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   outer_type  The outer content type of a DTLSCiphertext record
      carrying a CID is always set to tls12_cid(TBD2).  The real content
      type of the record is found in DTLSInnerPlaintext.real_type after

   cid  The CID value, cid_length bytes long, as agreed at the time the
      extension has been negotiated.  Recall that (as discussed
      previously) each peer chooses the CID value it will receive and
      use to identify the connection, so an implementation can choose to
      always recieve CIDs of a fixed length.  If, however, an
      implementation chooses to receive different lengths of CID, the
      assigned CID values must be self-delineating since there is no
      other mechanism available to determine what connection (and thus,
      what CID length) is in use.

   enc_content  The encrypted form of the serialized DTLSInnerPlaintext

   All other fields are as defined in RFC 6347.

5.  Record Payload Protection

   Several types of ciphers have been defined for use with TLS and DTLS
   and the MAC calculations for those ciphers differ slightly.

   This specification modifies the MAC calculation as defined in
   [RFC6347] and [RFC7366], as well as the definition of the additional
   data used with AEAD ciphers provided in [RFC6347], for records with
   content type tls12_cid.  The modified algorithm MUST NOT be applied
   to records that do not carry a CID, i.e., records with content type
   other than tls12_cid.

   The following fields are defined in this document; all other fields
   are as defined in the cited documents.

   cid  Value of the negotiated CID (variable length).

   cid_length  1 byte field indicating the length of the negotiated CID.

   length_of_DTLSInnerPlaintext  The length (in bytes) of the serialised
      DTLSInnerPlaintext (two-byte integer).
      The length MUST NOT exceed 2^14.

   Note "+" denotes concatenation.

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5.1.  Block Ciphers

   The following MAC algorithm applies to block ciphers that do not use
   the with Encrypt-then-MAC processing described in [RFC7366].

       MAC(MAC_write_key, seq_num +
           tls12_cid +
           DTLSCiphertext.version +
           cid +
           cid_length +
           length_of_DTLSInnerPlaintext +
           DTLSInnerPlaintext.content +
           DTLSInnerPlaintext.real_type +

5.2.  Block Ciphers with Encrypt-then-MAC processing

   The following MAC algorithm applies to block ciphers that use the
   with Encrypt-then-MAC processing described in [RFC7366].

       MAC(MAC_write_key, seq_num +
           tls12_cid +
           DTLSCipherText.version +
           cid +
           cid_length +
           length of (IV + DTLSCiphertext.enc_content) +
           IV +

5.3.  AEAD Ciphers

   For ciphers utilizing authenticated encryption with additional data
   the following modification is made to the additional data

       additional_data = seq_num +
                         tls12_cid +
                         DTLSCipherText.version +
                         cid +
                         cid_length +

6.  Peer Address Update

   When a record with a CID is received that has a source address
   different than the one currently associated with the DTLS connection,
   the receiver MUST NOT replace the address it uses for sending records

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   to its peer with the source address specified in the received
   datagram unless the following three conditions are met:

   -  The received datagram has been cryptographically verified using
      the DTLS record layer processing procedures.

   -  The received datagram is "newer" (in terms of both epoch and
      sequence number) than the newest datagram received.  Reordered
      datagrams that are sent prior to a change in a peer address might
      otherwise cause a valid address change to be reverted.  This also
      limits the ability of an attacker to use replayed datagrams to
      force a spurious address change, which could result in denial of
      service.  An attacker might be able to succeed in changing a peer
      address if they are able to rewrite source addresses and if
      replayed packets are able to arrive before any original.

   -  There is a strategy for ensuring that the new peer address is able
      to receive and process DTLS records.  No such test is defined in
      this specification.

   The conditions above are necessary to protect against attacks that
   use datagrams with spoofed addresses or replayed datagrams to trigger
   attacks.  Note that there is no requirement for use of the anti-
   replay window mechanism defined in Section of DTLS 1.2.  Both
   solutions, the "anti-replay window" or "newer" algorithm, will
   prevent address updates from replay attacks while the latter will
   only apply to peer address updates and the former applies to any
   application layer traffic.

   Note that datagrams that pass the DTLS cryptographic verification
   procedures but do not trigger a change of peer address are still
   valid DTLS records and are still to be passed to the application.

   Application protocols that implement protection against these attacks
   depend on being aware of changes in peer addresses so that they can
   engage the necessary mechanisms.  When delivered such an event, an
   application layer-specific address validation mechanism can be
   triggered, for example one that is based on successful exchange of a
   minimal amount of ping-pong traffic with the peer.  Alternatively, an
   DTLS-specific mechanism may be used, as described in

   DTLS implementations MUST silently discard records with bad MACs or
   that are otherwise invalid.

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7.  Examples

   Figure 4 shows an example exchange where a CID is used uni-
   directionally from the client to the server.  To indicate that a
   zero-length CID is present in the "connection_id" extension we use
   the notation 'connection_id=empty'.

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   Client                                             Server
   ------                                             ------

   ClientHello                 -------->

                               <--------      HelloVerifyRequest

   ClientHello                 -------->

                               <--------         ServerHelloDone

   Finished                    -------->

                               <--------                Finished

   Application Data            ========>

                               <========        Application Data


   <...> indicates that a connection id is used in the record layer
   (...) indicates an extension
   [...] indicates a payload other than a handshake message

               Figure 4: Example DTLS 1.2 Exchange with CID

   Note: In the example exchange the CID is included in the record layer
   once encryption is enabled.  In DTLS 1.2 only one handshake message
   is encrypted, namely the Finished message.  Since the example shows

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   how to use the CID for payloads sent from the client to the server,
   only the record layer payloads containing the Finished message or
   application data include a CID.

8.  Privacy Considerations

   The CID replaces the previously used 5-tuple and, as such, introduces
   an identifier that remains persistent during the lifetime of a DTLS
   connection.  Every identifier introduces the risk of linkability, as
   explained in [RFC6973].

   An on-path adversary observing the DTLS protocol exchanges between
   the DTLS client and the DTLS server is able to link the observed
   payloads to all subsequent payloads carrying the same ID pair (for
   bi-directional communication).  Without multi-homing or mobility, the
   use of the CID exposes the same information as the 5-tuple.

   With multi-homing, a passive attacker is able to correlate the
   communication interaction over the two paths and the sequence number
   makes it possible to correlate packets across CID changes.  The lack
   of a CID update mechanism in DTLS 1.2 makes this extension unsuitable
   for mobility scenarios where correlation must be considered.
   Deployments that use DTLS in multi-homing environments and are
   concerned about this aspects SHOULD refuse to use CIDs in DTLS 1.2
   and switch to DTLS 1.3 where a CID update mechanism is provided and
   sequence number encryption is available.

   The specification introduces record padding for the CID-enhanced
   record layer, which is a privacy feature not available with the
   original DTLS 1.2 specification.  Padding allows to inflate the size
   of the ciphertext making traffic analysis more difficult.  More
   details about record padding can be found in Section 5.4 and
   Appendix E.3 of RFC 8446.

   Finally, endpoints can use the CID to attach arbitrary per-connection
   metadata to each record they receive on a given connection.  This may
   be used as a mechanism to communicate per-connection information to
   on-path observers.  There is no straightforward way to address this
   concern with CIDs that contain arbitrary values.  Implementations
   concerned about this aspect SHOULD refuse to use CIDs.

9.  Security Considerations

   An on-path adversary can create reflection attacks against third
   parties because a DTLS peer has no means to distinguish a genuine
   address update event (for example, due to a NAT rebinding) from one
   that is malicious.  This attack is of concern when there is a large
   asymmetry of request/response message sizes.

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   Additionally, an attacker able to observe the data traffic exchanged
   between two DTLS peers is able to replay datagrams with modified IP
   address/port numbers.

   The topic of peer address updates is discussed in Section 6.

10.  IANA Considerations

   IANA is requested to allocate an entry to the existing TLS
   "ExtensionType Values" registry, defined in [RFC5246], for
   connection_id(TBD1) as described in the table below.  IANA is
   requested to add an extra column to the TLS ExtensionType Values
   registry to indicate whether an extension is only applicable to DTLS
   and to include this document as an additional reference for the

   Value   Extension Name  TLS 1.3  DTLS Only  Recommended  Reference
   TBD1    connection_id   CH, SH   Y          N           [[This doc]]

   Note: The value "N" in the Recommended column is set because this
   extension is intended only for specific use cases.  This document
   describes the behavior of this extension for DTLS 1.2 only; it is not
   applicable to TLS, and its usage for DTLS 1.3 is described in

   IANA is requested to allocate tls12_cid(TBD2) in the "TLS ContentType
   Registry".  The tls12_cid ContentType is only applicable to DTLS 1.2.

11.  References

11.1.  Normative References

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

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   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

11.2.  Informative References

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 (work in progress), May

              Fossati, T. and H. Tschofenig, "Return Routability Check
              for DTLS 1.2 and DTLS 1.3", draft-tschofenig-tls-dtls-
              rrc-01 (work in progress), March 2020.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

11.3.  URIs




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Appendix A.  History



   -  RRC draft moved from normative to informative.


   -  Wording changes in the security and privacy consideration and the
      peer address update sections.


   -  Updated IANA considerations

   -  Enhanced security consideration section to describe a potential
      man-in-the-middle attack concerning address validation.


   -  Restructed Section 5 "Record Payload Protection"


   -  Editorial simplifications to the 'Record Layer Extensions' and the
      'Record Payload Protection' sections.

   -  Added MAC calculations for block ciphers with and without Encrypt-
      then-MAC processing.


   -  Updated list of contributors

   -  Updated list of contributors and acknowledgements

   -  Updated example

   -  Changed record layer design

   -  Changed record payload protection

   -  Updated introduction and security consideration section

   -  Author- and affiliation changes

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   -  Move to internal content types a la DTLS 1.3.


   -  Remove 1.3 based on the WG consensus at IETF 101


   -  Initial working group version (containing a solution for DTLS 1.2
      and 1.3)


   -  Initial version

Appendix B.  Working Group Information


   The discussion list for the IETF TLS working group is located at the
   e-mail address [1].  Information on the group and
   information on how to subscribe to the list is at [2]

   Archives of the list can be found at:
   archive/web/tls/current/index.html [3]

Appendix C.  Contributors

   Many people have contributed to this specification and we would like
   to thank the following individuals for their contributions:

   * Yin Xinxing

   * Nikos Mavrogiannopoulos

   * Tobias Gondrom

   Additionally, we would like to thank the Connection ID task force
   team members:

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   -  Martin Thomson (Mozilla)

   -  Christian Huitema (Private Octopus Inc.)

   -  Jana Iyengar (Google)

   -  Daniel Kahn Gillmor (ACLU)

   -  Patrick McManus (Mozilla)

   -  Ian Swett (Google)

   -  Mark Nottingham (Fastly)

   The task force team discussed various design ideas, including
   cryptographically generated session
   ids using hash chains and public key encryption, but dismissed them
   due to their inefficiency.  The approach described in this
   specification is the simplest possible design that works given the
   limitations of DTLS 1.2.  DTLS 1.3 provides better privacy features
   and developers are encouraged to switch to the new version of DTLS.

   Finally, we want to thank the IETF TLS working group chairs, Chris
   Wood, Joseph Salowey, and Sean Turner, for their patience, support
   and feedback.

Appendix D.  Acknowledgements

   We would like to thank Achim Kraus for his review comments and
   implementation feedback.

Authors' Addresses

   Eric Rescorla (editor)
   RTFM, Inc.


   Hannes Tschofenig (editor)
   Arm Limited


Rescorla, et al.           Expires May 6, 2021                 [Page 17]

Internet-Draft           DTLS 1.2 Connection ID            November 2020

   Thomas Fossati
   Arm Limited


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