TLS                                                     E. Rescorla, Ed.
Internet-Draft                                                RTFM, Inc.
Updates: 6347 (if approved)                           H. Tschofenig, Ed.
Intended status: Standards Track                              T. Fossati
Expires: November 12, 2021                                   Arm Limited
                                                                A. Kraus
                                                           Bosch.IO GmbH
                                                            May 11, 2021

                  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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   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 November 12, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   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 . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Block Ciphers with Encrypt-then-MAC processing  . . . . .   8
     5.3.  AEAD Ciphers  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Peer Address Update . . . . . . . . . . . . . . . . . . . . .   9
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Extra Column to TLS ExtensionType Values Registry  . . .  13
     10.2.  Entry to the TLS ExtensionType Values Registry . . . . .  13
     10.3.  Entry to the TLS ContentType Registry  . . . . . . . . .  13
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Working Group Information  . . . . . . . . . . . . .  18
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  18
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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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 Connection ID
   (CID) to the DTLS record layer.  The presence of the CID is
   negotiated via a DTLS extension.

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

   This document assumes familiarity with DTLS 1.2 [RFC6347].  The
   presentation language used in this document is described in Section 4
   of [RFC8446].

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;

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   The extension_data field of this extension, when included in the
   ClientHello, MUST contain the ConnectionId structure.  This structure
   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 using a CID but does not wish the
   server to use one when sending.

     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 using the client's CID but does
   not wish the client to include a CID when sending.

   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 deployment-specific 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.  Since the CID length
   information is not included in the record itself, 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.

   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, or a zero-length
   CID has been advertised for a given direction, then the RFC
   6347-defined record format and content type MUST be used to send in
   the indicated direction(s).

   If DTLS peers have negotiated the use of a non-zero-length CID for a
   given direction, then once encryption is enabled they MUST send with
   the record format defined in Figure 3 with the new MAC computation
   defined in Section 5 and the content type tls12_cid.  Plaintext
   payloads never use the new record format or the CID content type.

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   When 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 a non-zero length CID is expected.  If a non-zero-
   length CID is expected for the retrieved association, then the
   datagram MUST be treated as invalid, as described in Section
   of [RFC6347].

   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 real 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.

        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

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   DTLSInnerPlaintext structure.  This newly introduced structure is
   shown in Figure 2.

        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 cleartext 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.)

   The DTLSInnerPlaintext byte sequence is then encrypted.  To create
   the DTLSCiphertext structure shown in Figure 3 the CID is added.

        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.

   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 receive CIDs of a fixed length.  If, however, an
      implementation chooses to receive different lengths of CID, the

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      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 serialized
      DTLSInnerPlaintext (two-byte integer).  The length MUST NOT exceed

   seq_num_placeholder  8 bytes of 0xff

   Note "+" denotes concatenation.

5.1.  Block Ciphers

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

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           seq_num_placeholder +
           tls12_cid +
           cid_length +
           tls12_cid +
           DTLSCiphertext.version +
           epoch +
           sequence_number +
           cid +
           length_of_DTLSInnerPlaintext +
           DTLSInnerPlaintext.content +
           DTLSInnerPlaintext.real_type +

   The rationale behind this construction is to separate the MAC input
   for DTLS without the connection ID from the MAC input with the
   connection ID.  The former always consists of a sequence number
   followed by some other content type than tls12_cid; the latter always
   consists of the seq_num_placeholder followed by tls12_cid.  Although
   2^64-1 is potentially a valid sequence number, tls12_cid will never
   be a valid content type when the connection ID is not in use.  In
   addition, the epoch and sequence_number are now fed into the MAC in
   the same order as they appear on the wire.

5.2.  Block Ciphers with Encrypt-then-MAC processing

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

           seq_num_placeholder +
           tls12_cid +
           cid_length +
           tls12_cid +
           DTLSCiphertext.version +
           epoch +
           sequence_number +
           cid +
           DTLSCiphertext.length +
           IV +
           ENC(content + padding + padding_length));

5.3.  AEAD Ciphers

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

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       additional_data = seq_num_placeholder +
                         tls12_cid +
                         cid_length +
                         tls12_cid +
                         DTLSCiphertext.version +
                         epoch +
                         sequence_number +
                         cid +

6.  Peer Address Update

   When a record with a CID is received that has a source address
   different from the one currently associated with the DTLS connection,
   the receiver MUST NOT replace the address it uses for sending records
   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 strategy is mandated by
      this specification but see note (*) below.

   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.

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   (*) Note: Application protocols that implement protection against
   spoofed addresses 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 [I-D.tschofenig-tls-dtls-rrc].

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

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.  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
   these 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 particular concern when the
   request is small and the response large.  See Section 6 for more on
   address updates.

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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

   This document requests three actions from IANA.

10.1.  Extra Column to TLS ExtensionType Values Registry

   IANA is requested to add an extra column named "DTLS-Only" 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 registry.

10.2.  Entry to the TLS ExtensionType Values Registry

   IANA is requested to allocate an entry to the existing "TLS
   ExtensionType Values" registry, for connection_id(TBD1) as described
   in the table below.  Although the value 53 has been allocated by
   early allocation for a previous version of this document, it is
   incompatible with this document.  Once this document is approved for
   publication, the early allocation will be deprecated in favor of this

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

   A new column "DTLS-Only" is added to the registry.  The valid entries
   are "Y" if the extension is only applicable to DTLS, "N" otherwise.
   All the pre-existing entries are given the value "N".

   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

10.3.  Entry to the TLS ContentType Registry

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

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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,

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

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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-dtls13-43 (work in progress), April

              Arm Limited and Arm Limited, "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,

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11.3.  URIs




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Appendix A.  History



   -  Improved peer address update text

   -  Editorial improvements

   -  Clarification regarding the use of the TLS ExtensionType Values


   -  Enhanced IANA considerations section

   -  Clarifications regarding CID negotiation and zero-length CIDs


   -  Clarify privacy impact.

   -  Have security considerations point to Section 6.


   -  Changed MAC/additional data calculation.

   -  Disallow sending MAC failure fatal alerts to non-validated peers.

   -  Incorporated editorial review comments by Ben Kaduk.


   -  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.

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   -  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


   -  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

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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:

   -  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

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   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.

Appendix D.  Acknowledgements

   We would like to thank Hanno Becker, Martin Duke, Lars Eggert, Ben
   Kaduk, Warren Kumari, Francesca Palombini, Tom Petch, John Scudder,
   Sean Turner, Eric Vyncke, and Robert Wilton for their review

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

Authors' Addresses

   Eric Rescorla (editor)
   RTFM, Inc.


   Hannes Tschofenig (editor)
   Arm Limited


   Thomas Fossati
   Arm Limited


   Achim Kraus
   Bosch.IO GmbH


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