TLS                                                     E. Rescorla, Ed.
Internet-Draft                                                RTFM, Inc.
Updates: 6347 (if approved)                           H. Tschofenig, Ed.
Intended status: Standards Track                             Arm Limited
Expires: January 3, 2019                                      T. Fossati
                                                              T. Gondrom
                                                           July 02, 2018

   The Datagram Transport Layer Security (DTLS) Connection Identifier


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

   A Connection ID 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.

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   This Internet-Draft will expire on January 3, 2019.

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Copyright Notice

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   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 . . . . . . . . . . . . . . . . . .   5
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security and Privacy Considerations . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Working Group Information  . . . . . . . . . . . . .  10
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   The Datagram Transport Layer Security (DTLS) 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 longer period of time since the established keys will only
   need to be updated once the key lifetime expires.

   In the current version of DTLS, 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 to add a connection ID to
   the DTLS record layer.  The presence of the connection ID 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 RFC
   2119 [RFC2119].

   The reader is assumed to be familiar with DTLS [RFC6347].

3.  The "connection_id" Extension

   This document defines a new extension type (connection_id(TBD)),
   which is used in ClientHello and ServerHello messages.

   The extension type is specified as follows.

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

   The extension_data field of this extension, when included in the
   ClientHello, MUST contain the CID structure, which carries the CID

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   which the client wishes the server to use when sending messages
   towards it.  A zero-length value indicates that the client is
   prepared to send with a connection ID but does not wish the server to
   use one when sending (alternately, this can be interpreted as the
   client wishes the server to use a zero-length CID; the result is the

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

   A server which is willing to use CIDs will respond with its own
   "connection_id" extension, 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 use a CID (or again, alternately, to use a zero-length

   When a session is resumed, the "connection_id" extension is
   negotiated afresh, not retained from previous connections in the

   This is effectively the simplest possible design that will work.
   Previous design ideas for using cryptographically generated session
   ids, either using hash chains or public key encryption, were
   dismissed due to their inefficient designs.  Note that a client
   always has the chance to fall back to a full handshake or more
   precisely to a handshake that uses session resumption.

   Because each party sends in the extension_data the value that it will
   receive as a connection identifier 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.  Implementations which 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 such
   implementations must still be able to send other length connection
   identifiers to other parties.

   In DTLS, connection ids are exchanged at the beginning of the DTLS
   session only.  There is no dedicated "connection id update" message
   that allows new connection ids to be established mid-session, because
   DTLS in general does not allow TLS 1.3-style post-handshake messages
   that do not themselves begin other handshakes.  DTLS peers switch to
   the new record layer format when encryption is enabled.

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4.  Record Layer Extensions

   This extension is applicable for use with DTLS 1.2 and below.
   Figure 1 illustrates the record format.  [I-D.ietf-tls-dtls13]
   specifies how to carry the CID in a DTLS 1.3 record.

      struct {
           ContentType type;
           ProtocolVersion version;
           uint16 epoch;
           uint48 sequence_number;
           opaque cid[cid_length];               // New field
           uint16 length;
           select (CipherSpec.cipher_type) {
               case block:  GenericBlockCipher;
               case aead:   GenericAEADCipher;
           } fragment;
      } DTLSCiphertext;

            Figure 1: DTLS 1.2 Record Format with Connection ID

   Note that for both record formats, it is not possible to parse the
   records without knowing how long the Connection ID is.

   In order to allow a receiver to determine whether a record has CID or
   not, connections which have negotiated this extension use new record
   types for all protected records.  Table 1 shows the record types to

                   | New ContentType           | Value |
                   | alert_with_cid            | 25    |
                   |                           |       |
                   | handshake_with_cid        | 26    |
                   |                           |       |
                   | application_data_with_cid | 27    |
                   |                           |       |
                   | heartbeat_with_cid        | 28    |

                                  Table 1

5.  Record Payload Protection

   The CID value, when present, is included in the MAC calculation for
   the DTLS record.  The MAC algorithm described in Section of
   [RFC6347] and Section of [RFC5246] is extended as follows:

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         MAC(MAC_write_key, DTLSCompressed.epoch +
                               DTLSCompressed.sequence_number +
                               DTLSCompressed.type +
                               DTLSCompressed.version +
                               connection_id + // New field
                               cid_length +        // New input
                               cid +               // New input
                               DTLSCompressed.length +
      where "+" denotes concatenation.

6.  Examples

   Figure 2 shows an example exchange where a connection id is used uni-
   directionally from the client to the server.

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


                               <--------      HelloVerifyRequest

   ClientHello                 -------->

                               <--------             ServerHello

   Certificate                 -------->
                               <--------      [ChangeCipherSpec]

   Application Data           ========>
                              <========         Application Data

          Figure 2: Example DTLS 1.2 Exchange with Connection IDs

7.  Security and Privacy Considerations

   The connection id 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].

   In addition, endpoints can use the connection ID to attach arbitrary
   metadata to each record they receive.  This may be used as a
   mechanism to communicate per-connection to on-path observers.  There
   is no straightforward way to address this with connection IDs that

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   contain arbitrary values; implementations concerned about this SHOULD
   refuse to use connection ids.

   An on-path adversary, who is able to observe 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 connection id pair (for bi-directional communication).  Without
   multi-homing or mobility, the use of the connection id is not
   different to the use of the 5-tuple.

   With multi-homing, an adversary is able to correlate the
   communication interaction over the two paths, which adds further
   privacy concerns.  In order to prevent this, implementations SHOULD
   attempt to use fresh connection IDs whenever they change local
   addresses or ports (though this is not always possible to detect).

   Importantly, the sequence number makes it possible for a passive
   attacker to correlate packets across CID changes.  Thus, even if a
   client/server pair do a rehandshake to change CID, that does not
   provide much privacy benefit.

   This document does not change the security properties of DTLS
   [RFC6347].  It merely provides a more robust mechanism for
   associating an incoming packet with a stored security context.

8.  IANA Considerations

   IANA is requested to allocate an entry to the existing TLS
   "ExtensionType Values" registry, defined in [RFC5246], for
   connection_id(TBD) defined in this document.

   IANA is requested to allocate the following new values in the "TLS
   ContentType Registry":

   -  alert_with_cid(25)

   -  handshake_with_cid(26)

   -  application_data_with_cid(27)

   -  heartbeat_with_cid(28)

9.  References

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9.1.  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, <>.

9.2.  Informative References

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-26 (work in progress), March

   [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,

9.3.  URIs




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Appendix A.  History



   -  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 since the
   functionality has been highly desired by the IoT community.  We would
   like to thank the following individuals for their contributions in
   earlier specifications:

   * Nikos Mavrogiannopoulos

   Additionally, we would like to thank Yin Xinxing (Huawei), Tobias
   Gondrom (Huawei), and the Connection ID task force team members:

   -  Martin Thomson (Mozilla)

   -  Christian Huitema (Private Octopus Inc.)

   -  Jana Iyengar (Google)

   -  Daniel Kahn Gillmor (ACLU)

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   -  Patrick McManus (Mozilla)

   -  Ian Swett (Google)

   -  Mark Nottingham (Fastly)

   Finally, we want to thank the IETF TLS working group chairs, 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


   Tobias Gondrom


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