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Versions: 00 01 02 draft-ietf-tls-dtls-connection-id

TLS                                                     E. Rescorla, Ed.
Internet-Draft                                                RTFM, Inc.
Obsoletes: 6347 (if approved)                         H. Tschofenig, Ed.
Intended status: Standards Track                             ARM Limited
Expires: May 18, 2018                                         T. Fossati
                                                                   Nokia
                                                              T. Gondrom
                                                                  Huawei
                                                       November 14, 2017


   The Datagram Transport Layer Security (DTLS) Connection Identifier
                draft-rescorla-tls-dtls-connection-id-02

Abstract

   This document specifies the "Connection ID" concept for the Datagram
   Transport Layer Security (DTLS) protocol, version 1.2 and version
   1.3.

   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.

   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 http://datatracker.ietf.org/drafts/current/.

   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 May 18, 2018.







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Copyright Notice

   Copyright (c) 2017 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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   This document may contain material from IETF Documents or IETF
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   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  The "connection_id" Extension . . . . . . . . . . . . . . . .   3
   4.  Post-Handshake Messages . . . . . . . . . . . . . . . . . . .   5
   5.  Record Layer Extensions . . . . . . . . . . . . . . . . . . .   5
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security and Privacy Considerations . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Working Group Information  . . . . . . . . . . . . .  11
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12







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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 to amortize the cost of the initial
   handshake to 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
   is 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 the device needs to
   enter extended sleep periods to increase the battery lifetime and is
   therefore subject to rebinding.  This 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
   each DTLS record.  The presence of the connection ID is negotiated
   via a DTLS extension.  It also defines a DTLS 1.3 post-handshake
   message to change connection ids.

2.  Conventions and Terminology

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

   The reader is assumed to be familiar with the DTLS specifications
   since this document defines an extension to DTLS 1.2 and DTLS 1.3.

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;




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   The extension_data field of this extension, when included in the
   ClientHello, MUST contain the CID structure, which contains the CID
   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
   same).

     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 which 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 CID).

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

   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 (DTLS 1.2
   language) or to a PSK-based handshake using the ticket-based
   approach.

   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.  Note that such implementations must still be able to
   send other length connection identifiers to other parties.

   In DTLS 1.2, 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 1.2 in general does not allow post-handshake messages
   that do not themselves begin other handshakes.  In DTLS 1.3, which
   does allow such messages, we use post-handshake message to update the
   connection ID Section 4 and to request new IDs.



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   DTLS 1.2 peers switch to the new record layer format when encryption
   is enabled.  The same is true for DTLS 1.3 but since the DTLS 1.3
   enables encryption early in the handshake phase the connection ID
   will be enabled earlier.  For this reason, the connection ID needs to
   go in the DTLS 1.3 ServerHello.

4.  Post-Handshake Messages

   In DTLS 1.3, if the client and server have negotiated the
   "connection_id" extension, either side can send a new connection ID
   which it wishes the other side to use in a NewConnectionId message:

      enum {
          cid_immediate(0), cid_spare(1), (255)
      } ConnectionIdUsage;

      struct {
          opaque cid<0..2^8-1>;
          ConnectionIdUsage usage;
      } NewConnectionId;

   cid  Indicates the CID which the sender wishes the peer to use.

   usage  Indicates whether the new CID should be used immediately or is
      a spare.  If usage is set to "cid_immediate", then the new CID
      MUST be used immediately for all future records.  If it is set to
      "cid_spare", then either CID MAY be used, as described in
      Section 7.

   If the client and server have negotiated the "connection_id"
   extension, either side can request a new CID using the
   RequestConnectionId message.

      struct {
      } RequestConnectionId;

   Endpoints SHOULD respond to RequestConnectionId by sending a
   NewConnectionId with usage "cid_spare" as soon as possible.  Note
   that an endpoint MAY ignore requests which it considers excessive
   (though they MUST be ACKed as usual).

5.  Record Layer Extensions

   This extension is applicable for use with DTLS 1.2 and DTLS 1.3.
   This extension can be used with the optimized DTLS 1.3 record layer
   format.





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   Figure 1 and Figure 2 illustrate the record formats of DTLS 1.2 and
   DTLS 1.3, respectively.

     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

     struct {
        opaque content[DTLSPlaintext.length];
        ContentType type;
        uint8 zeros[length_of_padding];
     } DTLSInnerPlaintext;

     struct {
        ContentType opaque_type = 23; /* application_data */
        ProtocolVersion legacy_record_version = {254,253); // DTLSv1.2
        uint16 epoch;                         // DTLS-related field
        uint48 sequence_number;               // DTLS-related field
        opaque cid[cid_length];               // New field
        uint16 length;
        opaque encrypted_record[length];
     } DTLSCiphertext;

            Figure 2: DTLS 1.3 Record Format with Connection ID

   Besides the "cid" field, all other fields are defined in the DTLS 1.2
   and DTLS 1.3 specifications.

   Note that for both record formats, it is not possible to parse the
   records without knowing if the connection ID is in use and how long
   it is.

6.  Examples

   Below is an example exchange for DTLS 1.3 using a single connection
   id in each direction.




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

   ClientHello
   (connection_id=5)
                               -------->


                               <--------       HelloRetryRequest
                                                        (cookie)

   ClientHello                 -------->
   (connection_id=5)
     +cookie

                               <--------             ServerHello
                                             (connection_id=100)
                                             EncryptedExtensions
                                                         (cid=5)
                                                     Certificate
                                                         (cid=5)
                                               CertificateVerify
                                                         (cid=5)
                                                        Finished
                                                         (cid=5)

   Certificate                -------->
   (cid=100)
   CertificateVerify
   (cid=100)
   Finished
   (cid=100)
                              <--------                      Ack
                                                         (cid=5)

   Application Data           ========>
   (cid=100)
                              <========         Application Data
                                                         (cid=5)

          Figure 3: Example DTLS 1.3 Exchange with Connection IDs

   Below is an example exchange for DTLS 1.2 using a connection id used
   uni-directionally from the client to the server.







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

   ClientHello
   (connection_id=empty)
                               -------->


                               <--------      HelloVerifyRequest
                                                        (cookie)

   ClientHello                 -------->
   (connection_id=empty)
     +cookie

                               <--------             ServerHello
                                             (connection_id=100)
                                                     Certificate
                                               ServerKeyExchange
                                              CertificateRequest
                                                 ServerHelloDone

   Certificate                 -------->
   ClientKeyExchange
   CertificateVerify
   [ChangeCipherSpec]
   Finished
   (cid=100)
                               <--------      [ChangeCipherSpec]
                                                        Finished

   Application Data           ========>
   (cid=100)
                              <========         Application Data

          Figure 4: 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 1.2 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).  In DTLS
   1.3, it is possible to provide new encrypted connection IDs, though
   of course those IDs are immediately used on the wire.  Without multi-
   homing and 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).
   In DTLS 1.3, The RequestConnectionId message can be used to ask for
   new IDs in order to ensure that you have a pool of suitable IDs.

   This document does not change the security properties of DTLS 1.2
   [RFC6347] and DTLS 1.3 [I-D.ietf-tls-dtls13].  It merely provides a
   more robust mechanism for associating an incoming packet with a
   stored security context.

   [[OPEN ISSUE: Sequence numbers leak connection IDs.  We need to
   update the document to address this.  One possibility would be the
   technique documented in https://quicwg.github.io/base-drafts/draft-
   ietf-quic-transport.html#packet-number-gap.]]

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 two values in the "TLS Handshake Type"
   registry, defined in [RFC5246], for request_connection_id (TBD), and
   new_connection_id (TBD), as defined in this document.

9.  References

9.1.  Normative References








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   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-02 (work in progress), October
              2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
              editor.org/info/rfc5246>.

   [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>.

9.2.  Informative References

   [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, <https://www.rfc-
              editor.org/info/rfc6973>.

9.3.  URIs

   [1] mailto:tls@ietf.org




















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Appendix A.  History

   RFC EDITOR: PLEASE REMOVE THE THIS SECTION

   draft-rescorla-tls-dtls-connection-id-00

   -  Initial version

Appendix B.  Working Group Information

   The discussion list for the IETF TLS working group is located at the
   e-mail address tls@ietf.org [1].  Information on the group and
   information on how to subscribe to the list is at
   https://www1.ietf.org/mailman/listinfo/tls

   Archives of the list can be found at: https://www.ietf.org/mail-
   archive/web/tls/current/index.html

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
     RedHat
     nmav@redhat.com

   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)

   -  Patrick McManus (Sole Proprietor)

   -  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.



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Authors' Addresses

   Eric Rescorla (editor)
   RTFM, Inc.

   EMail: ekr@rtfm.com


   Hannes Tschofenig (editor)
   ARM Limited

   EMail: hannes.tschofenig@arm.com


   Thomas Fossati
   Nokia

   EMail: thomas.fossati@nokia.com


   Tobias Gondrom
   Huawei

   EMail: tobias.gondrom@gondrom.org



























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