TLS                                                   H. Tschofenig, Ed.
Internet-Draft                                           T. Fossati, Ed.
Updates: 6347 (if approved)                                  Arm Limited
Intended status: Standards Track                       December 21, 2021
Expires: June 24, 2022


           Return Routability Check for DTLS 1.2 and DTLS 1.3
                       draft-ietf-tls-dtls-rrc-03

Abstract

   This document specifies a return routability check for use in context
   of the Connection ID (CID) construct for the Datagram Transport Layer
   Security (DTLS) protocol versions 1.2 and 1.3.

Status of This Memo

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   This Internet-Draft will expire on June 24, 2022.

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  RRC Extension . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  The Return Routability Check Message  . . . . . . . . . . . .   4
   5.  Path Validation Procedure . . . . . . . . . . . . . . . . . .   5
     5.1.  Path Challenge Requirements . . . . . . . . . . . . . . .   5
     5.2.  Path Response Requirements  . . . . . . . . . . . . . . .   6
     5.3.  Timer Choice  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security and Privacy Considerations . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   In "classical" DTLS, selecting a security context of an incoming DTLS
   record is accomplished with the help of the 5-tuple, i.e. source IP
   address, source port, transport protocol, destination IP address, and
   destination port.  Changes to this 5 tuple can happen for a variety
   reasons over the lifetime of the DTLS session.  In the IoT context,
   NAT rebinding is common with sleepy devices.  Other examples include
   end host mobility and multi-homing.  Without CID, 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.  As a result, the DTLS handshake has to be re-run.
   Of course, it is not necessary to re-run the full handshake if
   session resumption is supported and negotiated.




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   A CID is an identifier carried in the record layer header of a DTLS
   datagram that gives the receiver additional information for selecting
   the appropriate security context.  The CID mechanism has been
   specified in [I-D.ietf-tls-dtls-connection-id] for DTLS 1.2 and in
   [I-D.ietf-tls-dtls13] for DTLS 1.3.

   Section 6 of [I-D.ietf-tls-dtls-connection-id] describes how the use
   of CID increases the attack surface by providing both on-path and
   off-path attackers an opportunity for (D)DoS.  It then goes on
   describing the steps a DTLS principal must take when a record with a
   CID is received that has a source address (and/or port) different
   from the one currently associated with the DTLS connection.  However,
   the actual mechanism for ensuring that the new peer address is
   willing to receive and process DTLS records is left open.  This
   document standardizes a return routability check (RRC) as part of the
   DTLS protocol itself.

   The return routability check is performed by the receiving peer
   before the CID-to-IP address/port binding is updated in that peer's
   session state database.  This is done in order to provide more
   confidence to the receiving peer that the sending peer is reachable
   at the indicated address and port.

   Note however that, irrespective of CID, if RRC has been successfully
   negotiated by the peers, path validation can be used at any time by
   either endpoint.  For instance, an endpoint might use RRC to check
   that a peer is still in possession of its address after a period of
   quiescence.

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

   This document assumes familiarity with the CID format and protocol
   defined for DTLS 1.2 [I-D.ietf-tls-dtls-connection-id] and for DTLS
   1.3 [I-D.ietf-tls-dtls13].  The presentation language used in this
   document is described in Section 4 of [RFC8446].

   This document reuses the definition of "anti-amplification limit"
   from [RFC9000] to mean three times the amount of data received from
   an unvalidated address.  This includes all DTLS records originating
   from that source address, excluding discarded ones.





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3.  RRC Extension

   The use of RRC is negotiated via the "rrc" DTLS-only extension.  On
   connecting, the client includes the "rrc" extension in its
   ClientHello if it wishes to use RRC.  If the server is capable of
   meeting this requirement, it responds with a "rrc" extension in its
   ServerHello.  The "extension_type" value for this extension is TBD1
   and the "extension_data" field of this extension is empty.  The
   client and server MUST NOT use RRC unless both sides have
   successfully exchanged "rrc" extensions.

   Note that the RRC extension applies to both DTLS 1.2 and DTLS 1.3.

4.  The Return Routability Check Message

   When a record with CID is received that has the source address of the
   enclosing UDP datagram different from the one previously associated
   with that CID, the receiver MUST NOT update its view of the peer's IP
   address and port number with the source specified in the UDP datagram
   before cryptographically validating the enclosed record(s) but
   instead perform a return routability check.

   enum {
       invalid(0),
       change_cipher_spec(20),
       alert(21),
       handshake(22),
       application_data(23),
       heartbeat(24),  /* RFC 6520 */
       return_routability_check(TBD2), /* NEW */
       (255)
   } ContentType;

   uint64 Cookie;

   enum {
       path_challenge(0),
       path_response(1),
       reserved(2..255)
   } rrc_msg_type;

   struct {
       rrc_msg_type msg_type;
       select (return_routability_check.msg_type) {
           case path_challenge: Cookie;
           case path_response:  Cookie;
       };
   } return_routability_check;



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   The newly introduced "return_routability_check" message contains a
   cookie.  The cookie is a 8-byte field containing arbitrary data.

   The "return_routability_check" message MUST be authenticated and
   encrypted using the currently active security context.

5.  Path Validation Procedure

   The receiver that observes the peer's address or port update MUST
   stop sending any buffered application data (or limit the data sent to
   the unvalidated address to the anti-amplification limit) and initiate
   the return routability check that proceeds as follows:

   1.  An unpredictable cookie is placed in a "return_routability_check"
       message of type path_challenge;

   2.  The message is sent to the observed new address and a timer T
       (see Section 5.3) is started;

   3.  The peer endpoint, after successfully verifying the received
       "return_routability_check" message responds by echoing the cookie
       value in a "return_routability_check" message of type
       path_response;

   4.  When the initiator receives and verifies the
       "return_routability_check" message contains the sent cookie, it
       updates the peer address binding;

   5.  If T expires, or the address confirmation fails, the peer address
       binding is not updated.

   After this point, any pending send operation is resumed to the bound
   peer address.

   Section 5.1 and Section 5.2 contain the requirements for the
   initiator and responder roles, broken down per protocol phase.

5.1.  Path Challenge Requirements

   -  The initiator MAY send multiple "return_routability_check"
      messages of type path_challenge to cater for packet loss on the
      probed path.

      o  Each path_challenge SHOULD go into different transport packets.
         (Note that the DTLS implementation may not have control over
         the packetization done by the transport layer.)





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      o  The transmission of subsequent path_challenge messages SHOULD
         be paced to decrease the chance of loss.

      o  Each path_challenge message MUST contain random data.

   -  The initiator MAY use padding using the record padding mechanism
      available in DTLS 1.3 (and in DTLS 1.2, when CID is enabled on the
      sending direction) up to the anti-amplification limit to probe if
      the path MTU (PMTU) for the new path is still acceptable.

5.2.  Path Response Requirements

   -  The responder MUST NOT delay sending an elicited path_response
      message.

   -  The responder MUST send exactly one path_response messages for
      each received path_request.

   -  The responder MUST send the path_response on the network path
      where the corresponding path_challenge has been received, so that
      validation succeeds only if the path is functional in both
      directions.

      o  The initiator MUST NOT enforce this behaviour

   -  The initiator MUST silently discard any invalid path_response it
      receives.

   Note that RRC does not cater for PMTU discovery on the reverse path.
   If the responder wants to do PMTU discovery using RRC, it should
   initiate a new path validation procedure.

5.3.  Timer Choice

   When setting T, implementations are cautioned that the new path could
   have a longer round-trip time (RTT) than the original.

   In settings where there is external information about the RTT of the
   active path, implementations SHOULD use T = 3xRTT.

   If an implementation has no way to obtain information regarding the
   RTT of the active path, a value of 1s SHOULD be used.

   Profiles for specific deployment environments - for example,
   constrained networks [I-D.ietf-uta-tls13-iot-profile] - MAY specify a
   different, more suitable value.





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6.  Example

   The example TLS 1.3 handshake shown in Figure 1 shows a client and a
   server negotiating the support for CID and for the RRC extension.

       Client                                           Server

Key  ^ ClientHello
Exch | + key_share
     | + signature_algorithms
     | + rrc
     v + connection_id=empty
                               -------->
                                                  ServerHello  ^ Key
                                                 +  key_share  | Exch
                                          + connection_id=100  |
                                                        + rrc  v
                                        {EncryptedExtensions}  ^  Server
                                         {CertificateRequest}  v  Params
                                                {Certificate}  ^
                                          {CertificateVerify}  | Auth
                               <--------           {Finished}  v

     ^ {Certificate}
Auth | {CertificateVerify}
     v {Finished}              -------->
       [Application Data]      <------->  [Application Data]

              +  Indicates noteworthy extensions sent in the
                 previously noted message.

              *  Indicates optional or situation-dependent
                 messages/extensions that are not always sent.

              {} Indicates messages protected using keys
                 derived from a [sender]_handshake_traffic_secret.

              [] Indicates messages protected using keys
                 derived from [sender]_application_traffic_secret_N.

               Figure 1: Message Flow for Full TLS Handshake

   Once a connection has been established the client and the server
   exchange application payloads protected by DTLS with an unilaterally
   used CIDs.  In our case, the client is requested to use CID 100 for
   records sent to the server.





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   At some point in the communication interaction the IP address used by
   the client changes and, thanks to the CID usage, the security context
   to interpret the record is successfully located by the server.
   However, the server wants to test the reachability of the client at
   his new IP address.














































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

         Application Data            ========>
         <CID=100>
         Src-IP=A
         Dst-IP=Z
                                     <========        Application Data
                                                          Src-IP=Z
                                                          Dst-IP=A


                                 <<------------->>
                                 <<   Some      >>
                                 <<   Time      >>
                                 <<   Later     >>
                                 <<------------->>


         Application Data            ========>
         <CID=100>
         Src-IP=B
         Dst-IP=Z

                                                <<< Unverified IP
                                                    Address B >>

                                     <--------  Return Routability Check
                                                path_challenge(cookie)
                                                       Src-IP=Z
                                                       Dst-IP=B

         Return Routability Check    -------->
         path_response(cookie)
         Src-IP=B
         Dst-IP=Z

                                                <<< IP Address B
                                                    Verified >>


                                     <========        Application Data
                                                          Src-IP=Z
                                                          Dst-IP=B

                   Figure 2: Return Routability Example





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7.  Security and Privacy Considerations

   Note that the return routability checks do not protect against
   flooding of third-parties if the attacker is on-path, as the attacker
   can redirect the return routability checks to the real peer (even if
   those datagrams are cryptographically authenticated).  On-path
   adversaries can, in general, pose a harm to connectivity.

8.  IANA Considerations

   IANA is requested to allocate an entry to the TLS "ContentType"
   registry, for the "return_routability_check(TBD2)" defined in this
   document.  The "return_routability_check" content type is only
   applicable to DTLS 1.2 and 1.3.

   IANA is requested to allocate the extension code point (TBD1) for the
   "rrc" extension to the "TLS ExtensionType Values" registry as
   described in Table 1.

   +-------+-------------+-------+-----------+-------------+-----------+
   | Value | Extension   | TLS   | DTLS-Only | Recommended | Reference |
   |       | Name        | 1.3   |           |             |           |
   +-------+-------------+-------+-----------+-------------+-----------+
   | TBD1  | rrc         | CH,   | Y         | N           | RFC-THIS  |
   |       |             | SH    |           |             |           |
   +-------+-------------+-------+-----------+-------------+-----------+

        Table 1: rrc entry in the TLS ExtensionType Values registry

9.  Open Issues

   Issues against this document are tracked at https://github.com/tlswg/
   dtls-rrc/issues

10.  Acknowledgments

   We would like to thank Achim Kraus, Hanno Becker, Hanno Boeck, Manuel
   Pegourie-Gonnard, Mohit Sahni and Rich Salz for their input to this
   document.

11.  References

11.1.  Normative References

   [I-D.ietf-tls-dtls-connection-id]
              RTFM, Inc., Arm Limited, Arm Limited, and Bosch.IO GmbH,
              "Connection Identifiers for DTLS 1.2", draft-ietf-tls-
              dtls-connection-id-13 (work in progress), June 2021.



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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-dtls13-43 (work in progress), April
              2021.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

11.2.  Informative References

   [I-D.ietf-uta-tls13-iot-profile]
              Arm Limited and Arm Limited, "TLS/DTLS 1.3 Profiles for
              the Internet of Things", draft-ietf-uta-tls13-iot-
              profile-03 (work in progress), October 2021.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.





















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Appendix A.  History

   [[CREF1: RFC EDITOR: PLEASE REMOVE THIS SECTION]]

   draft-ietf-tls-dtls-rrc-02

   -  Undo the TLS flags extension for negotiating RRC, use a new
      extension type

   draft-ietf-tls-dtls-rrc-01

   -  Use the TLS flags extension for negotiating RRC

   -  Enhanced IANA consideration section

   -  Expanded example section

   -  Revamp message layout:

      o  Use 8-byte fixed size cookies

      o  Explicitly separate path challenge from response

   draft-ietf-tls-dtls-rrc-00

   -  Draft name changed after WG adoption

   draft-tschofenig-tls-dtls-rrc-01

   -  Removed text that overlapped with draft-ietf-tls-dtls-connection-
      id

   draft-tschofenig-tls-dtls-rrc-00

   -  Initial version

Authors' Addresses

   Hannes Tschofenig (editor)
   Arm Limited

   EMail: hannes.tschofenig@arm.com


   Thomas Fossati (editor)
   Arm Limited

   EMail: thomas.fossati@arm.com



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