Return Routability Check for DTLS 1.2 and DTLS 1.3
draft-ietf-tls-dtls-rrc-01

TLS                                                   H. Tschofenig, Ed.
Internet-Draft                                                T. Fossati
Updates: 6347 (if approved)                                  Arm Limited
Intended status: Standards Track                         25 October 2021
Expires: 28 April 2022

           Return Routability Check for DTLS 1.2 and DTLS 1.3
                       draft-ietf-tls-dtls-rrc-01

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.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Transport Layer
   Security Working Group mailing list (tls@ietf.org), which is archived
   at https://mailarchive.ietf.org/arch/browse/tls/.

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/dtls-rrc.

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
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   This Internet-Draft will expire on 28 April 2022.

Copyright Notice

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  RRC Extension . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  The Return Routability Check Message  . . . . . . . . . . . .   4
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security and Privacy Considerations . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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.

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   Of course, it is not necessary to re-run the full handshake if
   session resumption is supported and negotiated.

   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.

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].

3.  RRC Extension

   This specification uses the tls_flags extension defined in
   [I-D.ietf-tls-tlsflags] to allow a client and a server to negotiate
   support for this extension.

   The RRC flag is assigned the value (TBD1) and is used in the
   ClientHello (CH) and the ServerHello (SH).

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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(TBD), /* 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;

   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.

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

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   1.  A 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 timeout T
       is started;

   3.  The peer endpoint, after successfully verifying the received
       return_routability_check message echoes 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.

5.  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.

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       Client                                           Server

Key  ^ ClientHello
Exch | + key_share
     | + signature_algorithms
     | + tls_flags (RRC)
     v + connection_id=empty
                               -------->
                                                  ServerHello  ^ Key
                                                 +  key_share  | Exch
                                          + connection_id=100  |
                                            + tls_flags (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.

   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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6.  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.

7.  IANA Considerations

   IANA is requested to allocate an entry to the TLS "ContentType"
   registry, for the return_routability_check(TBD) defined in this
   document.

   IANA is requested to allocate an entry to the TLS Flags registry in
   the tls_flags type:

   *  Value: [[IANA please assign a value from the 32-63 value range.]]

   *  Flag Name: RRC

   *  Message: CH,SH

   *  Recommended: Y

   *  Reference: [[This document]]

8.  Open Issues

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

9.  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.

10.  Normative References

   [I-D.ietf-tls-dtls-connection-id]
              Rescorla, E., Tschofenig, H., Fossati, T., and A. Kraus,
              "Connection Identifiers for DTLS 1.2", Work in Progress,
              Internet-Draft, draft-ietf-tls-dtls-connection-id-13, 22
              June 2021, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tls-dtls-connection-id-13>.

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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", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              dtls13-43>.

   [I-D.ietf-tls-tlsflags]
              Nir, Y., "A Flags Extension for TLS 1.3", Work in
              Progress, Internet-Draft, draft-ietf-tls-tlsflags-06, 13
              July 2021, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tls-tlsflags-06>.

   [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/rfc/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/rfc/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/rfc/rfc8446>.

Appendix A.  History

   RFC EDITOR: PLEASE REMOVE THE THIS SECTION

   draft-ietf-tls-dtls-rrc-01

   *  Use the TLS flags extension for negotiating RRC

   *  Enhanced IANA consideration section

   *  Expanded example section

   *  Revamp message layout:

      -  Use 8-byte fixed size cookies

      -  Explicitly separate path challenge from response

   draft-ietf-tls-dtls-rrc-00

   *  Draft name changed after WG adoption

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   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
   Arm Limited

   Email: thomas.fossati@arm.com

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