Return Routability Check for DTLS 1.2 and DTLS 1.3

Document Type Active Internet-Draft (tls WG)
Authors Hannes Tschofenig  , Thomas Fossati 
Last updated 2021-06-10 (latest revision 2021-06-09)
Replaces draft-tschofenig-tls-dtls-rrc
Stream Internet Engineering Task Force (IETF)
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TLS                                                   H. Tschofenig, Ed.
Internet-Draft                                                T. Fossati
Updates: 6347 (if approved)                                  Arm Limited
Intended status: Standards Track                             9 June 2021
Expires: 11 December 2021

           Return Routability Check for DTLS 1.2 and DTLS 1.3


   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 (, which is archived

   Source for this draft and an issue tracker can be found at

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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 December 2021.

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

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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.

   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",
   "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.

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   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.  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 {
       heartbeat(24),  /* RFC 6520 */
       return_routability_check(TBD), /* NEW */
   } ContentType;

   struct {
       opaque cookie<1..2^16-1>;
   } Cookie;

   struct {
       Cookie cookie;
   } return_routability_check;

   The newly introduced return_routability_check message contains a
   cookie.  The semantic of the cookie is similar to the cookie used in
   the HelloRetryRequest message defined in [RFC8446].

   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 sending rate
   to a TBD threshold) and initiate the return routability check that
   proceeds as follows:

   1.  A cookie is placed in the return_routability_check message;

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   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 it back;

   4.  When the initiator receives and verifies the
       return_routability_check message, it updates the peer address

   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.

4.  Example

   The example shown in Figure 1 illustrates a client and a server
   exchanging application payloads protected by DTLS with an
   unilaterally used CIDs.  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, to avoid
   being abused (e.g., as an amplifier) by an attacker impersonating the

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

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

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

         Application Data            ========>

                                                <<< Unverified IP
                                                    Address B >>

                                     <--------  Return Routability Check

         Return Routability Check    -------->

                                                <<< IP Address B
                                                    Verified >>

                                     <========        Application Data

                    Figure 1: Return Routability Example

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5.  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.

6.  IANA Considerations

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

7.  Open Issues

   Issues against this document are tracked at

8.  Acknowledgments

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

9.  Normative References

              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-12, 11
              May 2021, <

              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,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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

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   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

Appendix A.  History



   *  Draft name changed after WG adoption


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


   *  Initial version

Authors' Addresses

   Hannes Tschofenig (editor)
   Arm Limited


   Thomas Fossati
   Arm Limited


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