DTLS return routability: Return Routability Check for DTLS 1.2 and 1.3

Internet Engineering Task Force (IETF)                H. Tschofenig, Ed.
Request for Comments: 9853                                      UniBw M.
Updates: 9146, 9147                                             A. Kraus
Category: Standards Track
ISSN: 2070-1721                                               T. Fossati
                                                                  Linaro
                                                              March 2026


             Return Routability Check for DTLS 1.2 and 1.3

Abstract

   This document specifies a Return Routability Check (RRC) subprotocol
   for use in the context of the Connection ID (CID) construct for the
   Datagram Transport Layer Security (DTLS) protocol versions 1.2 and
   1.3.

   Implementations offering the CID functionality described in RFCs 9146
   and 9147 are encouraged to also provide the RRC functionality
   described in this document.  For this reason, this document updates
   RFCs 9146 and 9147.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9853.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Terminology
   3.  RRC Extension
     3.1.  RRC and CID Interplay
   4.  Return Routability Check Message Types
   5.  Path Validation Procedure
     5.1.  Basic
     5.2.  Enhanced
     5.3.  Path Challenge Requirements
     5.4.  Path Response/Drop Requirements
     5.5.  Timer Choice
   6.  Example
   7.  Operational Considerations
     7.1.  Logging Anomalous Events
     7.2.  Middlebox Interference
   8.  Security Considerations
     8.1.  Attacker Model
       8.1.1.  Amplification
       8.1.2.  Off-Path Packet Forwarding
   9.  Privacy Considerations
   10. IANA Considerations
     10.1.  New TLS ContentType
     10.2.  New TLS ExtensionType
     10.3.  New "TLS RRC Message Type" Registry
       10.3.1.  Designated Expert Instructions
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   A Connection ID (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 [RFC9146] for DTLS 1.2 and in
   [RFC9147] for DTLS 1.3.

   Section 6 of [RFC9146] describes how the use of CID increases the
   attack surface of DTLS 1.2 and 1.3 by providing both on-path and off-
   path attackers an opportunity for DoS or DDoS.  It also describes the
   steps a DTLS principal must take when a record with a CID is received
   that has a source address 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.  To address the gap, this document defines a
   Return Routability Check (RRC) subprotocol for DTLS 1.2 and 1.3,
   inspired by the path validation procedure defined in Section 8.2 of
   [RFC9000].  As such, this document updates [RFC9146] and [RFC9147].

   The return routability check is performed by the receiving endpoint
   before the CID-address binding is updated in that endpoint's session
   state.  This is done in order to give the receiving endpoint
   confidence that the sending peer is in fact reachable at the source
   address indicated in the received datagram.  For an illustration of
   the handshake and address validation phases, see Section 6.

   Section 5.1 of this document explains the fundamental mechanism that
   aims to reduce the DDoS attack surface.  Additionally, Section 5.2
   discusses a more advanced address validation mechanism.  This
   mechanism is designed to counteract off-path attackers trying to
   place themselves on-path by racing packets that trigger address
   rebinding at the receiver.  To gain a detailed understanding of the
   attacker model, please refer to Section 8.1.

   Apart from of its use in the context of CID-address binding updates,
   the path validation capability offered by RRC can be used at any time
   by either endpoint.  For instance, an endpoint might use RRC to check
   that a peer is still reachable at its last known 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 [RFC9146] and for DTLS 1.3 [RFC9147].  The
   presentation language used in this document is described in Section 4
   of [RFC8446].

   In this document, the term "anti-amplification limit" means three
   times the amount of data received from an unvalidated address.  This
   includes all DTLS records originating from that source address,
   excluding those that have been discarded.  This follows the pattern
   of [RFC9000], applying a similar concept to DTLS.

   The term "address" is defined in Section 1.2 of [RFC9000].

   The terms "client", "server", "peer", and "endpoint" are defined in
   Section 1.1 of [RFC8446].

3.  RRC Extension

   The use of RRC is negotiated via the rrc extension.  The rrc
   extension is only defined for DTLS 1.2 and 1.3.  On connecting, a
   client wishing to use RRC includes the rrc extension in its
   ClientHello.  If the server is capable of meeting this requirement,
   it responds with an rrc extension in its ServerHello.  The
   extension_type value for this extension is 61, and the extension_data
   field of this extension is empty.  A client offering the rrc
   extension MUST also offer the connection_id extension [RFC9146].  If
   the client includes the rrc extension in its ClientHello but omits
   the connection_id extension, the server MUST NOT include the rrc
   extension in its ServerHello.  A client offering the connection_id
   extension SHOULD also offer the rrc extension, unless the application
   using DTLS has its own address validation mechanism.  The client and
   server MUST NOT use RRC unless both sides have successfully exchanged
   rrc extensions.

3.1.  RRC and CID Interplay

   RRC offers an in-protocol mechanism to perform peer address
   validation that complements the "peer address update" procedure
   described in Section 6 of [RFC9146].  Specifically, when both CID
   [RFC9146] and RRC have been successfully negotiated for the session,
   if a record with CID is received that has the source address of the
   enclosing UDP datagram different from what is currently associated
   with that CID value, the receiver SHOULD perform a return routability
   check as described in Section 5, unless an application-specific
   address validation mechanism can be triggered instead (e.g.,
   Constrained Application Protocol (CoAP) Echo [RFC9175]).

4.  Return Routability Check Message Types

   This document defines the return_routability_check content type
   (Figure 1) to carry Return Routability Check messages.

   The RRC subprotocol consists of three message types: path_challenge,
   path_response, and path_drop.  These message types are used for path
   validation and selection as described in Section 5.

   Each message carries a Cookie, an 8-byte field containing 64 bits of
   entropy (e.g., obtained from the cryptographically secure
   pseudorandom number generator (CSPRNG) used by the TLS
   implementation; see Appendix C.1 of [RFC8446]).

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

   enum {
       invalid(0),
       change_cipher_spec(20),
       alert(21),
       handshake(22),
       application_data(23),
       heartbeat(24),  /* RFC 6520 */
       tls12_cid(25),  /* RFC 9146, DTLS 1.2 only */
       return_routability_check(27), /* NEW */
       (255)
   } ContentType;

   uint64 Cookie;

   enum {
       path_challenge(0),
       path_response(1),
       path_drop(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;
           case path_drop:      Cookie;
       };
   } return_routability_check;

        Figure 1: Return Routability Check Message and Content Type

   Future extensions to the RRC subprotocol may define new message
   types.  Implementations MUST be able to parse and understand the
   three RRC message types defined in this document.  In addition,
   implementations MUST be able to parse and gracefully ignore messages
   with an unknown msg_type.

5.  Path Validation Procedure

   A receiver that observes the peer's address change MUST stop sending
   any buffered application data or limit the data sent to the
   unvalidated address to the anti-amplification limit.  It then
   initiates the return routability check.

   This document describes two kinds of checks: basic (Section 5.1) and
   enhanced (Section 5.2).  The choice of one or the other depends on
   whether the off-path attacker scenario described in Section 8.1.2 is
   to be considered.  (The decision on what strategy to choose depends
   mainly on the threat model but may also be influenced by other
   considerations.  Examples of impacting factors include the need to
   minimise implementation complexity, privacy concerns, and the need to
   reduce the time it takes to switch path.  The choice may be offered
   as a configuration option to the user of the TLS implementation.)

   After the path validation procedure is complete, any pending send
   operation is resumed to the bound peer address.

   Sections 5.3 and 5.4 list the requirements for the initiator and
   responder roles, broken down per protocol phase.

   Please note that the presented algorithms are not designed to handle
   nested rebindings, i.e. rebindings that may occur while a path is
   being validated following a previous rebinding.  This should rarely
   occur, but if it happens, the path_response message is dropped, the
   address validation times out, and the address will not be updated.  A
   new path validation will start when new data is received.

   Also, note that in the event of a NAT rebind, the initiator and
   responder will have different views of the path: The initiator will
   see a new path, while the responder will still see the old one.

5.1.  Basic

   The basic return routability check comprises the following steps:

   1.  The receiver (i.e., the initiator) creates a
       return_routability_check message of type path_challenge and
       places the unpredictable cookie into the message.

   2.  The message is sent to the observed new address and a timer T
       (see Section 5.5) is started.

   3.  The peer (i.e., the responder) cryptographically verifies the
       received return_routability_check message of type path_challenge
       and responds by echoing the cookie value in a
       return_routability_check message of type path_response.

   4.  When the initiator receives the return_routability_check message
       of type path_response and verifies that it contains the sent
       cookie, it updates the peer address binding.

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

5.2.  Enhanced

   The enhanced return routability check comprises the following steps:

   1.  The receiver (i.e., the initiator) creates a
       return_routability_check message of type path_challenge and
       places the unpredictable cookie into the message.

   2.  The message is sent to the previously valid address, which
       corresponds to the old path.  Additionally, a timer T is started
       (see Section 5.5).

   3.  If the path is still functional, the peer (i.e., the responder)
       cryptographically verifies the received return_routability_check
       message of type path_challenge.  The action to be taken depends
       on whether the path through which the message was received
       remains the preferred one.

       *  If the path through which the message was received is
          preferred, a return_routability_check message of type
          path_response MUST be returned.  (Note that, from the
          responder's perspective, the preferred path and the old path
          coincide in the event of a NAT rebind.)

       *  If the path through which the message was received is no
          longer preferred, a return_routability_check message of type
          path_drop MUST be returned.  (Note that the responder must
          have initiated a voluntary path migration in order to know
          that this path is no longer the preferred one.)

       In either case, the peer echoes the cookie value in the response.

   4.  The initiator receives and verifies that the
       return_routability_check message contains the previously sent
       cookie.  The actions taken by the initiator differ based on the
       received message:

       *  When a return_routability_check message of type path_response
          is received, the initiator MUST continue using the previously
          valid address, i.e., no switch to the new path takes place and
          the peer address binding is not updated.

       *  When a return_routability_check message of type path_drop is
          received, the initiator MUST perform a basic return
          routability check on the observed new address, as described in
          Section 5.1.

   5.  If T expires, the peer address binding is not updated.  In this
       case, the initiator MUST perform a basic return routability check
       on the observed new address, as described in Section 5.1.

5.3.  Path Challenge Requirements

   *  The initiator MAY send multiple return_routability_check messages
      of type path_challenge to account for packet loss on the probed
      path.

      -  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.)

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

      -  Each path_challenge message MUST contain random data.

      -  In general, the number of "backup" path_challenge messages
         depends on the application, since some are more sensitive than
         others to latency caused by changes in the path.  In the
         absence of application-specific requirements, the initiator can
         send a path_challenge message once per round-trip time (RTT),
         up to the anti-amplification limit.

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

5.4.  Path Response/Drop Requirements

   *  The responder MUST NOT delay sending an elicited path_response or
      path_drop messages.

   *  The responder MUST send exactly one path_response or path_drop
      message for each valid path_challenge it received.

   *  The responder MUST send the path_response or the path_drop to the
      address from which the corresponding path_challenge was received.
      This ensures that the path is functional in both directions.

   *  The initiator MUST silently discard any invalid path_response or
      path_drop it receives.

   Note that RRC does not account 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.

5.5.  Timer Choice

   When setting T, implementations are cautioned that the new path could
   have a longer RTT than the original.

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

   If an implementation has no way to obtain information regarding the
   RTT of the active path, T SHOULD be set to 1 second.

   Profiles for specific deployment environments -- for example,
   constrained networks [IOT-PROFILE] -- MAY specify a different, more
   suitable value for T.

6.  Example

   Figure 2 shows an example of a DTLS 1.3 handshake in which a client
   and a server successfully negotiate support for both the CID and RRC
   extensions.

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

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

            Figure 2: Message Flow for Full DTLS Handshake

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

   At some point in the communication interaction, the 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
   its new address.

   Figure 3 shows the server initiating a basic RRC exchange (see
   Section 5.1) that establishes reachability of the client at the new
   address.

         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 3: Basic Return Routability Example

7.  Operational Considerations

7.1.  Logging Anomalous Events

   Logging of RRC operations at both ends of the protocol can be
   generally useful for the users of an implementation.  In particular,
   for Security Information and Event Management (SIEM) and
   troubleshooting purposes, it is strongly advised that implementations
   collect statistics about any unsuccessful RRC operations, as they
   could represent security-relevant events when they coincide with
   attempts by an attacker to interfere with the end-to-end path.  It is
   also advisable to log instances where multiple responses to a single
   path_challenge are received, as this could suggest an off-path attack
   attempt.

   In some cases, the presence of frequent path probes could indicate a
   problem with the stability of the path.  This information can be used
   to identify any issues with the underlying connectivity service.

7.2.  Middlebox Interference

   Since the DTLS 1.3 encrypted packet's record type is opaque to on-
   path observers, RRC messages are immune to middlebox interference
   when using DTLS 1.3.  In contrast, DTLS 1.2 RRC messages that are not
   wrapped in the tls12_cid record (e.g., in the server-to-client
   direction if the server negotiated a zero-length CID) have the
   return_routability_check content type in plain text, making them
   susceptible to interference (e.g., dropping of path_challenge
   messages), which would hinder the RRC functionality altogether.
   Therefore, when RRC is used in DTLS 1.2 and middlebox interference is
   a concern, it is recommended to enable CID in both directions.

8.  Security 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.

   If the RRC challenger reuses a cookie that was previously used in the
   same connection and does not implement anti-replay protection (see
   Section 4.5.1 of [RFC9147] and Section 4.1.2.6 of [RFC6347]), an
   attacker could replay a previously sent path_response message
   containing the reused cookie to mislead the challenger into switching
   to a path of the attacker's choosing.  To prevent this, RRC cookies
   must be _freshly_ generated using a reliable source of entropy
   [RFC4086].  See Appendix C.1 of [RFC8446] for guidance.

8.1.  Attacker Model

   Two classes of attackers are considered, off-path and on-path, with
   increasing capabilities (see Figure 4).  The following descriptions
   of these attackers are based on those introduced in QUIC
   (Section 21.1 of [RFC9000]):

   *  An off-path attacker is not on the original path between the DTLS
      peers, but it is able to observe packets on the original path and
      has a faster forwarding path compared to the DTLS peers, which
      allows it to make copies of the observed packets, race its copies
      to either peer, and consistently win the race.

   *  An on-path attacker is on the original path between the DTLS peers
      and is therefore capable, compared to the off-path attacker, to
      also drop and delay records at will.

   Note that, in general, attackers cannot craft DTLS records in a way
   that would successfully pass verification, due to the cryptographic
   protections applied by the DTLS record layer.

              .--> .------------------------------------. <--.
              |    | Inspect unencrypted portions       |    |
              |    +------------------------------------+    |
              |    | Inject                             |    |
          off-path +------------------------------------+    |
              |    | Reorder                            |    |
              |    +------------------------------------+    |
              |    | Modify unauthenticated portions    | on-path
              '--> +------------------------------------+    |
                   | Delay                              |    |
                   +------------------------------------+    |
                   | Drop                               |    |
                   +------------------------------------+    |
                   | Manipulate the packetization layer |    |
                   '------------------------------------' <--'

                      Figure 4: Attacker Capabilities

   RRC is designed to defend against the following attacks:

   *  On-path and off-path attackers that try to create an amplification
      attack by spoofing the source address of the victim
      (Section 8.1.1).

   *  Off-path attackers that try to put themselves on-path
      (Section 8.1.2), provided that the enhanced path validation
      algorithm is used (Section 5.2).

8.1.1.  Amplification

   Both on-path and off-path attackers can send a packet (either by
   modifying it on the fly or by copying, injecting, and racing it,
   respectively) with the source address modified to that of a victim
   host.  If the traffic generated by the server in response is larger
   compared to the received packet (e.g., a CoAP server returning an
   MTU's worth of data from a 20-byte GET request [AMP-ATTACKS]), the
   attacker can use the server as a traffic amplifier toward the victim.

8.1.1.1.  Mitigation Strategy

   When receiving a packet with a known CID that has a source address
   different from the one currently associated with the DTLS connection,
   an RRC-capable endpoint will not send a (potentially large) response
   but instead a small path_challenge message to the victim host.  Since
   the host is not able to decrypt it and generate a valid
   path_response, the address validation fails, which in turn keeps the
   original address binding unaltered.

   Note that in the case of an off-path attacker, the original packet
   still reaches the intended destination; therefore, an implementation
   could use a different strategy to mitigate the attack.

8.1.2.  Off-Path Packet Forwarding

   An off-path attacker that can observe packets might forward copies of
   genuine packets to endpoints over a different path.  If the copied
   packet arrives before the genuine packet, this will appear as a path
   change, like in a genuine NAT rebinding occurrence.  Any genuine
   packet will be discarded as a duplicate.  If the attacker is able to
   continue forwarding packets, it might be able to cause migration to a
   path via the attacker.  This places the attacker on-path, giving it
   the ability to observe or drop all subsequent packets.

   This style of attack relies on the attacker using a path that has the
   same or better characteristics (e.g., due to a more favourable
   service level agreements) as the direct path between endpoints.  The
   attack is more effective if relatively few packets are sent or if
   packet loss coincides with the attempted attack.

   A data packet received on the original path that increases the
   maximum received packet number will cause the endpoint to move back
   to that path.  Therefore, eliciting packets on this path increases
   the likelihood that the attack is unsuccessful.  However, note that,
   unlike QUIC, DTLS has no "non-probing" packets so this would require
   application-specific mechanisms.

8.1.2.1.  Mitigation Strategy

   Figure 5 illustrates the case where a receiver receives a packet with
   a new source address.  In order to determine that this path change
   was not triggered by an off-path attacker, the receiver will send an
   RRC message of type path_challenge (1) on the old path.

                             new                  old
                             path  .----------.  path
                                   |          |
                             .-----+ Receiver +-----.
                             |     |          |     |
                             |     '----------'     |
                             |                      1
                             |                      |
                             |                      |
                        .----+------.               v
                       / Attacker? /                |
                      '------+----'                 |
                             |                      |
                             |                      |
                             |                      |
                             |     .----------.     |
                             |     |          |     |
                             '-----+  Sender  +-----'
                                   |          |
                                   '----------'

               Figure 5: Off-Path Packet Forwarding Scenario

   Three cases need to be considered:

   Case 1: The old path is dead (e.g., due to a NAT rebinding), which
   leads to a timeout of (1).

   As shown in Figure 6, a path_challenge (2) needs to be sent on the
   new path.  If the sender replies with a path_response on the new path
   (3), the switch to the new path is considered legitimate.

                       new                      old
                       path    .----------.    path
                       .------>|          +-------.
                       | .-----+ Receiver +...... |
                       | | .---+          |     . |
                       | | |   '----------'     . |
              path-    3 | |                    . 1 path-
              response | | |                    . | challenge
                       | | |                    . |
                    .--|-+-|----------------------v--.
                   /   |   |       NAT            X / timeout
                  '----|-+-|-----------------------'
                       | | |                    .
                       | | 2 path-              .
                       | | | challenge          .
                       | | |   .----------.     .
                       | | '-->|          |     .
                       | '-----+  Sender  +.....'
                       '-------+          |
                               '----------'

                         Figure 6: Old Path Is Dead

   Case 2: The old path is alive but not preferred.

   This case is shown in Figure 7 whereby the sender replies with a
   path_drop message (2) on the old path.  This triggers the receiver to
   send a path_challenge (3) on the new path.  The sender will reply
   with a path_response (4) on the new path, thus providing confirmation
   for the path migration.

                       new                      old
                       path    .----------.    path
                       .------>|          |<------.
                       | .-----+ Receiver +-----. |
                       | | .---+          +---. | |
                       | | |   '----------'   | | |
              path-    4 | |        path-     1 | |
              response | | |        challenge | | |
                       | | |                  | | |
             .---------|-+-|----.          .--|-+-|-----------.
            /    NAT A |   |   /          /   |   | NAT B    /
           '-----------|---|--'          '----|-+-|---------'
                       | | |                  | | |
                       | | 3 path-            | | 2 path-
                       | | | challenge        | | | drop
                       | | |   .----------.   | | |
                       | | '-->|          |<--' | |
                       | '-----+  Sender  +-----' |
                       '-------+          +-------'
                               '----------'

                    Figure 7: Old Path Is Not Preferred

   Case 3: The old path is alive and preferred.

   This is most likely the result of an off-path attacker trying to
   place itself on-path.  As shown in Figure 8, the receiver sends a
   path_challenge (1) on the old path, and the sender replies with a
   path_response (2) on the old path.  This results in the connection
   not being migrated to the new path, thus thwarting the attack.

                       new                    old
                       path  .----------.    path
                             |          +-------.
                       .-----+ Receiver +-----. |
                       |     |          |<--. | |
                       |     '----------'   | | |
                       |                    | | 1 path-
                       |                    | | | challenge
                       |                    | | |
                   .---+------.          .--|-+-|-----.
                  / off-path /          /   |NAT|    /
                 / attacker /          '----|-+-|---'
                '------+---'                | | |
                       |                    | | |
                       |           path-    2 | |
                       |           response | | |
                       |     .----------.   | | |
                       |     |          +---' | |
                       '-----+  Sender  +-----' |
                             |          |<------'
                             '----------'

                      Figure 8: Old Path Is Preferred

   Note that this defense is imperfect, but this is not considered a
   serious problem.  If the path via the attacker is reliably faster
   than the old path despite multiple attempts to use that old path, it
   is not possible to distinguish between an attack and an improvement
   in routing.

   An endpoint could also use heuristics to improve detection of this
   style of attack.  For instance, NAT rebinding is improbable if
   packets were recently received on the old path.  Endpoints can also
   look for duplicated packets.  Conversely, a change in CID is more
   likely to indicate an intentional migration rather than an attack.
   Note that changes in CIDs are supported in DTLS 1.3 but not in DTLS
   1.2.

9.  Privacy Considerations

   When using DTLS 1.3, peers SHOULD avoid using the same CID on
   multiple network paths, in particular when initiating connection
   migration or when probing a new network path, as described in
   Section 5, as an adversary can otherwise correlate the communication
   interaction across those different paths.  DTLS 1.3 provides
   mechanisms to ensure that a new CID can always be used.  In general,
   an endpoint should proactively send a RequestConnectionId message to
   ask for new CIDs as soon as the pool of spare CIDs is depleted (or
   goes below a threshold).  Also, in case a peer might have exhausted
   available CIDs, a migrating endpoint could include NewConnectionId in
   packets sent on the new path to make sure that the subsequent path
   validation can use fresh CIDs.

   Note that DTLS 1.2 does not offer the ability to request new CIDs
   during the session lifetime since CIDs have the same lifespan of the
   connection.  Therefore, deployments that use DTLS in multihoming
   environments SHOULD refuse to use CIDs with DTLS 1.2 and switch to
   DTLS 1.3 if the correlation privacy threat is a concern.

10.  IANA Considerations

10.1.  New TLS ContentType

   IANA has allocated an entry in the "TLS ContentType" registry within
   the "Transport Layer Security (TLS) Parameters" registry group
   [IANA.tls-parameters] for the return_routability_check (27) message
   defined in this document.  IANA set the DTLS_OK column to "Y" and
   added the following note to the registry:

   |  Note: The return_routability_check content type is only applicable
   |  to DTLS 1.2 and 1.3.

10.2.  New TLS ExtensionType

   IANA has allocated the extension code point (61) for rrc in the "TLS
   ExtensionType Values" registry as described in Table 1.

   +=====+=========+===+===========+=============+===========+=======+
   |Value|Extension|TLS| DTLS-Only | Recommended | Reference |Comment|
   |     |Name     |1.3|           |             |           |       |
   +=====+=========+===+===========+=============+===========+=======+
   |61   |rrc      |CH,| Y         | N           | RFC 9853  |       |
   |     |         |SH |           |             |           |       |
   +-----+---------+---+-----------+-------------+-----------+-------+

       Table 1: New Entry in the TLS ExtensionType Values Registry

10.3.  New "TLS RRC Message Type" Registry

   IANA has created the "TLS RRC Message Types" registry within the
   "Transport Layer Security (TLS) Parameters" registry group
   [IANA.tls-parameters].  This registration procedure is "Expert
   Review" (Section 4.5 of [RFC8126]).

   To submit registration requests, follow the procedures in Section 16
   of [RFC9847].

   Each entry in the registry must include the following fields:

   Value:
      A (decimal) number in the range 0 to 253.

   Description:
      A brief description of the RRC message.

   DTLS-Only:
      Indication of whether the message only applies to DTLS.  Since RRC
      is only available in DTLS, this column is set to "Y" for all the
      initial entries in this registry.  Future work may define new RRC
      message types that also apply to TLS.

   Recommended:
      Indication of whether the message is recommended for
      implementations to support.  The semantics for this field is
      defined in Section 5 of [RFC8447] and updated in Section 3 of
      [RFC9847].

   Reference:
      A reference to a publicly available specification for the value.

   Comment:
      Any relevant notes or comments that relate to this entry.

   Table 2 shows the initial contents of this registry:

   +=======+================+=========+=============+=========+=======+
   |Value  | Description    |DTLS-Only| Recommended |Reference|Comment|
   +=======+================+=========+=============+=========+=======+
   |0      | path_challenge |Y        | Y           |RFC 9853 |       |
   +-------+----------------+---------+-------------+---------+-------+
   |1      | path_response  |Y        | Y           |RFC 9853 |       |
   +-------+----------------+---------+-------------+---------+-------+
   |2      | path_drop      |Y        | Y           |RFC 9853 |       |
   +-------+----------------+---------+-------------+---------+-------+
   |3-253  | Unassigned     |         |             |         |       |
   +-------+----------------+---------+-------------+---------+-------+
   |254-255| Reserved for   |Y        |             |RFC 9853 |       |
   |       | Private Use    |         |             |         |       |
   +-------+----------------+---------+-------------+---------+-------+

        Table 2: Initial Entries in TLS RRC Message Type Registry

   IANA added the following note to provide additional information
   regarding the use of RRC message codepoints in experiments:

   |  Note: As specified in [RFC8126], assignments made in the Private
   |  Use space are not generally useful for broad interoperability.
   |  Those making use of the Private Use range are responsible for
   |  ensuring that no conflicts occur within the intended scope of use.
   |  For widespread experiments, provisional registrations
   |  (Section 4.13 of [RFC8126]) are available.

10.3.1.  Designated Expert Instructions

   To enable a broadly informed review of registration decisions, it is
   recommended that multiple designated experts be appointed to
   represent the perspectives of both the transport and security areas.

   In cases where a registration decision could be perceived as creating
   a conflict of interest for a particular expert, that expert SHOULD
   defer to the judgment of the other experts.

11.  References

11.1.  Normative References

   [IANA.tls-parameters]
              IANA, "Transport Layer Security (TLS) Parameters",
              <https://www.iana.org/assignments/tls-parameters>.

   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/info/rfc8447>.

   [RFC9146]  Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
              A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146,
              DOI 10.17487/RFC9146, March 2022,
              <https://www.rfc-editor.org/info/rfc9146>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9847]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 9847, DOI 10.17487/RFC9847, December 2025,
              <https://www.rfc-editor.org/info/rfc9847>.

11.2.  Informative References

   [AMP-ATTACKS]
              Preuß Mattsson, J., Selander, G., and C. Amsüss,
              "Amplification Attacks Using the Constrained Application
              Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
              irtf-t2trg-amplification-attacks-05, 18 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-irtf-t2trg-
              amplification-attacks-05>.

   [IOT-PROFILE]
              Tschofenig, H., Fossati, T., Richardson, M., and D.
              Migault, "TLS/DTLS 1.3 Profiles for the Internet of
              Things", Work in Progress, Internet-Draft, draft-ietf-uta-
              tls13-iot-profile-19, 20 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              tls13-iot-profile-19>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [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>.

   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/info/rfc9175>.

Acknowledgments

   We would like to thank Colin Perkins, Deb Cooley, Eric Rescorla, Éric
   Vyncke, Erik Kline, Hanno Becker, Hanno Böck, Joe Clarke, Manuel
   Pégourié-Gonnard, Marco Tiloca, Martin Thomson, Mike Bishop, Mike
   Ounsworth, Mohamed Boucadair, Mohit Sahni, Rich Salz, Russ Housley,
   Sean Turner, and Yaron Sheffer for their input to this document.

Authors' Addresses

   Hannes Tschofenig (editor)
   University of the Bundeswehr Munich
   Institute of Distributed Intelligent Systems
   Werner-Heisenberg-Weg 39
   85577 Neubiberg
   Germany
   Email: Hannes.Tschofenig@gmx.net


   Achim Kraus
   Email: achimkraus@gmx.net


   Thomas Fossati
   Linaro
   Email: thomas.fossati@linaro.org