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Return Routability Check for DTLS 1.2 and DTLS 1.3
draft-ietf-tls-dtls-rrc-07

Document Type Active Internet-Draft (tls WG)
Authors Hannes Tschofenig , Achim Kraus , Thomas Fossati
Last updated 2022-09-20 (Latest revision 2022-09-09)
Replaces draft-tschofenig-tls-dtls-rrc
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Formats
Stream WG state Waiting for WG Chair Go-Ahead
Document shepherd Sean Turner
IESG IESG state I-D Exists
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Send notices to sean@sn3rd.com
draft-ietf-tls-dtls-rrc-07
TLS                                                   H. Tschofenig, Ed.
Internet-Draft                                               Arm Limited
Updates: 6347, 9147 (if approved)                               A. Kraus
Intended status: Standards Track                                        
Expires: 13 March 2023                                        T. Fossati
                                                             Arm Limited
                                                        9 September 2022

           Return Routability Check for DTLS 1.2 and DTLS 1.3
                       draft-ietf-tls-dtls-rrc-07

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 13 March 2023.

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  RRC Extension . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Return Routability Check Message Types  . . . . . . . . . . .   4
   5.  RRC and CID Interplay . . . . . . . . . . . . . . . . . . . .   5
   6.  Attacker Model  . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Amplification . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Off-Path Packet Forwarding  . . . . . . . . . . . . . . .   7
   7.  Path Validation Procedure . . . . . . . . . . . . . . . . . .  11
     7.1.  Basic . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Enhanced  . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.3.   Path Challenge Requirements  . . . . . . . . . . . . . .  13
     7.4.  Path Response/Drop Requirements . . . . . . . . . . . . .  14
     7.5.  Timer Choice  . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Security and Privacy Considerations . . . . . . . . . . . . .  17
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  New TLS ContentType  . . . . . . . . . . . . . . . . . .  17
     10.2.   New TLS ExtensionType . . . . . . . . . . . . . . . . .  17
     10.3.  New RRC Message Type Sub-registry  . . . . . . . . . . .  18
   11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  19
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     13.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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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 [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 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.

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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].

   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.

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.  Return Routability Check Message Types

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

   The protocol consists of three message types: path_challenge,
   path_response and path_drop that are used for path validation and
   selection as described in Section 7.

   Each message carries a Cookie, a 8-byte field containing arbitrary
   data.

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

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   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(TBD2), /* 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

   Future extensions or additions to the Return Routability Check
   protocol may define new message types.  Implementations MUST be able
   to parse and ignore messages with an unknown msg_type.

5.  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 the one currently associated
   with that CID value, the receiver SHOULD perform a return routability
   check as described in Section 7, unless an application layer specific
   address validation mechanism can be triggered instead.

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6.  Attacker Model

   We define two classes of attackers, off-path and on-path, with
   increasing capabilities (see Figure 2) partly following terminology
   introduced in QUIC [RFC9000]:

   *  An off-path attacker is not on the original path between the DTLS
      peers, but is able to observe packets on the original path and has
      faster routing 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 un-encrypted portions      |    |
              |    +------------------------------------+    |
              |    | Inject                             |    |
          off-path +------------------------------------+    |
              |    | Reorder                            |    |
              |    +------------------------------------+    |
              |    | Modify un-authenticated portions   | on-path
              '--> +------------------------------------+    |
                   | Delay                              |    |
                   +------------------------------------+    |
                   | Drop                               |    |
                   +------------------------------------+    |
                   | Manipulate the packetization layer |    |
                   '------------------------------------' <--'

                      Figure 2: Attackers 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 6.1).

   *  Off-path attackers that try to put themselves on-path
      (Section 6.2), provided that the enhanced path validation
      algorithm is used (Section 7.2).

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6.1.  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-bytes GET request) the attacker can use
   the server as a traffic amplifier toward the victim.

   When receiving a packet with a known CID and a spoofed source
   address, 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 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.

6.2.  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 reliable 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.  Note however that,
   unlike QUIC, DTLS has no "non-probing" packets so this would require
   application specific mechanisms.

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   Figure 3 illustrates the case where a receiver receives a packet with
   a new source IP address and/or new port number.  In order to
   determine whether this path change was not triggered by an off-path
   attacker, the receiver will send a RRC message of type path_challenge
   (1) on the old path.

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

               Figure 3: 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 4, 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.

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

                         Figure 4: Old path is dead

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

   This case is shown in Figure 5 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.

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

                    Figure 5: 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.  The receiver sends a path_challenge on the old
   path and the sender replies with a path_response (2) on the old path.
   The interaction is shown in Figure 6.  This results in the connection
   not being migrated to the new path, thus thwarting the attack.

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

                      Figure 6: Old path is preferred

   Note that this defense is imperfect, but this is not considered a
   serious problem.  If the path via the attack 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; similarly, rebinding
   is rare on IPv6 paths.  Endpoints can also look for duplicated
   packets.  Conversely, a change in connection ID is more likely to
   indicate an intentional migration rather than an attack.  Note that
   changes in connection IDs are supported in DTLS 1.3 but not in DTLS
   1.2.

7.  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.

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   It then initiates the return routability check that proceeds as
   described either in Section 7.2 or Section 7.1, depending on whether
   the off-path attacker scenario described in Section 6.2 is to be
   taken into account or not.

   (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, the need to reduce the time it takes to
   switch path.  The choice may be offered as a configuration option to
   the user.)

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

   Section 7.3 and Section 7.4 list the requirements for the initiator
   and responder roles, broken down per protocol phase.

7.1.  Basic

   1.  The receiver 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 7.5) is started.

   3.  The peer endpoint 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.

7.2.  Enhanced

   1.  The receiver 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, see
       Section 7.5, is started.

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   3.  If the path is still functional, the peer endpoint
       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 is the
       preferred one or not anymore:

       *  If the path through which the message was received is
          preferred, a return_routability_check message of type
          path_response MUST be returned.

       *  If the path through which the message was received is not
          preferred, a return_routability_check message of type
          path_drop MUST be returned.  In either case, the peer endpoint
          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
          was 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 was
          received, the initiator MUST perform a return routability
          check on the observed new address, as described in
          Section 7.1.

   5.  If T expires the peer address binding is not updated.  In this
       case, the initiator MUST perform a return routability check on
       the observed new address, as described in Section 7.1.

7.3.   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.

      -  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.

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   *  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.

7.4.  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 received path_challenge.

   *  The responder MUST send the path_response or the path_drop on the
      path where the corresponding path_challenge has been received, so
      that validation succeeds only if the path is functional in both
      directions.  The initiator MUST NOT enforce this behaviour.

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

7.5.  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, T SHOULD be set to 1s.

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

8.  Example

   In the example TLS 1.3 handshake shown in Figure 7, a client and a
   server successfully negotiate support for CID as well as the RRC
   extension.

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       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 7: Message Flow for Full TLS Handshake

   Once a connection has been established the client and the server
   exchange application payloads protected by DTLS with a unilaterally
   used CID.  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
   its new IP address.

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   Figure 8 shows the server initiating a "basic" RRC exchange (see
   Section 7.1) that establishes reachability of the client at the new
   IP 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

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                Figure 8: "Basic" Return Routability Example

9.  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.

   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 7, 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 life-span 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

   // RFC Editor: please replace RFCthis with this RFC number and remove
   // this note.

10.1.  New TLS ContentType

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

10.2.   New TLS ExtensionType

   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.

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

      Table 1: rrc entry in the TLS ExtensionType Values registry

10.3.  New RRC Message Type Sub-registry

   IANA is requested to create a new sub-registry for RRC Message Types
   in the TLS Parameters registry [IANA.tls-parameters], with the policy
   "expert review" [RFC8126].

   Each entry in the registry must include:

   Value:
      A number in the range from 0 to 255 (decimal)

   Description:
      a brief description of the message

   DTLS-Only:
      RRC is only available in DTLS, therefore this column will be set
      to Y for all the entries in this registry

   Reference:
      a reference document

   The initial state of this sub-registry is as follows:

   +=======+================+===========+===========+
   | Value | Description    | DTLS-Only | Reference |
   +=======+================+===========+===========+
   | 0     | path_challenge | Y         | RFCthis   |
   +-------+----------------+-----------+-----------+
   | 1     | path_response  | Y         | RFCthis   |
   +-------+----------------+-----------+-----------+
   | 2     | path_drop      | Y         | RFCthis   |
   +-------+----------------+-----------+-----------+
   | 3-255 | Unassigned     |           |           |
   +-------+----------------+-----------+-----------+

      Table 2: Initial Entries in RRC Message Type
                        registry

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11.  Open Issues

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

12.  Acknowledgments

   We would like to thank Hanno Becker, Hanno Böck, Manuel Pégourié-
   Gonnard, Martin Thomson, Mohit Sahni and Rich Salz for their input to
   this document.

13.  References

13.1.  Normative References

   [IANA.tls-parameters]
              IANA, "Transport Layer Security (TLS) Parameters", 23
              August 2005,
              <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/rfc/rfc2119>.

   [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/rfc/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/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>.

   [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/rfc/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/rfc/rfc9147>.

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13.2.  Informative References

   [I-D.ietf-uta-tls13-iot-profile]
              Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
              the Internet of Things", Work in Progress, Internet-Draft,
              draft-ietf-uta-tls13-iot-profile-05, 6 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              tls13-iot-profile-05>.

   [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/rfc/rfc9000>.

Appendix A.  History

   // RFC EDITOR: PLEASE REMOVE THIS SECTION

   draft-ietf-tls-dtls-rrc-07

   *  Fix ambiguous wording around timer settings

   *  Clarify that the detailed protocol flow describes "basic" RRC

   draft-ietf-tls-dtls-rrc-06

   *  Add Achim as co-author

   *  Added IANA registry for RRC message types (#14)

   *  Small fix in the path validation algorithm (#15)

   *  Renamed path_delete to path_drop (#16)

   *  Added an "attacker model" section (#17, #31, #44, #45, #48)

   *  Add criteria for choosing between basic and enhanced path
      validation (#18)

   *  Reorganise Section 4 a bit (#19)

   *  Small fix in Path Response/Drop Requirements section (#20)

   *  Add privacy considerations wrt CID reuse (#30)

   draft-ietf-tls-dtls-rrc-05

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   *  Added text about off-path packet forwarding

   draft-ietf-tls-dtls-rrc-04

   *  Re-submitted draft to fix references

   draft-ietf-tls-dtls-rrc-03

   *  Added details for challenge-response exchange

   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:

      -  Use 8-byte fixed size cookies

      -  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

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   Achim Kraus
   Email: achimkraus@gmx.net

   Thomas Fossati
   Arm Limited
   Email: thomas.fossati@arm.com

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