TLS H. Tschofenig, Ed. Internet-Draft T. Fossati Updates: 6347 (if approved) Arm Limited Intended status: Standards Track 25 October 2021 Expires: 28 April 2022 Return Routability Check for DTLS 1.2 and DTLS 1.3 draft-ietf-tls-dtls-rrc-01 Abstract This document specifies a return routability check for use in context of the Connection ID (CID) construct for the Datagram Transport Layer Security (DTLS) protocol versions 1.2 and 1.3. Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the Transport Layer Security Working Group mailing list (tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/. Source for this draft and an issue tracker can be found at https://github.com/tlswg/dtls-rrc. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering 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 28 April 2022. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Tschofenig & Fossati Expires 28 April 2022 [Page 1] Internet-Draft DTLS Return Routability Check October 2021 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 3. RRC Extension . . . . . . . . . . . . . . . . . . . . . . . . 3 4. The Return Routability Check Message . . . . . . . . . . . . 4 5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Security and Privacy Considerations . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 10. Normative References . . . . . . . . . . . . . . . . . . . . 8 Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction In "classical" DTLS, selecting a security context of an incoming DTLS record is accomplished with the help of the 5-tuple, i.e. source IP address, source port, transport protocol, destination IP address, and destination port. Changes to this 5 tuple can happen for a variety reasons over the lifetime of the DTLS session. In the IoT context, NAT rebinding is common with sleepy devices. Other examples include end host mobility and multi-homing. Without CID, if the source IP address and/or source port changes during the lifetime of an ongoing DTLS session then the receiver will be unable to locate the correct security context. As a result, the DTLS handshake has to be re-run. Tschofenig & Fossati Expires 28 April 2022 [Page 2] Internet-Draft DTLS Return Routability Check October 2021 Of course, it is not necessary to re-run the full handshake if session resumption is supported and negotiated. A CID is an identifier carried in the record layer header of a DTLS datagram that gives the receiver additional information for selecting the appropriate security context. The CID mechanism has been specified in [I-D.ietf-tls-dtls-connection-id] for DTLS 1.2 and in [I-D.ietf-tls-dtls13] for DTLS 1.3. Section 6 of [I-D.ietf-tls-dtls-connection-id] describes how the use of CID increases the attack surface by providing both on-path and off-path attackers an opportunity for (D)DoS. It then goes on describing the steps a DTLS principal must take when a record with a CID is received that has a source address (and/or port) different from the one currently associated with the DTLS connection. However, the actual mechanism for ensuring that the new peer address is willing to receive and process DTLS records is left open. This document standardizes a return routability check (RRC) as part of the DTLS protocol itself. The return routability check is performed by the receiving peer before the CID-to-IP address/port binding is updated in that peer's session state database. This is done in order to provide more confidence to the receiving peer that the sending peer is reachable at the indicated address and port. 2. Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. This document assumes familiarity with the CID format and protocol defined for DTLS 1.2 [I-D.ietf-tls-dtls-connection-id] and for DTLS 1.3 [I-D.ietf-tls-dtls13]. The presentation language used in this document is described in Section 4 of [RFC8446]. 3. RRC Extension This specification uses the tls_flags extension defined in [I-D.ietf-tls-tlsflags] to allow a client and a server to negotiate support for this extension. The RRC flag is assigned the value (TBD1) and is used in the ClientHello (CH) and the ServerHello (SH). Tschofenig & Fossati Expires 28 April 2022 [Page 3] Internet-Draft DTLS Return Routability Check October 2021 4. The Return Routability Check Message When a record with CID is received that has the source address of the enclosing UDP datagram different from the one previously associated with that CID, the receiver MUST NOT update its view of the peer's IP address and port number with the source specified in the UDP datagram before cryptographically validating the enclosed record(s) but instead perform a return routability check. enum { invalid(0), change_cipher_spec(20), alert(21), handshake(22), application_data(23), heartbeat(24), /* RFC 6520 */ return_routability_check(TBD), /* NEW */ (255) } ContentType; uint64 Cookie; enum { path_challenge(0), path_response(1), reserved(2..255) } rrc_msg_type; struct { rrc_msg_type msg_type; select (return_routability_check.msg_type) { case path_challenge: Cookie; case path_response: Cookie; }; } return_routability_check; The newly introduced return_routability_check message contains a cookie. The cookie is a 8-byte field containing arbitrary data. The return_routability_check message MUST be authenticated and encrypted using the currently active security context. The receiver that observes the peer's address and or port update MUST stop sending any buffered application data (or limit the data sent to a TBD threshold) and initiate the return routability check that proceeds as follows: Tschofenig & Fossati Expires 28 April 2022 [Page 4] Internet-Draft DTLS Return Routability Check October 2021 1. A cookie is placed in a return_routability_check message of type path_challenge; 2. The message is sent to the observed new address and a timeout T is started; 3. The peer endpoint, after successfully verifying the received return_routability_check message echoes the cookie value in a return_routability_check message of type path_response; 4. When the initiator receives and verifies the return_routability_check message contains the sent cookie, it updates the peer address binding; 5. If T expires, or the address confirmation fails, the peer address binding is not updated. After this point, any pending send operation is resumed to the bound peer address. 5. Example The example TLS 1.3 handshake shown in Figure 1 shows a client and a server negotiating the support for CID and for the RRC extension. Tschofenig & Fossati Expires 28 April 2022 [Page 5] Internet-Draft DTLS Return Routability Check October 2021 Client Server Key ^ ClientHello Exch | + key_share | + signature_algorithms | + tls_flags (RRC) v + connection_id=empty --------> ServerHello ^ Key + key_share | Exch + connection_id=100 | + tls_flags (RRC) v {EncryptedExtensions} ^ Server {CertificateRequest} v Params {Certificate} ^ {CertificateVerify} | Auth <-------- {Finished} v ^ {Certificate} Auth | {CertificateVerify} v {Finished} --------> [Application Data] <-------> [Application Data] + Indicates noteworthy extensions sent in the previously noted message. * Indicates optional or situation-dependent messages/extensions that are not always sent. {} Indicates messages protected using keys derived from a [sender]_handshake_traffic_secret. [] Indicates messages protected using keys derived from [sender]_application_traffic_secret_N. Figure 1: Message Flow for Full TLS Handshake Once a connection has been established the client and the server exchange application payloads protected by DTLS with an unilaterally used CIDs. In our case, the client is requested to use CID 100 for records sent to the server. At some point in the communication interaction the IP address used by the client changes and, thanks to the CID usage, the security context to interpret the record is successfully located by the server. However, the server wants to test the reachability of the client at his new IP address. Tschofenig & Fossati Expires 28 April 2022 [Page 6] Internet-Draft DTLS Return Routability Check October 2021 Client Server ------ ------ Application Data ========> Src-IP=A Dst-IP=Z <======== Application Data Src-IP=Z Dst-IP=A <<------------->> << Some >> << Time >> << Later >> <<------------->> Application Data ========> Src-IP=B Dst-IP=Z <<< Unverified IP Address B >> <-------- Return Routability Check path_challenge(cookie) Src-IP=Z Dst-IP=B Return Routability Check --------> path_response(cookie) Src-IP=B Dst-IP=Z <<< IP Address B Verified >> <======== Application Data Src-IP=Z Dst-IP=B Figure 2: Return Routability Example Tschofenig & Fossati Expires 28 April 2022 [Page 7] Internet-Draft DTLS Return Routability Check October 2021 6. Security and Privacy Considerations Note that the return routability checks do not protect against flooding of third-parties if the attacker is on-path, as the attacker can redirect the return routability checks to the real peer (even if those datagrams are cryptographically authenticated). On-path adversaries can, in general, pose a harm to connectivity. 7. IANA Considerations IANA is requested to allocate an entry to the TLS "ContentType" registry, for the return_routability_check(TBD) defined in this document. IANA is requested to allocate an entry to the TLS Flags registry in the tls_flags type: * Value: [[IANA please assign a value from the 32-63 value range.]] * Flag Name: RRC * Message: CH,SH * Recommended: Y * Reference: [[This document]] 8. Open Issues Issues against this document are tracked at https://github.com/tlswg/ dtls-rrc/issues 9. Acknowledgments We would like to thank Achim Kraus, Hanno Becker, Hanno Boeck, Manuel Pegourie-Gonnard, Mohit Sahni and Rich Salz for their input to this document. 10. Normative References [I-D.ietf-tls-dtls-connection-id] Rescorla, E., Tschofenig, H., Fossati, T., and A. Kraus, "Connection Identifiers for DTLS 1.2", Work in Progress, Internet-Draft, draft-ietf-tls-dtls-connection-id-13, 22 June 2021, . Tschofenig & Fossati Expires 28 April 2022 [Page 8] Internet-Draft DTLS Return Routability Check October 2021 [I-D.ietf-tls-dtls13] Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", Work in Progress, Internet-Draft, draft-ietf-tls- dtls13-43, 30 April 2021, . [I-D.ietf-tls-tlsflags] Nir, Y., "A Flags Extension for TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-tlsflags-06, 13 July 2021, . [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, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . Appendix A. History RFC EDITOR: PLEASE REMOVE THE THIS SECTION draft-ietf-tls-dtls-rrc-01 * Use the TLS flags extension for negotiating RRC * Enhanced IANA consideration section * Expanded example section * Revamp message layout: - Use 8-byte fixed size cookies - Explicitly separate path challenge from response draft-ietf-tls-dtls-rrc-00 * Draft name changed after WG adoption Tschofenig & Fossati Expires 28 April 2022 [Page 9] Internet-Draft DTLS Return Routability Check October 2021 draft-tschofenig-tls-dtls-rrc-01 * Removed text that overlapped with draft-ietf-tls-dtls-connection- id draft-tschofenig-tls-dtls-rrc-00 * Initial version Authors' Addresses Hannes Tschofenig (editor) Arm Limited Email: hannes.tschofenig@arm.com Thomas Fossati Arm Limited Email: thomas.fossati@arm.com Tschofenig & Fossati Expires 28 April 2022 [Page 10]