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Secure Negotiation of Incompatible Protocols in TLS

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Martin Thomson
Last updated 2020-07-13
Replaced by draft-ietf-tls-snip
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Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                              13 July 2020
Expires: 14 January 2021

          Secure Negotiation of Incompatible Protocols in TLS


   An extension is defined for TLS that allows a client and server to
   detect an attempt to force the use of less-preferred application
   protocol even where protocol options are incompatible.  This
   supplements application-layer protocol negotiation, which allows
   choices between compatible protocols to be authenticated.

Discussion Venues

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

   Discussion of this document takes place on the TLS Working Group
   mailing list (, which is archived at

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 14 January 2021.

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Copyright Notice

   Copyright (c) 2020 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 (
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Incompatible Protocols and SVCB . . . . . . . . . . . . . . .   4
   4.  Authenticating Incompatible Protocols . . . . . . . . . . . .   4
   5.  Protocol Authentication Scope . . . . . . . . . . . . . . . .   5
     5.1.  SVCB Discovery Scope  . . . . . . . . . . . . . . . . . .   6
     5.2.  QUIC Version Negotiation  . . . . . . . . . . . . . . . .   6
     5.3.  Alternative Services  . . . . . . . . . . . . . . . . . .   6
     5.4.  Scope for Other Discovery Methods . . . . . . . . . . . .   7
   6.  Incompatible Protocol Selection . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   With increased diversity in protocol choice, some applications are
   able to use one of several semantically-equivalent protocols to
   achieve their goals.  This is particularly notable in HTTP where
   there are currently three distinct protocols: HTTP/1.1 [HTTP11],
   HTTP/2 [HTTP2], and HTTP/3 [HTTP3].  This is also true for protocols
   that support variants based on both TLS [TLS] and DTLS [DTLS].

   For protocols that are mutually compatible, Application-Layer
   Protocol Negotiation (ALPN; [ALPN]) provides a secure way to
   negotiate protocol selection.

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   In ALPN, the client offers a list of options in a TLS ClientHello and
   the server chooses the option that it most prefers.  A downgrade
   attack occurs where both client and server support a protocol that
   the server prefers more than than the selected protocol.  ALPN
   protects against this attack by ensuring that the server is aware of
   all options the client supports and including those options and the
   server choice under the integrity protection provided by the TLS

   This downgrade protection functions because protocol negotiation is
   part of the TLS handshake.  The introduction of semantically-
   equivalent protocols that use incompatible handshakes introduces new
   opportunities for downgrade attack.  For instance, it is not possible
   to negotiate the use of HTTP/2 based on an attempt to connect using
   HTTP/3.  The former relies on TCP, whereas the latter uses UDP.
   These protocols are therefore mutually incompatible.

   This document defines an extension to TLS that allows clients to
   discover when servers support alternative protocols that are
   incompatible with the currently-selected TLS version.  This might be
   used to avoid downgrade attack caused by interference in protocol
   discovery mechanisms.

   This extension is motivated by the addition of new mechanisms, such
   as [SVCB].  SVCB enables the discovery of servers that support
   multiple different protocols, some of which are incompatible.  The
   extension can also be used to authenticate protocol choices that are
   discovered by other means.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Two protocols are consider "compatible" if it is possible to
   negotiate either using the same connection attempt.  In comparison,
   protocols are "incompatible" if they require separate attempts to
   establish a connection.

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3.  Incompatible Protocols and SVCB

   The SVCB record [SVCB] allows a client to learn about services
   associated with a domain name.  This includes how to locate a server,
   along with supplementary information about the server, including
   protocols that the server supports.  This allows a client to start
   using a protocol of their choice without added latency, as the lookup
   can be performed concurrently with other name resolution.  The added
   cost of the additional DNS queries is minimal.

   However, SVCB provides no protection against a downgrade attack
   between incompatible protocols.  An attacker could remove DNS records
   for client-preferred protocols, leaving the client to believe that
   only less-prefered, mutually-incompatible options are available.  The
   client only offers compatible options to a server in its TLS
   handshake.  Even if a client were to inform the server that it
   supports a more preferred protocol, the server would not be able to
   act upon it.

   Authenticating all of the information presented in SVCB records might
   provide clients with complete information about server support, but
   this is impractical for several reasons:

   *  it is not possible to ensure that all server instances in a
      deployment have the same protocol configuration, as deployments
      for a single name routinely include multiple providers that cannot
      coordinate closely;

   *  the ability to provide a subset of valid DNS records is integral
      to many strategies for managing servers; and

   *  it is difficult to ensure that cached DNS records are synchronized
      with server state.

   Overall, an authenticated TLS handshake is a better source of
   authoritative information about the protocols that are supported.

4.  Authenticating Incompatible Protocols

   The incompatible_protocols(TBD) TLS extension provides clients with
   information about the incompatible protocols that are supported by

   enum {
       incompatible_protocols(TBD), (65535)
   } ExtensionType;

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   A client that supports the extension advertises an empty extension.
   In response, a server that supports this extension includes a list of
   application protocol identifiers.  The "extension_data" field of the
   value server extension uses the "ProtocolNameList" format defined in
   [ALPN].  This syntax is shown in Figure 1.

   struct {
     select (Handshake.msg_type) {
       case client_hello:
       case encrypted_extensions:
         ProtocolNameList incompatible_protocols;
   } IncompatibleProtocols;

         Figure 1: TLS Syntax for incompatible_protocols Extension

   This extension only applies to the ClientHello and
   EncryptedExtensions messages.  An implementation that receives this
   extension in any other handshake message MUST send a fatal
   illegal_parameter alert.

   A server deployment that supports multiple incompatible protocols MAY
   advertise all protocols that are supported.  A server MAY limit this
   to protocols that it considers to have similar semantics to protocols
   that the client lists in its application_layer_protocol_negotiation

   The definition of what a server includes is intentionally loose.  It
   is better that a server offer more information than less as the needs
   of a client are not necessarily well reflected in its ALPN extension.
   However, it is not reasonable to require that a server advertise all
   potential protocols as that is unlikely to be practical.

   A server MUST omit any compatible protocols from this extension on
   the understanding that the client will include compatible protocols
   in the application_layer_protocol_negotiation extension.

   A server needs to ensure that protocols advertised in this fashion
   are available to the client within the same protocol authentication

5.  Protocol Authentication Scope

   The protocol authentication scope is the set of protocol endpoints at
   a server that share a protocol configuration.  A client learns of
   this scope as part of the process it follows to discover the server.

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   By default, the protocol authentication scope is a single protocol
   endpoint.  The default protocol authentication scope offers no means
   to authenticate incompatible protocols as it is not possible for a
   client to access any endpoint that supports those protocols.  A
   client cannot use information from the incompatible_protocols
   extension unless a wider scope is used.

   [[TODO: This likely needs some discussion.]]

5.1.  SVCB Discovery Scope

   For SVCB records, the protocol authentication scope is defined by the
   set of ServiceForm SVCB records with the same SvcDomainName.

   This ensures that the final choice a client makes between ServiceForm
   SVCB records is protected by this extension.  If the client does not
   receive a SVCB record for a protocol that the server includes in its
   incompatible_protocols extension, then it can assume that this
   omission was caused by an error or attack.

   Thus, for SVCB, a choice between AliasForm records (or CNAME or DNAME
   records) is not authenticated, but choices between ServiceForm
   records is.  This allows for server deployments for the same name to
   have different administrative control and protocol configurations.

5.2.  QUIC Version Negotiation

   TODO: define how this can be used to authenticate protocol choices
   where there are incompatible QUIC versions.

5.3.  Alternative Services

   It is possible to negotiate protocols based on an established
   connection without exposure to downgrade.  The Alternative Services
   [ALTSVC] bootstrapping in HTTP/3 does just that.  Assuming that
   HTTP/2 or HTTP/1.1 are not vulnerable to attacks that would
   compromise integrity, a server can advertise the presence of an
   endpoint that supports HTTP/3.

   Under these assumptions Alternative Services is secure, but it has
   performance trade-offs.  A client could attempt the protocol it
   prefers most, but that comes at a risk that this protocol is not
   supported by a server.  A client could implement a fallback, which
   might even be performed concurrently (see [HAPPY-EYEBALLS]), but this
   costs time and resources.  A client avoids these costs by attempting
   the protocol it believes to be most widely supported, though this
   comes with a performance penalty in cases where the most-preferred
   protocol is supported.

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   A server that is discovered using Alternative Services uses the
   default protocol authentication scope.  As use of Alternative
   Services is discretionary for both client and server, a client cannot
   expect to receive information about incompatible protocols.  To avoid
   downgrade, a client only has to avoid using an Alternative Service
   that offers a less-preferred protocol.

5.4.  Scope for Other Discovery Methods

   For other discovery methods, a definition for protocol authentication
   scope is needed before a client can act on what is learned using the
   incompatible_protocols extension.  That definition needs to define
   how to discover server instances that support all incompatible
   protocols in the scope.

   In particular, a server that is discovered using forms of DNS-based
   name resolution other than SVCB uses the default protocol
   authentication scope.  This discovery method does not provide enough
   information to locate other incompatible protocols.

   For instance, an HTTPS server that is discovered using purely A or
   AAAA records (and CNAME or DNAME records) might advertise support for
   incompatible protocols, but as there is no way to determine where
   those protocols are supported, a client cannot act on the
   information.  Note that Alternative Services do not change the
   protocol authentication scope.

   Deployments of discovery methods that define a protocol
   authentication scope larger than the default need to ensure that
   every server provides information that is consistent with every
   protocol authentication scope that includes that server.  A server
   that fails to indicate support for a protocol that is within a
   protocol authentication scope does not offer any protection against
   attack; a server that advertises a protocol that the client cannot
   discover risks this misconfiguration being identified as an attack by

6.  Incompatible Protocol Selection

   This represents a different model for protocol selection than the one
   used by ALPN.  In ALPN, the client presents a set of (compatible)
   options and the server chooses its most preferred.

   In comparison, as the client makes a selection between incompatible
   protocols before making a connection attempt, this design only
   provides the client with information about other incompatible
   protocols that the server might support.  Any choice to attempt a
   connection using those protocols is left to the client.

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   In summary:

   *  For compatible protocols, the server chooses

   *  For incompatible protocols, the client chooses

   Detecting a potential downgrade between incompatible protocols does
   not automatically imply that a client abandon a connection attempt.
   This is left to client policy.

   For a protocol like HTTP/3, this might not result in the client
   choosing to use HTTP/3, even if the server prefers that protocol.
   Blocking of UDP or QUIC is known to be widespread.  As a result,
   clients might adopt a policy of tolerating a downgrade to a TCP-based
   protocol, even if HTTP/3 were preferred.  However, as blocking of UDP
   is highly correlated by access network, clients that are able to
   establish HTTP/3 connections to some servers might choose to apply a
   stricter response when a server that indicates HTTP/3 support is

7.  Security Considerations

   This design depends on the integrity of the TLS handshake across all
   forms, including TLS [RFC8446], DTLS [DTLS], and QUIC [QUIC-TLS].  An
   attacker that can modify a TLS handshake in any one of these
   protocols can cause a client to believe that other options do not

   A server deployment that uses AliasForm SVCB records and does not
   uniformly support a client-preferred protocol is vulnerable to
   downgrade attacks that steer clients toward instances that lack
   support for that protocol.  This attack is ineffective for protocols
   that are consistently supported by all server instances.

8.  IANA Considerations

   TODO: register the extension

9.  References

9.1.  Normative References

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

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   [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, <>.

9.2.  Informative References

   [ALTSVC]   Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

   [DTLS]     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-
              dtls13-38, 29 May 2020, <

              Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <>.

   [HTTP11]   Fielding, R., Nottingham, M., and J. Reschke, "HTTP/1.1
              Messaging", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-10, 12 July 2020, <

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-29, 9 June 2020, <

   [QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-29,
              9 June 2020, <

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

   [SVCB]     Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPSSVC)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-httpssvc-03, 11 June 2020,

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

Appendix A.  Acknowledgments

Author's Address

   Martin Thomson


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