Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                            25 August 2021
Expires: 26 February 2022

          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 (ALPN), 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

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   This Internet-Draft will expire on 26 February 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Incompatible Protocols and SVCB . . . . . . . . . . . . . . .   4
   4.  Authenticating Incompatible Protocols . . . . . . . . . . . .   4
   5.  Incompatible Protocol Selection . . . . . . . . . . . . . . .   5
   6.  Logical Servers . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Validation Process  . . . . . . . . . . . . . . . . . . .   7
     6.2.  QUIC Version Negotiation  . . . . . . . . . . . . . . . .   7
     6.3.  Alternative Services  . . . . . . . . . . . . . . . . . .   7
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Defining Logical Servers . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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 of 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-preferred options are available.  If those options are not
   compatible with the client-preferred option, the client will not know
   to attempt these.  The client then only offers options compatible
   with the less-preferred options when attempting a 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 by

4.  Authenticating Incompatible Protocols

   The incompatible_protocols(TBD) TLS extension provides clients with
   information about the incompatible protocols that are supported by
   the same logical server; see Section 6 for a definition of a logical

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

   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 "ProtocolName" type defined in
   [ALPN], which is repeated here.  This syntax is shown in Figure 1.

   opaque ProtocolName<1..2^8-1>;
   ProtocolName IncompatibleProtocol;

   struct {
     select (Handshake.msg_type) {
       case client_hello:
       case encrypted_extensions:
         IncompatibleProtocol incompatible_protocols<3..2^16-1>;
   } 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 by the same logical
   server.  A server needs to ensure that protocols advertised in this
   fashion are available to the client.

   A server MUST omit any compatible protocols from this extension.
   That is, any protocol that the server might be able to select, had
   the client offered the protocol in the
   application_layer_protocol_negotiation extension.  In comparison,
   clients are expected to include all compatible protocols in the
   application_layer_protocol_negotiation extension.

5.  Incompatible Protocol Selection

   This document expands the definition of protocol negotiation to
   include both compatible and incompatible protocols and provide
   protection against downgrade for both types of selection.  ALPN
   [ALPN] only considers compatible protocols: the client presents a set
   of compatible options and the server chooses its most preferred.

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   With an selection of protocols that includes incompatible options,
   the client makes a selection between incompatible options before
   making a connection attempt.  Therefore, this design does not enable
   negotiation, it instead provides the client with information about
   other incompatible protocols that the server might support.

   Detecting a potential downgrade between incompatible protocols does
   not automatically imply that a client abandon a connection attempt.
   It only provides the client with authenticated information about its
   options.  What a client does with this information is left to client

   In brief:

   *  For compatible protocols, the client offers all acceptable options
      and the server selects its most preferred

   *  For incompatible protocols, information the server offers is
      authenticated and the client is able to act on that

   For a protocol like HTTP/3, this might not result in the client
   choosing to use HTTP/3, even if HTTP/3 is preferred and the server
   indicates that a service endpoint supporting HTTP/3 is available.
   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 policy when a server that indicates HTTP/3 support is

6.  Logical Servers

   The set of endpoints over which clients can assume availability of
   incompatible protocols is the set of endpoints that share an IP
   version, IP address, and port number with the TLS server that
   provides the incompatible_protocols extension.

   This definition includes a port number that is independent of the
   protocol that is used.  Any protocol that defines a port number is
   considered to be equivalent.  In particular, incompatible protocols
   can be deployed to TCP, UDP, SCTP, or DCCP ports as long as the IP
   address and port number is the same.

   This determination is made from the perspective of a client.  This
   means that servers need to be aware of all instances that might
   answer to the same IP address and port; see Section 7.

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6.1.  Validation Process

   The type of protocol authentication scope describes how a client
   might learn of all of the service endpoints that a server offers in
   that scope.  If a client has attempted to discover service endpoints
   using the methods defined by the protocol authentication scope,
   receiving an incompatible_protocols extension from a server is a
   strong indication of a potential downgrade attack.

   A client considers that a downgrade attack might have occurred if a
   server advertises that there are endpoints that support a protocol
   that the client prefers over the protocol that is currently in use.

   In response to detecting a potential downgrade attack, a client might
   abandon the current connection attempt and report an error.  A client
   that supports discovery of incompatible protocols, but chooses not to
   make a discovery attempt under normal conditions might instead not
   fail, but it could use what it learns as cause to initiate discovery.

6.2.  QUIC Version Negotiation

   QUIC enables the definition of incompatible protocols that share a
   port.  This mechanism can be used to authenticate the choice of
   application protocol in QUIC.  QUIC version negotiation [QUIC-VN] is
   used to authenticate the choice of QUIC version.

   As there are two potentially competing sets of preferences, clients
   need to set preferences for QUIC version and application protocol
   that do not result in inconsistent outcomes.  For example, if
   application protocol A exclusively uses QUIC version X and
   application protocol B exclusively uses QUIC version Y, setting a
   preference for both A and Y will lead to a failure condition that
   cannot be reconciled.

6.3.  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 [HTTP3] 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

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   costs time and resources.  A client avoids these costs by attempting
   the protocol it believes to be most widely supported, though this
   might come with a performance penalty in cases where the most-
   preferred protocol is supported.

   A client can choose to ignore incompatible protocols when attempting
   to use an alternative service.

7.  Operational Considerations

   By listing incompatible protocols a server needs reliable knowledge
   of the existence of these alternatives.  This depends on some
   coordination of deployments.  In particular, coordination is
   important if a load balancer distributes load for a single IP address
   to multiple server instances.  Ensuring consistent configuration of
   servers could present operational difficulties as it requires that
   incompatible protocols are only listed when those protocols are
   deployed across all server instances.

   Server deployments can choose not to provide information about
   incompatible protocols, which denies clients information about
   downgrade attacks but might avoid the operational complexity of
   providing accurate information.

   During rollout of a new, incompatible protocol, until the deployment
   is stable and not at risk of being disabled, servers SHOULD NOT
   advertise the existence of the new protocol.  Protocol deployments
   that are disabled, first need to be removed from the
   incompatible_protocols extension or there could be some loss of
   service.  Though the incompatible_protocols extension only applies at
   the time of the TLS handshake, clients might take some time to act on
   the information.  If an incompatible protocol is removed from
   deployment between when the client completes a handshake and when it
   acts, this could be treated as an error by the client.

   If a server does not list incompatible protocols, clients cannot
   learn about other services and so cannot detect downgrade attacks
   against those protocols.

8.  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

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9.  IANA Considerations

   TODO: register the extension

10.  References

10.1.  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, <>.

   [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, <>.

10.2.  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-
              dtls13-43, 30 April 2021,

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

   [HTTP11]   Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-18, 18 August 2021,

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   [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-34, 2 February 2021,

   [QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-34,
              14 January 2021, <

   [QUIC-VN]  Schinazi, D. and E. Rescorla, "Compatible Version
              Negotiation for QUIC", Work in Progress, Internet-Draft,
              draft-ietf-quic-version-negotiation-04, 26 May 2021,

   [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,

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

Appendix A.  Acknowledgments

   Benjamin Schwartz provided significant input into the design of the
   mechanism and helped simplify the overall design.

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Appendix B.  Defining Logical Servers

   As incompatible protocols use different protocol stacks, they also
   use different endpoints.  In other words, it is in many cases
   impossible for the exactly same endpoint to support multiple
   incompatible protocols.  Thus, it is necessary to understand the set
   of endpoints at a server that offer the incompatible protocols.

   A number of choices are possible here:

   *  The set of endpoints that are authoritative for the same domain

   *  The set of endpoints that are authoritative for the same
      "authority" as defined in RFC 3986 [URI], which is in effect
      domain name plus port number.

   *  The set of endpoints that are referenced by the same SVCB
      ServiceMode record.

   *  The set of endpoints that share an IP address.

   *  The set of endpoints that share an IP address and port number.

   The challenge with options based on domain name is that it might
   prevent the use of multiple service providers.  This is a common
   practice for HTTP, where the same domain name can be operated by
   multiple CDN operators.

   Having multiple service operators also rules out using SVCB
   ServiceMode records also as different records might be used to
   identify different operators.

   Hosts on the same IP address might work, but common deployment
   practices include use of different ports for entirely different
   services, which can have different operational constraints such as
   deployment schedules.  Including different ports in the same scope
   could force all services on the same host to support a consistent set
   of protocols.

   This leaves IP and port.  There is always a risk that the same port
   number is used for completely different purposes depending on the
   choice of protocol, but this practice is sufficiently rare that it is
   not anticipated to be a problem.

Author's Address

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   Martin Thomson


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