Application-Layer Protocol Negotiation (ALPN) for WebRTC
RFC 8833



Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8833                                       Mozilla
Category: Standards Track                                   January 2021
ISSN: 2070-1721

        Application-Layer Protocol Negotiation (ALPN) for WebRTC

Abstract

   This document specifies two Application-Layer Protocol Negotiation
   (ALPN) labels for use with Web Real-Time Communication (WebRTC).  The
   "webrtc" label identifies regular WebRTC: a DTLS session that is used
   to establish keys for the Secure Real-time Transport Protocol (SRTP)
   or to establish data channels using the Stream Control Transmission
   Protocol (SCTP) over DTLS.  The "c-webrtc" label describes the same
   protocol, but the peers also agree to maintain the confidentiality of
   the media by not sharing it with other applications.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8833.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Conventions
   2.  ALPN Labels for WebRTC
   3.  Media Confidentiality
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Author's Address

1.  Introduction

   Web Real-Time Communication (WebRTC) [RFC8825] uses Datagram
   Transport Layer Security (DTLS) [RFC6347] to secure all peer-to-peer
   communications.

   Identifying WebRTC protocol usage with Application-Layer Protocol
   Negotiation (ALPN) [RFC7301] enables an endpoint to positively
   identify WebRTC uses and distinguish them from other DTLS uses.

   Different WebRTC uses can be advertised and behavior can be
   constrained to what is appropriate to a given use.  In particular,
   this allows for the identification of sessions that require
   confidentiality protection from the application that manages the
   signaling for the session.

1.1.  Conventions

   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.

2.  ALPN Labels for WebRTC

   The following identifiers are defined for use in ALPN:

   webrtc:  The DTLS session is used to establish keys for the Secure
      Real-time Transport Protocol (SRTP) -- known as DTLS-SRTP -- as
      described in [RFC5764].  The DTLS record layer is used for WebRTC
      data channels [RFC8831].

   c-webrtc:  The DTLS session is used for confidential WebRTC, where
      peers agree to maintain the confidentiality of the media, as
      described in Section 3.  The confidentiality protections ensure
      that media is protected from other applications, but the
      confidentiality protections do not extend to messages on data
      channels.

   Both identifiers describe the same basic protocol: a DTLS session
   that is used to provide keys for an SRTP session in combination with
   WebRTC data channels.  Either SRTP or data channels could be absent.
   The data channels send the Stream Control Transmission Protocol
   (SCTP) [RFC4960] over the DTLS record layer, which can be multiplexed
   with SRTP on the same UDP flow.  WebRTC requires the use of
   Interactive Connectivity Establishment (ICE) [RFC8445] to establish
   UDP flow, but this is not covered by the identifier.

   A more thorough definition of what WebRTC entails is included in
   [RFC8835].

   There is no functional difference between the identifiers except that
   an endpoint negotiating "c-webrtc" makes a promise to preserve the
   confidentiality of the media it receives.

   A peer that is not aware of whether it needs to request
   confidentiality can use either identifier.  A peer in the client role
   MUST offer both identifiers if it is not aware of a need for
   confidentiality.  A peer in the server role SHOULD select "webrtc" if
   it does not prefer either.

   An endpoint that requires media confidentiality might negotiate a
   session with a peer that does not support this specification.  An
   endpoint MUST abort a session if it requires confidentiality but does
   not successfully negotiate "c-webrtc".  A peer that is willing to
   accept "webrtc" SHOULD assume that a peer that does not support this
   specification has negotiated "webrtc" unless signaling provides other
   information; however, a peer MUST NOT assume that "c-webrtc" has been
   negotiated unless explicitly negotiated.

3.  Media Confidentiality

   Private communications in WebRTC depend on separating control (i.e.,
   signaling) capabilities and access to media [RFC8827].  In this way,
   an application can establish a session that is end-to-end
   confidential, where the ends in question are user agents (or
   browsers) and not the signaling application.  This allows an
   application to manage signaling for a session without having access
   to the media that is exchanged in the session.

   Without some form of indication that is securely bound to the
   session, a WebRTC endpoint is unable to properly distinguish between
   a session that requires this confidentiality protection and one that
   does not.  The ALPN identifier provides that signal.

   A browser is required to enforce this confidentiality protection
   using isolation controls similar to those used in content cross-
   origin protections (see the "Origin" section of [HTML5]).  These
   protections ensure that media is protected from applications, which
   are not able to read or modify the contents of a protected flow of
   media.  Media that is produced from a session using the "c-webrtc"
   identifier MUST only be displayed to users.

   The promise to apply confidentiality protections do not apply to data
   that is sent using data channels.  Confidential data depends on
   having both data sources and consumers that are exclusively browser
   or user based.  No mechanisms currently exist to take advantage of
   data confidentiality, though some use cases suggest that this could
   be useful, for example, confidential peer-to-peer file transfer.
   Alternative labels might be provided in the future to support these
   use cases.

   This mechanism explicitly does not define a specific authentication
   method; a WebRTC endpoint that accepts a session with this ALPN
   identifier MUST respect confidentiality no matter what identity is
   attributed to a peer.

   RTP middleboxes and entities that forward media or data cannot
   promise to maintain confidentiality.  Any entity that forwards
   content, or records content for later access by entities other than
   the authenticated peer, MUST NOT offer or accept a session with the
   "c-webrtc" identifier.

4.  Security Considerations

   Confidential communications depend on more than just an agreement
   from browsers.

   Information is not confidential if it is displayed to others than for
   whom it is intended.  Peer authentication [RFC8827] is necessary to
   ensure that data is only sent to the intended peer.

   This is not a digital rights management mechanism.  A user is not
   prevented from using other mechanisms to record or forward media.
   This means that (for example) screen-recording devices, tape
   recorders, portable cameras, or a cunning arrangement of mirrors
   could variously be used to record or redistribute media once
   delivered.  Similarly, if media is visible or audible (or otherwise
   accessible) to others in the vicinity, there are no technical
   measures that protect the confidentiality of that media.

   The only guarantee provided by this mechanism and the browser that
   implements it is that the media was delivered to the user that was
   authenticated.  Individual users will still need to make a judgment
   about how their peer intends to respect the confidentiality of any
   information provided.

   On a shared computing platform like a browser, other entities with
   access to that platform (i.e., web applications) might be able to
   access information that would compromise the confidentiality of
   communications.  Implementations MAY choose to limit concurrent
   access to input devices during confidential communications sessions.

   For instance, another application that is able to access a microphone
   might be able to sample confidential audio that is playing through
   speakers.  This is true even if acoustic echo cancellation, which
   attempts to prevent this from happening, is used.  Similarly, an
   application with access to a video camera might be able to use
   reflections to obtain all or part of a confidential video stream.

5.  IANA Considerations

   The following two entries have been added to the "TLS Application-
   Layer Protocol Negotiation (ALPN) Protocol IDs" registry established
   by [RFC7301]:

   webrtc:
      The "webrtc" label identifies mixed media and data communications
      using SRTP and data channels:

      Protocol:  WebRTC Media and Data

      Identification Sequence:  0x77 0x65 0x62 0x72 0x74 0x63 ("webrtc")

      Specification:  RFC 8833 (this document)

   c-webrtc:
      The "c-webrtc" label identifies WebRTC with a promise to protect
      media confidentiality:

      Protocol:  Confidential WebRTC Media and Data

      Identification Sequence:  0x63 0x2d 0x77 0x65 0x62 0x72 0x74 0x63
         ("c-webrtc")

      Specification:  RFC 8833 (this document)

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7301]  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, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8827]  Rescorla, E., "WebRTC Security Architecture", RFC 8827,
              DOI 10.17487/RFC8827, January 2021,
              <https://www.rfc-editor.org/info/rfc8827>.

   [RFC8831]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
              Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,
              <https://www.rfc-editor.org/info/rfc8831>.

6.2.  Informative References

   [HTML5]    WHATWG, "HTML - Living Standard", Section 7.5, January
              2021, <https://html.spec.whatwg.org/#origin>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8825]  Alvestrand, H., "Overview: Real-Time Protocols for
              Browser-Based Applications", RFC 8825,
              DOI 10.17487/RFC8825, January 2021,
              <https://www.rfc-editor.org/info/rfc8825>.

   [RFC8835]  Alvestrand, H., "Transports for WebRTC", RFC 8835,
              DOI 10.17487/RFC8835, January 2021,
              <https://www.rfc-editor.org/info/rfc8835>.

Author's Address

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com