Network Working Group                                         A. Langley
Internet-Draft                                                Google Inc
Intended status: Standards Track                              P. Hoffman
Expires: November 15, 2011                                          VPNC
                                                            May 14, 2011


         Transport Layer Security (TLS) Next Protocol Extension
                       draft-agl-tls-nextproto-00

Abstract

   This document describes a Transport Layer Security (TLS) extension
   for application layer protocol probing and announcement.  This allows
   the application client to specify which protocol will be performed
   over the secure connection.

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Table of Contents

   1.  Introduction
     1.1.  Design of the Protocol
   2.  Requirements Notation
   3.  Next Protocol Extension
     3.1.  Handshake Summary
     3.2.  Session Resumption and Renegotiation
   4.  Security Considerations
   5.  IANA Considerations
   6.  Acknowledgements
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Authors' Addresses


.  Introduction

   As the Internet has evolved, it has become commonplace for hosts to
   initiate connections based on untrusted and possibly hostile data.
   HTTP [RFC2616] clients are currently the most widespread example of
   this as they will fetch URLs based on the contents of untrusted
   webpages.

   Any time that a connection is initiated based on untrusted data there
   is the possibility of a cross-protocol attack.  If the attacker can
   control the contents of the connection in any way (for example, the
   requested URL in an HTTP connection) they may be able to encode a
   valid message in another protocol.  The connecting host believes that
   it is speaking one protocol but the server understands it to be
   another.  The application of Postel's Law exacerbates the issue as
   many servers will permit gross violations of the expected protocol in
   order to achieve maximum compatibility with clients.

   The WebSockets [websockets] protocol seeks to allow low-latency,
   full-duplex communication between browsers and HTTP servers.
   However, it also permits an unprecedented amount of attacker control
   over the contents of the connection.  In order to prevent cross-
   protocol attacks, a mechanism to assure that both client and server
   are speaking the same protocol is required.  To this end, the Next
   Protocol extension described in this document extends the TLS
   [RFC5246] handshake to allow the client to tell the server the
   intended application protocol.

1.1.  Design of the Protocol

   The basic design of the extension is that the client expresses that
   it knows how to specify which application protocol it will use after
   the TLS session is established.  The server responds with some or all
   the types of application protocols that it knows.  The client
   responds with the protocol that it intends to use; this might even be
   one that was not listed by the server.

   Note that this protocol is not a negotiation in the classic sense of
   "client says what it wants and server picks one choice".  Instead, it
   allows the client to not reveal in the clear which application
   protocol it intends to use after TLS is established.  Intermediaries
   who might prevent TLS from being established for a particular
   application cannot determine which protocol will be used; this, in
   turn, leads to secure connections for more protocols.


.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


.  Next Protocol Extension

   This document defines a new extension type, "next_protocol(TBD)".

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

   The next_protocol extension MAY be included by the client in its
   "ClientHello" message.  If, and only if, the server sees this
   extension in the "ClientHello", it MAY choose to include the
   extension in its "ServerHello".

   The "extension_data" field of a "next_protocol" in a "ClientHello"
   MUST be empty.  The "extension_data" field of a "next_protocol" in a
   "ServerHello" contains the list of application-layer protocols that
   the server wishes to advertise that it supports.  That list is a set
   of two-octet (uint16) values, in network byte order, taken from the
   ports numbers assigned by IANA; see
   <http://www.iana.org/assignments/port-numbers>.

   This document also defines a new handshake message type,
   "next_protocol_ann(TBD)").

      struct {
        uint16 announced_protocol;
      } NextProtocolAnnounce;

   If, and only if, the server included a "next_protocol" extension in
   its ServerHello message, the client MUST send a
   "NextProtocolAnnounce" message after its "ChangeCipherSpec" and
   before its "Finished" message.  The NextProtocolAnnounce message
   contains the single application-layer protocol that the client will
   use in this connection after the TLS handshake completes; that value
   does not need to match any of those given by the server in the
   next_protocol extension.

3.1.  Handshake Summary

      -> ClientHello (contains next_protocol extension
              with empty extension_data )
      <- ServerHello (contains next_protocol extension
              with list of protocols)
      <- ...
      <- ServerHelloDone
      -> ClientKeyExchange
      -> ...
      -> ChangeCipherSpec
      -> NextProtocolAnnounce (contains announced_protocol)
      -> Finished
      <- ChangeCipherSpec
      <- Finished

3.2.  Session Resumption and Renegotiation

   Unlike many other TLS extensions, this extension does not establish
   properties of the session, only of the connection.  When session
   resumption or session tickets [RFC5077] are used, the previous
   contents of this extension are irrelevant and only the values in the
   new handshake messages are considered.

   For the same reasons, after a handshake has been performed for a
   given connection, renegotiations on the same connection MUST NOT
   include the "next_protocol" extension.


.  Security Considerations

   This extension sends the server's list of supported protocols in the
   clear.  This may be undesirable for certain protocols (such as Tor
   [tor]) where one could imagine that hostile networks would terminate
   any TLS connection with a server that advertised such a capability.
   Thus, if a client knows through out-of-band methods that a server
   supports a particular protocol, it can specify that protocol in the
   NextProtocolAnnounce message and use that protocol after TLS is set
   up.


.  IANA Considerations

   This document requires IANA to update its registry of TLS extensions
   to assign an entry, referred herein as "next_protocol".

   This document also requires IANA to update its registry of TLS
   handshake types to assign an entry, referred herein as
   "next_protocol_ann".


.  Acknowledgements

   This document benefitted specifically from discussions with Wan-Teh
   Chang and Nagendra Modadugu.


.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

7.2.  Informative References

   [websockets]
              Fette, I., "The Web Socket protocol",
              draft-ietf-hybi-thewebsocketprotocol (work in progress),
              2011.

   [tor]      "Tor Onion Router", www.tor.org , 2011.


Authors' Addresses

   Adam Langley
   Google Inc

   Email: agl@google.com


   Paul Hoffman
   VPNC

   Email: paul.hoffman@vpnc.org