Network Working Group                                          S. Friedl
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                                A. Popov
Expires: August 25, 2013                                 Microsoft Corp.
                                                       February 21, 2013

 Transport Layer Security (TLS) Application Layer Protocol Negotiation


   This document describes a Transport Layer Security (TLS) extension
   for application layer protocol negotiation within the TLS handshake.
   For instances in which the TLS connection is established over a well
   known TCP/IP port not associated with the desired application layer
   protocol, this extension allows the application layer to negotiate
   which protocol will be used within the TLS session.

Status of this Memo

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Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Application Layer Protocol Negotiation
     3.1.  The Application Layer Protocol Negotiation Extension
     3.2.  Protocol Selection
   4.  Design Considerations
   5.  Security Considerations
   6.  IANA Considerations
   7.  Acknowledgements
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

1.  Introduction

   Currently, the Next Protocol Negotiation extension (NPN) is used to
   establish a SPDY [spdy] protocol session within a TLS RFC 5246
   [RFC5246] session on port 443.  NPN is not specific to SPDY and can
   be used to negotiate sessions for a wide variety of protocols within
   the TLS handshake.

   NPN seeks to provide a reliable mechanism for application developers
   to establish secure sessions for arbitrary protocols without
   interference from firewalls, HTTP proxies and MITM proxies.  It
   addresses this goal by introducing a protocol negotiation process
   into the TLS handshake under the constraints that no additional
   roundtrips be added to the handshake and that the final protocol
   selection be opaque to the network carrying the TLS session.  Within
   the NPN extension, it is the server that first generates and
   transmits an offer of supported protocols to the client.  The offer
   is sent as part of the TLS ServerHello message before the
   [ChangeCipherSpec] subprotocol has been started, therefore the list
   of protocols supported by the server is transmitted in plaintext.
   The client chooses a protocol which may or may not appear in the
   offer from the server and then responds with the definitive protocol
   selection answer.  The client response is sent after the
   [ChangeCipherSpec] subprotocol has been initiated, so the protocol
   selected is encrypted in the client response.

   In many other application layer protocol negotiation processes, it is
   the client that first sends an offer of protocols it supports to the
   server.  The server then selects the protocol to be used in the
   session and includes this answer in the response.  RFC 3264 [RFC3264]
   describes a SDP based offer/answer model which is not proscriptive in
   terms of which party generates the offer, however in practice it is
   typically the client generating the offer and the server replying
   with the answer.  This permits the server to act as the definitive
   entity for selection of the application layer protocol.

   This draft proposes an alternative formulation of the NPN protocol
   which 1) brings the offer/answer negotiation into alignment with the
   majority of other application layer protocol negotiation standards,
   2) allows certificate selection based on the application protocol and
   3) makes the definitive protocol selection answer from the server
   visible to the network, when the parties so desire.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Application Layer Protocol Negotiation

3.1.  The Application Layer Protocol Negotiation Extension

   A new extension type ("application_layer_protocol_negotiation(TBD)")
   is defined and MAY be included by the client in its "ClientHello"

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

   The "extension_data" field of the
   ("application_layer_protocol_negotiation(TBD)") extension SHALL
   contain a "ProtocolNameList" value.

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

   struct {
           ProtocolName protocol_name_list<2..2^16-1>
   } ProtocolNameList;

   "ProtocolNameList" contains the list of protocols advertised by the
   client, in descending order of preference.  Protocols are named by
   IANA registered, opaque, non-empty byte strings.  Implementations
   MUST ensure that an empty string is not included and that no byte
   strings are truncated.

   Experimental protocol names, which are not registered by IANA, will
   start with the following sequence of bytes: 0x65, 0x78, 0x70 ("exp").

   Servers that receive a client hello containing the
   "application_layer_protocol_negotiation" extension, MAY return a
   suitable protocol selection response to the client.  The server will
   ignore any protocol name that it does not recognize.  A new
   ServerHello extension type
   ("application_layer_protocol_negotiation(TBD)") MAY be returned to
   the client within the extended ServerHello message.  The
   "extension_data" field of the
   ("application_layer_protocol_negotiation(TBD)") extension SHALL be
   structured the same as described above for the client
   "extension_data", except that the "ProtocolNameList" MUST contain
   exactly one "ProtocolName".

   The additional content associated with this extension MUST be
   included in the hash calculations associated with the "Finished"

   Therefore, a full handshake with the
   "application_layer_protocol_negotiation" extension in the ClientHello
   and ServerHello messages has the following flow (contrast with
   section 7.3 of RFC 5246 [RFC5246]):

   Client                                              Server

   ClientHello                     -------->       ServerHello
     (ALPN extension &                               (ALPN extension &
      list of protocols)                              selected protocol)
                                   <--------       ServerHelloDone
   Finished                        -------->
                                   <--------       Finished
   Application Data                <------->       Application Data

                                 Figure 1

   An abbreviated handshake with the
   "application_layer_protocol_negotiation" extension has the following

   Client                                              Server

   ClientHello                     -------->       ServerHello
     (ALPN extension &                               (ALPN extension &
      list of protocols)                              selected protocol)
                                   <--------       Finished
   Finished                        -------->
   Application Data                <------->       Application Data

                                 Figure 2

   Unlike many other TLS extensions, this extension does not establish
   properties of the session, only of the connection.  When session
   resumption or session tickets RFC 5077 [RFC5077] are used, the
   previous contents of this extension are irrelevant and only the
   values in the new handshake messages are considered.

3.2.  Protocol Selection

   It is expected that a server will have a list of protocols that it
   supports, in preference order, and will only select a protocol if the
   client supports it.  In that case, the server SHOULD select the most
   highly preferred protocol it supports which is also advertised by the
   client.  In the event that the server supports no protocols that the
   client advertises, then the server SHALL respond with a fatal
   "no_application_protocol" alert.

   enum {
   } AlertDescription;

   The "no_application_protocol" fatal alert is only defined for the
   "application_layer_protocol_negotiation" extension and MUST NOT be
   sent unless the server has received a ClientHello message containing
   this extension.

   The protocol identified in the
   "application_layer_protocol_negotiation" extension type in the
   ServerHello SHALL be definitive for the connection.  The server SHALL
   NOT respond with a selected protocol and subsequently use a different
   protocol for application data exchange.

4.  Design Considerations

   The ALPN extension is intended to follow the typical design of TLS
   protocol extensions.  Specifically, the negotiation is performed
   entirely within the client/server hello exchange in accordance with
   established TLS architecture.  The
   "application_layer_protocol_negotiation" ServerHello extension is
   intended to be definitive for the connection and is sent in plaintext
   to permit network elements to provide differentiated service for the
   connection when the TCP/IP port number is not definitive for the
   application layer protocol to be used in the connection.  By placing
   ownership of protocol selection on the server, ALPN facilitates
   scenarios in which certificate selection or connection rerouting may
   be based on the negotiated protocol.

   Finally, by managing protocol selection in the clear as part of the
   handshake, ALPN avoids introducing false confidence with respect to
   the the ability to hide the negotiated protocol in advance of
   establishing the connection.  If hiding the protocol is required,
   then renegotiation after connection establishment, which would
   provide true TLS security guarantees, would be a preferred

5.  Security Considerations

   The ALPN extension does not impact the security of TLS session
   establishment or application data exchange.  ALPN serves to provide
   an externally visible marker for the application layer protocol
   associated with the TLS connection.  Historically, the application
   layer protocol associated with a connection could be ascertained from
   the TCP/IP port number in use.

   Encrypting the selected application protocol information and sending
   it before the Finished messages are exchanged, as done in NPN, does
   not provide confidentiality guarantees due to the possibility of man-
   in-the-middle attacks.

6.  IANA Considerations

   This document requires the IANA to update its registry of TLS
   extensions to assign an entry referred to here as
   "application_layer_protocol_negotiation" for extended ClientHello and
   ServerHello messages.

   This document also requires the IANA to create a registry of
   Application Layer Protocol Negotiation protocol byte strings,
   initially containing the following entries:

   - "http/1.1": HTTP/1.1 [RFC2616];

   - "http/2.0": HTTP/2.0;

   - "spdy/1": (obsolete) SPDY version 1;

   - "spdy/2": SPDY version 2;

   - "spdy/3": SPDY version 3.

   A namespace will be assigned for experimental protocols, comprising
   byte strings which start with the following sequence of bytes: 0x65,
   0x78, 0x70 ("exp").  Assignments in this namespace do not need IANA

7.  Acknowledgements

   This document benefitted specifically from the NPN extension draft
   authored by Adam Langley of Google and from discussions with Tom
   Wesselman and Cullen Jennings both of Cisco.

8.  References

8.1.  Normative References

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

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

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

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

8.2.  Informative References

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

   [spdy]     Belshe, M. and R. Peon, "SPDY Protocol (Internet Draft)",

Authors' Addresses

   Stephan Friedl
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: (720)562-6785

   Andrei Popov
   Microsoft Corp.
   One Microsoft Way
   Redmond, WA  98052