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Network Working Group                                         A. Langley
Internet-Draft                                                Google Inc
Expires: October 31, 2012                                       May 2012

   Transport Layer Security (TLS) Next Protocol Negotiation Extension
                     draft-agl-tls-nextprotoneg-04

Abstract

   This document describes a Transport Layer Security (TLS) extension
   for application layer protocol negotiation.  This allows the
   application layer to negotiate which protocol should be performed
   over the secure connection.

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
   and may be updated, replaced, or obsoleted by other documents at any
   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 October 31, 2012.

Copyright Notice

   Copyright (c) 2012 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 (http://trustee.ietf.org/
   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.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  2
   3.  Next Protocol Negotiation Extension  . . . . . . . . . . . . .  2
   4.  Protocol selection . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Design discussion  . . . . . . . . . . . . . . . . . . . . . .  5

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   6.  Security considerations  . . . . . . . . . . . . . . . . . . .  5
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  5
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  6
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  6
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  6

1.  Introduction

   The Next Protocol Negotiation extension (NPN) is currently used to
   negotiate the use of SPDY [spdy] as an application level protocol on
   port 443, and to perform SPDY version negotiation.  However, it is
   not SPDY specific in any way.

   Designers of new application level protocols are faced with a
   problem: there are no good options for establishing a clean transport
   for a new protocol and negotiating its use.  Negotiations on port 80
   will run afoul of intercepting proxies.  Ports other than 80 and 443
   are likely to be firewalled without any fast method of detection, and
   are also unlikely to traverse HTTP proxies with CONNECT. Negotiating
   on port 443 is possible, but may run afoul of MITM proxies and also
   uses a round trip for negotiation on top of the round trips for
   establishing the TLS connection.  Negotiation at that level is also
   dependent on the application level protocol, i.e.  the real world
   tolerance of servers to HTTP Upgrade requests.

   Next Protocol Negotiation allows application level protocols to be
   negotiated without additional round trips and with clean fallback in
   the case of an unsupportive MITM proxy.

2.  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].

3.  Next Protocol Negotiation Extension

   A new extension type ("next_protocol_negotiation(TBD)") is defined
   and 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 echo the extension in its "ServerHello".

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

   The "extension_data" field of a "next_protocol_negotiation" extension
   in a "ClientHello" MUST be empty.







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   The "extension_data" field of a "next_protocol_negotiation" extension
   in a "ServerHello" contains an optional list of protocols advertised
   by the server.  Protocols are named by opaque, non-empty byte strings
   and the list of protocols is serialized as a concatenation of 8-bit,
   length prefixed byte strings.  Implementations MUST ensure that the
   empty string is not included and that no byte strings are truncated.

   A new handshake message type ("encrypted_extensions(TBD)") is
   defined.  If the server included a "next_protocol_negotiation"
   extension in its "ServerHello" message, the client MUST send a
   "EncryptedExtensions" message after its "ChangeCipherSpec" and before
   its "Finished" message.

   enum {
     encrypted_extensions(TBD), (65535)
   } HandshakeType;

   Therefore a full handshake with "EncryptedExtensions" has the
   following flow (contrast with section 7.3 of RFC 5246 [RFC5246]):

   Client                                               Server

   ClientHello (NPN extension)   -------->
                                                   ServerHello
                                                     (NPN extension &
                                                      list of protocols)
                                                  Certificate*
                                            ServerKeyExchange*
                                           CertificateRequest*
                                <--------      ServerHelloDone
   Certificate*
   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   EncryptedExtensions
   Finished                     -------->
                                            [ChangeCipherSpec]
                                <--------             Finished
   Application Data             <------->     Application Data

   An abbreviated handshake with "EncryptedExtensions" has the following
   flow:












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   Client                                                Server

   ClientHello (NPN extension)    -------->
                                                   ServerHello
                                                     (NPN extension &
                                                      list of protocols)
                                            [ChangeCipherSpec]
                                 <--------            Finished
   [ChangeCipherSpec]
   EncryptedExtensions
   Finished                      -------->
   Application Data              <------->    Application Data

   The "EncryptedExtensions" message contains a series of "Extension"
   structures (see section 7.4.1.4 of RFC 5246 [RFC5246]

   If the server included a "next_protocol_negotiation" extension in its
   "ServerHello" message, the client MUST include an "Extension" with
   "extension_type" equal to "next_protocol_negotiation(TBD)".  The
   "extension_data" of which has the following format:

   struct {
     opaque selected_protocol<0..255>;
     opaque padding<0..255>;
   } NextProtocolNegotiationEncryptedExtension;

   The contents of "selected_protocol" are an opaque protocol string,
   but need not have been advertised by the server.  The length of
   "padding" SHOULD be 32 - ((len(selected_protocol) + 2) % 32). Note
   that len(selected_protocol) does not include its length prefix.

   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_negotiation" extension.

4.  Protocol selection

   It's expected that a client will have a list of protocols that it
   supports, in preference order, and will only select a protocol if the
   server supports it.  In that case, the client SHOULD select the first
   protocol advertised by the server that it also supports.  In the
   event that the client doesn't support any of server's protocols, or
   the server doesn't advertise any, it SHOULD select the first protocol
   that it supports.




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   There may be cases where the client knows, via other means, that a
   server supports an unadvertised protocol.  In these cases the client
   can simply select that protocol.

5.  Design discussion

   NPN is an outlier from TLS in several respects: firstly that it
   introduces a handshake message between the "ChangeCipherSpec" and
   "Finished" message, that the handshake message is padded, and that
   the negotiation isn't done purely with the hello messages.  All these
   aspects of the protocol are intended to prevent middle-ware
   discrimination based on the negotiated protocol and follow the
   general principle that anything that can be encrypted, should be
   encrypted.  The server's list of advertised protocols is in the clear
   as a compromise between performance and robustness.

6.  Security considerations

   The server's list of supported protocols is still advertised in the
   clear with this extension.  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.  In this case, clients may wish to
   opportunistically select a protocol that wasn't advertised by the
   server.  However, the workings of such a scheme are outside the scope
   of this document.

7.  IANA Considerations

   This document requires IANA to update its registry of TLS extensions
   to assign an entry referred to here as "next_protocol_negotiation".

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

   This document also requires IANA to create a registry of TLS Next
   Protocol Negotiation protocol strings on a first come, first served
   basis, initially containing the following entries:

   o  "http/1.1": HTTP/1.1 [RFC2616]

   o  "spdy/1": (obsolete) SPDY version 1

   o  "spdy/2": SPDY version 2

   o  "spdy/3": SPDY version 3

8.  Acknowledgments

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


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9.  References

9.1.  Normative References

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

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

9.2.  Informative References

   [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.

   [tor]      Dingledine, R., Matthewson, N. and P. Syverson, "Tor: The
              Second-Generation Onion Router", August 2004.

   [spdy]     Belshe, M. and R. Peon, "SPDY Protocol (Internet Draft)",
              Feb 2012.

Author's Address

   Adam Langley
   Google Inc

   Email: agl@google.com





















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