TitleTransport Layer Security (TLS) Application-Layer Protocol Negotiation Extension
AuthorS. Friedl, A. Popov, A. Langley, E. Stephan
DateJuly 2014
Format:TXT, HTML
Updated byRFC8447

Internet Engineering Task Force (IETF)                         S. Friedl
Request for Comments: 7301                           Cisco Systems, Inc.
Category: Standards Track                                       A. Popov
ISSN: 2070-1721                                          Microsoft Corp.
                                                              A. Langley
                                                             Google Inc.
                                                              E. Stephan
                                                               July 2014

                     Transport Layer Security (TLS)
            Application-Layer Protocol Negotiation Extension


   This document describes a Transport Layer Security (TLS) extension
   for application-layer protocol negotiation within the TLS handshake.
   For instances in which multiple application protocols are supported
   on the same TCP or UDP port, this extension allows the application
   layer to negotiate which protocol will be used within the TLS

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

Copyright Notice

   Copyright (c) 2014 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 Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Application-Layer Protocol Negotiation  . . . . . . . . . . .   3
     3.1.  The Application-Layer Protocol Negotiation Extension  . .   3
     3.2.  Protocol Selection  . . . . . . . . . . . . . . . . . . .   5
   4.  Design Considerations . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Increasingly, application-layer protocols are encapsulated in the TLS
   protocol [RFC5246].  This encapsulation enables applications to use
   the existing, secure communications links already present on port 443
   across virtually the entire global IP infrastructure.

   When multiple application protocols are supported on a single server-
   side port number, such as port 443, the client and the server need to
   negotiate an application protocol for use with each connection.  It
   is desirable to accomplish this negotiation without adding network
   round-trips between the client and the server, as each round-trip
   will degrade an end-user's experience.  Further, it would be
   advantageous to allow certificate selection based on the negotiated
   application protocol.

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

   This document specifies a TLS extension that permits the application
   layer to negotiate protocol selection within the TLS handshake.  This
   work was requested by the HTTPbis WG to address the negotiation of
   HTTP/2 ([HTTP2]) over TLS; however, ALPN facilitates negotiation of
   arbitrary application-layer protocols.

   With ALPN, the client sends the list of supported application
   protocols as part of the TLS ClientHello message.  The server chooses
   a protocol and sends the selected protocol as part of the TLS
   ServerHello message.  The application protocol negotiation can thus
   be accomplished within the TLS handshake, without adding network
   round-trips, and allows the server to associate a different
   certificate with each application protocol, if desired.

2.  Requirements Language

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

3.  Application-Layer Protocol Negotiation

3.1.  The Application-Layer Protocol Negotiation Extension

   A new extension type ("application_layer_protocol_negotiation(16)")
   is defined and MAY be included by the client in its "ClientHello"

   enum {
       application_layer_protocol_negotiation(16), (65535)
   } ExtensionType;

   The "extension_data" field of the
   ("application_layer_protocol_negotiation(16)") 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, as described further
   in Section 6 ("IANA Considerations") of this document.  Empty strings
   MUST NOT be included and byte strings MUST NOT be truncated.

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

   Servers that receive a ClientHello 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(16)") MAY be returned to the
   client within the extended ServerHello message.  The "extension_data"
   field of the ("application_layer_protocol_negotiation(16)") extension
   is structured the same as described above for the client
   "extension_data", except that the "ProtocolNameList" MUST contain
   exactly one "ProtocolName".

   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 [RFC5246]):

   Client                                              Server

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

                                 Figure 1

   * Indicates optional or situation-dependent messages that are not
   always sent.

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

   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 [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 that it supports and that 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 protocol identified in the
   "application_layer_protocol_negotiation" extension type in the
   ServerHello SHALL be definitive for the connection, until
   renegotiated.  The server SHALL NOT respond with a selected protocol
   and subsequently use a different protocol for application data

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

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
   the established TLS architecture.  The
   "application_layer_protocol_negotiation" ServerHello extension is
   intended to be definitive for the connection (until the connection is
   renegotiated) and is sent in plaintext to permit network elements to
   provide differentiated service for the connection when the TCP or UDP
   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 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 or UDP port number in use.

   Implementers and document editors who intend to extend the protocol
   identifier registry by adding new protocol identifiers should
   consider that in TLS versions 1.2 and below the client sends these
   identifiers in the clear.  They should also consider that, for at
   least the next decade, it is expected that browsers would normally
   use these earlier versions of TLS in the initial ClientHello.

   Care must be taken when such identifiers may leak personally
   identifiable information, or when such leakage may lead to profiling
   or to leaking of sensitive information.  If any of these apply to
   this new protocol identifier, the identifier SHOULD NOT be used in
   TLS configurations where it would be visible in the clear, and
   documents specifying such protocol identifiers SHOULD recommend
   against such unsafe use.

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

6.  IANA Considerations

   The IANA has updated its "ExtensionType Values" registry to include
   the following entry:

      16 application_layer_protocol_negotiation

   This document establishes a registry for protocol identifiers
   entitled "Application-Layer Protocol Negotiation (ALPN) Protocol IDs"
   under the existing "Transport Layer Security (TLS) Extensions"

   Entries in this registry require the following fields:

   o  Protocol: The name of the protocol.
   o  Identification Sequence: The precise set of octet values that
      identifies the protocol.  This could be the UTF-8 encoding
      [RFC3629] of the protocol name.
   o  Reference: A reference to a specification that defines the

   This registry operates under the "Expert Review" policy as defined in
   [RFC5226].  The designated expert is advised to encourage the
   inclusion of a reference to a permanent and readily available
   specification that enables the creation of interoperable
   implementations of the identified protocol.

   The initial set of registrations for this registry is as follows:

   Protocol:  HTTP/1.1
   Identification Sequence:
      0x68 0x74 0x74 0x70 0x2f 0x31 0x2e 0x31 ("http/1.1")
   Reference:  [RFC7230]

   Protocol:  SPDY/1
   Identification Sequence:
      0x73 0x70 0x64 0x79 0x2f 0x31 ("spdy/1")

   Protocol:  SPDY/2
   Identification Sequence:
      0x73 0x70 0x64 0x79 0x2f 0x32 ("spdy/2")

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

   Protocol:  SPDY/3
   Identification Sequence:
      0x73 0x70 0x64 0x79 0x2f 0x33 ("spdy/3")

7.  Acknowledgements

   This document benefitted specifically from the Next Protocol
   Negotiation (NPN) extension document authored by Adam Langley and
   from discussions with Tom Wesselman and Cullen Jennings, both of

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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June

8.2.  Informative References

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", Work in Progress, June 2014.

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

RFC 7301         TLS App-Layer Protocol Negotiation Ext        July 2014

Authors' Addresses

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

   Phone: (720)562-6785
   EMail: sfriedl@cisco.com

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

   EMail: andreipo@microsoft.com

   Adam Langley
   Google Inc.

   EMail: agl@google.com

   Emile Stephan
   2 avenue Pierre Marzin
   Lannion  F-22307

   EMail: emile.stephan@orange.com