RFC 7639

Internet Engineering Task Force (IETF)                         A. Hutton
Request for Comments: 7639                                         Unify
Category: Standards Track                                      J. Uberti
ISSN: 2070-1721                                                   Google
                                                              M. Thomson
                                                             August 2015

                       The ALPN HTTP Header Field


   This specification allows HTTP CONNECT requests to indicate what
   protocol is intended to be used within the tunnel once established,
   using the ALPN header field.

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

Copyright Notice

   Copyright (c) 2015 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.

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RFC 7639                     The ALPN Header                 August 2015

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  The ALPN HTTP Header Field  . . . . . . . . . . . . . . . . .   3
     2.1.  Header Field Values . . . . . . . . . . . . . . . . . . .   3
     2.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The HTTP CONNECT method (Section 4.3.6 of [RFC7231]) requests that
   the recipient establish a tunnel to the identified origin server and
   thereafter forward packets, in both directions, until the tunnel is
   closed.  Such tunnels are commonly used to create end-to-end virtual
   connections through one or more proxies.

   The ALPN HTTP header field identifies the protocol or protocols that
   the client intends to use within a tunnel that is established using
   CONNECT.  This uses the Application-Layer Protocol Negotiation (ALPN)
   identifier [RFC7301].

   For a tunnel that is then secured using Transport Layer Security
   (TLS) [RFC5246], the header field carries the same application
   protocol label as will be carried within the TLS handshake [RFC7301].
   If there are multiple possible application protocols, all of those
   application protocols are indicated.

   The ALPN header field carries an indication of client intent only.
   An ALPN identifier is used here only to identify the application
   protocol or suite of protocols that the client intends to use in the
   tunnel.  No negotiation takes place using this header field.  In TLS,
   the final choice of application protocol is made by the server from
   the set of choices presented by the client.  Other substrates could
   negotiate the application protocol differently.

   Proxies do not implement the tunneled protocol, though they might
   choose to make policy decisions based on the value of the header
   field.  For example, a proxy could use the application protocol to
   select appropriate traffic prioritization.

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RFC 7639                     The ALPN Header                 August 2015

1.1.  Requirements Language

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

2.  The ALPN HTTP Header Field

   Clients include the ALPN header field in an HTTP CONNECT request to
   indicate the application-layer protocol that a client intends to use
   within the tunnel, or a set of protocols that might be used within
   the tunnel.

2.1.  Header Field Values

   Valid values for the protocol field are taken from the "Application-
   Layer Protocol Negotiation (ALPN) Protocol ID" registry [ALPN-IDS]
   established by [RFC7301].

2.2.  Syntax

   The ABNF (Augmented Backus-Naur Form) syntax for the ALPN header
   field value is given below.  It uses the syntax defined in
   Section 1.2 of [RFC7230].

   ALPN            = 1#protocol-id
   protocol-id     = token ; percent-encoded ALPN protocol identifier

   ALPN protocol names are octet sequences with no additional
   constraints on format.  Octets not allowed in tokens ([RFC7230],
   Section 3.2.6) MUST be percent-encoded as per 2.1">Section 2.1 of
   [RFC3986].  Consequently, the octet representing the percent
   character "%" (hex 25) MUST be percent-encoded as well.

   In order to have precisely one way to represent any ALPN protocol
   name, the following additional constraints apply:

   o  Octets in the ALPN protocol MUST NOT be percent-encoded if they
      are valid token characters except "%".

   o  When using percent-encoding, uppercase hex digits MUST be used.

   With these constraints, recipients can apply simple string comparison
   to match protocol identifiers.

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RFC 7639                     The ALPN Header                 August 2015

   For example:

     CONNECT www.example.com HTTP/1.1
     Host: www.example.com
     ALPN: h2, http%2F1.1

2.3.  Usage

   When used in the ALPN header field, an ALPN identifier is used to
   identify an entire application protocol stack, not a single protocol
   layer or component.

   For a CONNECT tunnel that conveys a protocol secured with TLS, the
   value of the ALPN header field contains the same list of ALPN
   identifiers that will be sent in the TLS ClientHello message

   Where no protocol negotiation is expected to occur, such as in
   protocols that do not use TLS, the ALPN header field contains a
   single ALPN protocol identifier corresponding to the application
   protocol that is intended to be used.  If an alternative form of
   protocol negotiation is possible, the ALPN header field contains the
   set of protocols that might be negotiated.

   A proxy can use the value of the ALPN header field to more cleanly
   and efficiently reject requests for a CONNECT tunnel.  Exposing
   protocol information at the HTTP layer allows a proxy to deny
   requests earlier, with better error reporting (such as a 403 status
   code).  The ALPN header field can be falsified and therefore is not a
   sufficient basis for authorizing a request.

   A proxy could attempt to inspect packets to determine the protocol in
   use.  This requires that the proxy understand each ALPN identifier.
   Protocols like TLS could hide negotiated protocols, or protocol
   negotiation details could change over time.  Proxies SHOULD NOT break
   a CONNECT tunnel solely on the basis of a failure to recognize the

   A proxy can use the ALPN header field value to change how it manages
   or prioritizes connections.

3.  IANA Considerations

   HTTP header fields are registered within the "Permanent Message
   Header Field Names" registry maintained by IANA [MSG-HDRS].  This
   document defines and registers the ALPN header field, according to
   [RFC3864] as follows:

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RFC 7639                     The ALPN Header                 August 2015

   Header Field Name:  ALPN

   Protocol:  http

   Status:  Standard

   Reference:  Section 2 of this document (RFC 7639)

   Change Controller:  IETF (iesg@ietf.org) - Internet Engineering Task

4.  Security Considerations

   In case of using HTTP CONNECT to a TURN (Traversal Using Relays
   around NAT, [RFC5766]) server, the security considerations of
   Section 4.3.6 of [RFC7231] apply.  It states that there "are
   significant risks in establishing a tunnel to arbitrary servers,
   particularly when the destination is a well-known or reserved TCP
   port that is not intended for Web traffic. ... Proxies that support
   CONNECT SHOULD restrict its use to a limited set of known ports or a
   configurable whitelist of safe request targets."

   The ALPN header field described in this document is OPTIONAL.
   Clients and HTTP proxies could choose not to support it and therefore
   either fail to provide it or ignore it when present.  If the header
   field is not available or is ignored, a proxy cannot identify the
   purpose of the tunnel and use this as input to any authorization
   decision regarding the tunnel.  This is indistinguishable from the
   case where either client or proxy does not support the ALPN header

   There is no confidentiality protection for the ALPN header field.
   ALPN identifiers that might expose confidential or sensitive
   information SHOULD NOT be sent, as described in Section 5 of

   The value of the ALPN header field could be falsified by a client.
   If the data being sent through the tunnel is encrypted (for example,
   with TLS [RFC5246]), then the proxy might not be able to directly
   inspect the data to verify that the claimed protocol is the one which
   is actually being used, though a proxy might be able to perform
   traffic analysis [TRAFFIC].  Therefore, a proxy cannot rely on the
   value of the ALPN header field as a policy input in all cases.

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RFC 7639                     The ALPN Header                 August 2015

5.  References

5.1.  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,

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230,
              DOI 10.17487/RFC7230, June 2014,

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [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, <http://www.rfc-editor.org/info/rfc7301>.

5.2.  Informative References

   [ALPN-IDS] IANA, "Application-Layer Protocol Negotiation (ALPN)
              Protocol ID", <http://www.iana.org/assignments/

   [MSG-HDRS] IANA, "Permanent Message Header Field Names>",

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RFC 7639                     The ALPN Header                 August 2015

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,

   [TRAFFIC]  Pironti, A., Strub, P-Y., and K. Bhargavan, "Identifying
              Website Users by TLS Traffic Analysis: New Attacks and
              Effective Countermeasures, Revision 1", 2012,

Authors' Addresses

   Andrew Hutton
   Technology Drive
   Nottingham  NG9 1LA
   United Kingdom

   Email: andrew.hutton@unify.com

   Justin Uberti
   747 6th Street South
   Kirkland, WA  98033
   United States

   Email: justin@uberti.name

   Martin Thomson
   331 East Evelyn Avenue
   Mountain View, CA  94041
   United States

   Email: martin.thomson@gmail.com

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