HTTP Working Group                                             A. Hutton
Internet-Draft                                                     Unify
Intended status: Standards Track                               J. Uberti
Expires: November 21, December 13, 2015                                        Google
                                                              M. Thomson
                                                            May 20,
                                                           June 11, 2015

                       The ALPN HTTP Header Field


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 21, December 13, 2015.

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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 . . . . . . . . . . . . . . . . . . .   4   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6   7
   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 HTTP ALPN header field identifies the protocol or protocols that will be used
   the tunnel, client intends to use within a tunnel that is established using
   CONNECT.  This uses the Application Layer Protocol Negotiation
   identifier (ALPN, [RFC7301]).

   When the CONNECT method is used to establish a tunnel, the ALPN
   header field can be used to identify the protocol that the client
   intends to use with that tunnel.

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

   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.

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 will be used a client intends to use
   within the tunnel, or the 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 ([1])
   established by [RFC7301].

2.2.  Syntax

   The ABNF (Augmented Backus-Naur Form) syntax for the ALPN header
   field value is given below.  It is based on uses the Generic Grammar syntax defined in
   Section 2 1.2 of [RFC7230].

   ALPN = "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 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 "%", and

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

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

   For example:

     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 TLS session that in turn
   encapsulates another protocol, 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.

   When used in

   A proxy can use the value of the ALPN header field, 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 identifier header field can be falsified and registry
   are used to identify an entire application protocol stack, is therefore not
   sufficient basis for authorizing a
   single request.

   A proxy could attempt to inspect packets to determine the protocol layer in
   use.  This requires that the proxy understand each ALPN identifier.
   Protocols like TLS could hide negotiated protocols, or component. 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 at [2].  This document
   defines and registers the ALPN header field, according to [RFC3864]
   as follows:

   Header Field Name:  ALPN

   Protocol:  http

   Status:  Standard

   Reference:  Section 2

   Change Controller:  IETF ( - Internet Engineering Task

4.  Security Considerations

   In case of using HTTP CONNECT to a TURN server ("Traversal Using
   Relays around NAT", [RFC5766]) 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 an OPTIONAL
   header field. OPTIONAL.
   Clients and HTTP proxies could choose to not support
   the header it and therefore
   either fail to provide it, or ignore it when present.  If the header
   field is not available or 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].  A proxy therefore cannot rely on the
   value of the ALPN header field as a policy input in all cases.

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, <>.

5.2.  Informative References

   [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, "Website Users
              by TLS Traffic Analysis: New Attacks and Effective
              Countermeasures, Revision 1", 2012,

5.3.  URIs



Authors' Addresses

   Andrew Hutton
   Technology Drive
   Nottingham  NG9 1LA


   Justin Uberti
   747 6th Ave S
   Kirkland, WA  98033


   Martin Thomson
   331 E Evelyn Street
   Mountain View, CA  94041