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The ALPN HTTP Header Field

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7639.
Authors Andrew Hutton , Justin Uberti , Martin Thomson
Last updated 2015-06-11 (Latest revision 2015-05-20)
Replaces draft-hutton-httpbis-connect-protocol
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Mark Nottingham
Shepherd write-up Show Last changed 2015-04-29
IESG IESG state Became RFC 7639 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass.
Responsible AD Barry Leiba
Send notices to,,,,
IANA IANA review state IANA OK - Actions Needed
HTTP Working Group                                             A. Hutton
Internet-Draft                                                     Unify
Intended status: Standards Track                               J. Uberti
Expires: November 21, 2015                                        Google
                                                              M. Thomson
                                                            May 20, 2015

                       The ALPN HTTP Header Field


   This specification allows HTTP CONNECT requests to indicate what
   protocol will 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 November 21, 2015.

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
   ( 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6

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 that will be used
   within the tunnel, using 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.  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

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   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 within the
   tunnel, or the 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 is given below.  It is based on the Generic Grammar defined in
   Section 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.

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   For example:

     ALPN: h2, http%2F1.1

2.3.  Usage

   For a CONNECT tunnel that conveys a TLS session that in turn
   encapsulates another protocol, the value of the ALPN header field
   contains the same list of ALPN identifiers that will be sent in the
   TLS ClientHello message [RFC7301].

   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 the ALPN header field, the ALPN identifier and registry
   are used to identify an entire application protocol stack, not a
   single protocol layer or component.

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

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   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.  Clients and HTTP proxies could choose to not support
   the header and therefore fail to provide it, or ignore it when
   present.  If the header 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 field.

   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, March 1997,

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

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

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

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

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   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, July 2014,

5.2.  Informative References

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


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   Martin Thomson
   331 E Evelyn Street
   Mountain View, CA  94041


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