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


                      The ALPN HTTP Header Field

Abstract

  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
  http://www.rfc-editor.org/info/rfc7639.

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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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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1.1.  Requirements Language

  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].

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 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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  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
  [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.

  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
  protocol.

  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 ([email protected]) - Internet Engineering Task
     Force

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
  field.

  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
  [RFC7301].

  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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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,
             <http://www.rfc-editor.org/info/rfc2119>.

  [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
             Procedures for Message Header Fields", BCP 90, RFC 3864,
             DOI 10.17487/RFC3864, September 2004,
             <http://www.rfc-editor.org/info/rfc3864>.

  [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,
             <http://www.rfc-editor.org/info/rfc3986>.

  [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
             (HTTP/1.1): Message Syntax and Routing", RFC 7230,
             DOI 10.17487/RFC7230, June 2014,
             <http://www.rfc-editor.org/info/rfc7230>.

  [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
             (HTTP/1.1): Semantics and Content", RFC 7231,
             DOI 10.17487/RFC7231, June 2014,
             <http://www.rfc-editor.org/info/rfc7231>.

  [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/
             tls-extensiontype-values>.

  [MSG-HDRS] IANA, "Permanent Message Header Field Names>",
             <https://www.iana.org/assignments/message-headers>.









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  [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246,
             DOI 10.17487/RFC5246, August 2008,
             <http://www.rfc-editor.org/info/rfc5246>.

  [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,
             <http://www.rfc-editor.org/info/rfc5766>.

  [TRAFFIC]  Pironti, A., Strub, P-Y., and K. Bhargavan, "Identifying
             Website Users by TLS Traffic Analysis: New Attacks and
             Effective Countermeasures, Revision 1", 2012,
             <https://alfredo.pironti.eu/research/publications/full/
             identifying-website-users-tls-traffic-analysis-new-
             attacks-and-effective-counterme>.

Authors' Addresses

  Andrew Hutton
  Unify
  Technology Drive
  Nottingham  NG9 1LA
  United Kingdom

  Email: [email protected]


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

  Email: [email protected]


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

  Email: [email protected]






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