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Internet Engineering Task Force (IETF)                         A. HuttonRequest for Comments: 7639                                         UnifyCategory: Standards Track                                      J. UbertiISSN: 2070-1721                                                   Google                                                              M. Thomson                                                                 Mozilla                                                             August 2015The ALPN HTTP Header FieldAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Hutton, et al.               Standards Track                    [Page 1]

RFC 7639                     The ALPN Header                 August 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.  The ALPN HTTP Header Field  . . . . . . . . . . . . . . . . .32.1.  Header Field Values . . . . . . . . . . . . . . . . . . .32.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .32.3.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . .43.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .44.  Security Considerations . . . . . . . . . . . . . . . . . . .55.  References  . . . . . . . . . . . . . . . . . . . . . . . . .65.1.  Normative References  . . . . . . . . . . . . . . . . . .65.2.  Informative References  . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .71.  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.Hutton, et al.               Standards Track                    [Page 2]

RFC 7639                     The ALPN Header                 August 20151.1.  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 inRFC 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 inSection 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 perSection 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.Hutton, et al.               Standards Track                    [Page 3]

RFC 7639                     The ALPN Header                 August 2015   For example:     CONNECT www.example.com HTTP/1.1     Host: www.example.com     ALPN: h2, http%2F1.12.3.  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:Hutton, et al.               Standards Track                    [Page 4]

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      Force4.  Security Considerations   In case of using HTTP CONNECT to a TURN (Traversal Using Relays   around NAT, [RFC5766]) server, the security considerations ofSection 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 inSection 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.Hutton, et al.               Standards Track                    [Page 5]

RFC 7639                     The ALPN Header                 August 20155.  References5.1.  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>.Hutton, et al.               Standards Track                    [Page 6]

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,              <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: andrew.hutton@unify.com   Justin Uberti   Google   747 6th Street South   Kirkland, WA  98033   United States   Email: justin@uberti.name   Martin Thomson   Mozilla   331 East Evelyn Avenue   Mountain View, CA  94041   United States   Email: martin.thomson@gmail.comHutton, et al.               Standards Track                    [Page 7]