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IETF RFC 7639
Last modified on Thursday, August 27th, 2015
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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/RFC 7639.
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.
Hutton, et al. Standards Track PAGE 1 
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 [RFC 7231]) 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 [RFC 7301].
For a tunnel that is then secured using Transport Layer Security
(TLS) [RFC 5246], the header field carries the same application
protocol label as will be carried within the TLS handshake [RFC 7301].
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 2015
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 [RFC 2119].
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 [RFC 7301].
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 [RFC 7230].
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 ([RFC 7230],
Section 3.2.6) MUST be percent-encoded as per Section 2.1 of
[RFC 3986]. 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%2F1.1
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
[RFC 7301].
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
[RFC 3864] 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
Force
4. Security Considerations
In case of using HTTP CONNECT to a TURN (Traversal Using Relays
around NAT, [RFC 5766]) server, the security considerations of
Section 4.3.6 of [RFC 7231] 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
[RFC 7301].
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 [RFC 5246]), 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 2015
5. References
5.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC 3864, September 2004,
<http://www.rfc-editor.org/info/RFC 3864>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC 3986, January 2005,
<http://www.rfc-editor.org/info/RFC 3986>.
[RFC 7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230,
DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
[RFC 7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC 7231, June 2014,
<http://www.rfc-editor.org/info/RFC 7231>.
[RFC 7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC 7301,
July 2014, <http://www.rfc-editor.org/info/RFC 7301>.
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
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 5766] 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/RFC 5766, April 2010,
<http://www.rfc-editor.org/info/RFC 5766>.
[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.com
Hutton, et al. Standards Track PAGE 7
RFC TOTAL SIZE: 14455 bytes
PUBLICATION DATE: Thursday, August 27th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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