HTTP Working Group A. Hutton
Internet-Draft Unify
Intended status: Standards Track J. Uberti
Expires: October 20, 2015 Google
M. Thomson
Mozilla
April 18, 2015
The ALPN HTTP Header Field
draft-ietf-httpbis-tunnel-protocol-03
Abstract
This specification allows HTTP CONNECT requests to indicate what
protocol will be used within the tunnel once established, using the
ALPN header field.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at [1].
Working Group information can be found at [2] and [3]; source code
and issues list for this draft can be found at [4].
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
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This Internet-Draft will expire on October 20, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 5
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
spoken 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.
In TLS, the final choice of application protocol is made by the
server from the set of choices presented by the client. Other
protocols could negotiate protocols differently.
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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",
"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 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 ([5])
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.
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With these constraints, recipients can apply simple string comparison
to match protocol identifiers.
For example:
CONNECT www.example.com HTTP/1.1
Host: www.example.com
ALPN: h2, http%2F1.1
3. IANA Considerations
HTTP header fields are registered within the "Message Headers"
registry maintained at [6]. 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 (iesg@ietf.org) - Internet Engineering Task
Force
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. 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.
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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,
<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,
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, 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, 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, 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, July 2014,
<http://www.rfc-editor.org/info/rfc7301>.
5.2. Informative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, 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, April 2010,
<http://www.rfc-editor.org/info/rfc5766>.
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[TRAFFIC] Pironti, A., Strub, P-Y., and K. Bhargavan, "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>.
5.3. URIs
[1] http://www.iana.org/assignments/tls-extensiontype-values/#alpn-
protocol-ids
[2] https://www.iana.org/assignments/message-headers
Authors' Addresses
Andrew Hutton
Unify
Technology Drive
Nottingham NG9 1LA
UK
EMail: andrew.hutton@unify.com
Justin Uberti
Google
747 6th Ave S
Kirkland, WA 98033
US
EMail: justin@uberti.name
Martin Thomson
Mozilla
331 E Evelyn Street
Mountain View, CA 94041
US
EMail: martin.thomson@gmail.com
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