The ALPN HTTP Header Field
A header that lets an HTTP CONNECT request announce which application protocol will run inside the tunnel.
Hutton, A., Uberti, J., and M. Thomson, "The ALPN HTTP Header Field", RFC 7639, DOI 10.17487/RFC7639, August 2015, <https://www.rfc-editor.org/info/rfc7639>.
@misc{rfc7639,
series = {Request for Comments},
number = 7639,
howpublished = {RFC 7639},
publisher = {RFC Editor},
doi = {10.17487/RFC7639},
url = {https://www.rfc-editor.org/info/rfc7639},
author = {A. Hutton and J. Uberti and M. Thomson},
title = {{The ALPN HTTP Header Field}},
pagetotal = 7,
year = 2015,
month = aug,
} RFC text
Contents
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.
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.
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.
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:
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, [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.
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>.
[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.com
AI ELI5
On this page
§1Why CONNECT Needs a Hint
The HTTP CONNECT method asks the recipient to set up a tunnel to an origin server and then 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.
RFC 7639 adds an ALPN header field to CONNECT requests. It names the protocol, or the set of protocols, that the client intends to use once the tunnel exists. It reuses the identifiers from Application-Layer Protocol Negotiation, defined in RFC 7301.
Proxies do not implement the tunneled protocol. They might still make policy decisions from the header value, for example choosing traffic prioritization.
When you ask a gatekeeper to open a tunnel, you can also tell them what you will carry through it. That helps them decide how to treat you.
§2Intent, Not Negotiation
The header carries an indication of client intent only. No negotiation takes place using it. In TLS, the final choice of application protocol is made by the server from the set of choices the client presents. Other substrates could negotiate the protocol differently.
When the tunnel will be secured with TLS, the header carries the same application protocol labels that will appear in the TLS handshake. If several protocols are possible, all of them are listed. An ALPN identifier here names an entire application protocol stack, not a single layer or component.
Where no negotiation is expected, such as protocols that do not use TLS, the header holds a single identifier for the intended protocol. If some other form of negotiation is possible, it holds the set of protocols that might be negotiated.
The sign on the truck says what you hope to carry. The actual deal is still made with whoever is at the other end of the tunnel.
§3Header Syntax and Encoding
Valid values come from the ALPN Protocol ID registry created by RFC 7301. The value is a comma-separated list of one or more protocol-id items, each a HTTP token.
ALPN protocol names are octet sequences with no format constraints, so octets that are not allowed in a token MUST be percent-encoded, as in RFC 3986. That includes the percent sign itself (hex 25). To give each name exactly one representation, octets that are valid token characters (other than %) MUST NOT be percent-encoded, and percent-encoding MUST use uppercase hex digits. Recipients can then match identifiers with simple string comparison.
The slash in http/1.1 is not a token character, which is why the example encodes it.
Example from the RFC: ALPN: h2, http%2F1.1 on a CONNECT www.example.com HTTP/1.1 request.
Protocol names go on the sign as a list. Some characters can not be written on the sign directly, so they are swapped for a special code, always written the same way so everyone reads it alike.
CONNECT www.example.com HTTP/1.1
Host: www.example.com
ALPN: h2, http%2F1.1§4How Clients and Proxies Use It
For a tunnel carrying TLS, the header holds the same list of identifiers that will go in the TLS ClientHello message.
A proxy can use the value to reject CONNECT requests more cleanly and efficiently. Seeing the protocol at the HTTP layer lets it deny a request earlier and report better errors, such as a 403 status code. It can also use the value to change how it manages or prioritizes connections.
The alternative is to inspect packets to guess the protocol, which requires understanding every ALPN identifier. Protocols like TLS can hide the negotiated protocol, and negotiation details can change over time. For this reason proxies SHOULD NOT break a CONNECT tunnel solely because they fail to recognize the protocol.
The guard can read the sign and say no right away, or move your truck to the right lane. Guessing from what is inside the truck is much harder.
§5Security Considerations
The header is OPTIONAL. Clients and proxies may not support it, and if it is missing or ignored the proxy cannot learn the tunnel's purpose for authorization. That case looks the same as a client or proxy that simply lacks support.
There is no confidentiality protection for the header, so ALPN identifiers that might expose sensitive information SHOULD NOT be sent (see Section 5 of RFC 7301).
A client can falsify the value. If the tunnel data is encrypted, the proxy may be unable to check the claim, although it might use traffic analysis. A proxy therefore cannot rely on the header as a policy input in all cases. When CONNECT is used to reach a TURN server (RFC 5766), the warnings in Section 4.3.6 of RFC 7231 apply: proxies SHOULD restrict CONNECT to a limited set of known ports or a configurable whitelist of safe targets.
Anyone can write anything on a sign, and the sign is visible to everyone. So the guard may use it as a hint but must not treat it as proof.
§6IANA Registration
The RFC registers the header in the Permanent Message Header Field Names registry, following RFC 3864. The entry is: header field name ALPN, protocol http, status Standard, with the IETF (iesg@ietf.org) as change controller and Section 2 of this document as the reference.
The new sign type was added to the official list of allowed signs, so everyone knows it exists.