Unify

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Google

747 6th Street South Kirkland WA 98033 United States justin@uberti.name

Mozilla

331 East Evelyn Avenue Mountain View CA 94041 United States martin.thomson@gmail.com

Applications HTTP Working Group HTTP CONNECT Firewall HTTP proxy 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.

The HTTP CONNECT method () 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 . For a tunnel that is then secured using Transport Layer Security (TLS), 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. 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.

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.

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.

Valid values for the protocol field are taken from the "Application-Layer Protocol Negotiation (ALPN) Protocol ID" registry established by .

The ABNF (Augmented Backus-Naur Form) syntax for the ALPN header field value is given below. It uses the syntax defined in .

ALPN protocol names are octet sequences with no additional constraints on format. Octets not allowed in tokens () MUST be percent-encoded as per . 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: Octets in the ALPN protocol MUST NOT be percent-encoded if they are valid token characters except "%". 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:

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 . 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.

HTTP header fields are registered within the "Permanent Message Header Field Names" registry maintained by IANA . This document defines and registers the ALPN header field, according to as follows: ALPN http Standard of this document (RFC 7639) IETF (iesg@ietf.org) - Internet Engineering Task Force

In case of using HTTP CONNECT to a TURN (Traversal Using Relays around NAT, ) server, the security considerations of 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 . 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), 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 . Therefore, a proxy cannot rely on the value of the ALPN header field as a policy input in all cases.

Harvard University

1350 Massachusetts Avenue Cambridge MA 02138 - +1 617 495 3864 sob@harvard.edu

General keyword In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. Authors who follow these guidelines should incorporate this phrase near the beginning of their document: 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. Note that the force of these words is modified by the requirement level of the document in which they are used. This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. World Wide Web Consortium

Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge MA 02139 USA +1-617-253-5702 +1-617-258-5999 timbl@w3.org http://www.w3.org/People/Berners-Lee/

Day Software

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Adobe Systems Incorporated

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Applications uniform resource identifier URI URL URN WWW resource A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection. IANA IANA This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] If a host is located behind a NAT, then in certain situations it can be impossible for that host to communicate directly with other hosts (peers). In these situations, it is necessary for the host to use the services of an intermediate node that acts as a communication relay. This specification defines a protocol, called TURN (Traversal Using Relays around NAT), that allows the host to control the operation of the relay and to exchange packets with its peers using the relay. TURN differs from some other relay control protocols in that it allows a client to communicate with multiple peers using a single relay address. [STANDARDS-TRACK]