The Tunnel-Protocol HTTP Request 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, which may then be secured using TLS (Transport Layer Security, ). The HTTP Tunnel-Protocol header field identifies the protocol that will be spoken within the tunnel, using the application layer next protocol identifier specified for TLS . When CONNECT is used to establish a TLS tunnel, the Tunnel-Protocol header field may be used to carry 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 Tunnel-Protocol header field carries an indication only. In TLS, the final choice of application protocol is made by the server. Proxies do not implement the tunneled protocol, though they might choose to make policy decisions based on the value of the header field.

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 `Tunnel-Protocol` Request Header field in an HTTP CONNECT request to indicate the application layer protocol will be used within the tunnel, or the set of protocols that might be used within the tunnel.

Valid values for the protocol field are taken from the registry established in .

The ABNF (Augmented Backus-Naur Form) syntax for the `Tunnel-Protocol` header field is given below. It is based on the Generic Grammar 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 "%", and 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 Tunnel-Protocol: h2, http%2F1.1

HTTP header fields are registered within the "Message Headers" registry maintained at . This document defines and registers the `Tunnel-Protocol` header field, according to as follows: Tunnel-Protocol http Standard IETF (iesg@ietf.org) - Internet Engineering Task Force

In case of using HTTP CONNECT to a TURN 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 `Tunnel-Protocol` request header field described in this document is an optional header. 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 `Tunnel-Protocol` header.