The CONNECT-UDP HTTP Method

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Table of Contents

1. Introduction

This document describes the CONNECT-UDP HTTP method. CONNECT-UDP is similar to the HTTP CONNECT method (see section 4.3.6 of [RFC7231]), but it uses UDP [UDP] instead of TCP [TCP].¶

2. Supported HTTP Versions

The CONNECT-UDP method is defined for all versions of HTTP. UDP payloads are sent using HTTP Datagrams [HTTP-DGRAM]. Note that, when the HTTP version in use does not support multiplexing streams (such as HTTP/1.1), then any reference to "stream" in this document is meant to represent the entire connection.¶

3. The CONNECT-UDP Method

The CONNECT-UDP method requests that the recipient establish a tunnel over a single HTTP stream to the destination origin server identified by the request-target and, if successful, thereafter restrict its behavior to blind forwarding of packets, in both directions, until the tunnel is closed. Tunnels are commonly used to create an end-to-end virtual connection, which can then be secured using QUIC [QUIC] or another protocol running over UDP.¶

The request-target of a CONNECT-UDP request is a URI [RFC3986] which uses the "masque" scheme and an immutable path of "/". For example:¶

     CONNECT-UDP masque://target.example.com:443/ HTTP/1.1
     Host: target.example.com:443

When using HTTP/2 [H2] or later, CONNECT-UDP requests use HTTP pseudo-headers with the following requirements:¶

• The ":method" pseudo-header field is set to "CONNECT-UDP".¶
• The ":scheme" pseudo-header field is set to "masque".¶
• The ":path" pseudo-header field is set to "/".¶
• The ":authority" pseudo-header field contains the host and port to connect to (similar to the authority-form of the request-target of CONNECT requests; see [RFC7230], Section 5.3).¶

A CONNECT-UDP request that does not conform to these restrictions is malformed (see [H2], Section 8.1.2.6).¶

The recipient proxy establishes a tunnel by directly opening a UDP socket to the request-target. Any 2xx (Successful) response indicates that the proxy has opened a socket to the request-target and is willing to proxy UDP payloads. Any response other than a successful response indicates that the tunnel has not yet been formed.¶

A proxy MUST NOT send any Transfer-Encoding or Content-Length header fields in a 2xx (Successful) response to CONNECT-UDP. A client MUST treat a response to CONNECT-UDP containing any Content-Length or Transfer-Encoding header fields as malformed.¶

A payload within a CONNECT-UDP request message has no defined semantics; a CONNECT-UDP request with a non-empty payload is malformed.¶

Responses to the CONNECT-UDP method are not cacheable.¶

4. Encoding of Proxied UDP Packets

UDP packets are encoded using HTTP Datagrams [HTTP-DGRAM]. The payload of a UDP packet (referred to as "data octets" in [UDP]) is sent unmodified in the "HTTP Datagram Payload" field of an HTTP Datagram. In order to use HTTP Datagrams, the CONNECT-UDP client will first decide whether or not to use HTTP Datagram Contexts and then register its context ID (or lack thereof) using the corresponding registration capsule, see [HTTP-DGRAM].¶

Since HTTP Datagrams require prior negotiation (for example, in HTTP/3 it is necessary to both send and receive the H3_DATAGRAM SETTINGS Parameter), clients MUST NOT send any HTTP Datagrams until they have established support on a given connection. If negotiation of HTTP Datagrams fails (for example if an HTTP/3 SETTINGS frame was received without the H3_DATAGRAM SETTINGS Parameter), the client MUST consider its CONNECT-UDP request as failed.¶

The proxy that is creating the UDP socket to the destination responds to the CONNECT-UDP request with a 2xx (Successful) response, and indicates it supports HTTP Datagrams by sending the corresponding registration capsule.¶

Clients MAY optimistically start sending proxied UDP packets before receiving the response to its CONNECT-UDP request, noting however that those may not be processed by the proxy if it responds to the CONNECT-UDP request with a failure, or if the datagrams arrive before the CONNECT-UDP request.¶

Extensions to CONNECT-UDP MAY leverage the "Context Extensions" field of registration capsules in order to negotiate different semantics or encoding for UDP payloads.¶

5. Proxy Handling

Unlike TCP, UDP is connection-less. The proxy that opens the UDP socket has no way of knowing whether the destination is reachable. Therefore it needs to respond to the CONNECT-UDP request without waiting for a TCP SYN-ACK.¶

Proxies can use connected UDP sockets if their operating system supports them, as that allows the proxy to rely on the kernel to only send it UDP packets that match the correct 5-tuple. If the proxy uses a non-connected socket, it MUST validate the IP source address and UDP source port on received packets to ensure they match the client's CONNECT-UDP request. Packets that do not match MUST be discarded by the proxy.¶

The lifetime of the socket is tied to the CONNECT-UDP stream. The proxy MUST keep the socket open while the CONNECT-UDP stream is open. Proxies MAY choose to close sockets due to a period of inactivity, but they MUST close the CONNECT-UDP stream before closing the socket.¶

6. Performance Considerations

Proxies SHOULD strive to avoid increasing burstiness of UDP traffic: they SHOULD NOT queue packets in order to increase batching.¶

When the protocol running over UDP that is being proxied uses congestion control (e.g., [QUIC]), the proxied traffic will incur at least two nested congestion controllers. This can reduce performance but the underlying HTTP connection MUST NOT disable congestion control unless it has an out-of-band way of knowing with absolute certainty that the inner traffic is congestion-controlled.¶

If a client or proxy with a connection containing a CONNECT-UDP stream disables congestion control, it MUST NOT signal ECN support on that connection. That is, it MUST mark all IP headers with the Not-ECT codepoint. It MAY continue to report ECN feedback via ACK_ECN frames, as the peer may not have disabled congestion control.¶

When the protocol running over UDP that is being proxied uses loss recovery (e.g., [QUIC]), and the underlying HTTP connection runs over TCP, the proxied traffic will incur at least two nested loss recovery mechanisms. This can reduce performance as both can sometimes independently retransmit the same data. To avoid this, HTTP/3 datagrams SHOULD be used.¶

7. Security Considerations

There are significant risks in allowing arbitrary clients to establish a tunnel to arbitrary servers, as that could allow bad actors to send traffic and have it attributed to the proxy. Proxies that support CONNECT-UDP SHOULD restrict its use to authenticated users.¶

Because the CONNECT method creates a TCP connection to the target, the target has to indicate its willingness to accept TCP connections by responding with a TCP SYN-ACK before the proxy can send it application data. UDP doesn't have this property, so a CONNECT-UDP proxy could send more data to an unwilling target than a CONNECT proxy. However, in practice denial of service attacks target open TCP ports so the TCP SYN-ACK does not offer much protection in real scenarios. Proxies MUST NOT introspect the contents of UDP payloads as that would lead to ossification of UDP-based protocols by proxies.¶

8. IANA Considerations

  +-------------+------+------------+---------------+
  | Method Name | Safe | Idempotent |   Reference   |
  +-------------+------+------------+---------------+
  | CONNECT-UDP |  no  |     no     | This document |
  +-------------+------+------------+---------------+

masque-URI = "masque:" "//" host ":" port "/"

Acknowledgments

This document is a product of the MASQUE Working Group, and the author thanks all MASQUE enthusiasts for their contibutions. This proposal was inspired directly or indirectly by prior work from many people. In particular, the author would like to thank Eric Rescorla for suggesting to use an HTTP method to proxy UDP. Thanks to Lucas Pardue for their inputs on this document.¶

Author's Address