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].¶

Discussion of this work is encouraged to happen on the MASQUE IETF mailing list masque@ietf.org or on the GitHub repository which contains the draft: https://github.com/DavidSchinazi/masque-drafts.¶

2. Supported HTTP Versions

The CONNECT-UDP method is defined for all versions of HTTP. When the HTTP version used runs over QUIC [QUIC], UDP payloads can be sent over QUIC DATAGRAM frames [DGRAM]. Otherwise they are sent on the stream where the CONNECT-UDP request was made. Note that when multiple proxies are involved in a CONNECT-UDP request, all the HTTP connections along the path need to be using HTTP/3 [H3] or later in order for UDP payloads to be sent over QUIC DATAGRAM frames. Additionally, 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, through one or more proxies, which can then be secured using QUIC or another protocol running over UDP.¶

A client sending a CONNECT-UDP request MUST send the authority form of request-target (Section 5.3 of [RFC7230]); i.e., the request-target consists of only the host name and port number of the tunnel destination, separated by a colon. For example,¶

     CONNECT-UDP server.example.com:443 HTTP/1.1
     Host: server.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" and ":path" pseudo-header fields MUST be omitted.¶
• The ":authority" pseudo-header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT-UDP 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 can establish a tunnel either by directly opening a UDP socket to the request-target or, if configured to use another proxy, by forwarding the CONNECT-UDP request to the next inbound proxy. Any 2xx (Successful) response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the blank line that concludes the successful response's header section; data received after that blank line is from the server identified by the request-target. Any response other than a successful response indicates that the tunnel has not yet been formed and that the connection remains governed by HTTP.¶

A tunnel is closed when a tunnel intermediary detects that either side has closed its connection: the intermediary MUST attempt to send any outstanding data that came from the closed side to the other side, close both connections, and then discard any remaining data left undelivered.¶

A server MUST NOT send any Transfer-Encoding or Content-Length header fields in a 2xx (Successful) response to CONNECT. 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

When the HTTP connection between client and proxy supports HTTP/3 datagrams [H3DGRAM], UDP packets can be encoded using QUIC DATAGRAM frames. This support is ascertained by checking receipt of the H3_DATAGRAM SETTINGS Parameter. Note that when there are multiple proxies involved, this support needs to be ascertained on all the HTTP connections that will carry proxied UDP packets.¶

If the client supports HTTP/3 datagrams and has received the H3_DATAGRAM SETTINGS Parameter on this connection, it SHOULD attempt to use HTTP/3 datagrams. This is accomplished by requesting a datagram flow identifier from the flow identifier allocation service [H3DGRAM]. That service generates an even flow identifier, and the client sends it to the server by using the "Datagram-Flow-Id" header (see Section 5).¶

If there are multiple proxies involved, proxies along the chain MUST check whether their upstream connection supports HTTP/3 datagrams. If it does not, that proxy MUST remove the "Datagram-Flow-Id" header before forwarding the CONNECT-UDP request.¶

The proxy that is creating the UDP socket to the destination responds to the CONNECT-UDP request with a 2xx (Successful) response, and MUST echo the "Datagram-Flow-Id" header. Once the client has received the "Datagram-Flow-Id" header on the successful response, it knows that it can use the HTTP/3 datagram encoding to send proxied UDP packets for this particular destination. It then encodes the payload of UDP datagrams into the payload of HTTP/3 datagrams.¶

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 they arrive before the CONNECT-UDP request.¶

If HTTP/3 datagrams are not supported, the stream is used to convey UDP payloads, by prefixing them with a 16-bit length.¶

5. Datagram-Flow-Id Header Definition

"Datagram-Flow-Id" is a Item Structured Header [STRUCT-HDR]. Its value MUST be an Integer. Its ABNF is:¶

  Datagram-Flow-Id = sh-integer

6. Server Handling

Unlike TCP, UDP is connection-less. The HTTP server 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.¶

Servers can use connected UDP sockets if their operating system supports them, as that allows the HTTP server to rely on the kernel to only send it UDP packets that match the correct 5-tuple. If the server 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 server.¶

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.¶

8. IANA Considerations

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

  +-------------------+----------+--------+---------------+
  | Header Field Name | Protocol | Status |   Reference   |
  +-------------------+----------+--------+---------------+
  | Datagram-Flow-Id  |   http   |  exp   | This document |
  +-------------------+----------+--------+---------------+

Acknowledgments

This proposal was inspired directly or indirectly by prior work from many people. The author would like to thank Eric Rescorla for suggesting to use an HTTP method to proxy UDP.¶

Author's Address