MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                            12 July 2021
Expires: 13 January 2022

                      The CONNECT-UDP HTTP Method


   This document describes the CONNECT-UDP HTTP method.  CONNECT-UDP is
   similar to the HTTP CONNECT method, but it uses UDP instead of TCP.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the MASQUE WG mailing list
   (, which is archived at

   Source for this draft and an issue tracker can be found at

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   This Internet-Draft will expire on 13 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   2
   2.  Supported HTTP Versions . . . . . . . . . . . . . . . . . . .   3
   3.  The CONNECT-UDP Method  . . . . . . . . . . . . . . . . . . .   3
   4.  Encoding of Proxied UDP Packets . . . . . . . . . . . . . . .   4
   5.  Proxy Handling  . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Performance Considerations  . . . . . . . . . . . . . . . . .   5
     6.1.  Tunneling of ECN Marks  . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  HTTP Method . . . . . . . . . . . . . . . . . . . . . . .   6
     8.2.  URI Scheme Registration . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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].

1.1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   In this document, we use the term "proxy" to refer to the HTTP server
   that opens the UDP socket and responds to the CONNECT-UDP request.
   If there are HTTP intermediaries (as defined in Section 2.3 of
   [RFC7230]) between the client and the proxy, those are referred to as
   "intermediaries" in this document.

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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:// HTTP/1.1

   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

   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

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   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

   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.

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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.

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6.1.  Tunneling of ECN Marks

   CONNECT-UDP does not create an IP-in-IP tunnel, so the guidance in
   [RFC6040] about transferring ECN marks between inner and outer IP
   headers does not apply.  There is no inner IP header in CONNECT-UDP

   Note that CONNECT-UDP clients do not have the ability in this
   specification to control the ECN codepoints on UDP packets the proxy
   sends to the server, nor can proxies communicate the markings of each
   UDP packet from server to proxy.

   A CONNECT-UDP proxy MUST ignore ECN bits in the IP header of UDP
   packets received from the server, and MUST set the ECN bits to Not-
   ECT on UDP packets it sends to the server.  These do not relate to
   the ECN markings of packets sent between client and proxy in any way.

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

   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

8.1.  HTTP Method

   This document will request IANA to register "CONNECT-UDP" in the HTTP
   Method Registry (IETF review) maintained at

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

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8.2.  URI Scheme Registration

   This document will request IANA to register the URI scheme "masque".

   The syntax definition below uses Augmented Backus-Naur Form (ABNF)
   [RFC5234].  The definitions of "host" and "port" are adopted from
   [RFC3986].  The syntax of a MASQUE URI is:

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

   The "host" and "port" component MUST NOT be empty, and the "port"
   component MUST NOT be 0.

9.  References

9.1.  Normative References

   [H2]       Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

              Schinazi, D. and L. Pardue, "Using Datagrams with HTTP",
              Work in Progress, Internet-Draft, draft-ietf-masque-h3-
              datagram-03, 12 July 2021,

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

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   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7231]  "*** BROKEN REFERENCE ***".

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [UDP]      Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

9.2.  Informative References

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <>.


   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

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043,
   United States of America


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