Network Working Group                                        D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                        30 December 2020
Expires: 3 July 2021

                      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 of this work is encouraged to happen on the MASQUE IETF
   mailing list or on the GitHub repository which
   contains the draft:

Discussion Venues

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

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 July 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
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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.  Datagram Encoding of Proxied UDP Packets  . . . . . . . . . .   4
   5.  Stream Chunks . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Stream Encoding of Proxied UDP Packets  . . . . . . . . . . .   6
   7.  Proxy Handling  . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  HTTP Intermediaries . . . . . . . . . . . . . . . . . . . . .   6
   9.  Performance Considerations  . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  HTTP Method  . . . . . . . . . . . . . . . . . . . . . .   8
     11.2.  URI Scheme Registration  . . . . . . . . . . . . . . . .   8
     11.3.  Stream Chunk Type Registration . . . . . . . . . . . . .   8
   12. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 or on the GitHub repository which
   contains the draft:

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.

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   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.

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 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 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

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   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

   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.  Note that the CONNECT-UDP stream is used to convey UDP
   packets, but they are not semantically part of the request or
   response themselves.

   Responses to the CONNECT-UDP method are not cacheable.

4.  Datagram Encoding of Proxied UDP Packets

   When the HTTP connection supports HTTP/3 datagrams [H3DGRAM], UDP
   packets can be encoded using QUIC DATAGRAM frames.  This support is
   ascertained by checking the received value of the H3_DATAGRAM
   SETTINGS Parameter.

   If the client has both sent and received the H3_DATAGRAM SETTINGS
   Parameter with value 1 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 proxy by using the "Datagram-Flow-Id" header; see
   [H3DGRAM].  A CONNECT-UDP request with an odd flow identifier is

   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 datagram encoding by echoing 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 request.  It then encodes the payload of UDP datagrams
   into the payload of HTTP/3 datagrams.  Is the CONNECT-UDP response
   does not carry the "Datagram-Flow-Id" header, then the datagram
   encoding is not available for this request.  A CONNECT-UDP response
   that carries the "Datagram-Flow-Id" header but with a different flow
   identifier than the one sent on the request is malformed.

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   When the proxy processes a new CONNECT-UDP request, it MUST ensure
   that the datagram flow identifier is not equal to flow identifiers
   from other requests: if it is, the proxy MUST reject the request with
   a 4xx (Client Error) status code.  Extensions MAY weaken or remove
   this requirement.

   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 without echoing the "Datagram-
   Flow-Id" header, or if the datagrams arrive before the CONNECT-UDP

   Note that a proxy can send the H3_DATAGRAM SETTINGS Parameter with a
   value of 1 while disabling datagrams on a particular request by not
   echoing the "Datagram-Flow-Id" header.  If the proxy does this, it
   MUST NOT treat receipt of datagrams as an error, because the client
   could have sent them optimistically before receiving the response.
   In this scenario, the proxy MUST discard those datagrams.

   Extensions to CONNECT-UDP MAY leverage parameters on the "Datagram-
   Flow-Id" header (parameters are defined in Section 3.1.2 of
   [STRUCT-HDR]).  Proxies MUST NOT echo parameters on the "Datagram-
   Flow-Id" header if it does not understand their semantics.

5.  Stream Chunks

   The bidirectional stream that the CONNECT-UDP request was sent on is
   a sequence of CONNECT-UDP Stream Chunks, which are defined as a
   sequence of type-length-value tuples using the following format
   (using the notation from the "Notational Conventions" section of

   CONNECT-UDP Stream {
     CONNECT-UDP Stream Chunk (..) ...,

                    Figure 1: CONNECT-UDP Stream Format

   CONNECT-UDP Stream Chunk {
     CONNECT-UDP Stream Chunk Type (i),
     CONNECT-UDP Stream Chunk Length (i),
     CONNECT-UDP Stream Chunk Value (..),

                 Figure 2: CONNECT-UDP Stream Chunk Format

   CONNECT-UDP Stream Chunk Type:  A variable-length integer indicating

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      the Type of the CONNECT-UDP Stream Chunk.  Endpoints that receive
      a chunk with an unknown CONNECT-UDP Stream Chunk Type MUST
      silently skip over that chunk.

   CONNECT-UDP Stream Chunk Length:  The length of the CONNECT-UDP
      Stream Chunk Value field following this field.  Note that this
      field can have a value of zero.

   CONNECT-UDP Stream Chunk Value:  The payload of this chunk.  Its
      semantics are determined by the value of the CONNECT-UDP Stream
      Chunk Type field.

6.  Stream Encoding of Proxied UDP Packets

   CONNECT-UDP Stream Chunks can be used to convey UDP payloads, by
   using a CONNECT-UDP Stream Chunk Type of UDP_PACKET (value 0x00).
   The payload of UDP packets is encoded in its unmodified entirety in
   the CONNECT-UDP Stream Chunk Value field.  This is necessary when the
   version of HTTP in use does not support QUIC DATAGRAM frames, but MAY
   also be used when datagrams are supported.  Note that empty UDP
   payloads are allowed.

7.  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.

8.  HTTP Intermediaries

   HTTP/3 DATAGRAM flow identifiers are specific to a given HTTP/3
   connection.  However, in some cases, an HTTP request may travel
   across multiple HTTP connections if there are HTTP intermediaries
   involved; see Section 2.3 of [RFC7230].

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   Intermediaries that support both CONNECT-UDP and HTTP/3 datagrams
   MUST negotiate flow identifiers separately on the client-facing and
   server-facing connections.  This is accomplished by having the
   intermediary parse the "Datagram-Flow-Id" header on all CONNECT-UDP
   requests it receives, and sending the same value in the "Datagram-
   Flow-Id" header on the response.  The intermediary then ascertains
   whether it can use datagrams on the server-facing connection.  If
   they are supported (as indicated by the H3_DATAGRAM SETTINGS
   parameter), the intermediary uses its own flow identifier allocation
   service to allocate a flow identifier for the server-facing
   connection, and waits for the server's reply to see if the server
   sent the "Datagram-Flow-Id" header on the response.  The intermediary
   then translates datagrams between the two connections by using the
   flow identifier specific to that connection.  An intermediary MAY
   also choose to use datagrams on only one of the two connections, and
   translate between datagrams and streams.

9.  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.

   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.

10.  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

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   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.

11.  IANA Considerations

11.1.  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 |

11.2.  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.

11.3.  Stream Chunk Type Registration

   This document will request IANA to create a "CONNECT-UDP Stream Chunk
   Type" registry.  This registry governs a 62-bit space, and follows
   the registration policy for QUIC registries as defined in [QUIC].  In
   addition to the fields required by the QUIC policy, registrations in
   this registry MUST include the following fields:

   Type:  A short mnemonic for the type.

   Description:  A brief description of the type semantics, which MAY be
      a summary if a specification reference is provided.

   The initial contents of this registry are:

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     | Value |    Type    |      Description      |   Reference   |
     | 0x00  | UDP_PACKET | Payload of UDP packet | This document |

   Each value of the format "37 * N + 23" for integer values of N (that
   is, 23, 60, 97, ...) are reserved; these values MUST NOT be assigned
   by IANA and MUST NOT appear in the listing of assigned values.

12.  Normative References

   [DGRAM]    Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", Work in Progress, Internet-
              Draft, draft-ietf-quic-datagram-01, 24 August 2020,

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

   [H3DGRAM]  Schinazi, D. and L. Pardue, "Using QUIC Datagrams with
              HTTP/3", Work in Progress, Internet-Draft, draft-schinazi-
              masque-h3-datagram-02, 14 December 2020,

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-33, 13 December 2020,

   [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,

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

   [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]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

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

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-header-structure-19, 3 June 2020,

   [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,


   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

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   Mountain View, California 94043,
   United States of America


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