MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                          7                         25 October 2021
Expires: 10 28 April 2022

                     UDP Proxying Support for HTTP


   This document describes how to proxy UDP over HTTP.  Similar to how
   the CONNECT method allows proxying TCP over HTTP, this document
   defines a new mechanism to proxy UDP.  It is built using HTTP
   Extended CONNECT.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Configuration of Clients  . . . . . . . . . . . . . . . . . .   3
   3.  HTTP Exchanges  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Proxy Handling  . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  HTTP Request over HTTP/1.1  . . . . . . . . . . . . . . .   5
     3.3.  HTTP Response over HTTP/1.1 . . . . . . . . . . . . . . .   5
     3.4.  HTTP Request over HTTP/2 and HTTP/3 . . . . . . . . . . .   6
     3.5.  HTTP Response over HTTP/2 and HTTP/3  . . . . . . . . . .   7
     3.6.  Note About Draft Versions . . . . . . . . . . . . . . . .   7
   4.  Encoding of Proxied UDP Packets . . . . . . . . . . . . . . .   7
   5.  Performance Considerations  . . . . . . . . . . . . . . . . .   8
     5.1.  MTU Considerations  . . . . . . . . . . . . . . . . . . .   9
     5.2.  Tunneling of ECN Marks  . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  HTTP Upgrade Token  . . . . . . . . . . . . . . . . . . .  10
     7.2.  Datagram Format Type  . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   This document describes how to proxy UDP over HTTP.  Similar to how
   the CONNECT method (see Section 9.3.6 of [SEMANTICS]) allows proxying
   TCP [TCP] over HTTP, this document defines a new mechanism to proxy
   UDP [UDP].

   UDP Proxying supports all versions of HTTP and uses HTTP Datagrams
   [HTTP-DGRAM].  When using HTTP/2 or HTTP/3, UDP proxying uses HTTP
   Extended CONNECT as described in [EXT-CONNECT2] and [EXT-CONNECT3].
   When using HTTP/1.x, UDP proxying uses HTTP Upgrade as defined in
   Section 7.8 of [SEMANTICS].

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 UDP proxying request.
   If there are HTTP intermediaries (as defined in Section 3.7 of
   [SEMANTICS]) between the client and the proxy, those are referred to
   as "intermediaries" in this document.

   Note that, when the HTTP version in use does not support multiplexing
   streams (such as HTTP/1.1), any reference to "stream" in this
   document represents the entire connection.

2.  Configuration of Clients

   Clients are configured to use UDP Proxying over HTTP via an URI
   Template [TEMPLATE].  The URI template MUST contain exactly two
   variables: "target_host" and "target_port".  Examples are shown

                      Figure 1: URI Template Examples

   Since the original HTTP CONNECT method allowed conveying the target
   host and port but not the scheme, proxy authority, path, nor query,
   there exist proxy configuration interfaces that only allow the user
   to configure the proxy host and the proxy port.  Client
   implementations of this specification that are constrained by such
   limitations MUST use the default template which is defined as:
   "https://$PROXY_HOST:$PROXY_PORT/{target_host}/{target_port}/" where
   $PROXY_HOST and $PROXY_PORT are the configured host and port of the
   proxy respectively.  Proxy deployments SHOULD use the default
   template to facilitate interoperability with such clients.

3.  HTTP Exchanges

   This document defines the "connect-udp" HTTP Upgrade Token. "connect-
   udp" uses the Capsule Protocol as defined in [HTTP-DGRAM].

   A "connect-udp" request requests that the recipient establish a
   tunnel over a single HTTP stream to the destination target server
   identified by the "target_host" and "target_port" variables of the
   URI template (see Section 2).  If the request is successful, the
   proxy commits to converting received HTTP Datagrams into UDP packets
   and vice versa 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.

   When sending its UDP proxying request, the client SHALL perform URI
   template expansion to determine the path and query of its request.
   target_host supports using DNS names, IPv6 literals and IPv4
   literals.  Note that this URI template expansion requires using pct-
   encoding, so for example if the target_host is "2001:db8::42", it
   will be encoded in the URI as "2001%3Adb8%3A%3A42".

   A payload within a UDP proxying request message has no defined
   semantics; a UDP proxying request with a non-empty payload is

   Responses to UDP proxying requests are not cacheable.

3.1.  Proxy Handling

   Upon receiving a UDP proxying request, the recipient proxy extracts
   the "target_host" and "target_port" variables from the URI it has
   reconstructed from the request headers, and establishes a tunnel by
   directly opening a UDP socket to the requested target.

   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 request without waiting for a
   packet from the target.  However, if the target_host is a DNS name,
   the proxy MUST perform DNS resolution before replying to the HTTP
   request.  If DNS resolution fails, the proxy MUST fail the request
   and SHOULD send details using the Proxy-Status header [PROXY-STATUS].

   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 request.  Packets that do not match MUST be discarded by the

   The lifetime of the socket is tied to the request stream.  The proxy
   MUST keep the socket open while the request stream is open.  If a
   proxy is notified by its operating system that its socket is no
   longer usable, it MUST close the request stream.  Proxies MAY choose
   to close sockets due to a period of inactivity, but they MUST close
   the request stream before closing the socket.  Proxies that close
   sockets after a period of inactivity SHOULD NOT use a period lower
   than two minutes, see Section 4.3 of [BEHAVE].

   A successful response (as defined in Section 3.3 and Section 3.5)
   indicates that the proxy has opened a socket to the requested target
   and is willing to proxy UDP payloads.  Any response other than a
   successful response indicates that the request has failed, and the
   client MUST therefore abort the request.

3.2.  HTTP Request over HTTP/1.1

   When using HTTP/1.1, a UDP proxying request will meet the following

   *  the method SHALL be "CONNECT".

   *  the request-target SHALL use absolute-form (see Section 3.2.2 of

   *  the request SHALL include a single Host header containing the
      origin of the proxy.

   *  the request SHALL include a single "Connection" header with value

   *  the request SHALL include a single "Upgrade" header with value

   For example, if the client is configured with URI template
   "{target_host}/{target_port}/" and wishes
   to open a UDP proxying tunnel to target, it could send
   the following request:

   Connection: upgrade
   Upgrade: connect-udp

                Figure 2: Example HTTP Request over HTTP/1.1

3.3.  HTTP Response over HTTP/1.1

   The proxy SHALL indicate a successful response by replying with the
   following requirements:

   *  the HTTP status code on the response SHALL be 101 (Switching

   *  the reponse SHALL include a single "Connection" header with value

   *  the response SHALL include a single "Upgrade" header with value

   *  the response SHALL NOT include any Transfer-Encoding or Content-
      Length header fields.

   If any of these requirements are not met, the client MUST treat this
   proxying attempt as failed and abort the connection.

   For example, the proxy could respond with:

   HTTP/1.1 101 Switching Protocols
   Connection: upgrade
   Upgrade: connect-udp

               Figure 3: Example HTTP Response over HTTP/1.1

3.4.  HTTP Request over HTTP/2 and HTTP/3

   When using HTTP/2 [H2] or HTTP/3 [H3], UDP proxying requests use HTTP
   pseudo-headers with the following requirements:

   *  The ":method" pseudo-header field SHALL be "CONNECT".

   *  The ":protocol" pseudo-header field SHALL be "connect-udp".

   *  The ":authority" pseudo-header field SHALL contain the authority
      of the proxy.

   *  The ":path" and ":scheme" pseudo-header fields SHALL NOT be empty.
      Their values SHALL contain the scheme and path from the URI
      template after the URI template expansion process has been

   A UDP proxying request that does not conform to these restrictions is
   malformed (see Section of [H2]).

   For example, if the client is configured with URI template
   "{target_host}/{target_port}/" and wishes
   to open a UDP proxying tunnel to target, it could send
   the following request:

   :method = CONNECT
   :protocol = connect-udp
   :scheme = https
   :path = /
   :authority =

                 Figure 4: Example HTTP Request over HTTP/2

3.5.  HTTP Response over HTTP/2 and HTTP/3

   The proxy SHALL indicate a successful response by replying with any
   2xx (Successful) HTTP status code, without any Transfer-Encoding or
   Content-Length header fields.

   If any of these requirements are not met, the client MUST treat this
   proxying attempt as failed and abort the request.

   For example, the proxy could respond with:

   :status = 200

                Figure 5: Example HTTP Response over HTTP/2

3.6.  Note About Draft Versions

   [[RFC editor: please remove this section before publication.]]

   In order to allow implementations to support multiple draft versions
   of this specification during its development, we introduce the
   "connect-udp-version" header.  When sent by the client, it contains a
   list of draft numbers supported by the client (e.g., "connect-udp-
   version: 0, 2").  When sent by the proxy, it contains a single draft
   number selected by the proxy from the list provided by the client
   (e.g., "connect-udp-version: 2").  Sending this header is RECOMMENDED
   but not required.

4.  Encoding of Proxied UDP Packets

   UDP packets are encoded using HTTP Datagrams [HTTP-DGRAM] with the
   UDP_PAYLOAD HTTP Datagram Format Type (see value in Section 7.2).
   When using the UDP_PAYLOAD HTTP Datagram Format Type, 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 client will first decide whether
   or not it will attempt to use HTTP Datagram Contexts and then
   register its context ID (or lack thereof) using the corresponding
   registration capsule, see [HTTP-DGRAM].

   REGISTER_DATAGRAM_NO_CONTEXT registration capsule using the "Datagram Format Type"
   set to UDP_PAYLOAD, the "Datagram Format Additional Data" field SHALL
   be empty.  Servers MUST NOT register contexts using the UDP_PAYLOAD
   HTTP Datagram Format Type.  Clients MUST NOT register more than one
   context using the UDP_PAYLOAD HTTP Datagram Format Type.  Endpoints
   MUST NOT close contexts using the UDP_PAYLOAD HTTP Datagram Format
   Type.  If an endpoint detects a violation of any of these
   requirements, it MUST abort the stream.

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

   Extensions to this mechanism MAY define new HTTP Datagram Format
   Types in order to use different semantics or encodings for UDP

5.  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 UDP proxying
   request 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

   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.

5.1.  MTU Considerations

   When using HTTP/3 with the QUIC Datagram extension [DGRAM], UDP
   payloads are transmitted in QUIC DATAGRAM frames.  Since those cannot
   be fragmented, they can only carry payloads up to a given length
   determined by the QUIC connection configuration and the path MTU.  If
   a proxy is using QUIC DATAGRAM frames and it receives a UDP payload
   from the target that will not fit inside a QUIC DATAGRAM frame, the
   proxy SHOULD NOT send the UDP payload in a DATAGRAM capsule, as that
   defeats the end-to-end unreliability characteristic that methods such
   as Datagram Packetization Layer Path MTU Discovery (DPLPMTUD) depend
   on [RFC8899].  In this scenario, the proxy SHOULD drop the UDP
   payload and send an ICMP "Packet Too Big" message to the target

5.2.  Tunneling of ECN Marks

   UDP proxying 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 UDP proxying

   Note that UDP proxying 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 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.

6.  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 UDP proxying 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 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.

7.  IANA Considerations

7.1.  HTTP Upgrade Token

   This document will request IANA to register "connect-udp" in the HTTP
   Upgrade Token Registry maintained at

   Value:  connect-udp

   Description:  Proxying of UDP Payloads.

   Expected Version Tokens:  None.

   Reference:  This document.

7.2.  Datagram Format Type

   This document will request IANA to register UDP_PAYLOAD in the "HTTP
   Datagram Format Types" registry established by [HTTP-DGRAM].

                | Type        | Value    | Specification |
                | UDP_PAYLOAD | 0xff6f00 | This Document |

                 Table 1: Registered Datagram Format Type

8.  References

8.1.  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-06, 5 October 2021,

              McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,

              Hamilton, R., "Bootstrapping WebSockets with HTTP/3", Work
              in Progress, Internet-Draft, draft-ietf-httpbis-h3-
              websockets-00, 9 September 2021,

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

   [H3]       Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,

              Schinazi, D. and L. Pardue, "Using Datagrams with HTTP",
              Work in Progress, Internet-Draft, draft-ietf-masque-h3-
              datagram-04, 6
              datagram-05, 25 October 2021,

              Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-19, 12 September 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,

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

              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,

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

   [TEMPLATE] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,

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

8.2.  Informative References

   [BEHAVE]   Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <>.

              Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
              Response Header Field", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-proxy-status-06, 16 August
              draft-ietf-httpbis-proxy-status-08, 13 October 2021,

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,

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

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <>.


   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