UDP Proxying Support for HTTP
draft-ietf-masque-connect-udp-08

MASQUE                                                       D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Standards Track                           21 March 2022
Expires: 22 September 2022

                     UDP Proxying Support for HTTP
                    draft-ietf-masque-connect-udp-08

Abstract

   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.

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
   (masque@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/masque/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-masque/draft-ietf-masque-connect-udp.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 22 September 2022.

Copyright Notice

   Copyright (c) 2022 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Configuration of Clients  . . . . . . . . . . . . . . . . . .   3
   3.  HTTP Exchanges  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Proxy Handling  . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  HTTP Request over HTTP/1.1  . . . . . . . . . . . . . . .   5
     3.3.  HTTP Response over HTTP/1.1 . . . . . . . . . . . . . . .   6
     3.4.  HTTP Request over HTTP/2 and HTTP/3 . . . . . . . . . . .   7
     3.5.  HTTP Response over HTTP/2 and HTTP/3  . . . . . . . . . .   7
     3.6.  Note About Draft Versions . . . . . . . . . . . . . . . .   8
   4.  Context Identifiers . . . . . . . . . . . . . . . . . . . . .   8
   5.  HTTP Datagram Payload Format  . . . . . . . . . . . . . . . .   9
   6.  Performance Considerations  . . . . . . . . . . . . . . . . .  10
     6.1.  MTU Considerations  . . . . . . . . . . . . . . . . . . .  10
     6.2.  Tunneling of ECN Marks  . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  HTTP Upgrade Token  . . . . . . . . . . . . . . . . . . .  11
     8.2.  Well-Known URI  . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document describes how to proxy UDP over HTTP.  Similar to how
   the CONNECT method (see Section 9.3.6 of [HTTP]) 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 [HTTP].

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1.1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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
   [HTTP]) 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] with the variables "target_host" and
   "target_port".  Examples are shown below:

https://masque.example.org/.well-known/masque/udp/{target_host}/{target_port}/
https://proxy.example.org:4443/masque?h={target_host}&p={target_port}
https://proxy.example.org:4443/masque{?target_host,target_port}

                   Figure 1: URI Template Examples

   The URI template MUST be a level 3 template or lower.  The URI
   template MUST be in absolute form, and MUST include non-empty scheme,
   authority and path components.  The path component of the URI
   template MUST start with a slash "/".  All template variables MUST be
   within the path component of the URI.  The URI template MUST contain
   the two variables "target_host" and "target_port" and MAY contain
   other variables.  The URI template MUST NOT contain any non-ASCII
   unicode characters and MUST only contain ASCII characters in the
   range 0x21-0x7E inclusive (note that percent-encoding is allowed).
   The URI template MUST NOT use Reserved Expansion ("+" operator),
   Fragment Expansion ("#" operator), Label Expansion with Dot-Prefix,
   Path Segment Expansion with Slash-Prefix, nor Path-Style Parameter
   Expansion with Semicolon-Prefix.  If any of the requirements above
   are not met by a URI template, the client MUST reject its
   configuration and fail the request without sending it to the proxy.

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   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/.well-known/masque/
   udp/{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 proxy establish a
   tunnel over a single HTTP stream to the destination target 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
   malformed.

   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.

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   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
   proxy.

   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 (for example, this can happen when an ICMP "Destination
   Unreachable" message is received, see Section 3.1 of [ICMP6]), 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
   when 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.

   Proxies MUST NOT introduce fragmentation at the IP layer when
   forwarding HTTP Datagrams onto a UDP socket.  In IPv4, the Don't
   Fragment (DF) bit MUST be set if possible, to prevent fragmentation
   on the path.  Future extensions MAY remove these requirements.

3.2.  HTTP Request over HTTP/1.1

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

   *  the method SHALL be "GET".

   *  the request-target SHALL use absolute-form (see Section 3.2.2 of
      [H1]).

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

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

   *  the request SHALL include a single "Upgrade" header with value
      "connect-udp".

   For example, if the client is configured with URI template
   "https://proxy.example.org/.well-known/masque/
   udp/{target_host}/{target_port}/" and wishes to open a UDP proxying
   tunnel to target 192.0.2.42:443, it could send the following request:

GET https://proxy.example.org/.well-known/masque/udp/192.0.2.42/443/ HTTP/1.1
Host: proxy.example.org
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
      Protocols).

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

   *  the response SHALL include a single "Upgrade" header with value
      "connect-udp".

   *  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

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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
      completed.

   A UDP proxying request that does not conform to these restrictions is
   malformed (see Section 8.1.1 of [H2]).

   For example, if the client is configured with URI template
   "https://proxy.example.org/{target_host}/{target_port}/" and wishes
   to open a UDP proxying tunnel to target 192.0.2.42:443, it could send
   the following request:

   HEADERS
   :method = CONNECT
   :protocol = connect-udp
   :scheme = https
   :path = /.well-known/masque/udp/192.0.2.42/443/
   :authority = proxy.example.org

                 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:

   HEADERS
   :status = 200

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                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.  Its ABNF is:

   connect-udp-version = sf-list

4.  Context Identifiers

   This protocol allows future extensions to exchange HTTP Datagrams
   which carry different semantics from UDP payloads.  Some of these
   extensions can augment UDP payloads with additional data, while
   others can exchange data that is completely separate from UDP
   payloads.  In order to accomplish this, all HTTP Datagrams associated
   with UDP Proxying request streams start with a context ID, see
   Section 5.

   Context IDs are 62-bit integers (0 to 2^62-1).  Context IDs are
   encoded as variable-length integers, see Section 16 of [QUIC].  The
   context ID value of 0 is reserved for UDP payloads, while non-zero
   values are dynamically allocated: non-zero even-numbered context IDs
   are client-allocated, and odd-numbered context IDs are proxy-
   allocated.  The context ID namespace is tied to a given HTTP request:
   it is possible for a context ID with the same numeric value to be
   simultaneously assigned different semantics in distinct requests,
   potentially with different semantics.  Context IDs MUST NOT be re-
   allocated within a given HTTP namespace but MAY be allocated in any
   order.  Once allocated, any context ID can be used by both client and
   proxy - only allocation carries separate namespaces to avoid
   requiring synchronization.

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   Registration is the action by which an endpoint informs its peer of
   the semantics and format of a given context ID.  This document does
   not define how registration occurs.  Future extensions MAY use HTTP
   headers or capsules to register contexts.  Depending on the method
   being used, it is possible for datagrams to be received with Context
   IDs which have not yet been registered, for instance due to
   reordering of the datagram and the registration packets during
   transmission.

5.  HTTP Datagram Payload Format

   When associated with UDP proxying request streams, the HTTP Datagram
   Payload field of HTTP Datagrams (see [HTTP-DGRAM]) has the format
   defined in Figure 6.  Note that when HTTP Datagrams are encoded using
   QUIC DATAGRAM frames, the Context ID field defined below directly
   follows the Quarter Stream ID field which is at the start of the QUIC
   DATAGRAM frame payload:

   UDP Proxying HTTP Datagram Payload {
     Context ID (i),
     Payload (..),
   }

                Figure 6: UDP Proxying HTTP Datagram Format

   Context ID:  A variable-length integer that contains the value of the
      Context ID.  If an HTTP/3 datagram which carries an unknown
      Context ID is received, the receiver SHALL either drop that
      datagram silently or buffer it temporarily (on the order of a
      round trip) while awaiting the registration of the corresponding
      Context ID.

   Payload:  The payload of the datagram, whose semantics depend on
      value of the previous field.  Note that this field can be empty.

   UDP packets are encoded using HTTP Datagrams with the Context ID set
   to zero.  When the Context ID is set to zero, the Payload field
   contains the unmodified payload of a UDP packet (referred to as "data
   octets" in [UDP]).

   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.

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   Endpoints MUST NOT send HTTP Datagrams with payloads longer than
   65527 using Context ID zero.  An endpoint that receives a DATAGRAM
   capsule using Context ID zero whose payload is longer than 65527 MUST
   abort the stream.  If a proxy knows it can only send out UDP packets
   of a certain length due to its underlying link MTU, it SHOULD discard
   incoming DATAGRAM capsules using Context ID zero whose payload is
   longer than that limit without buffering the capsule contents.

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 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
   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, UDP proxying
   SHOULD be performed over HTTP/3 to allow leveraging the QUIC DATAGRAM
   frame.

6.1.  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 [DPLPMTUD].  In this scenario, the proxy SHOULD drop the UDP
   payload and send an ICMP "Packet Too Big" message to the target, see

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   Section 3.2 of [ICMP6].

6.2.  Tunneling of ECN Marks

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

   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 target, nor can proxies communicate the markings of each
   UDP packet from target to proxy.

   A UDP proxy MUST ignore ECN bits in the IP header of UDP packets
   received from the target, and MUST set the ECN bits to Not-ECT on UDP
   packets it sends to the target.  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 targets, 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
   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 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.

8.  IANA Considerations

8.1.  HTTP Upgrade Token

   This document will request IANA to register "connect-udp" in the HTTP
   Upgrade Token Registry maintained at
   <https://www.iana.org/assignments/http-upgrade-tokens>.

   Value:  connect-udp

   Description:  Proxying of UDP Payloads

   Expected Version Tokens:  None

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   Reference:  This document

8.2.  Well-Known URI

   This document will request IANA to register "masque/udp" in the Well-
   Known URIs Registry maintained at <https://www.iana.org/assignments/
   well-known-uris/well-known-uris.xhtml>.

   URI Suffix:  masque/udp

   Change Controller:  IETF

   Reference:  This document

   Status:  permanent (if this document is approved)

   Related Information:  Includes all resources identified with the path
      prefix "/.well-known/masque/udp/"

9.  References

9.1.  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-10, 4 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              datagram-10>.

   [EXT-CONNECT2]
              McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,
              <https://www.rfc-editor.org/rfc/rfc8441>.

   [EXT-CONNECT3]
              Hamilton, R., "Bootstrapping WebSockets with HTTP/3", Work
              in Progress, Internet-Draft, draft-ietf-httpbis-h3-
              websockets-04, 8 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              h3-websockets-04>.

   [H1]       Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              messaging-19>.

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   [H2]       Thomson, M. and C. Benfield, "HTTP/2", Work in Progress,
              Internet-Draft, draft-ietf-httpbis-http2bis-07, 24 January
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              httpbis-http2bis-07>.

   [H3]       Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [HTTP-DGRAM]
              Schinazi, D. and L. Pardue, "Using Datagrams with HTTP",
              Work in Progress, Internet-Draft, draft-ietf-masque-h3-
              datagram-07, 21 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-masque-
              h3-datagram-07>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/rfc/rfc793>.

   [TEMPLATE] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/rfc/rfc6570>.

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   [UDP]      Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/rfc/rfc768>.

9.2.  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, <https://www.rfc-editor.org/rfc/rfc4787>.

   [DPLPMTUD] 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, <https://www.rfc-editor.org/rfc/rfc8899>.

   [ECN-TUNNEL]
              Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <https://www.rfc-editor.org/rfc/rfc6040>.

   [ICMP6]    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,
              <https://www.rfc-editor.org/rfc/rfc4443>.

   [PROXY-STATUS]
              Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
              Response Header Field", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-proxy-status-08, 13 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              proxy-status-08>.

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.  The extensibility design
   in this document came out of the HTTP Datagrams Design Team, whose
   members were Alan Frindell, Alex Chernyakhovsky, Ben Schwartz, Eric
   Rescorla, Lucas Pardue, Marcus Ihlar, Martin Thomson, Mike Bishop,
   Tommy Pauly, Victor Vasiliev, and the author of this document.

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Author's Address

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043,
   United States of America
   Email: dschinazi.ietf@gmail.com

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