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rfc:rfc9298



Internet Engineering Task Force (IETF) D. Schinazi Request for Comments: 9298 Google LLC Category: Standards Track August 2022 ISSN: 2070-1721

                        Proxying UDP in HTTP

Abstract

 This document describes how to proxy UDP in HTTP, similar to how the
 HTTP CONNECT method allows proxying TCP in HTTP.  More specifically,
 this document defines a protocol that allows an HTTP client to create
 a tunnel for UDP communications through an HTTP server that acts as a
 proxy.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9298.

Copyright Notice

 Copyright (c) 2022 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 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 and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1.  Introduction
   1.1.  Conventions and Definitions
 2.  Client Configuration
 3.  Tunneling UDP over HTTP
   3.1.  UDP Proxy Handling
   3.2.  HTTP/1.1 Request
   3.3.  HTTP/1.1 Response
   3.4.  HTTP/2 and HTTP/3 Requests
   3.5.  HTTP/2 and HTTP/3 Responses
 4.  Context Identifiers
 5.  HTTP Datagram Payload Format
 6.  Performance Considerations
   6.1.  MTU Considerations
   6.2.  Tunneling of ECN Marks
 7.  Security Considerations
 8.  IANA Considerations
   8.1.  HTTP Upgrade Token
   8.2.  Well-Known URI
 9.  References
   9.1.  Normative References
   9.2.  Informative References
 Acknowledgments
 Author's Address

1. Introduction

 While HTTP provides the CONNECT method (see Section 9.3.6 of [HTTP])
 for creating a TCP [TCP] tunnel to a proxy, it lacked a method for
 doing so for UDP [UDP] traffic prior to this specification.
 This document describes a protocol for tunneling UDP to a server
 acting as a UDP-specific proxy over HTTP.  UDP 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.
 Unlike the HTTP CONNECT method, the UDP proxy itself is identified
 with an absolute URL containing the traffic's destination.  Clients
 generate those URLs using a URI Template [TEMPLATE], as described in
 Section 2.
 This protocol supports all existing versions of HTTP by using HTTP
 Datagrams [HTTP-DGRAM].  When using HTTP/2 [HTTP/2] or HTTP/3
 [HTTP/3], it uses HTTP Extended CONNECT as described in
 [EXT-CONNECT2] and [EXT-CONNECT3].  When using HTTP/1.x [HTTP/1.1],
 it uses HTTP Upgrade as defined in Section 7.8 of [HTTP].

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 "UDP proxy" to refer to the HTTP
 server that acts upon the client's UDP tunneling request to open a
 UDP socket to a target server and that generates the response to this
 request.  If there are HTTP intermediaries (as defined in Section 3.7
 of [HTTP]) between the client and the UDP 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. Client Configuration

 HTTP clients are configured to use a UDP proxy with a URI Template
 [TEMPLATE] that has the variables "target_host" and "target_port".
 Examples are shown below:

https://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 following requirements apply to the URI Template:
  • 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 or query components

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; see Section 2.1
    of [URI]).
  • 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.
 Clients SHOULD validate the requirements above; however, clients MAY
 use a general-purpose URI Template implementation that lacks this
 specific validation.  If a client detects that any of the
 requirements above are not met by a URI Template, the client MUST
 reject its configuration and abort the request without sending it to
 the UDP proxy.
 The original HTTP CONNECT method allowed for the conveyance of the
 target host and port, but not the scheme, proxy authority, path, or
 query.  Thus, clients with proxy configuration interfaces that only
 allow the user to configure the proxy host and the proxy port exist.
 Client implementations of this specification that are constrained by
 such limitations MAY attempt to access UDP proxying capabilities
 using 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 UDP proxy, respectively.  UDP
 proxy deployments SHOULD offer service at this location if they need
 to interoperate with such clients.

3. Tunneling UDP over HTTP

 To allow negotiation of a tunnel for UDP over HTTP, this document
 defines the "connect-udp" HTTP upgrade token.  The resulting UDP
 tunnels use the Capsule Protocol (see Section 3.2 of [HTTP-DGRAM])
 with HTTP Datagrams in the format defined in Section 5.
 To initiate a UDP tunnel associated with a single HTTP stream, a
 client issues a request containing the "connect-udp" upgrade token.
 The target of the tunnel is indicated by the client to the UDP proxy
 via the "target_host" and "target_port" variables of the URI
 Template; see Section 2.
 "target_host" supports using DNS names, IPv6 literals and IPv4
 literals.  Note that IPv6 scoped addressing zone identifiers are not
 supported.  Using the terms IPv6address, IPv4address, reg-name, and
 port from [URI], the "target_host" and "target_port" variables MUST
 adhere to the format in Figure 2, using notation from [ABNF].
 Additionally:
  • both the "target_host" and "target_port" variables MUST NOT be

empty.

  • if "target_host" contains an IPv6 literal, the colons (":") MUST

be percent-encoded. For example, if the target host is

    "2001:db8::42", it will be encoded in the URI as
    "2001%3Adb8%3A%3A42".
  • "target_port" MUST represent an integer between 1 and 65535

inclusive.

 target_host = IPv6address / IPv4address / reg-name
 target_port = port
                 Figure 2: URI Template Variable Format
 When sending its UDP proxying request, the client SHALL perform URI
 Template expansion to determine the path and query of its request.
 If the request is successful, the UDP proxy commits to converting
 received HTTP Datagrams into UDP packets, and vice versa, until the
 tunnel is closed.
 By virtue of the definition of the Capsule Protocol (see Section 3.2
 of [HTTP-DGRAM]), UDP proxying requests do not carry any message
 content.  Similarly, successful UDP proxying responses also do not
 carry any message content.

3.1. UDP Proxy Handling

 Upon receiving a UDP proxying request:
  • if the recipient is configured to use another HTTP proxy, it will

act as an intermediary by forwarding the request to another HTTP

    server.  Note that such intermediaries may need to re-encode the
    request if they forward it using a version of HTTP that is
    different from the one used to receive it, as the request encoding
    differs by version (see below).
  • otherwise, the recipient will act as a UDP proxy. It extracts the

"target_host" and "target_port" variables from the URI it has

    reconstructed from the request headers, decodes their percent-
    encoding, and establishes a tunnel by directly opening a UDP
    socket to the requested target.
 Unlike TCP, UDP is connectionless.  The UDP 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 UDP proxy MUST perform DNS resolution before replying to the HTTP
 request.  If errors occur during this process, the UDP proxy MUST
 reject the request and SHOULD send details using an appropriate
 Proxy-Status header field [PROXY-STATUS].  For example, if DNS
 resolution returns an error, the proxy can use the dns_error Proxy
 Error Type from Section 2.3.2 of [PROXY-STATUS].
 UDP proxies can use connected UDP sockets if their operating system
 supports them, as that allows the UDP proxy to rely on the kernel to
 only send it UDP packets that match the correct 5-tuple.  If the UDP
 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 UDP proxy.
 The lifetime of the socket is tied to the request stream.  The UDP
 proxy MUST keep the socket open while the request stream is open.  If
 a UDP proxy is notified by its operating system that its socket is no
 longer usable, it MUST close the request stream.  For example, this
 can happen when an ICMP Destination Unreachable message is received;
 see Section 3.1 of [ICMP6].  UDP proxies MAY choose to close sockets
 due to a period of inactivity, but they MUST close the request stream
 when closing the socket.  UDP 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 Sections 3.3 and 3.5) indicates
 that the UDP 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; thus, the client MUST
 abort the request.
 UDP proxies MUST NOT introduce fragmentation at the IP layer when
 forwarding HTTP Datagrams onto a UDP socket; overly large datagrams
 are silently dropped.  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.
 Implementers of UDP proxies will benefit from reading the guidance in
 [UDP-USAGE].

3.2. HTTP/1.1 Request

 When using HTTP/1.1 [HTTP/1.1], a UDP proxying request will meet the
 following requirements:
  • the method SHALL be "GET".
  • the request SHALL include a single Host header field containing

the origin of the UDP proxy.

  • the request SHALL include a Connection header field with value

"Upgrade" (note that this requirement is case-insensitive as per

    Section 7.6.1 of [HTTP]).
  • the request SHALL include an Upgrade header field with value

"connect-udp".

 A UDP proxying request that does not conform to these restrictions is
 malformed.  The recipient of such a malformed request MUST respond
 with an error and SHOULD use the 400 (Bad Request) status code.
 For example, if the client is configured with URI Template
 "https://example.org/.well-known/masque/
 udp/{target_host}/{target_port}/" and wishes to open a UDP proxying
 tunnel to target 192.0.2.6:443, it could send the following request:
GET https://example.org/.well-known/masque/udp/192.0.2.6/443/ HTTP/1.1
Host: example.org
Connection: Upgrade
Upgrade: connect-udp
Capsule-Protocol: ?1
                  Figure 3: Example HTTP/1.1 Request
 In HTTP/1.1, this protocol uses the GET method to mimic the design of
 the WebSocket Protocol [WEBSOCKET].

3.3. HTTP/1.1 Response

 The UDP 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 response SHALL include a Connection header field with value

"Upgrade" (note that this requirement is case-insensitive as per

    Section 7.6.1 of [HTTP]).
  • the response SHALL include a single Upgrade header field with

value "connect-udp".

  • the response SHALL meet the requirements of HTTP responses that

start the Capsule Protocol; see Section 3.2 of [HTTP-DGRAM].

 If any of these requirements are not met, the client MUST treat this
 proxying attempt as failed and abort the connection.
 For example, the UDP proxy could respond with:
 HTTP/1.1 101 Switching Protocols
 Connection: Upgrade
 Upgrade: connect-udp
 Capsule-Protocol: ?1
                  Figure 4: Example HTTP/1.1 Response

3.4. HTTP/2 and HTTP/3 Requests

 When using HTTP/2 [HTTP/2] or HTTP/3 [HTTP/3], UDP proxying requests
 use HTTP Extended CONNECT.  This requires that servers send an HTTP
 Setting as specified in [EXT-CONNECT2] and [EXT-CONNECT3] and that
 requests use HTTP pseudo-header fields 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 UDP 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 [HTTP/2] and Section 4.1.2 of
 [HTTP/3]).
 For example, if the client is configured with URI Template
 "https://example.org/.well-known/masque/
 udp/{target_host}/{target_port}/" and wishes to open a UDP proxying
 tunnel to target 192.0.2.6:443, it could send the following request:
 HEADERS
 :method = CONNECT
 :protocol = connect-udp
 :scheme = https
 :path = /.well-known/masque/udp/192.0.2.6/443/
 :authority = example.org
 capsule-protocol = ?1
                    Figure 5: Example HTTP/2 Request

3.5. HTTP/2 and HTTP/3 Responses

 The UDP proxy SHALL indicate a successful response by replying with
 the following requirements:
  • the HTTP status code on the response SHALL be in the 2xx

(Successful) range.

  • the response SHALL meet the requirements of HTTP responses that

start the Capsule Protocol; see Section 3.2 of [HTTP-DGRAM].

 If any of these requirements are not met, the client MUST treat this
 proxying attempt as failed and abort the request.
 For example, the UDP proxy could respond with:
 HEADERS
 :status = 200
 capsule-protocol = ?1
                   Figure 6: Example HTTP/2 Response

4. Context Identifiers

 The mechanism for proxying UDP in HTTP defined in this document
 allows future extensions to exchange HTTP Datagrams that 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 field; 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 allocated 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.  The Context
 ID allocation restrictions to the use of even-numbered and odd-
 numbered Context IDs exist in order to avoid the need for
 synchronization between endpoints.  However, once a Context ID has
 been allocated, those restrictions do not apply to the use of the
 Context ID; it can be used by any client or UDP proxy, independent of
 which endpoint initially allocated it.
 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
 header fields or capsules to register Context IDs.  Depending on the
 method being used, it is possible for datagrams to be received with
 Context IDs that have not yet been registered.  For instance, this
 can be due to reordering of the packet containing the datagram and
 the packet containing the registration message during transmission.

5. HTTP Datagram Payload Format

 When HTTP Datagrams (see Section 2 of [HTTP-DGRAM]) are associated
 with UDP Proxying request streams, the HTTP Datagram Payload field
 has the format defined in Figure 7, using notation from Section 1.3
 of [QUIC].  Note that when HTTP Datagrams are encoded using QUIC
 DATAGRAM frames [QUIC-DGRAM], the Context ID field defined below
 directly follows the Quarter Stream ID field, which is at the start
 of the QUIC DATAGRAM frame payload; see Section 2.1 of [HTTP-DGRAM].
 UDP Proxying HTTP Datagram Payload {
   Context ID (i),
   UDP Proxying Payload (..),
 }
              Figure 7: UDP Proxying HTTP Datagram Format
 Context ID:  A variable-length integer (see Section 16 of [QUIC])
    that contains the value of the Context ID.  If an HTTP/3 Datagram
    that 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.
 UDP Proxying Payload:  The payload of the datagram, whose semantics
    depend on the value of the previous field.  Note that this field
    can be empty.
 UDP packets are encoded using HTTP Datagrams with the Context ID
 field set to zero.  When the Context ID field is set to zero, the UDP
 Proxying Payload field contains the unmodified payload of a UDP
 packet (referred to as data octets in [UDP]).
 By virtue of the definition of the UDP header [UDP], it is not
 possible to encode UDP payloads longer than 65527 bytes.  Therefore,
 endpoints MUST NOT send HTTP Datagrams with a UDP Proxying Payload
 field longer than 65527 using Context ID zero.  An endpoint that
 receives an HTTP Datagram using Context ID zero whose UDP Proxying
 Payload field is longer than 65527 MUST abort the corresponding
 stream.  If a UDP proxy knows it can only send out UDP packets of a
 certain length due to its underlying link MTU, it has no choice but
 to discard incoming HTTP Datagrams using Context ID zero whose UDP
 Proxying Payload field is longer than that limit.  If the discarded
 HTTP Datagram was transported by a DATAGRAM capsule, the receiver
 SHOULD discard that capsule without buffering the capsule contents.
 If a UDP proxy receives an HTTP Datagram before it has received the
 corresponding request, it SHALL either drop that HTTP Datagram
 silently or buffer it temporarily (on the order of a round trip)
 while awaiting the corresponding request.
 Note that buffering datagrams (either because the request was not yet
 received or because the Context ID is not yet known) consumes
 resources.  Receivers that buffer datagrams SHOULD apply buffering
 limits in order to reduce the risk of resource exhaustion occurring.
 For example, receivers can limit the total number of buffered
 datagrams or the cumulative size of buffered datagrams on a per-
 stream, per-context, or per-connection basis.
 A client MAY optimistically start sending UDP packets in HTTP
 Datagrams before receiving the response to its UDP proxying request.
 However, implementers should note that such proxied packets may not
 be processed by the UDP proxy if it responds to the request with a
 failure or if the proxied packets are received by the UDP proxy
 before the request and the UDP proxy chooses to not buffer them.

6. Performance Considerations

 Bursty traffic can often lead to temporally correlated packet losses;
 in turn, this can lead to suboptimal responses from congestion
 controllers in protocols running over UDP.  To avoid this, UDP
 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.  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 UDP proxy with a connection containing a UDP Proxying
 request stream disables congestion control, it MUST NOT signal
 Explicit Congestion Notification (ECN) [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 QUIC ACK_ECN
 frames or the TCP ECE bit, 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 [QUIC-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
 (PMTU).  If a UDP 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 UDP 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 PMTU
 Discovery (DPLPMTUD) depend on [DPLPMTUD].  In this scenario, the UDP
 proxy SHOULD drop the UDP payload and send an ICMP Packet Too Big
 message to the target; see 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.
 In this specification, note that UDP proxying clients do not have the
 ability to control the ECN codepoints on UDP packets the UDP proxy
 sends to the target, nor can UDP proxies communicate the markings of
 each UDP packet from target to UDP proxy.
 A UDP proxy MUST ignore ECN bits in the IP header of UDP packets
 received from the target, and it 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 UDP 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 UDP proxy.  HTTP
 servers that support UDP proxying ought to restrict its use to
 authenticated users.
 There exist software and network deployments that perform access
 control checks based on the source IP address of incoming requests.
 For example, some software allows unauthenticated configuration
 changes if they originated from 127.0.0.1.  Such software could be
 running on the same host as the UDP proxy or in the same broadcast
 domain.  Proxied UDP traffic would then be received with a source IP
 address belonging to the UDP proxy.  If this source address is used
 for access control, UDP proxying clients could use the UDP proxy to
 escalate their access privileges beyond those they might otherwise
 have.  This could lead to unauthorized access by UDP proxying clients
 unless the UDP proxy disallows UDP proxying requests to vulnerable
 targets, such as the UDP proxy's own addresses and localhost, link-
 local, multicast, and broadcast addresses.  UDP proxies can use the
 destination_ip_prohibited Proxy Error Type from Section 2.3.5 of
 [PROXY-STATUS] when rejecting such requests.
 UDP proxies share many similarities with TCP CONNECT proxies when
 considering them as infrastructure for abuse to enable denial-of-
 service (DoS) attacks.  Both can obfuscate the attacker's source
 address from the attack target.  In the case of a stateless
 volumetric attack (e.g., a TCP SYN flood or a UDP flood), both types
 of proxies pass the traffic to the target host.  With stateful
 volumetric attacks (e.g., HTTP flooding) being sent over a TCP
 CONNECT proxy, the proxy will only send data if the target has
 indicated its willingness to accept data by responding with a TCP
 SYN-ACK.  Once the path to the target is flooded, the TCP CONNECT
 proxy will no longer receive replies from the target and will stop
 sending data.  Since UDP does not establish shared state between the
 UDP proxy and the target, the UDP proxy could continue sending data
 to the target in such a situation.  While a UDP proxy could
 potentially limit the number of UDP packets it is willing to forward
 until it has observed a response from the target, that provides
 limited protection against DoS attacks when attacks target open UDP
 ports where the protocol running over UDP would respond and that
 would be interpreted as willingness to accept UDP by the UDP proxy.
 Such a packet limit could also cause issues for valid traffic.
 The security considerations described in Section 4 of [HTTP-DGRAM]
 also apply here.  Since it is possible to tunnel IP packets over UDP,
 the guidance in [TUNNEL-SECURITY] can apply.

8. IANA Considerations

8.1. HTTP Upgrade Token

 IANA has registered "connect-udp" in the "HTTP Upgrade Tokens"
 registry maintained at <https://www.iana.org/assignments/http-
 upgrade-tokens>.
 Value:  connect-udp
 Description:  Proxying of UDP Payloads
 Expected Version Tokens:  None
 Reference:  RFC 9298

8.2. Well-Known URI

 IANA has registered "masque" in the "Well-Known URIs" registry
 maintained at <https://www.iana.org/assignments/well-known-uris>.
 URI Suffix:  masque
 Change Controller:  IETF
 Reference:  RFC 9298
 Status:  permanent
 Related Information:  Includes all resources identified with the path
    prefix "/.well-known/masque/udp/"

9. References

9.1. Normative References

 [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", RFC 2234, DOI 10.17487/RFC2234,
            November 1997, <https://www.rfc-editor.org/info/rfc2234>.
 [ECN]      Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP",
            RFC 3168, DOI 10.17487/RFC3168, September 2001,
            <https://www.rfc-editor.org/info/rfc3168>.
 [EXT-CONNECT2]
            McManus, P., "Bootstrapping WebSockets with HTTP/2",
            RFC 8441, DOI 10.17487/RFC8441, September 2018,
            <https://www.rfc-editor.org/info/rfc8441>.
 [EXT-CONNECT3]
            Hamilton, R., "Bootstrapping WebSockets with HTTP/3",
            RFC 9220, DOI 10.17487/RFC9220, June 2022,
            <https://www.rfc-editor.org/info/rfc9220>.
 [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP Semantics", STD 97, RFC 9110,
            DOI 10.17487/RFC9110, June 2022,
            <https://www.rfc-editor.org/info/rfc9110>.
 [HTTP-DGRAM]
            Schinazi, D. and L. Pardue, "HTTP Datagrams and the
            Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August
            2022, <https://www.rfc-editor.org/info/rfc9297>.
 [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
            June 2022, <https://www.rfc-editor.org/info/rfc9112>.
 [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
            DOI 10.17487/RFC9113, June 2022,
            <https://www.rfc-editor.org/info/rfc9113>.
 [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
            June 2022, <https://www.rfc-editor.org/info/rfc9114>.
 [PROXY-STATUS]
            Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
            Response Header Field", RFC 9209, DOI 10.17487/RFC9209,
            June 2022, <https://www.rfc-editor.org/info/rfc9209>.
 [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/info/rfc9000>.
 [QUIC-DGRAM]
            Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
            Datagram Extension to QUIC", RFC 9221,
            DOI 10.17487/RFC9221, March 2022,
            <https://www.rfc-editor.org/info/rfc9221>.
 [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/info/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/info/rfc8174>.
 [TCP]      Eddy, W., Ed., "Transmission Control Protocol (TCP)",
            STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
            <https://www.rfc-editor.org/info/rfc9293>.
 [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/info/rfc6570>.
 [UDP]      Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            DOI 10.17487/RFC0768, August 1980,
            <https://www.rfc-editor.org/info/rfc768>.
 [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <https://www.rfc-editor.org/info/rfc3986>.

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/info/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/info/rfc8899>.
 [ECN-TUNNEL]
            Briscoe, B., "Tunnelling of Explicit Congestion
            Notification", RFC 6040, DOI 10.17487/RFC6040, November
            2010, <https://www.rfc-editor.org/info/rfc6040>.
 [HELIUM]   Schwartz, B. M., "Hybrid Encapsulation Layer for IP and
            UDP Messages (HELIUM)", Work in Progress, Internet-Draft,
            draft-schwartz-httpbis-helium-00, 25 June 2018,
            <https://datatracker.ietf.org/doc/html/draft-schwartz-
            httpbis-helium-00>.
 [HiNT]     Pardue, L., "HTTP-initiated Network Tunnelling (HiNT)",
            Work in Progress, Internet-Draft, draft-pardue-httpbis-
            http-network-tunnelling-00, 2 July 2018,
            <https://datatracker.ietf.org/doc/html/draft-pardue-
            httpbis-http-network-tunnelling-00>.
 [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/info/rfc4443>.
 [MASQUE-ORIGINAL]
            Schinazi, D., "The MASQUE Protocol", Work in Progress,
            Internet-Draft, draft-schinazi-masque-00, 28 February
            2019, <https://datatracker.ietf.org/doc/html/draft-
            schinazi-masque-00>.
 [TUNNEL-SECURITY]
            Krishnan, S., Thaler, D., and J. Hoagland, "Security
            Concerns with IP Tunneling", RFC 6169,
            DOI 10.17487/RFC6169, April 2011,
            <https://www.rfc-editor.org/info/rfc6169>.
 [UDP-USAGE]
            Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
            Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
            March 2017, <https://www.rfc-editor.org/info/rfc8085>.
 [WEBSOCKET]
            Fette, I. and A. Melnikov, "The WebSocket Protocol",
            RFC 6455, DOI 10.17487/RFC6455, December 2011,
            <https://www.rfc-editor.org/info/rfc6455>.

Acknowledgments

 This document is a product of the MASQUE Working Group, and the
 author thanks all MASQUE enthusiasts for their contributions.  This
 proposal was inspired directly or indirectly by prior work from many
 people, in particular [HELIUM] by Ben Schwartz, [HiNT] by Lucas
 Pardue, and the original MASQUE Protocol [MASQUE-ORIGINAL] by the
 author of this document.
 The author would like to thank Eric Rescorla for suggesting the use
 of an HTTP method to proxy UDP.  The author is indebted to Mark
 Nottingham and Lucas Pardue for the many improvements they
 contributed to 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.

Author's Address

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