Proxying IP in HTTP
draft-ietf-masque-connect-ip-05

MASQUE                                                     T. Pauly, Ed.
Internet-Draft                                                Apple Inc.
Updates: 9298 (if approved)                                  D. Schinazi
Intended status: Standards Track                       A. Chernyakhovsky
Expires: 24 July 2023                                         Google LLC
                                                           M. Kuehlewind
                                                           M. Westerlund
                                                                Ericsson
                                                         20 January 2023

                          Proxying IP in HTTP
                    draft-ietf-masque-connect-ip-05

Abstract

   This document describes how to proxy IP packets in HTTP.  This
   protocol is similar to UDP proxying in HTTP, but allows transmitting
   arbitrary IP packets.  More specifically, this document defines a
   protocol that allows an HTTP client to create an IP tunnel through an
   HTTP server that acts as a proxy.  This document updates RFC 9298.

About This Document

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

   The latest revision of this draft can be found at https://ietf-wg-
   masque.github.io/draft-ietf-masque-connect-ip/draft-ietf-masque-
   connect-ip.html.  Status information for this document may be found
   at https://datatracker.ietf.org/doc/draft-ietf-masque-connect-ip/.

   Discussion of this document takes place on the MASQUE Working Group
   mailing list (mailto:masque@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/masque/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/masque/.

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

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/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Configuration of Clients  . . . . . . . . . . . . . . . . . .   4
   4.  Tunnelling IP over HTTP . . . . . . . . . . . . . . . . . . .   5
     4.1.  HTTP/1.1 Request  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  HTTP/1.1 Response . . . . . . . . . . . . . . . . . . . .   6
     4.3.  HTTP/2 and HTTP/3 Requests  . . . . . . . . . . . . . . .   7
     4.4.  HTTP/2 and HTTP/3 Responses . . . . . . . . . . . . . . .   7
     4.5.  Limiting Request Scope  . . . . . . . . . . . . . . . . .   7
     4.6.  Capsules  . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.6.1.  ADDRESS_ASSIGN Capsule  . . . . . . . . . . . . . . .   9
       4.6.2.  ADDRESS_REQUEST Capsule . . . . . . . . . . . . . . .  10
       4.6.3.  ROUTE_ADVERTISEMENT Capsule . . . . . . . . . . . . .  12
   5.  Context Identifiers . . . . . . . . . . . . . . . . . . . . .  13
   6.  HTTP Datagram Payload Format  . . . . . . . . . . . . . . . .  14
   7.  Error Signalling  . . . . . . . . . . . . . . . . . . . . . .  16
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Remote Access VPN . . . . . . . . . . . . . . . . . . . .  17
     8.2.  IP Flow Forwarding  . . . . . . . . . . . . . . . . . . .  19
     8.3.  Proxied Connection Racing . . . . . . . . . . . . . . . .  22
   9.  Extensibility Considerations  . . . . . . . . . . . . . . . .  23
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  HTTP Upgrade Token . . . . . . . . . . . . . . . . . . .  24
     11.2.  Creation of the MASQUE URI Suffixes Registry . . . . . .  24

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     11.3.  Updates to masque Well-Known URI . . . . . . . . . . . .  25
     11.4.  Capsule Type Registrations . . . . . . . . . . . . . . .  25
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     12.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   HTTP provides the CONNECT method (see Section 9.3.6 of [HTTP]) for
   creating a TCP [TCP] tunnel to a destination and a similar mechanism
   for UDP [CONNECT-UDP].  However, these mechanisms cannot tunnel other
   protocols nor convey fields of the IP header.

   This document describes a protocol for tunnelling IP to an HTTP
   server acting as an IP-specific proxy over HTTP.  This can be used
   for various use cases such as point-to-network VPN, secure point-to-
   point communication, or general-purpose packet tunnelling.

   IP proxying operates similarly to UDP proxying [CONNECT-UDP], whereby
   the proxy itself is identified with an absolute URL, optionally
   containing the traffic's destination.  Clients generate these URLs
   using a URI Template [TEMPLATE], as described in Section 3.

   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].

2.  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 "IP proxy" to refer to the HTTP
   server that responds to the IP 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.

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3.  Configuration of Clients

   Clients are configured to use IP proxying over HTTP via an URI
   Template [TEMPLATE].  The URI template MAY contain two variables:
   "target" and "ipproto" (Section 4.5).  The optionality of the
   variables needs to be considered when defining the template so that
   either the variable is self-identifying or it is possible to exclude
   it in the syntax.

   Examples are shown below:

   https://example.org/.well-known/masque/ip/{target}/{ipproto}/
   https://proxy.example.org:4443/masque/ip?t={target}&i={ipproto}
   https://proxy.example.org:4443/masque/ip{?target,ipproto}
   https://masque.example.org/?user=bob

                      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 MAY contain the two variables "target" and
      "ipproto" and MAY contain other variables.  If the "target" or
      "ipproto" variables are included, their values MUST NOT be empty.
      Clients can instead use "*" to indicate wildcard or no-preference
      values; see Section 4.5.

   *  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.

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   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 IP proxy.

   As with UDP proxying, some client configurations for IP proxies will
   only allow the user to configure the proxy host and proxy port.
   Clients with such limitations MAY attempt to access IP proxying
   capabilities using the default template, which is defined as:
   "https://$PROXY_HOST:$PROXY_PORT/.well-known/masque/
   ip/{target}/{ipproto}/", where $PROXY_HOST and $PROXY_PORT are the
   configured host and port of the IP proxy, respectively.  IP proxy
   deployments SHOULD offer service at this location if they need to
   interoperate with such clients.

4.  Tunnelling IP over HTTP

   To allow negotiation of a tunnel for IP over HTTP, this document
   defines the "connect-ip" HTTP Upgrade Token.  The resulting IP
   tunnels use the Capsule Protocol (see Section 3.2 of [HTTP-DGRAM])
   with HTTP Datagrams in the format defined in Section 6.

   To initiate an IP tunnel associated with a single HTTP stream, a
   client issues a request containing the "connect-ip" upgrade token.
   The target of the tunnel is indicated by the client to the IP proxy
   via the "target_host" and "target_port" variables of the URI
   Template; see Section 3.

   When sending its IP proxying request, the client SHALL perform URI
   template expansion to determine the path and query of its request,
   see Section 3.

   A successful response indicates that the IP proxy is willing to open
   an IP forwarding tunnel between it and the client.  Any response
   other than a successful response indicates that the tunnel has not
   been formed.

   The lifetime of the IP forwarding tunnel is tied to the IP proxying
   request stream.  Closing that stream (in HTTP/3 via the FIN bit on a
   QUIC STREAM frame, or a QUIC RESET_STREAM frame) closes the
   associated IP tunnel.

   Along with a successful response, the IP proxy can send capsules to
   assign addresses and advertise routes to the client (Section 4.6).
   The client can also assign addresses and advertise routes to the IP
   proxy for network-to-network routing.

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   By virtue of the definition of the Capsule Protocol (see Section 3.2
   of [HTTP-DGRAM]), IP proxying requests do not carry any message
   content.  Similarly, successful IP proxying responses also do not
   carry any message content.

4.1.  HTTP/1.1 Request

   When using HTTP/1.1 [HTTP/1.1], an IP 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 IP 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-ip".

   An IP 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.

4.2.  HTTP/1.1 Response

   The IP 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-ip".

   *  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.

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4.3.  HTTP/2 and HTTP/3 Requests

   When using HTTP/2 [HTTP/2] or HTTP/3 [HTTP/3], IP 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-ip".

   *  The :authority pseudo-header field SHALL contain the authority of
      the IP 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; see Section 3.  Variables in the URI template can
      determine the scope of the request, such as requesting full-tunnel
      IP packet forwarding, or a specific proxied flow; see Section 4.5.

   An IP 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]).

4.4.  HTTP/2 and HTTP/3 Responses

   The IP 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.

4.5.  Limiting Request Scope

   Unlike UDP proxying requests, which require specifying a target host,
   IP proxying requests can allow endpoints to send arbitrary IP packets
   to any host.  The client can choose to restrict a given request to a
   specific IP prefix or IP protocol by adding parameters to its
   request.  When the IP proxy knows that a request is scoped to a
   target prefix or protocol, it can leverage this information to

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   optimize its resource allocation; for example, the IP proxy can
   assign the same public IP address to two IP proxying requests that
   are scoped to different prefixes and/or different protocols.

   The scope of the request is indicated by the client to the IP proxy
   via the "target" and "ipproto" variables of the URI Template; see
   Section 3.  Both the "target" and "ipproto" variables are optional;
   if they are not included, they are considered to carry the wildcard
   value "*".

   target:  The variable "target" contains a hostname or IP prefix of a
      specific host to which the client wants to proxy packets.  If the
      "target" variable is not specified or its value is "*", the client
      is requesting to communicate with any allowable host. "target"
      supports using DNS names, IPv6 prefixes and IPv4 prefixes.  Note
      that IPv6 scoped addressing zone identifiers are not supported.
      If the target is an IP prefix (IP address optionally followed by a
      percent-encoded slash followed by the prefix length in bits), the
      request will only support a single IP version.  If the target is a
      hostname, the IP proxy is expected to perform DNS resolution to
      determine which route(s) to advertise to the client.  The IP proxy
      SHOULD send a ROUTE_ADVERTISEMENT capsule that includes routes for
      all addresses that were resolved for the requested hostname, that
      are accessible to the IP proxy, and belong to an address family
      for which the IP proxy also sends an Assigned Address.
   ipproto:  The variable "ipproto" contains an IP protocol number, as
      defined in the "Assigned Internet Protocol Numbers" IANA registry
      maintained at <https://www.iana.org/assignments/protocol-numbers/
      protocol-numbers.xhtml>.  If present, it specifies that a client
      only wants to proxy a specific IP protocol for this request.  If
      the value is "*", or the variable is not included, the client is
      requesting to use any IP protocol.

   Using the terms IPv6address, IPv4address, and reg-name from [URI],
   the "target" and "ipproto" variables MUST adhere to the format in
   Figure 2, using notation from [ABNF].  Additionally:

   *  if "target" contains an IPv6 literal or prefix, 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".

   *  If present, the IP prefix length in "target" SHALL be preceded by
      a percent-encoded slash ("/"): "%2F".  The IP prefix length MUST
      represent an integer between 0 and the length of the IP address in
      bits, inclusive.

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   *  "ipproto" MUST represent an integer between 0 and 255 inclusive,
      or the wildcard value "*".

   target = IPv6prefix / IPv4prefix / reg-name / "*"
   IPv6prefix = IPv6address ["%2F" 1*3DIGIT]
   IPv4prefix = IPv4address ["%2F" 1*2DIGIT]
   ipproto = 1*3DIGIT / "*"

                   Figure 2: URI Template Variable Format

4.6.  Capsules

   This document defines multiple new capsule types that allow endpoints
   to exchange IP configuration information.  Both endpoints MAY send
   any number of these new capsules.

4.6.1.  ADDRESS_ASSIGN Capsule

   The ADDRESS_ASSIGN capsule (see Section 11.4 for the value of the
   capsule type) allows an endpoint to inform its peer of the list of IP
   addresses or prefixes it has assigned to it.  Every capsule contains
   the full list of IP prefixes currently assigned to the receiver.  Any
   of these addresses can be used as the source address on IP packets
   originated by the receiver of this capsule.

   ADDRESS_ASSIGN Capsule {
     Type (i) = ADDRESS_ASSIGN,
     Length (i),
     Assigned Address (..) ...,
   }

                  Figure 3: ADDRESS_ASSIGN Capsule Format

   The ADDRESS_ASSIGN capsule contains a sequence of zero or more
   Assigned Addresses.

   Assigned Address {
     Request ID (i),
     IP Version (8),
     IP Address (32..128),
     IP Prefix Length (8),
   }

                     Figure 4: Assigned Address Format

   Request ID:  Request identifier, encoded as a variable-length

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      integer.  If this address assignment is in response to an Address
      Request (see Section 4.6.2), then this field SHALL contain the
      value of the corresponding field in the request.  Otherwise, this
      field SHALL be zero.
   IP Version:  IP Version of this address assignment, encoded as an
      unsigned 8-bit integer.  MUST be either 4 or 6.
   IP Address:  Assigned IP address.  If the IP Version field has value
      4, the IP Address field SHALL have a length of 32 bits.  If the IP
      Version field has value 6, the IP Address field SHALL have a
      length of 128 bits.
   IP Prefix Length:  The number of bits in the IP Address that are used
      to define the prefix that is being assigned, encoded as an
      unsigned 8-bit integer.  This MUST be less than or equal to the
      length of the IP Address field, in bits.  If the prefix length is
      equal to the length of the IP Address, the receiver of this
      capsule is only allowed to send packets from a single source
      address.  If the prefix length is less than the length of the IP
      address, the receiver of this capsule is allowed to send packets
      from any source address that falls within the prefix.

   If an ADDRESS_ASSIGN capsule does not contain an address that was
   previously transmitted in another ADDRESS_ASSIGN capsule, that
   indicates that the address has been removed.  An ADDRESS_ASSIGN
   capsule can also be empty, indicating that all addresses have been
   removed.

   In some deployments of IP proxying in HTTP, an endpoint needs to be
   assigned an address by its peer before it knows what source address
   to set on its own packets.  For example, in the Remote Access case
   (Section 8.1) the client cannot send IP packets until it knows what
   address to use.  In these deployments, the endpoint that is expecting
   an address assignment MUST send an ADDRESS_REQUEST capsule.  This
   isn't required if the endpoint does not need any address assignment,
   for example when it is configured out-of-band with static addresses.

   While ADDRESS_ASSIGN capsules are commonly sent in response to
   ADDRESS_REQUEST capsules, endpoints MAY send ADDRESS_ASSIGN capsules
   unprompted.

4.6.2.  ADDRESS_REQUEST Capsule

   The ADDRESS_REQUEST capsule (see Section 11.4 for the value of the
   capsule type) allows an endpoint to request assignment of IP
   addresses from its peer.  The capsule allows the endpoint to
   optionally indicate a preference for which address it would get
   assigned.

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   ADDRESS_REQUEST Capsule {
     Type (i) = ADDRESS_REQUEST,
     Length (i),
     Requested Address (..) ...,
   }

                  Figure 5: ADDRESS_REQUEST Capsule Format

   The ADDRESS_REQUEST capsule contains a sequence of one or more
   Requested Addresses.

   Requested Address {
     Request ID (i),
     IP Version (8),
     IP Address (32..128),
     IP Prefix Length (8),
   }

                     Figure 6: Requested Address Format

   Request ID:  Request identifier, encoded as a variable-length
      integer.  This is the identifier of this specific address request.
      Each request from a given endpoint carries a different identifier.
      Request IDs MUST NOT be reused by an endpoint, and MUST NOT be
      zero.
   IP Version:  IP Version of this address request, encoded as an
      unsigned 8-bit integer.  MUST be either 4 or 6.
   IP Address:  Requested IP address.  If the IP Version field has value
      4, the IP Address field SHALL have a length of 32 bits.  If the IP
      Version field has value 6, the IP Address field SHALL have a
      length of 128 bits.
   IP Prefix Length:  Length of the IP Prefix requested, in bits,
      encoded as an unsigned 8-bit integer.  MUST be lesser or equal to
      the length of the IP Address field, in bits.

   If the IP Address is all-zero (0.0.0.0 or ::), this indicates that
   the sender is requesting an address of that address family but does
   not have a preference for a specific address.  In that scenario, the
   prefix length still indicates the sender's preference for the prefix
   length it is requesting.

   Upon receiving the ADDRESS_REQUEST capsule, an endpoint SHOULD assign
   an IP address to its peer, and then respond with an ADDRESS_ASSIGN
   capsule to inform the peer of the assignment.  Note that the receiver
   of the ADDRESS_REQUEST capsule is not required to assign the
   requested address, and that it can also assign some requested
   addresses but not others.

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   If an endpoint receives an ADDRESS_REQUEST capsule that contains zero
   Requested Addresses, it MUST abort the IP proxying request stream.

4.6.3.  ROUTE_ADVERTISEMENT Capsule

   The ROUTE_ADVERTISEMENT capsule (see Section 11.4 for the value of
   the capsule type) allows an endpoint to communicate to its peer that
   it is willing to route traffic to a set of IP address ranges.  This
   indicates that the sender has an existing route to each address
   range, and notifies its peer that if the receiver of the
   ROUTE_ADVERTISEMENT capsule sends IP packets for one of these ranges
   in HTTP Datagrams, the sender of the capsule will forward them along
   its preexisting route.  Any address which is in one of the address
   ranges can be used as the destination address on IP packets
   originated by the receiver of this capsule.

   ROUTE_ADVERTISEMENT Capsule {
     Type (i) = ROUTE_ADVERTISEMENT,
     Length (i),
     IP Address Range (..) ...,
   }

                Figure 7: ROUTE_ADVERTISEMENT Capsule Format

   The ROUTE_ADVERTISEMENT capsule contains a sequence of IP Address
   Ranges.

   IP Address Range {
     IP Version (8),
     Start IP Address (32..128),
     End IP Address (32..128),
     IP Protocol (8),
   }

                     Figure 8: IP Address Range Format

   IP Version:  IP Version of this range, encoded as an unsigned 8-bit
      integer.  MUST be either 4 or 6.
   Start IP Address and End IP Address:  Inclusive start and end IP
      address of the advertised range.  If the IP Version field has
      value 4, these fields SHALL have a length of 32 bits.  If the IP
      Version field has value 6, these fields SHALL have a length of 128
      bits.  The Start IP Address MUST be lesser or equal to the End IP
      Address.
   IP Protocol:  The Internet Protocol Number for traffic that can be
      sent to this range, encoded as an unsigned 8-bit integer.  If the
      value is 0, all protocols are allowed.  ICMP traffic is always
      allowed, regardless of the value of this field.

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   Upon receiving the ROUTE_ADVERTISEMENT capsule, an endpoint MAY start
   routing IP packets in these ranges to its peer.

   Each ROUTE_ADVERTISEMENT contains the full list of address ranges.
   If multiple ROUTE_ADVERTISEMENT capsules are sent in one direction,
   each ROUTE_ADVERTISEMENT capsule supersedes prior ones.  In other
   words, if a given address range was present in a prior capsule but
   the most recently received ROUTE_ADVERTISEMENT capsule does not
   contain it, the receiver will consider that range withdrawn.

   If multiple ranges using the same IP protocol were to overlap, some
   routing table implementations might reject them.  To prevent overlap,
   the ranges are ordered; this places the burden on the sender and
   makes verification by the receiver much simpler.  If an IP Address
   Range A precedes an IP address range B in the same
   ROUTE_ADVERTISEMENT capsule, they MUST follow these requirements:

   *  IP Version of A MUST be lesser or equal than IP Version of B

   *  If the IP Version of A and B are equal, the IP Protocol of A MUST
      be lesser or equal than IP Protocol of B.

   *  If the IP Version and IP Protocol of A and B are both equal, the
      End IP Address of A MUST be strictly less than the Start IP
      Address of B.

   If an endpoint receives a ROUTE_ADVERTISEMENT capsule that does not
   meet these requirements, it MUST abort the IP proxying request
   stream.

5.  Context Identifiers

   The mechanism for proxying IP in HTTP defined in this document allows
   future extensions to exchange HTTP Datagrams that carry different
   semantics from IP payloads.  Some of these extensions can augment IP
   payloads with additional data or compress IP header fields, while
   others can exchange data that is completely separate from IP
   payloads.  In order to accomplish this, all HTTP Datagrams associated
   with IP proxying request streams start with a Context ID field; see
   Section 6.

   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 IP 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

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   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 IP 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.

6.  HTTP Datagram Payload Format

   When associated with IP proxying request streams, the HTTP Datagram
   Payload field of HTTP Datagrams (see [HTTP-DGRAM]) has the format
   defined in Figure 9.  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:

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

                 Figure 9: IP 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.

   IP 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 a full IP packet (from the IP Version field until the last
   byte of the IP Payload).

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   Clients MAY optimistically start sending proxied IP packets before
   receiving the response to its IP proxying request, noting however
   that those may not be processed by the IP proxy if it responds to the
   request with a failure, or if the datagrams are received by the IP
   proxy before the request.  Since receiving addresses and routes is
   required in order to know that a packet can be sent through the
   tunnel, such optimistic packets might be dropped by the IP proxy if
   it chooses to provide different addressing or routing information
   than what the client assumed.

   When an endpoint receives an HTTP Datagram containing an IP packet,
   it will parse the packet's IP header, perform any local policy checks
   (e.g., source address validation), check their routing table to pick
   an outbound interface, and then send the IP packet on that interface
   or pass it to a local application.

   In the other direction, when an endpoint receives an IP packet, it
   checks to see if the packet matches the routes mapped for an IP
   tunnel, and performs the same forwarding checks as above before
   transmitting the packet over HTTP Datagrams.

   Note that endpoints will decrement the IP Hop Count (or TTL) upon
   encapsulation but not decapsulation.  In other words, the Hop Count
   is decremented right before an IP packet is transmitted in an HTTP
   Datagram.  This prevents infinite loops in the presence of routing
   loops, and matches the choices in IPsec [IPSEC].

   IPv6 requires that every link have an MTU of at least 1280 bytes
   [IPv6].  Since IP proxying in HTTP conveys IP packets in HTTP
   Datagrams and those can in turn be sent in QUIC DATAGRAM frames which
   cannot be fragmented [DGRAM], the MTU of an IP tunnel can be limited
   by the MTU of the QUIC connection that IP proxying is operating over.
   This can lead to situations where the IPv6 minimum link MTU is
   violated.  IP proxying endpoints that support IPv6 MUST ensure that
   the IP tunnel link MTU is at least 1280 (i.e., that they can send
   HTTP Datagrams with payloads of at least 1280 bytes).  This can be
   accomplished using various techniques:

   *  if both IP proxying endpoints know for certain that HTTP
      intermediaries are not in use, the endpoints can pad the QUIC
      INITIAL packets of the underlying QUIC connection that IP proxying
      is running over.  (Assuming QUIC version 1 is in use, the overhead
      is 1 byte type, 20 bytes maximal connection ID length, 4 bytes
      maximal packet number length, 1 byte DATAGRAM frame type, 8 bytes
      maximal quarter stream ID, one byte for the zero Context ID, and
      16 bytes for the AEAD authentication tag, for a total of 51 bytes
      of overhead which corresponds to padding QUIC INITIAL packets to
      1331 bytes or more.)

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   *  IP proxying endpoints can also send ICMPv6 echo requests with 1232
      bytes of data to ascertain the link MTU and tear down the tunnel
      if they do not receive a response.  Unless endpoints have an out
      of band means of guaranteeing that the previous techniques is
      sufficient, they MUST use this method.

   If an endpoint is using QUIC DATAGRAM frames to convey IPv6 packets,
   and it detects that the QUIC MTU is too low to allow sending 1280
   bytes, it MUST abort the IP proxying request stream.

   Endpoints MAY implement additional filtering policies on the IP
   packets they forward.

7.  Error Signalling

   Since IP proxying endpoints often forward IP packets onwards to other
   network interfaces, they need to handle errors in the forwarding
   process.  For example, forwarding can fail if the endpoint does not
   have a route for the destination address, or if it is configured to
   reject a destination prefix by policy, or if the MTU of the outgoing
   link is lower than the size of the packet to be forwarded.  In such
   scenarios, IP proxying endpoints SHOULD use ICMP [ICMP] [ICMPv6] to
   signal the forwarding error to its peer.

   Endpoints are free to select the most appropriate ICMP errors to
   send.  Some examples that are relevant for IP proxying include:

   *  For invalid source addresses, send Destination Unreachable
      Section 3.1 of [ICMPv6] with code 5, "Source address failed
      ingress/egress policy".

   *  For unroutable destination addresses, send Destination Unreachable
      Section 3.1 of [ICMPv6] with a code 0, "No route to destination",
      or code 1, "Communication with destination administratively
      prohibited".

   *  For packets that cannot fit within the MTU of the outgoing link,
      send Packet Too Big Section 3.2 of [ICMPv6].

   In order to receive these errors, endpoints need to be prepared to
   receive ICMP packets.  If an endpoint sends ROUTE_ADVERTISEMENT
   capsules, its routes SHOULD include an allowance for receiving ICMP
   messages.  If an endpoint does not send ROUTE_ADVERTISEMENT capsules,
   such as a client opening an IP flow through an IP proxy, it SHOULD
   process proxied ICMP packets from its peer in order to receive these
   errors.  Note that ICMP messages can originate from a source address
   different from that of the IP proxying peer.

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8.  Examples

   IP proxying in HTTP enables many different use cases that can benefit
   from IP packet proxying and tunnelling.  These examples are provided
   to help illustrate some of the ways in which IP proxying in HTTP can
   be used.

8.1.  Remote Access VPN

   The following example shows a point-to-network VPN setup, where a
   client receives a set of local addresses, and can send to any remote
   host through the IP proxy.  Such VPN setups can be either full-tunnel
   or split-tunnel.

   +--------+ IP A          IP B +--------+           +---> IP D
   |        |--------------------|   IP   | IP C      |
   | Client | IP Subnet C <--> ? |  Proxy |-----------+---> IP E
   |        |--------------------|        |           |
   +--------+                    +--------+           +---> IP ...

                        Figure 10: VPN Tunnel Setup

   In this case, the client does not specify any scope in its request.
   The IP proxy assigns the client an IPv4 address (192.0.2.11) and a
   full-tunnel route of all IPv4 addresses (0.0.0.0/0).  The client can
   then send to any IPv4 host using a source address in its assigned
   prefix.

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   [[ From Client ]]             [[ From IP Proxy ]]

   SETTINGS
     H3_DATAGRAM = 1

                                 SETTINGS
                                   ENABLE_CONNECT_PROTOCOL = 1
                                   H3_DATAGRAM = 1

   STREAM(44): HEADERS
   :method = CONNECT
   :protocol = connect-ip
   :scheme = https
   :path = /vpn
   :authority = proxy.example.com
   capsule-protocol = ?1

                                 STREAM(44): HEADERS
                                 :status = 200
                                 capsule-protocol = ?1

                                 STREAM(44): CAPSULE
                                 Capsule Type = ADDRESS_ASSIGN
                                 (Request ID = 0
                                  IP Version = 4
                                  IP Address = 192.0.2.11
                                  IP Prefix Length = 32)

                                 STREAM(44): CAPSULE
                                 Capsule Type = ROUTE_ADVERTISEMENT
                                 (IP Version = 4
                                  Start IP Address = 0.0.0.0
                                  End IP Address = 255.255.255.255
                                  IP Protocol = 0) // Any

   DATAGRAM
   Quarter Stream ID = 11
   Context ID = 0
   Payload = Encapsulated IP Packet

                                 DATAGRAM
                                 Quarter Stream ID = 11
                                 Context ID = 0
                                 Payload = Encapsulated IP Packet

                     Figure 11: VPN Full-Tunnel Example

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   A setup for a split-tunnel VPN (the case where the client can only
   access a specific set of private subnets) is quite similar.  In this
   case, the advertised route is restricted to 192.0.2.0/24, rather than
   0.0.0.0/0.

   [[ From Client ]]             [[ From IP Proxy ]]

                                 STREAM(44): CAPSULE
                                 Capsule Type = ADDRESS_ASSIGN
                                 (Request ID = 0
                                  IP Version = 4
                                  IP Address = 192.0.2.42
                                  IP Prefix Length = 32)

                                 STREAM(44): CAPSULE
                                 Capsule Type = ROUTE_ADVERTISEMENT
                                 (IP Version = 4
                                  Start IP Address = 192.0.2.0
                                  End IP Address = 192.0.2.255
                                  IP Protocol = 0) // Any

                Figure 12: VPN Split-Tunnel Capsule Example

8.2.  IP Flow Forwarding

   The following example shows an IP flow forwarding setup, where a
   client requests to establish a forwarding tunnel to
   target.example.com using SCTP (IP protocol 132), and receives a
   single local address and remote address it can use for transmitting
   packets.  A similar approach could be used for any other IP protocol
   that isn't easily proxied with existing HTTP methods, such as ICMP,
   ESP, etc.

   +--------+ IP A         IP B +--------+
   |        |-------------------|   IP   | IP C
   | Client |    IP C <--> D    |  Proxy |---------> IP D
   |        |-------------------|        |
   +--------+                   +--------+

                       Figure 13: Proxied Flow Setup

   In this case, the client specfies both a target hostname and an IP
   protocol number in the scope of its request, indicating that it only
   needs to communicate with a single host.  The IP proxy is able to
   perform DNS resolution on behalf of the client and allocate a
   specific outbound socket for the client instead of allocating an
   entire IP address to the client.  In this regard, the request is
   similar to a traditional CONNECT proxy request.

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   The IP proxy assigns a single IPv6 address to the client
   (2001:db8:1234::a) and a route to a single IPv6 host
   (2001:db8:3456::b), scoped to SCTP.  The client can send and receive
   SCTP IP packets to the remote host.

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   [[ From Client ]]             [[ From IP Proxy ]]

   SETTINGS
     H3_DATAGRAM = 1

                                 SETTINGS
                                   ENABLE_CONNECT_PROTOCOL = 1
                                   H3_DATAGRAM = 1

   STREAM(44): HEADERS
   :method = CONNECT
   :protocol = connect-ip
   :scheme = https
   :path = /proxy?target=target.example.com&ipproto=132
   :authority = proxy.example.com
   capsule-protocol = ?1

                                 STREAM(44): HEADERS
                                 :status = 200
                                 capsule-protocol = ?1

                                 STREAM(44): CAPSULE
                                 Capsule Type = ADDRESS_ASSIGN
                                 (Request ID = 0
                                  IP Version = 6
                                  IP Address = 2001:db8:1234::a
                                  IP Prefix Length = 128)

                                 STREAM(44): CAPSULE
                                 Capsule Type = ROUTE_ADVERTISEMENT
                                 (IP Version = 6
                                  Start IP Address = 2001:db8:3456::b
                                  End IP Address = 2001:db8:3456::b
                                  IP Protocol = 132)

   DATAGRAM
   Quarter Stream ID = 11
   Context ID = 0
   Payload = Encapsulated SCTP/IP Packet

                                 DATAGRAM
                                 Quarter Stream ID = 11
                                 Context ID = 0
                                 Payload = Encapsulated SCTP/IP Packet

                    Figure 14: Proxied SCTP Flow Example

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8.3.  Proxied Connection Racing

   The following example shows a setup where a client is proxying UDP
   packets through an IP proxy in order to control connection
   establishment racing through an IP proxy, as defined in Happy
   Eyeballs [HEv2].  This example is a variant of the proxied flow, but
   highlights how IP-level proxying can enable new capabilities even for
   TCP and UDP.

   +--------+ IP A         IP B +--------+ IP C
   |        |-------------------|        |<------------> IP E
   | Client |  IP C <--> E      |   IP   |
   |        |     D <--> F      |  Proxy |
   |        |-------------------|        |<------------> IP F
   +--------+                   +--------+ IP D

                 Figure 15: Proxied Connection Racing Setup

   As with proxied flows, the client specfies both a target hostname and
   an IP protocol number in the scope of its request.  When the IP proxy
   performs DNS resolution on behalf of the client, it can send the
   various remote address options to the client as separate routes.  It
   can also ensure that the client has both IPv4 and IPv6 addresses
   assigned.

   The IP proxy assigns the client both an IPv4 address (192.0.2.3) and
   an IPv6 address (2001:db8:1234::a) to the client, as well as an IPv4
   route (198.51.100.2) and an IPv6 route (2001:db8:3456::b), which
   represent the resolved addresses of the target hostname, scoped to
   UDP.  The client can send and recieve UDP IP packets to the either of
   the IP proxy addresses to enable Happy Eyeballs through the IP proxy.

   [[ From Client ]]             [[ From IP Proxy ]]

   SETTINGS
     H3_DATAGRAM = 1

                                 SETTINGS
                                   ENABLE_CONNECT_PROTOCOL = 1
                                   H3_DATAGRAM = 1

   STREAM(44): HEADERS
   :method = CONNECT
   :protocol = connect-ip
   :scheme = https
   :path = /proxy?ipproto=17
   :authority = proxy.example.com
   capsule-protocol = ?1

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                                 STREAM(44): HEADERS
                                 :status = 200
                                 capsule-protocol = ?1

                                 STREAM(44): CAPSULE
                                 Capsule Type = ADDRESS_ASSIGN
                                 (Request ID = 0
                                  IP Version = 4
                                  IP Address = 192.0.2.3
                                  IP Prefix Length = 32),
                                 (Request ID = 0
                                  IP Version = 6
                                  IP Address = 2001:db8::1234:1234
                                  IP Prefix Length = 128)

                                 STREAM(44): CAPSULE
                                 Capsule Type = ROUTE_ADVERTISEMENT
                                 (IP Version = 4
                                  Start IP Address = 198.51.100.2
                                  End IP Address = 198.51.100.2
                                  IP Protocol = 17),
                                 (IP Version = 6
                                  Start IP Address = 2001:db8:3456::b
                                  End IP Address = 2001:db8:3456::b
                                  IP Protocol = 17)
   ...

   DATAGRAM
   Quarter Stream ID = 11
   Context ID = 0
   Payload = Encapsulated IPv6 Packet

   DATAGRAM
   Quarter Stream ID = 11
   Context ID = 0
   Payload = Encapsulated IPv4 Packet

                Figure 16: Proxied Connection Racing Example

9.  Extensibility Considerations

   Extensions to IP proxying in HTTP can define behavior changes to this
   mechanism.  Such extensions SHOULD define new capsule types to
   exchange configuration information if needed.  It is RECOMMENDED for
   extensions that modify addressing to specify that their extension
   capsules be sent before the ADDRESS_ASSIGN capsule and that they do
   not take effect until the ADDRESS_ASSIGN capsule is parsed.  This
   allows modifications to address assignement to operate atomically.

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   Similarly, extensions that modify routing SHOULD behave similarly
   with regards to the ROUTE_ADVERTISEMENT capsule.

10.  Security Considerations

   There are significant risks in allowing arbitrary clients to
   establish a tunnel that permits sending to arbitrary hosts, as that
   could allow bad actors to send traffic and have it attributed to the
   IP proxy.  IP proxies SHOULD restrict its use to authenticated users.
   The HTTP Authorization header [HTTP] MAY be used to authenticate
   clients.  More complex authentication schemes are out of scope for
   this document but can be implemented using extensions.

   Falsifying IP source addresses in sent traffic has been common for
   denial of service attacks.  Implementations of this mechanism need to
   ensure that they do not facilitate such attacks.  In particular,
   there are scenarios where an endpoint knows that its peer is only
   allowed to send IP packets from a given prefix.  For example, that
   can happen through out of band configuration information, or when
   allowed prefixes are shared via ADDRESS_ASSIGN capsules.  In such
   scenarios, endpoints MUST follow the recommendations from [BCP38] to
   prevent source address spoofing.

11.  IANA Considerations

11.1.  HTTP Upgrade Token

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

   Value:  connect-ip
   Description:  Proxying of IP Payloads
   Expected Version Tokens:  None
   References:  This document

11.2.  Creation of the MASQUE URI Suffixes Registry

   This document requests that IANA create a new "MASQUE URI Suffixes"
   registry maintained at IANA_URL_TBD.  This new registry governs the
   path segment that immediately follows "masque" in paths that start
   with "/.well-known/masque/", see <https://www.iana.org/assignments/
   well-known-uris> for the registration of "masque" in the "Well-Known
   URIs" registry.  This new registry contains three columns:

   Path Segment:  An ASCII string containing only characters allowed in
      tokens; see Section 5.6.2 of [HTTP].  Entries in this registry
      MUST all have distinct entries in this column.

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   Description:  A description of the entry.

   Reference:  An optional reference defining the use of the entry.

   The registration policy for this registry is Expert Review; see
   Section 4.5 of [IANA-POLICY].

   There are initially two entries in this registry:

              +==============+==============+===============+
              | Path Segment | Description  | Reference     |
              +==============+==============+===============+
              | udp          | UDP Proxying | RFC 9298      |
              +--------------+--------------+---------------+
              | ip           | IP Proxying  | This Document |
              +--------------+--------------+---------------+

                      Table 1: New MASQUE URI Suffixes

11.3.  Updates to masque Well-Known URI

   This document will request IANA to update the entry for the "masque"
   URI suffix in the "Well-Known URIs" registry maintained at
   <https://www.iana.org/assignments/well-known-uris>.

   IANA is requested to update the "Reference" field to include this
   document in addition to previous values from that field.

   IANA is requested to replace the "Related Information" field with
   "For sub-suffix allocations, see registry at IANA_URL_TBD." where
   IANA_URL_TBD is the URL of the new registry described in
   Section 11.2.

11.4.  Capsule Type Registrations

   This document will request IANA to add the following values to the
   "HTTP Capsule Types" registry maintained at
   <https://www.iana.org/assignments/http-capsule-protocol/http-capsule-
   protocol.xhtml>.

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   +============+=====================+====================+===========+
   | Value      | Type                | Description        | Reference |
   +============+=====================+====================+===========+
   | 0x1ECA6A00 | ADDRESS_ASSIGN      | Address            | This      |
   |            |                     | Assignment         | Document  |
   +------------+---------------------+--------------------+-----------+
   | 0x1ECA6A01 | ADDRESS_REQUEST     | Address            | This      |
   |            |                     | Request            | Document  |
   +------------+---------------------+--------------------+-----------+
   | 0x1ECA6A02 | ROUTE_ADVERTISEMENT | Route              | This      |
   |            |                     | Advertisement      | Document  |
   +------------+---------------------+--------------------+-----------+

                           Table 2: New Capsules

12.  References

12.1.  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/rfc/rfc2234>.

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/rfc/rfc2827>.

   [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/rfc/rfc9221>.

   [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",
              RFC 9220, DOI 10.17487/RFC9220, June 2022,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc9110>.

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   [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/rfc/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/rfc/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/rfc/rfc9113>.

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

   [IANA-POLICY]
              Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [ICMP]     Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/rfc/rfc792>.

   [ICMPv6]   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>.

   [IPv6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/rfc/rfc8200>.

   [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>.

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   [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", 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>.

   [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/rfc/rfc3986>.

12.2.  Informative References

   [CONNECT-UDP]
              Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
              DOI 10.17487/RFC9298, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9298>.

   [HEv2]     Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8305>.

   [IPSEC]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/rfc/rfc4301>.

   [PROXY-REQS]
              Chernyakhovsky, A., McCall, D., and D. Schinazi,
              "Requirements for a MASQUE Protocol to Proxy IP Traffic",
              Work in Progress, Internet-Draft, draft-ietf-masque-ip-
              proxy-reqs-03, 27 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-masque-
              ip-proxy-reqs-03>.

Acknowledgments

   The design of this method was inspired by discussions in the MASQUE
   working group around [PROXY-REQS].  The authors would like to thank
   participants in those discussions for their feedback.  Additionally,
   Alejandro Sedeño provided valuable feedback on the document.

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   Most of the text on client configuration is based on the
   corresponding text in [CONNECT-UDP].

Authors' Addresses

   Tommy Pauly (editor)
   Apple Inc.
   Email: tpauly@apple.com

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

   Alex Chernyakhovsky
   Google LLC
   Email: achernya@google.com

   Mirja Kuehlewind
   Ericsson
   Email: mirja.kuehlewind@ericsson.com

   Magnus Westerlund
   Ericsson
   Email: magnus.westerlund@ericsson.com

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