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QUIC-Aware Proxying Using HTTP
draft-pauly-masque-quic-proxy-05

Document Type Active Internet-Draft (individual)
Authors Tommy Pauly , David Schinazi
Last updated 2022-10-21
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draft-pauly-masque-quic-proxy-05
MASQUE                                                          T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Experimental                                D. Schinazi
Expires: 24 April 2023                                        Google LLC
                                                         21 October 2022

                     QUIC-Aware Proxying Using HTTP
                    draft-pauly-masque-quic-proxy-05

Abstract

   This document defines an extension to UDP Proxying over HTTP that
   adds specific optimizations for proxied QUIC connections.  This
   extension allows a proxy to reuse UDP 4-tuples for multiple
   connections.  It also defines a mode of proxying in which QUIC short
   header packets can be forwarded using an HTTP/3 proxy rather than
   being re-encapsulated and re-encrypted.

Discussion Venues

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

   Source for this draft and an issue tracker can be found at
   https://github.com/tfpauly/quic-proxy (https://github.com/tfpauly/
   quic-proxy).

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 24 April 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Virtual Connection IDs  . . . . . . . . . . . . . . . . .   5
   2.  Required Proxy State  . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Stream Mapping  . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Virtual Target Connection ID Mapping  . . . . . . . . . .   6
     2.3.  Client Connection ID Mappings . . . . . . . . . . . . . .   7
     2.4.  Detecting Connection ID Conflicts . . . . . . . . . . . .   7
     2.5.  Stateless Resets for Forwarded Mode QUIC Packets  . . . .   8
   3.  Connection ID Capsule Types . . . . . . . . . . . . . . . . .   8
   4.  Client Request Behavior . . . . . . . . . . . . . . . . . . .  11
     4.1.  New Proxied Connection Setup  . . . . . . . . . . . . . .  11
     4.2.  Adding New Client Connection IDs  . . . . . . . . . . . .  12
     4.3.  Sending With Forwarded Mode . . . . . . . . . . . . . . .  12
     4.4.  Receiving With Forwarded Mode . . . . . . . . . . . . . .  13
   5.  Proxy Response Behavior . . . . . . . . . . . . . . . . . . .  14
     5.1.  Removing Mapping State  . . . . . . . . . . . . . . . . .  16
     5.2.  Handling Connection Migration . . . . . . . . . . . . . .  16
   6.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Packet Size Considerations  . . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  HTTP Header . . . . . . . . . . . . . . . . . . . . . . .  19
     9.2.  Capsule Types . . . . . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   UDP Proxying over HTTP [CONNECT-UDP] defines a way to send datagrams
   through an HTTP proxy, where UDP is used to communicate between the
   proxy and a target server.  This can be used to proxy QUIC
   connections [QUIC], since QUIC runs over UDP datagrams.

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   This document uses the term "target" to refer to the server that a
   client is accessing via a proxy.  This target may be an origin
   hosting content, or another proxy.

   This document extends the UDP proxying protocol to add signalling
   about QUIC Connection IDs.  QUIC Connection IDs are used to identify
   QUIC connections in scenarios where there is not a strict
   bidirectional mapping between one QUIC connection and one UDP 4-tuple
   (pairs of IP addresses and ports).  A proxy that is aware of
   Connection IDs can reuse UDP 4-tuples between itself and a target for
   multiple proxied QUIC connections.

   Awareness of Connection IDs also allows a proxy to avoid re-
   encapsulation and re-encryption of proxied QUIC packets once a
   connection has been established.  When this functionality is present,
   the proxy can support two modes for handling QUIC packets:

   1.  Tunnelled, in which client <-> target QUIC packets are
       encapsulated inside client <-> proxy QUIC packets.  These packets
       use multiple layers of encryption and congestion control.  QUIC
       long header packets MUST use this mode.  QUIC short header
       packets MAY use this mode.  This is the default mode for UDP
       proxying.

   2.  Forwarded, in which client <-> target QUIC packets are sent
       directly over the client <-> proxy UDP socket.  These packets are
       only encrypted using the client-target keys, and use the client-
       target congestion control.  This mode MUST only be used for QUIC
       short header packets.

   Forwarded mode is defined as an optimization to reduce CPU processing
   on clients and proxies, as well as avoiding MTU overhead for packets
   on the wire.  This makes it suitable for deployment situations that
   otherwise relied on cleartext TCP proxies, which cannot support QUIC
   and have inferior security and privacy properties.

   The properties provided by the forwarded mode are as follows:

   *  All packets sent between the client and the target traverse
      through the proxy device.

   *  The target server cannot know the IP address of the client solely
      based on the proxied packets the target receives.

   *  Observers of either or both of the client <-> proxy link and the
      proxy <-> target are not able to learn more about the client <->
      target communication than if no proxy was used.

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   It is not a goal of forwarded mode to prevent correlation between
   client <-> proxy and the proxy <-> target packets from an entity that
   can observe both links.  See Section 8 for further discussion.

   Both clients and proxies can unilaterally choose to disable forwarded
   mode for any client <-> target connection.

   The forwarded mode of this extension is only defined for HTTP/3
   [HTTP3] and not any earlier versions of HTTP.

   QUIC proxies only need to understand the Header Form bit, and the
   connection ID fields from packets in client <-> target QUIC
   connections.  Since these fields are all in the QUIC invariants
   header [INVARIANTS], QUIC proxies can proxy all versions of QUIC.

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.

1.2.  Terminology

   This document uses the following terms:

   *  Client: the client of all QUIC connections discussed in this
      document.

   *  Proxy: the endpoint that responds to the UDP proxying request.

   *  Target: the server that a client is accessing via a proxy.

   *  Client <-> Proxy HTTP stream: a single HTTP stream established
      from the client to the proxy.

   *  Socket: a UDP 4-tuple (local IP address, local UDP port, remote IP
      address, remote UDP port).  In some implementations, this is
      referred to as a "connected" socket.

   *  Client-facing socket: the socket used to communicate between the
      client and the proxy.

   *  Target-facing socket: the socket used to communicate between the
      proxy and the target.

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   *  Client Connection ID: a QUIC Connection ID that is chosen by the
      client, and is used in the Destination Connection ID field of
      packets from the target to the client.

   *  Target Connection ID: a QUIC Connection ID that is chosen by the
      target, and is used in the Destination Connection ID field of
      packets from the client to the target.

   *  Virtual Client Connection ID: a fake QUIC Connection ID that is
      chosen by the client that the proxy MUST use when sending QUIC
      packets in forwarded mode.

   *  Virtual Target Connection ID: a fake QUIC Connection ID that is
      chosen by the proxy that the client MUST use when sending QUIC
      packets in forwarded mode.

1.3.  Virtual Connection IDs

   QUIC allows each endpoint of a connection to choose the connection
   IDs it receives with.  Servers receiving QUIC packets can employ load
   balancing strategies such as those described in [QUIC-LB] that encode
   routing information in the connection ID.  When operating in
   forwarded mode, clients send QUIC packets destined for the Target
   directly to the Proxy.  Since these packets are generated using the
   Target Connection ID, load balancers would not be able to route
   packets to the correct Proxy if the packets were sent with the Target
   Connection ID.  The Virtual Target Connection ID is a connection ID
   chosen by the Proxy that the Client uses when sending forwarded mode
   packets.  The Proxy replaces the Virtual Target Connection ID with
   the Target Connection ID prior to forwarding the packet to the
   Target.

   Similarly, QUIC requires that connection IDs aren't reused over
   multiple network paths to avoid linkability.  The Virtual Client
   Connection ID is a connection ID chosen by the Client that the Proxy
   uses when sending forwarded mode packets.  The Proxy replaces the
   Client Connection ID with the Virtual Client Connection ID prior to
   forwarding the packet to the Client.  Clients take advantage of this
   to avoid linkability when migrating a client to proxy network path.
   The Virtual Client Connection ID allows the connection ID bytes to
   change on the wire without requiring the connection IDs on the client
   to target connection change.

   Clients and Proxies not implementing forwarded mode do not need to
   consider Virtual Connection IDs since all Client<->Target datagrams
   will be encapsulated within the Client<->Proxy connection.

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2.  Required Proxy State

   In the methods defined in this document, the proxy is aware of the
   QUIC Connection IDs being used by proxied connections, along with the
   sockets used to communicate with the client and the target.  Tracking
   Connection IDs in this way allows the proxy to reuse target-facing
   sockets for multiple connections and support the forwarded mode of
   proxying.

   QUIC packets can be either tunnelled within an HTTP proxy connection
   using HTTP Datagram frames [HTTP-DGRAM], or be forwarded directly
   alongside an HTTP/3 proxy connection on the same set of IP addresses
   and UDP ports.  The use of forwarded mode requires the consent of
   both the client and the proxy.

   In order to correctly route QUIC packets in both tunnelled and
   forwarded modes, the proxy needs to maintain mappings between several
   items.  There are three required unidirectional mappings, described
   below.

2.1.  Stream Mapping

   Each client <-> proxy HTTP stream MUST be mapped to a single target-
   facing socket.

   (Client <-> Proxy HTTP Stream) => Target-facing socket

   Multiple streams can map to the same target-facing socket, but a
   single stream cannot be mapped to multiple target-facing sockets.

   This mapping guarantees that any HTTP Datagram using a stream sent
   from the client to the proxy in tunnelled mode can be sent to the
   correct target.

2.2.  Virtual Target Connection ID Mapping

   Each pair of Virtual Target Connection ID and client-facing socket
   MUST map to a single target-facing socket and Target Connection ID.

   (Client-facing socket + Virtual Target Connection ID)
       => (Target-facing socket + Target Connection ID)

   Multiple pairs of Connection IDs and client-facing sockets can map to
   the same target-facing socket.

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   This mapping guarantees that any QUIC packet containing the Virtual
   Target Connection ID sent from the client to the proxy in forwarded
   mode can be sent to the correct target with the correct Target
   Connection ID.  Thus, a proxy that does not allow forwarded mode does
   not need to maintain this mapping.

2.3.  Client Connection ID Mappings

   Each pair of Client Connection ID and target-facing socket MUST map
   to a single stream on a single client <-> proxy HTTP stream.
   Additionally, when supporting forwarded mode, the pair of Client
   Connection ID and target-facing socket MUST map to a single client-
   facing socket and Virtual Client Connection ID.

(Target-facing socket + Client Connection ID) => (Client <-> Proxy HTTP Stream)
(Target-facing socket + Client Connection ID)
    => (Client-facing socket + Virtual Client Connection ID)

   Multiple pairs of Connection IDs and target-facing sockets can map to
   the same HTTP stream or client-facing socket.

   These mappings guarantee that any QUIC packet sent from a target to
   the proxy can be sent to the correct client, in either tunnelled or
   forwarded mode.  Note that this mapping becomes trivial if the proxy
   always opens a new target-facing socket for every client request with
   a unique stream.  The mapping is critical for any case where target-
   facing sockets are shared or reused.

2.4.  Detecting Connection ID Conflicts

   In order to be able to route packets correctly in both tunnelled and
   forwarded mode, proxies check for conflicts before creating a new
   mapping.  If a conflict is detected, the proxy will reject the
   client's request, as described in Section 5.

   Two sockets conflict if and only if all members of the 4-tuple (local
   IP address, local UDP port, remote IP address, and remote UDP port)
   are identical.

   Two Connection IDs conflict if and only if one Connection ID is equal
   to or a prefix of another.  For example, a zero-length Connection ID
   conflicts with all connection IDs.  This definition of a conflict
   originates from the fact that QUIC short headers do not carry the
   length of the Destination Connection ID field, and therefore if two
   short headers with different Destination Connection IDs are received
   on a shared socket, one being a prefix of the other prevents the
   receiver from identifying which mapping this corresponds to.

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   The proxy treats two mappings as being in conflict when a conflict is
   detected for all elements on the left side of the mapping diagrams
   above.

   Since very short Connection IDs are more likely to lead to conflicts,
   particularly zero-length Connection IDs, a proxy MAY choose to reject
   all requests for very short Connection IDs as conflicts, in
   anticipation of future conflicts.

2.5.  Stateless Resets for Forwarded Mode QUIC Packets

   While the lifecycle of forwarding rules are bound to the lifecycle of
   the client<->proxy HTTP stream, a peer may not be aware that the
   stream has terminated.  If the above mappings are lost or removed
   without the peer's knowledge, they may send forwarded mode packets
   even though the Client or Proxy no longer has state for that
   connection.  To allow the Client or Proxy to reset the
   client<->target connection in the absence the mappings above, a
   stateless reset token corresponding to the Virtual Connection ID can
   be provided.

   Consider a proxy that initiates closure of a client<->proxy QUIC
   connection.  If the client is temporarily unresponsive or
   unreachable, the proxy might have considered the connection closed
   and removed all connection state (including the stream mappings used
   for forwarding).  If the client never learned about the closure, it
   might send forwarded mode packets to the proxy, assuming the stream
   mappings and client<->proxy connection are still intact.  The proxy
   will receive these forwarded mode packets, but won't have any state
   corresponding to the destination connection ID in the packet.  If the
   proxy has provided a stateless reset token for the Virtual Target
   Connection ID, it can send a stateless reset packet to quickly notify
   the client that the client<->target connection is broken.

3.  Connection ID Capsule Types

   Proxy awareness of QUIC Connection IDs relies on using capsules
   ([HTTP-DGRAM]) to signal the addition and removal of Client and
   Target Connection IDs.

   Note that these capsules do not register contexts.  QUIC packets are
   encoded using HTTP Datagrams with the context ID set to zero as
   defined in [CONNECT-UDP].

   The capsules used for QUIC-aware proxying allow a client to register
   connection IDs with the proxy, and for the proxy to acknowledge or
   reject the connection ID mappings.

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   The REGISTER_CLIENT_CID and REGISTER_TARGET_CID capsule types (see
   Section 9.2 for the capsule type values) allow a client to inform the
   proxy about a new Client Connection ID or a new Target Connection ID,
   respectively.  These capsule types MUST only be sent by a client.

   The ACK_CLIENT_CID and ACK_TARGET_CID capsule types (see Section 9.2
   for the capsule type values) are sent by the proxy to the client to
   indicate that a mapping was successfully created for a registered
   connection ID as well as provide the Virtual Target Connection ID
   that may be used in forwarded mode.  These capsule types MUST only be
   sent by a proxy.

   The CLOSE_CLIENT_CID and CLOSE_TARGET_CID capsule types (see
   Section 9.2 for the capsule type values) allow either a client or a
   proxy to remove a mapping for a connection ID.  These capsule types
   MAY be sent by either a client or the proxy.  If a proxy sends a
   CLOSE_CLIENT_CID without having sent an ACK_CLIENT_CID, or if a proxy
   sends a CLOSE_TARGET_CID without having sent an ACK_TARGET_CID, it is
   rejecting a Connection ID registration.

   REGISTER_TARGET_CID, ACK_CLIENT_CID, CLOSE_CLIENT_CID, and
   CLOSE_TARGET_CID capsule types are formatted as follows:

   Connection ID Capsule {
     Type (i) = 0xffe301, 0xffe302, 0xffe304, 0xffe305
     Length (i),
     Connection ID (0..2040),
   }

                   Figure 1: Connection ID Capsule Format

   Connection ID:  A connection ID being registered or acknowledged,
      which is between 0 and 255 bytes in length.  The length of the
      connection ID is implied by the length of the capsule.  Note that
      in QUICv1, the length of the Connection ID is limited to 20 bytes,
      but QUIC invariants allow up to 255 bytes.

   The REGISTER_CLIENT_CID and ACK_TARGET_CID capsule types include a
   Virtual Connection ID and Stateless Reset Token.

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   Virtual Connection ID Capsule {
     Type (i) = 0xffe300, 0xffe303
     Length (i)
     Connection ID Length (i)
     Connection ID (0..2040),
     Virtual Connection ID Length (i)
     Virtual Connection ID (0..2040),
     Stateless Reset Token Length (i),
     Stateless Reset Token (..),
   }

               Figure 2: Virtual Connection ID Capsule Format

   Connection ID Length  The length of the connection ID being
      acknowledged, which is between 0 and 255.  Note that in QUICv1,
      the length of the Connection ID is limited to 20 bytes, but QUIC
      invariants allow up to 255 bytes.

   Connection ID  A connection ID being acknowledged whose length is
      equal to Connection ID Length.  This is the real Target or Client
      Connection ID.

   Virtual Connection ID Length  The length of the virtual connection ID
      being provided.  This MUST be a valid connection ID length for the
      QUIC version used in the client<->proxy QUIC connection.  When
      forwarded mode is not negotiated, the length MUST be zero.  The
      Virtual Connection ID Length and Connection ID Length SHOULD be
      equal when possible to avoid the need to resize packets during
      replacement.

   Virtual Connection ID  The peer-chosen connection ID that the sender
      of forwarded mode packets MUST use when sending.  The endpoint
      rewrites forwarded mode packets to contain the correct Connection
      ID prior to sending them.

   Stateless Reset Token Length  The length of the stateless reset token
      that may be sent by the client or proxy in response to forwarded
      mode packets in order to reset the Client<->Target QUIC
      connection.  When forwarded mode is not negotiated, the length
      MUST be zero.  Proxies or Clients choosing not to support
      stateless resets MAY set the length to zero.  Clients or Proxies
      receiving a zero-length stateless reset token MUST ignore it.

   Stateless Reset Token  A Stateless Reset Token allowing reset of the
      Client<->Target connection in response to Client<->Target
      forwarded mode packets.

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4.  Client Request Behavior

   A client initiates UDP proxying via a CONNECT request as defined in
   [CONNECT-UDP].  Within its request, it includes the "Proxy-QUIC-
   Forwarding" header to indicate whether or not the request should
   support forwarding.  If this header is not included, the client MUST
   NOT send any connection ID capsules.

   The "Proxy-QUIC-Forwarding" is an Item Structured Header [RFC8941].
   Its value MUST be a Boolean.  Its ABNF is:

       Proxy-QUIC-Forwarding = sf-boolean

   If the client wants to enable QUIC packet forwarding for this
   request, it sets the value to "?1".  If it doesn't want to enable
   forwarding, but instead only provide information about QUIC
   Connection IDs for the purpose of allowing the proxy to share a
   target-facing socket, it sets the value to "?0".

   If the proxy supports QUIC-aware proxying, it will include the
   "Proxy-QUIC-Forwarding" header in successful HTTP responses.  The
   value indicates whether or not the proxy supports forwarding.  If the
   client does not receive this header in responses, the client SHALL
   assume that the proxy does not understand how to parse Connection ID
   capsules, and MUST NOT send any Connection ID capsules.

   The client sends a REGISTER_CLIENT_CID capsule whenever it advertises
   a new Client Connection ID to the target, and a REGISTER_TARGET_CID
   capsule when it has received a new Target Connection ID for the
   target.  Note that the initial REGISTER_CLIENT_CID capsule MAY be
   sent prior to receiving an HTTP response from the proxy.

4.1.  New Proxied Connection Setup

   To initiate QUIC-aware proxying, the client sends a
   REGISTER_CLIENT_CID capsule containing the initial Client Connection
   ID that the client has advertised to the target as well as a Virtual
   Connection ID that the proxy MUST use when sending forwarded mode
   packets.  If forwarded mode is not supported, the Virtual Connection
   ID Length MUST be zero.

   If the mapping is created successfully, the client will receive a
   ACK_CLIENT_CID capsule that contains the same Client Connection ID
   that was requested.

   Since clients are always aware whether or not they are using a QUIC
   proxy, clients are expected to cooperate with proxies in selecting
   Client Connection IDs.  A proxy detects a conflict when it is not

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   able to create a unique mapping using the Client Connection ID
   (Section 2.4).  It can reject requests that would cause a conflict
   and indicate this to the client by replying with a CLOSE_CLIENT_CID
   capsule.  In order to avoid conflicts, clients SHOULD select Client
   Connection IDs of at least 8 bytes in length with unpredictable
   values.  A client also SHOULD NOT select a Client Connection ID that
   matches the ID used for the QUIC connection to the proxy, as this
   inherently creates a conflict.

   If the rejection indicated a conflict due to the Client Connection
   ID, the client MUST select a new Connection ID before sending a new
   request, and generate a new packet.  For example, if a client is
   sending a QUIC Initial packet and chooses a Connection ID that
   conflicts with an existing mapping to the same target server, it will
   need to generate a new QUIC Initial.

4.2.  Adding New Client Connection IDs

   Since QUIC connection IDs are chosen by the receiver, an endpoint
   needs to communicate its chosen connection IDs to its peer before the
   peer can start using them.  In QUICv1, this is performed using the
   NEW_CONNECTION_ID frame.

   Prior to informing the target of a new chosen client connection ID,
   the client MUST send a REGISTER_CLIENT_CID capsule request containing
   the new Client Connection ID and Virtual Client Connection ID.

   The client should only inform the target of the new Client Connection
   ID once an ACK_CLIENT_CID capsule is received that contains the
   echoed connection ID.

4.3.  Sending With Forwarded Mode

   Support for forwarded mode is determined by the "Proxy-QUIC-
   Forwarding" header, see Section 5.

   Once the client has learned the target server's Connection ID, such
   as in the response to a QUIC Initial packet, it can send a
   REGISTER_TARGET_CID capsule containing the Target Connection ID to
   request the ability to forward packets.

   The client MUST wait for an ACK_TARGET_CID capsule that contains the
   echoed connection ID and Virtual Target Connection ID before using
   forwarded mode.

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   Prior to receiving the proxy server response, the client MUST send
   short header packets tunnelled in HTTP Datagram frames.  The client
   MAY also choose to tunnel some short header packets even after
   receiving the successful response.

   If the Target Connection ID registration is rejected, for example
   with a CLOSE_TARGET_CID capsule, it MUST NOT forward packets to the
   requested Target Connection ID, but only use tunnelled mode.  The
   request might also be rejected if the proxy does not support
   forwarded mode or has it disabled by policy.

   QUIC long header packets MUST NOT be forwarded.  These packets can
   only be tunnelled within HTTP Datagram frames to avoid exposing
   unnecessary connection metadata.

   When forwarding, the client sends a QUIC packet with the Virtual
   Target Connection ID in the QUIC short header, using the same socket
   between client and proxy that was used for the main QUIC connection
   between client and proxy.

   When forwarding, the proxy sends a QUIC packet with the Virtual
   Client Target Connection ID in the QUIC short header, using the same
   socket between client and proxy that was used for the main QUIC
   connection between client and proxy.

   Prior to sending a forwarded mode packet, the sender MUST replace the
   Connection ID with the Virtual Connection ID.  If the Virtual
   Connection ID is larger than the Connection ID, the sender MUST
   extend the length of the packet by the difference between the two
   lengths, to include the entire Virtual Connection ID.  If the Virtual
   Connection ID is smaller than the Connection ID, the sender MUST
   shrink the length of the packet by the difference between the two
   lengths.

   Clients and proxies supporting forwarded mode MUST be able to handle
   Virtual Connection IDs of different lengths than the corresponding
   Connection IDs.

4.4.  Receiving With Forwarded Mode

   If the client has indicated support for forwarded mode with the
   "Proxy-QUIC-Forwarding" header, the proxy MAY use forwarded mode for
   any Client Connection ID for which it has a valid mapping.

   Once a client has sent "Proxy-QUIC-Forwarding" with a value of "?1",
   it MUST be prepared to receive forwarded short header packets on the
   socket between itself and the proxy for any Virtual Client Connection
   ID that it has registered with a REGISTER_CLIENT_CID capsule.  The

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   client uses the Destination Connection ID field of the received
   packet to determine if the packet was originated by the proxy, or
   merely forwarded from the target.  The client replaces the Virtual
   Client Connection ID with the real Client Connection ID before
   processing the packet further.

5.  Proxy Response Behavior

   Upon receipt of a CONNECT request that includes the "Proxy-QUIC-
   Forwarding" header, the proxy indicates to the client that it
   supports QUIC-aware proxying by including a "Proxy-QUIC-Forwarding"
   header in a successful response.  If it supports QUIC packet
   forwarding, it sets the value to "?1"; otherwise, it sets it to "?0".

   Upon receipt of a REGISTER_CLIENT_CID or REGISTER_TARGET_CID capsule,
   the proxy validates the registration, tries to establish the
   appropriate mappings as described in Section 2.

   The proxy MUST reply to each REGISTER_CLIENT_CID capsule with either
   an ACK_CLIENT_CID or CLOSE_CLIENT_CID capsule containing the
   Connection ID that was in the registration capsule.

   Similarly, the proxy MUST reply to each REGISTER_TARGET_CID capsule
   with either an ACK_TARGET_CID or CLOSE_TARGET_CID capsule containing
   the Connection ID that was in the registration capsule.

   The proxy then determines the target-facing socket to associate with
   the client's request.  This will generally involve performing a DNS
   lookup for the target hostname in the CONNECT request, or finding an
   existing target-facing socket to the authority.  The target-facing
   socket might already be open due to a previous request from this
   client, or another.  If the socket is not already created, the proxy
   creates a new one.  Proxies can choose to reuse target-facing sockets
   across multiple UDP proxying requests, or have a unique target-facing
   socket for every UDP proxying request.

   If a proxy reuses target-facing sockets, it SHOULD store which
   authorities (which could be a domain name or IP address literal) are
   being accessed over a particular target-facing socket so it can avoid
   performing a new DNS query and potentially choosing a different
   target server IP address which could map to a different target
   server.

   Target-facing sockets MUST NOT be reused across QUIC and non-QUIC UDP
   proxy requests, since it might not be possible to correctly
   demultiplex or direct the traffic.  Any packets received on a target-
   facing socket used for proxying QUIC that does not correspond to a
   known Connection ID MUST be dropped.

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   When the proxy recieves a REGISTER_CLIENT_CID capsule, it is
   receiving a request to be able to route traffic matching the Client
   Connection ID back to the client using the Virtual Client Connection
   ID.  If the pair of this Client Connection ID and the selected
   target-facing socket does not create a conflict, the proxy creates
   the mapping and responds with a ACK_CLIENT_CID capsule.  After this
   point, any packets received by the proxy from the target-facing
   socket that match the Client Connection ID can to be sent to the
   client after the proxy has replaced the Connection ID with the
   Virtual Client Connection ID.  The proxy MUST use tunnelled mode
   (HTTP Datagram frames) for any long header packets.  The proxy SHOULD
   forward directly to the client for any matching short header packets
   if forwarding is supported by the client, but the proxy MAY tunnel
   these packets in HTTP Datagram frames instead.  If the mapping would
   create a conflict, the proxy responds with a CLOSE_CLIENT_CID
   capsule.

   When the proxy recieves a REGISTER_TARGET_CID capsule, it is
   receiving a request to allow the client to forward packets to the
   target.  The proxy generates a Virtual Target Connection ID for the
   client to use when sending packets in forwarded mode.  If forwarded
   mode is not supported, the proxy MUST NOT send a Virtual Target
   Connection ID by setting the length to zero.  If forwarded mode is
   supported, the proxy MUST use a Virtual Target Connection ID that
   does not introduce a conflict with any other Connection ID on the
   client-facing socket.  The proxy creates the mapping and responds
   with an ACK_TARGET_CID capsule.  Once the successful response is
   sent, the proxy will forward any short header packets received on the
   client-facing socket that use the Virtual Target Connection ID using
   the correct target-facing socket after first rewriting the Virtual
   Target Connection ID to be the correct Target Connection ID.

   A proxy that supports forwarded mode but chooses not to support
   rewriting the Virtual Target Connection ID to the Target Connection
   ID may opt to simply let them be equal.  If the proxy does wish to
   choose a Virtual Target Connection ID, it MUST be able to replace the
   Virtual Target Connection ID with the Target Connection ID and
   correctly handle length differences between the two.  Regardless of
   whether or not the proxy chooses to support rewriting of the Virtual
   Target Connection ID, it MUST be able to support rewriting the Client
   Connection ID to the Virtual Client Connection ID.

   If the proxy does not support forwarded mode, or does not allow
   forwarded mode for a particular client or authority by policy, it can
   reject all REGISTER_TARGET_CID requests with CLOSE_TARGET_CID
   capsule.

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   The proxy MUST only forward non-tunnelled packets from the client
   that are QUIC short header packets (based on the Header Form bit) and
   have mapped Virtual Target Connection IDs.  Packets sent by the
   client that are forwarded SHOULD be considered as activity for
   restarting QUIC's Idle Timeout [QUIC].

5.1.  Removing Mapping State

   For any registration capsule for which the proxy has sent an
   acknowledgement, any mappings last until either endpoint sends a
   close capsule or the either side of the HTTP stream closes.

   A client that no longer wants a given Connection ID to be forwarded
   by the proxy sends a CLOSE_CLIENT_CID or CLOSE_TARGET_CID capsule.

   If a client's connection to the proxy is terminated for any reason,
   all mappings associated with all requests are removed.

   A proxy can close its target-facing socket once all UDP proxying
   requests mapped to that socket have been removed.

5.2.  Handling Connection Migration

   If a proxy supports QUIC connection migration, it needs to ensure
   that a migration event does not end up sending too many tunnelled or
   proxied packets on a new path prior to path validation.

   Specifically, the proxy MUST limit the number of packets that it will
   proxy to an unvalidated client address to the size of an initial
   congestion window.  Proxies additionally SHOULD pace the rate at
   which packets are sent over a new path to avoid creating
   unintentional congestion on the new path.

   When operating in forwarded mode, the proxy MUST NOT send forwarded
   mode packets with the same Destination Connection ID over multiple
   network paths.

   After switching to a new network path, the proxy MUST tunnel Target
   to Client packets instead of forwarding.  Only once a new Virtual
   Client Connection ID has been communicated to the proxy via a
   REGISTER_CLIENT_CID capsule may the proxy begin forwarding packets to
   the client.  Similarly, when a client actively migrates, it MUST NOT
   send any forwarded mode packets until it has registered a new Virtual
   Target Connection ID.  In both cases, reusing a connection ID would
   increase linkability of the connection between network paths.  Note
   that the Client Connection ID and Target Connection ID may stay the
   same while the Virtual Target Connection ID and Virtual Client
   Connection ID change.

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6.  Example

   Consider a client that is establishing a new QUIC connection through
   the proxy.  It has selected a Client Connection ID of 0x31323334.  In
   order to inform a proxy of the new QUIC Client Connection ID, the
   client also sends a REGISTER_CLIENT_CID capsule.

   The client will also send the initial QUIC packet with the Long
   Header form in an HTTP datagram.

   Client                                             Server

   STREAM(44): HEADERS             -------->
     :method = CONNECT
     :protocol = connect-udp
     :scheme = https
     :path = /target.example.com/443/
     :authority = proxy.example.org
     proxy-quic-forwarding = ?1
     capsule-protocol = ?1

   STREAM(44): DATA                -------->
     Capsule Type = REGISTER_CLIENT_CID
     Connection ID = 0x31323334
     Virtual CID = 0x62646668
     Stateless Reset Token = Token

   DATAGRAM                        -------->
     Quarter Stream ID = 11
     Context ID = 0
     Payload = Encapsulated QUIC initial

              <--------  STREAM(44): HEADERS
                           :status = 200
                           proxy-quic-forwarding = ?1
                           capsule-protocol = ?1

              <--------  STREAM(44): DATA
                           Capsule Type = ACK_CLIENT_CID
                           Connection ID = 0x31323334

   /* Wait for target server to respond to UDP packet. */

              <--------  DATAGRAM
                           Quarter Stream ID = 11
                           Context ID = 0
                           Payload = Encapsulated QUIC initial

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   Immediately after sending the REGISTER_CLIENT_CID capsule, the client
   may receive forwarded mode packets from the proxy with a Virtual
   Client Connection ID of 0x62646668 which it will replace with the
   real Client Connection ID of 0x31323334.  All forwarded mode packets
   sent by the proxy will have been modified to contain the Virtual
   Client Connection ID instead of the Client Connection ID.

   Once the client learns which Connection ID has been selected by the
   target server, it can send a new request to the proxy to establish a
   mapping for forwarding.  In this case, that ID is 0x61626364.  The
   client sends the following capsule:

   STREAM(44): DATA                -------->
     Capsule Type = REGISTER_TARGET_CID
     Connection ID = 0x61626364

              <--------  STREAM(44): DATA
                           Capsule Type = ACK_TARGET_CID
                           Connection ID = 0x61626364
                           Virtual Target Connection ID = 0x123412341234
                           Stateless Reset Token = Token

   Upon receiving an ACK_TARGET_CID capsule, the client starts sending
   Short Header packets with a Destination Connection ID of
   0x123412341234 directly to the proxy (not tunnelled), and these are
   rewritten by the proxy to have the Destination Connection ID
   0x61626364 prior to being forwarded directly to the target.  In the
   reverse direction, Short Header packets from the target with a
   Destination Connection ID of 0x31323334 are modified to replace the
   Destination Connection ID with the Virtual Client Connection ID of
   0x62646668 and forwarded directly to the client.

7.  Packet Size Considerations

   Since Initial QUIC packets must be at least 1200 bytes in length, the
   HTTP Datagram frames that are used for a QUIC-aware proxy MUST be
   able to carry at least 1200 bytes.

   Additionally, clients that connect to a proxy for purpose of proxying
   QUIC SHOULD start their connection with a larger packet size than
   1200 bytes, to account for the overhead of tunnelling an Initial QUIC
   packet within an HTTP Datagram frame.  If the client does not begin
   with a larger packet size than 1200 bytes, it will need to perform
   Path MTU (Maximum Transmission Unit) discovery to discover a larger
   path size prior to sending any tunnelled Initial QUIC packets.

   Once a proxied QUIC connections moves into forwarded mode, the client
   SHOULD initiate Path MTU discovery to increase its end-to-end MTU.

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8.  Security Considerations

   Proxies that support this extension SHOULD provide protections to
   rate-limit or restrict clients from opening an excessive number of
   proxied connections, so as to limit abuse or use of proxies to launch
   Denial-of-Service attacks.

   Sending QUIC packets by forwarding through a proxy without tunnelling
   exposes some QUIC header metadata to onlookers, and can be used to
   correlate packet flows if an attacker is able to see traffic on both
   sides of the proxy.  Tunnelled packets have similar inference
   problems.  An attacker on both sides of the proxy can use the size of
   ingress and egress packets to correlate packets belonging to the same
   connection.  (Absent client-side padding, tunnelled packets will
   typically have a fixed amount of overhead that is removed before
   their HTTP Datagram contents are written to the target.)

   Since proxies that forward QUIC packets do not perform any
   cryptographic integrity check, it is possible that these packets are
   either malformed, replays, or otherwise malicious.  This may result
   in proxy targets rate limiting or decreasing the reputation of a
   given proxy.

9.  IANA Considerations

9.1.  HTTP Header

   This document registers the "Proxy-QUIC-Forwarding" header in the
   "Permanent Message Header Field Names"
   <https://www.iana.org/assignments/message-headers>.

       +-----------------------+----------+--------+---------------+
       | Header Field Name     | Protocol | Status |   Reference   |
       +-----------------------+----------+--------+---------------+
       | Proxy-QUIC-Forwarding |   http   |  exp   | This document |
       +-----------------------+----------+--------+---------------+

                      Figure 3: Registered HTTP Header

9.2.  Capsule Types

   This document registers six new values in the "HTTP Capsule Types"
   registry established by [HTTP-DGRAM].

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            +=====================+==========+===============+
            | Capule Type         | Value    | Specification |
            +=====================+==========+===============+
            | REGISTER_CLIENT_CID | 0xffe300 | This Document |
            +---------------------+----------+---------------+
            | REGISTER_TARGET_CID | 0xffe301 | This Document |
            +---------------------+----------+---------------+
            | ACK_CLIENT_CID      | 0xffe302 | This Document |
            +---------------------+----------+---------------+
            | ACK_TARGET_CID      | 0xffe303 | This Document |
            +---------------------+----------+---------------+
            | CLOSE_CLIENT_CID    | 0xffe304 | This Document |
            +---------------------+----------+---------------+
            | CLOSE_TARGET_CID    | 0xffe305 | This Document |
            +---------------------+----------+---------------+

                    Table 1: Registered Capsule Types

10.  References

10.1.  Normative References

   [CONNECT-UDP]
              Schinazi, D., "Proxying UDP in HTTP", Work in Progress,
              Internet-Draft, draft-ietf-masque-connect-udp-15, 17 June
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              masque-connect-udp-15>.

   [HTTP-DGRAM]
              Schinazi, D. and L. Pardue, "HTTP Datagrams and the
              Capsule Protocol", Work in Progress, Internet-Draft,
              draft-ietf-masque-h3-datagram-11, 17 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-masque-
              h3-datagram-11>.

   [HTTP3]    Bishop, M., "HTTP/3", Work in Progress, Internet-Draft,
              draft-ietf-quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,
              <https://www.rfc-editor.org/rfc/rfc8999>.

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

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

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

   [RFC8941]  Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
              <https://www.rfc-editor.org/rfc/rfc8941>.

10.2.  Informative References

   [QUIC-LB]  Duke, M., Banks, N., and C. Huitema, "QUIC-LB: Generating
              Routable QUIC Connection IDs", Work in Progress, Internet-
              Draft, draft-ietf-quic-load-balancers-14, 11 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              load-balancers-14>.

Acknowledgments

   Thanks to Lucas Pardue, Ryan Hamilton, and Mirja Kuehlewind for their
   inputs on this document.

Authors' Addresses

   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America
   Email: tpauly@apple.com

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

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