]> Apple Inc.

One Apple Park Way Cupertino, California 95014 United States of America tpauly@apple.com

Apple Inc.

One Apple Park Way Cupertino, California 95014 United States of America eric_rosenberg@apple.com

Google LLC

1600 Amphitheatre Parkway Mountain View, California 94043 United States of America dschinazi.ietf@gmail.com

Transport MASQUE quic http datagram udp proxy tunnels quic in quic turtles all the way down masque http-ng 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. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the MASQUE Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction UDP Proxying over HTTP 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 , since QUIC runs over UDP datagrams. 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 separately over the client <-> proxy UDP socket, using a special-purpose transform instead of full QUIC encapsulation. This mode MUST only be used for QUIC short header packets.

Forwarded mode is defined as an optimization to reduce CPU and memory cost to 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.

Forwarded mode does not prevent correlation of client <-> proxy and proxy <-> target packets by an entity that can observe both links. The precise risks depend on the negotiated transform (). See 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 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 , QUIC proxies can proxy all versions of QUIC.

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 when, and only when, they appear in all capitals, as shown here.

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.
• 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.
• Packet Transform: the procedure used to modify packets before they enter the client-proxy link.

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 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.

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 , 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.

Stream Mapping Each client <-> proxy HTTP stream MUST be mapped to a single target-facing socket. 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. Each stream MUST also be associated with a single Packet Transform. 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.

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. (Target-facing socket + Target Connection ID) ]]> Multiple pairs of Connection IDs and client-facing sockets can map to the same target-facing socket. 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.

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. (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.

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 . 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. 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.

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 of 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.

Stateless Resets from the Target Reuse of target-facing sockets is only possible because QUIC connection IDs allow distinguishing packets for multiple QUIC connections received with the same 5-tuple. One exception to this is Stateless Reset packets, in which the connection ID is not used, but rather populated with unpredictable bits followed by a Stateless Reset token, to make it indistinguishable from a regular packet with a short header. In order for the proxy to correctly recognize Stateless Reset packets, the client SHOULD share the Stateless Reset token for each registered Target Connection ID. When the proxy receives a Stateless Reset packet, it can send the packet to the client as a tunnelled datagram. Although Stateless Reset packets look like short header packets, they are not technically short header packets and do not contain negotiated connection IDs, and thus are not eligible for forwarded mode.

Connection ID Capsule Types Proxy awareness of QUIC Connection IDs relies on using capsules () 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 . 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. The REGISTER_CLIENT_CID and REGISTER_TARGET_CID capsule types (see 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 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 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. ACK_CLIENT_CID, CLOSE_CLIENT_CID, and CLOSE_TARGET_CID capsule types are formatted as follows:

Connection ID Capsule Format

Connection ID:

A connection ID being 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_TARGET_CID capsule includes the target-provided connection ID and Stateless Reset Token.

Register Target Connection ID Capsule Format

Connection ID Length

The length of the connection ID being registered, 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 registered whose length is equal to Connection ID Length. This is the real Target or Client Connection ID.

Stateless Reset Token Length

The length of the target-provided Stateless Reset Token.

Stateless Reset Token

The target-provided Stateless Reset token allowing the proxy to correctly recognize Stateless Reset packets to be tunnelled to the client.

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

Virtual Connection ID Capsule Format

Connection ID Length

The length of the connection ID being registered or 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 registered or 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.

Client Behavior A client initiates UDP proxying via a CONNECT request as defined in . 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. "Proxy-QUIC-Forwarding" is an Item Structured Header . Its value MUST be a 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". The client MUST add an "accept-transform" parameter whose value is an sf-string containing the supported packet transforms () in order of descending preference, separated by commas. If the proxy receives a "Proxy-QUIC-Forwarding" header without the "accept-transform" parameters, it MUST ignore the header and respond as if the client had not sent the "Proxy-QUIC-Forwarding" header. 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 support this extension. 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. In order to change the connection ID bytes on the wire, a client can update a previously advertised Virtual Client Connection ID by sending a new REGISTER_CLIENT_CID with the same Connection ID, but a different Virtual Connection ID. Similarly, the client may solicit a new Virtual Target Connection ID by sending a REGISTER_TARGET_CID capsule with a previously registered Target Connection ID. Clients are responsible for changing Virtual Connection IDs when the HTTP stream's network path changes to avoid linkability across network paths. Note that initial REGISTER_CLIENT_CID capsules MAY be sent prior to receiving an HTTP response from the proxy.

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 able to create a unique mapping using the Client Connection ID (). 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.

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.

Sending With Forwarded Mode Support for forwarded mode is determined by the "Proxy-QUIC-Forwarding" header, see . 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. 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.

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 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.

Connection Maintenance in Forwarded Mode When a client and proxy are using forwarded mode, it is possible that there can be long periods of time in which no ack-eliciting packets (see ) are exchanged between the client and proxy. If these periods extend beyond the effective idle timeout for the client-to-proxy QUIC connection (see ), the QUIC connection might be closed by the proxy if the proxy does not use forwarded packets as an explicit liveness signal. To avoid this, clients SHOULD send keepalive packets to the proxy before the idle timeouts would be reached, which can be done using a PING frame or another ack-eliciting frame as described in .

Proxy 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 proxying in forwarded mode, it sets the value to "?1"; otherwise, it sets it to "?0". The proxy MUST include a "transform" parameter whose value is an sf-string indicating the selected transform. If the proxy does not recognize or accept any of the transforms offered by the client, it MUST omit this parameter and set the header field value to "?0", or omit the header entirely. Upon receipt of a REGISTER_CLIENT_CID or REGISTER_TARGET_CID capsule, the proxy validates the registration and tries to establish the appropriate mappings as described in . 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. 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. 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 .

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.

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 reconfigures or removes forwarding rules as the network path between the client and proxy changes. In the event of passive migration, the proxy automatically reconfigures forwarding rules to use the latest active and validated network path for the HTTP stream. In the event of active migration, the proxy removes forwarding rules in order to not send packets with the same connection ID bytes over multiple network paths. After initiating active migration, clients are no longer able to send forwarded mode packets since the proxy will have removed forwarding rules. Clients can proceed with tunnelled mode or can request new forwarding rules via REGISTER_CLIENT_CID and REGISTER_TARGET_CID capsules. Each of these capsules will contain new virtual connection IDs to prevent packets with the same connection ID bytes being used over multiple network paths. Note that the Client Connection ID and Target Connection ID can stay the same while the Virtual Target Connection ID and Virtual Client Connection ID change.

Handling ECN Marking Explicit Congestion Notification marking uses two bits in the IP header to signal congestion from a network to endpoints. When using forwarded mode, the proxy replaces IP headers for packets exchanged between the client and target; these headers can include ECN markings. Proxies SHOULD preserve ECN markings on forwarded packets in both directions, to allow ECN to function end-to-end. If the proxy does not preserve ECN markings, it MUST set ECN marks to zero on the IP headers it generates. Forwarded mode does not create an IP-in-IP tunnel, so the guidance in about transferring ECN markings between inner and outer IP headers does not apply. A proxy MAY additionally add ECN markings to signal congestion being experienced on the proxy itself.

Packet Transforms A packet transform is the procedure applied to encode packets as they are sent on the link between the client and proxy, along with the inverse decode step applied on receipt. Simple transforms can be modeled as a function as follows: Inputs:

1. A QUIC short header packet (after Connection ID remapping).
2. The mode (encode or decode).
3. The direction (client-to-proxy or proxy-to-client).
4. Any configuration information negotiated at startup.

Output:

• A UDP payload that conforms to the QUIC invariants and does not modify the Connection ID.

More complex transform behaviors could have internal state, but no such transforms are presented here. Packet transforms are identified by an IANA-registered name, and negotiated in the HTTP headers (see ). This document defines two initial transforms: the identity transform and the scramble transform.

The identify transform The identity transform does not modify the packet in any way. When this transform is in use, a global passive adversary can trivially correlate pairs of packets that crossed the forwarder, providing a compact proof that a specific client was communicating to a specific target. The identity transform is identified by the value "identity" . Use of this transform is NOT RECOMMENDED if the scramble transform is supported by both the client and the proxy. Implementations MAY choose to not implement or support the identity transform, depending on the use cases and privacy requirements of the deployment.

The scramble transform The scramble transform implements length-preserving unauthenticated re-encryption of QUIC packets while preserving the QUIC invariants. When the scramble transform is in use, a global passive adversary cannot simply compare the packet contents on both sides of the proxy to link the client and target. However, the scramble transform does not defend against analysis of packet sizes and timing, nor does it protect privacy against an active attacker. Deployments that implement the version of the scramble transform defined in this document MUST use the value "scramble-dt". The finalized version is expected to use the reserved value "scramble" . The scramble transform is initialized using a 32-byte random symmetric key. When offering or selecting this transform, the client and server each generate the key that they will use to encrypt scrambled packets and MUST add it to the Proxy-QUIC-Transform header in an sf-binary parameter named "scramble-key". If either side receives a scramble transform without the "scramble-key" parameter, forwarding mode MUST be disabled. This transform relies on the AES-128 block cipher, which is represented by the syntax AES-ECB(key, plaintext_block) as in . The corresponding decryption operation is written here as AES-ECB-inv(key, ciphertext_block). It also uses AES in Counter Mode (, Section 6.5), which is represented by the syntax AES-CTR(key, iv, input) for encryption and decryption (which are identical). In this syntax, iv is an array of 16 bytes containing the initial counter block. The counter is incremented by the standard incrementing function (, Appendix B.1) on the full block width. In brief, the transform applies AES in counter mode (AES-CTR) using an initialization vector drawn from the packet, then encrypts the initialization vector with AES-ECB. The detailed procedure is as follows:

1. Let k1, k2 = scramble_key[:16], scramble_key[16:32].
2. Let L be the Connection ID length.
3. Let cid = packet[1:L+1], i.e., the Connection ID.
4. Let iv = packet[L+1:L+17], i.e., the 16 bytes following the Connection ID.
5. Let ctr_input = packet[0] | packet[L+17:].
6. Let ctr_output = AES-CTR(k1, iv, ctr_input).
7. Let header = ctr_output[0] & 0x7F. This ensures that the Header Form bit is zero, as required by the QUIC invariants (, Section 5.2).
8. Encrypt iv with the block cipher: encrypted_iv = AES-ECB(k2, iv).
9. Produce the output packet as:
   header | cid | encrypted_iv | ctr_output[1:].

The inverse transform operates as follows:

1. Decrypt the AES-CTR initialization vector:
   iv = AES-ECB-inv(k2, packet[L+1:L+17]).
2. Compute the other variables exactly as in the forward transform. (AES-CTR encryption and decryption are identical.)
3. Produce the output: header | cid | iv | ctr_output[1:].

The encryption keys used in this procedure do not depend on the packet contents, so each party only needs to perform AES initialization once for each connection. NOTE: The security of this arrangement relies on every short-header QUIC packet containing a distinct 16 bytes following the Connection ID. This is true for the original ciphersuites of QUICv1, but it is not guaranteed by the QUIC Invariants. Future ciphersuites and QUIC versions could in principle produce packets that are too short or repeat the values at this location. When using the scramble transform, clients MUST NOT offer any configuration that could cause the client or target to violate this requirement.

Example Consider a client that is establishing a new QUIC connection through the proxy. In this example, the client prefers the scramble transform, but also offers the identity transform. 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. :method = CONNECT :protocol = connect-udp :scheme = https :path = /target.example.com/443/ :authority = proxy.example.org proxy-quic-forwarding = ?1; accept-transform=scramble,identity; \ scramble-key=:abc...789=: 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; \ transform=scramble; \ scramble-key=:ABC...321=: 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 /* All Client -> Target QUIC packets must still be encapsulated */ DATAGRAM --------> Quarter Stream ID = 11 Context ID = 0 Payload = Encapsulated QUIC packet /* Forwarded mode packets possible in Target -> Client direction */ <-------- UDP Datagram Payload = Forwarded QUIC SH packet ]]> 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, and processed by the negotiated "scramble" packet transform. However, in the unlikely event that a forwarded packet arrives before the proxy's HTTP response, the client will not know which transform the proxy selected. In this case, the client will have to ignore the packet or buffer it until the HTTP response is received. 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: 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 /* Client -> Target QUIC short header packets may use forwarding mode */ UDP Datagram --------> Payload = Forwarded QUIC SH packet ]]> 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.

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.

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 clients to additional information exposure and deanonymization attacks which need to be carefully considered. Analysis should consider both passive and active attackers which may be global or localized to the network paths used on one side of the proxy. The following sections highlight deanonymization risks with using forwarded mode.

Passive Attacks A passive attacker aims to deanonymize a client by correlating traffic across both sides of the proxy. When using forwarded mode with the identity packet transform (see ), such correlation is trivial by matching a subset of QUIC packet bytes as packets enter the proxy on one side and exit on the other. Packet transforms such as scramble mitigate this by cryptographically preventing such byte comparisons (see ). Regardless of which packet transform is used, both tunnelled and forwarded mode are still vulnerable to size and timing attacks, without the addition of techniques that go beyond the analysis in this document, such as padding and adding chaff packets. Such techniques could be supported in future packet transforms, subject to additional security analysis. Unlike tunnelled mode where packets are fully encapsulated in the client-to-proxy connection, clients using forwarded mode to access multiple target servers over the same client-to-proxy connection expose the number of target servers they are communicating with on each connection to passive attackers that can observe the client-to-proxy traffic. This additional metadata revealed on each packet simplifies size and timing attacks.

Active Attacks An active attacker is an adversary that can inject, modify, drop, and view packets in the network. Some active attacks have different effects between forwarded mode and tunnelled mode, but active attacks can be used to correlate flows in either mode. Both tunnelled mode and forwarded mode (regardless of packet transform) are vulnerable to packet injection in the target-to-client direction. An attacker can inject a burst of packets with a known QUIC Connection ID and see which Connection ID is used for the corresponding burst on the proxy-to-client network path. Packet injection with a known QUIC Connection ID can also happen in the client-to-proxy direction, which only affects forwarded mode since tunnelled mode sends packets within an authenticated and integrity protected QUIC connection to the proxy (see ). None of the packet transforms defined in this document provide integrity protection. Even if a packet transform did provide integrity protection, attackers can inject replayed packets. Protection against replayed packets is similarly provided by QUIC in tunnelled mode, but not provided by any of the forwarded mode packet transforms defined in this document. An active attacker can modify packets in the client-to-proxy direction, which would cause a tunnelling proxy to silently drop packets, while a forwarding proxy would forward the packets. In this way, forwarded mode is less vulnerable to flow recognition based on corrupting a portion of packets in a burst.

IANA Considerations

HTTP Header This document registers the "Proxy-QUIC-Forwarding" header in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" <>.

Registered HTTP Header

Proxy QUIC Forwarding Parameter Names This document establishes a new registry, "Proxy QUIC Forwarding Parameter Names", for parameter names to use with the Proxy-QUIC-Forwarding header field, in <>. Registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [IANA-POLICY]).

Initial Proxy QUIC Forwarding Parameter Names

Packet Transform Names This document establishes a new registry for packet transform names in <> and defines two initial transforms: "identity" and "scramble". Prior to finalization, deployments that implement the version of the scramble transform defined in this document should use the value "scramble-dt". Once the design team proposal is adopted and a new draft is submitted, the wire identifier will become "scramble-XX" where XX is the draft number. Registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [IANA-POLICY]). Initial Packet Transform Names

Transform Name	Description	Specification	Notes
identity	no transformation	This Document	Section
scramble	Reserved (will be used for final version)	This Document	Section

Capsule Types This document registers six new values in the "HTTP Capsule Types" registry established by . Note that the codepoints below will be replaced with lower values before publication. Registered Capsule Types

Capule Type	Value
REGISTER_CLIENT_CID	0xffe400
REGISTER_TARGET_CID	0xffe401
ACK_CLIENT_CID	0xffe402
ACK_TARGET_CID	0xffe403
CLOSE_CLIENT_CID	0xffe404
CLOSE_TARGET_CID	0xffe405

All of these new entries use the following values for these fields:

Status:

provisional (permanent when this document is published)

Reference:

This document

Change Controller:

IETF

Contact:

masque@ietf.org

Notes:

None

References Normative References This document describes how to proxy UDP in HTTP, similar to how the HTTP CONNECT method allows proxying TCP in HTTP. More specifically, this document defines a protocol that allows an HTTP client to create a tunnel for UDP communications through an HTTP server that acts as a proxy. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC and describes how HTTP/2 extensions can be ported to HTTP/3. This document defines the properties of the QUIC transport protocol that are common to all versions of the protocol. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes HTTP Datagrams, a convention for conveying multiplexed, potentially unreliable datagrams inside an HTTP connection. In HTTP/3, HTTP Datagrams can be sent unreliably using the QUIC DATAGRAM extension. When the QUIC DATAGRAM frame is unavailable or undesirable, HTTP Datagrams can be sent using the Capsule Protocol, which is a more general convention for conveying data in HTTP connections. HTTP Datagrams and the Capsule Protocol are intended for use by HTTP extensions, not applications. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. This document describes loss detection and congestion control mechanisms for QUIC. This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. [STANDARDS-TRACK] Informative References Google Microsoft Private Octopus Inc. QUIC address migration allows clients to change their IP address while maintaining connection state. To reduce the ability of an observer to link two IP addresses, clients and servers use new connection IDs when they communicate via different client addresses. This poses a problem for traditional "layer-4" load balancers that route packets via the IP address and port 4-tuple. This specification provides a standardized means of securely encoding routing information in the server's connection IDs so that a properly configured load balancer can route packets with migrated addresses correctly. As it proposes a structured connection ID format, it also provides a means of connection IDs self-encoding their length to aid some hardware offloads. This document redefines how the explicit congestion notification (ECN) field of the IP header should be constructed on entry to and exit from any IP-in-IP tunnel. On encapsulation, it updates RFC 3168 to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301 IPsec ECN processing. On decapsulation, it updates both RFC 3168 and RFC 4301 to add new behaviours for previously unused combinations of inner and outer headers. The new rules ensure the ECN field is correctly propagated across a tunnel whether it is used to signal one or two severity levels of congestion; whereas before, only one severity level was supported. Tunnel endpoints can be updated in any order without affecting pre-existing uses of the ECN field, thus ensuring backward compatibility. Nonetheless, operators wanting to support two severity levels (e.g., for pre-congestion notification -- PCN) can require compliance with this new specification. A thorough analysis of the reasoning for these changes and the implications is included. In the unlikely event that the new rules do not meet a specific need, RFC 4774 gives guidance on designing alternate ECN semantics, and this document extends that to include tunnelling issues. [STANDARDS-TRACK] This document defines the properties of the QUIC transport protocol that are common to all versions of the protocol. This document describes how Transport Layer Security (TLS) is used to secure QUIC.

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