QUIC-Aware Proxying Using HTTP

1.1. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

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-to-proxy 4-tuple: the UDP 4-tuple (client IP address, client UDP port, proxy IP address, proxy UDP port) used to communicate between the client and the proxy.¶

• Proxy-to-target 4-tuple: the UDP 4-tuple (proxy IP address, proxy UDP port, target IP address, target UDP port) used to communicate between the proxy and the target.¶

• Client Connection ID (CID): 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 (CID): 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 Connection ID (VCID): a fake QUIC Connection ID chosen by the proxy that is used on the client-to-proxy 4-tuple in forwarded mode.¶

• Client VCID: a VCID used by the proxy to send forwarded packets from the target to the client.¶

• Target VCID: a VCID used by the client to send forwarded packets to the target via the proxy.¶

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

2.1. Connection ID Awareness

For a proxy to be aware of proxied QUIC connection IDs, it needs to know and correlate three values:¶

1. The HTTP stream used to proxy a client-to-target QUIC connection¶

2. The client-chosen connection ID on the client-to-target QUIC connection, or the "client CID"¶

3. The target-chosen connection ID on the client-to-target QUIC connection, or the "target CID"¶

For example, consider a proxy using HTTP/3 that has two clients (A and B) connected simultaneously, each client coming from a different IP address and port. Each client makes a request to proxy a UDP flow to "target.example.net" on port 443. If the proxy knows that client A's connection to the target has negotiated a client CID AAAA0000 and a target CID 0000AAAA, and client B's connection to the target has negotiated a client CID BBBB0000 and a target CID 0000BBBB, then the proxy would be able to use the same proxy-to-target 4-tuple for both connections, because it can route packets from the target to the correct client based on CID AAAA0000 or BBBB0000.¶

In order to share a proxy-to-target 4-tuple between multiple proxied connections, the proxy MUST guarantee that the client CIDs do not conflict. See Section 3.3.1 for more discussion of handlng CID conflicts.¶

2.2. Virtual Connection IDs

Virtual Connection IDs (VCIDs) are QUIC Connection IDs used on the link between a client and proxy that do not belong to the QUIC connection between the client and proxy, but instead are aliases for particular client-to-target connections. VCIDs are only used in forwarded mode. They are established using HTTP capsules [HTTP-CAPSULES] as described in Section 3.¶

For example, consider a proxy using HTTP/3 that has a single client connected to it. The client-to-proxy QUIC connection has CCCC0000 as the client CID and 0000CCCC as the proxy CID. The client has connected to a single target through the proxy, and the client-to-target QUIC connection has CCCC1111 as the client CID and 1111CCCC as the target CID. In order to use forwarded mode, the proxy assigns VCIDs to use on the client-to-proxy link to represent the client-to-target connection. In this case, the VCIDs could be CCCC2222 for the client's VCID and 2222CCCC for the proxy's VCID that it uses to forward to the target.¶

In order for a proxy to correctly route packets using VCIDs from client-to-target and target-to-client, the proxy MUST guarantee that the mappings between VCIDs, CIDs, and 4-tuples are unique. Specifically, in order to route packets sent by the client, the proxy needs to be able to observe the VCID and the client-to-proxy 4-tuple, and map them to a specific target CID and proxy-to-target 4-tuple. In order to route packets sent by a target, the proxy needs to be able to observe the client CID and the proxy-to-target 4-tuple, and map them to a specific VCID and client-to-proxy 4-tuple. Since proxies choose the VCID values, they can ensure that the VCIDs are distinguishable.¶

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 CID, load balancers may not have the necessary information to route packets to the correct proxy. The target VCID is a VCID chosen by the proxy that the client uses when sending forwarded mode packets. The proxy replaces the target VCID with the target CID 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 client VCID is a connection ID chosen by the proxy that the proxy uses when sending forwarded mode packets. The proxy replaces the client CID with the client VCID 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 CID allows the connection ID bytes to change on the wire without requiring the connection IDs on the client to target connection change. To reduce the likelihood of connection ID conflicts, the proxy MUST choose a client VCID that is at least as long as the original client CID. Similarly, clients multiplexing connections on the same UDP 4-tuple SHOULD choose a client CID that's sufficiently long to reduce the likelihood of a conflict with the proxy-chosen client VCID. For proxies that share a client-facing IP and port with other proxies, support target-facing port sharing, and cannot guarantee there are no proxies being proxied to, the client VCID MUST be constructed such that it is unpredictable to the client or to guarantee no conflicts among all proxies sharing an IP address and port. For such proxies, a non-empty client VCID MUST NOT be equal to the original client CID. See Section 8 for more discussion on client VCID construction.¶

Clients and Proxies not implementing forwarded mode do not need to consider VCIDs since all client-to-target datagrams will be encapsulated within the client-to-proxy connection.¶

2.3. Negotiating Modes

In order to support QUIC-aware proxying, both clients and proxies need to support capsules [HTTP-CAPSULES], which is indicated by including the "Capsule-Protocol" header field in requests and responses. If this header field is not included, none of the functionality in this document can be used.¶

  capsule-protocol = ?1

To permit the proxy to share target-facing ports, the client needs to include the "Proxy-QUIC-Port-Sharing" header field, as defined in Section 2.3.2. This indicates that the proxy may share target-facing 4-tuples concurrently with other QUIC connections. Clients that do not want the target-facing 4-tuple shared will not include this header or will provide it with a value of "?0".¶

  proxy-quic-port-sharing = ?1

To support forwarded mode, both clients and proxies need to include the "Proxy-QUIC-Forwarding" header field, as defined in Section 2.3.1. This indicates support for forwarded mode, and allows negotiation of packet transforms to apply when forwarding (Section 4.3), along with any parameters for specific transforms. Clients or proxies that don't support forwarded mode will not include this header field.¶

  proxy-quic-forwarding = ?1; accept-transform="scramble,identity"; \
      scramble-key=:abc...789=:

If neither header is supplied with a value of "?1", none of the functionality in this document can be used and handling reduces to normal connect-udp.¶

After negotiating support with header fields, clients and proxies use the capsules defined in Section 3 to communicate information about CIDs and VCIDs.¶

3.1. Capsule Types and Formats

Register Client CID Capsule {
  Type (i) = see {{iana}} for the value of the capsule type
  Length (i),
  Reason (i),
  Connection ID (0..2040),
}

Register Target CID Capsule {
  Type (i) = see {{iana}} for the value of the capsule type
  Length (i),
  Reason (i),
  Connection ID Length (i)
  Connection ID (0..2040),
  Stateless Reset Token Length (i),
  Stateless Reset Token (..),
}

Acknowledge Client CID Capsule {
  Type (i) = see {{iana}} for the value of the capsule type
  Length (i)
  Connection ID Length (i)
  Connection ID (0..2040),
  Virtual Connection ID Length (i)
  Virtual Connection ID (0..2040),
}

Acknowledge Target CID Capsule {
  Type (i) = see {{iana}} for the value of the capsule type
  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 (..),
}

Acknowledge Client VCID Capsule {
  Type (i) = see {{iana}} for the value of the capsule type
  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 (..),
}

Reject CID Capsule {
  Type (i) = see {{iana}} for the values of the capsule types
  Length (i),
  Reason (i),
  Connection ID (0..2040),
}

Close CID Capsule {
  Type (i) = see {{iana}} for the values of the capsule types
  Length (i),
  Reason (i),
  Connection ID (0..2040),
}

Maximum Connection IDs Capsule {
  Type (i) = see {{iana}} for the value of the capsule type
  Length (i)
  Maximum Connection IDs (i)
}

3.2. Client Considerations

The client sends a REGISTER_CLIENT_CID capsule before it advertises a new client CID to the target, and a REGISTER_TARGET_CID capsule when it has received a new target CID for the target. In order to change the connection ID bytes on the wire, a client can solicit new virtual connection IDs by re-registering the same connection IDs. Note that re-registrations consume sequence numbers like any other registration. The client may solicit a new target VCID by sending a REGISTER_TARGET_CID capsule with a previously registered target CID. Similarly, the client may solicit a new client VCID by sending a REGISTER_CLIENT_CID with a previously registered client CID. The client MUST acknowledge the new client VCID with an ACK_CLIENT_VCID capsule or close the registration. The proxy MUST NOT send in forwarded mode until ACK_CLIENT_VCID has been received. 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.¶

If the client receives a VCID it cannot use, it may re-register the same client CID with a reason code indicating why the previous VCID was unusable. The proxy SHOULD use this information to select a more suitable VCID. If the reason is TOO_SHORT, the proxy MUST either select a longer VCID or close the registration. If the reason is CONFLICT, the proxy MUST select a different VCID.¶

Connection ID registrations are subject to a proxy-advertised limit. Each registration has a corresponding sequence number. The client MUST NOT send a registration capsule with a sequence number greater than what the proxy advertises via the MAX_CONNECTION_IDS capsule. The initial MAX_CONNECTION_IDS value is 1, allowing both sequence numbers 0 and 1 for a total of two registrations without receiving a MAX_CONNECTION_IDS capsule from the proxy.¶

Clients that cannot register new connection IDs within a reasonable time due to the MAX_CONNECTION_IDS limit SHOULD abort the proxied connection by resetting the HTTP stream with error code NO_ERROR. This may happen, for example, if the target server sends a NEW_CONNECTION_ID frame with Sequence Number and Retire Prior To equal to the same value.¶

Clients can cease receiving with forwarded mode over an existing tunnel while retaining the same client-to-target connection by creating a new tunnel with "Proxy-QUIC-Forwarding" set to "?0" and migrating the client-to-target connection.¶

3.3. Proxy Considerations

The proxy MUST reply to each REGISTER_CLIENT_CID capsule with either an ACK_CLIENT_CID or REJECT_CLIENT_CID capsule containing the Connection ID that was in the registration capsule. Capsules MUST be responded to in the order in which they are received.¶

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

When a proxy receives a REGISTER_CLIENT_CID with a non-zero reason code, it indicates the client is requesting a new VCID because the previous one was unusable. The proxy SHOULD attempt to address the issue indicated by the reason code when selecting the new VCID.¶

The proxy then determines the proxy-to-target 4-tuple 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 proxy-to-target 4-tuple to the authority. The proxy-to-target 4-tuple might already be open due to a previous request from this client, or another. If the 4-tuple is not already created, the proxy creates a new one. Proxies can choose to reuse proxy-to-target 4-tuples across multiple UDP proxying requests, or have a unique proxy-to-target 4-tuple for every UDP proxying request. If the client did not send a value of "?1" for the "Proxy-QUIC-Port-Sharing" header, port reuse is not permitted and the proxy MUST allocate a new UDP 4-tuple.¶

If a proxy reuses proxy-to-target 4-tuples, it SHOULD store which authorities (which could be a domain name or IP address literal) are being accessed over a particular proxy-to-target 4-tuple 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.¶

Proxy-to-target 4-tuples 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 proxy-to-target 4-tuple used for proxying QUIC that does not correspond to a known CID 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 CID back to the client using. If the pair of this client CID and the selected proxy-to-target 4-tuple does not create a conflict, the proxy creates the mapping and responds with an ACK_CLIENT_CID capsule. If forwarded mode is enabled, the capsule contains a proxy-chosen client VCID. If forwarded mode is enabled, and after receiving an ACK_CLIENT_VCID capsule from the client, any packets received by the proxy from the proxy-to-target 4-tuple that match the client CID can be sent to the client after the proxy has replaced the CID with the client VCID and executed the negotiated transform (Section 4.3). If forwarded mode is not supported, the proxy MUST NOT send a client VCID by setting the length to zero. 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 REJECT_CLIENT_CID capsule with the CONFLICT reason code.¶

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 target VCID for the client to use when sending packets in forwarded mode. If forwarded mode is not supported, the proxy MUST NOT send a target VCID by setting the length to zero. If forwarded mode is supported, the proxy MUST use a target VCID that does not introduce a conflict with any other Connection ID on the client-to-proxy 4-tuple. 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-to-proxy 4-tuple that use the target VCID using the correct proxy-to-target 4-tuple after first rewriting the target VCID to be the correct target CID and executing the negotiated transform.¶

Proxies MUST choose unpredictable client and target VCIDs to avoid forwarding loop attacks.¶

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 target VCIDs. Packets sent by the client that are forwarded SHOULD be considered as activity for restarting QUIC's Idle Timeout [QUIC].¶

In order to permit the client to change client-to-target connection IDs, the proxy SHOULD send MAX_CONNECTION_IDS capsules allowing the client additional connection ID registrations.¶

4.1. Sending With Forwarded Mode

Support for forwarded mode is determined by the "Proxy-QUIC-Forwarding" header, see Section 2.3.1.¶

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 CID to request the ability to forward packets.¶

The client MUST wait for an ACK_TARGET_CID capsule that contains the echoed connection ID and target VCID 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 CID registration is rejected, for example with a REJECT_TARGET_CID capsule, it MUST NOT forward packets to the requested target CID, but only use tunnelled mode. The registration 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 target VCID in the QUIC short header, using the same 4-tuple 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 client VCID in the QUIC short header, using the same 4-tuple 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.¶

4.2. 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 CID for which it has a valid mapping.¶

Once a client has sent an ACK_CLIENT_VCID capsule to the proxy, it MUST be prepared to receive forwarded short header packets on the 4-tuple between itself and the proxy for the specified client VCID.¶

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 client VCID with the real client CID before processing the packet further.¶

4.3. 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. When sending packets, the packet transform is applied after replacing a CID with a VCID. When receiving packets, the packet transform is applied before replacing a VCID with a CID.¶

Simple transforms can be modeled as a function as follows:¶

Inputs:¶

1. A QUIC short header packet with a VCID.¶

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 [RFC8999] 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 Section 2.3.1). This document defines two initial transforms: the identity transform and the scramble transform.¶

4.4. 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 Section 2 of [QUIC-RETRANSMISSION]) are exchanged between the client and proxy. If these periods extend beyond the effective idle timeout for the client-to-proxy QUIC connection (see Section 10.1 of [QUIC]), 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 Section 10.1.1 of [QUIC].¶

4.5. 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 forwarded packets on a new path prior to path validation.¶

Specifically, the proxy MUST NOT forward packets to an unvalidated client address. 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 (i.e., the mappings for VCIDs) as the network path between the client and proxy changes.¶

In the event of passive migration, the proxy automatically reconfigures forwarding rules according to the HTTP stream's active network path and its validation status. Specifically, the proxy MUST NOT forward packets to the client until the new network path is validated. The proxy SHOULD continue forwarding packets to the target to avoid disruption of the client-to-target forwarding packet flow.¶

In the event of active migration, the proxy MUST remove forwarding rules in order to not send packets with the same connection ID bytes over multiple network paths, and MUST NOT apply the new forwarding rule in the target-to-client direction until the new network path is validated. 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. Requesting new forwarding rules in this way (registering the same client-to-target CIDs again) is a re-registration and does increment the sequence number and is dependent on sufficient MAX_CONNECTION_IDs. Each of the acknowledging 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 CID and target CID can stay the same while the target VCID and client VCID change. Importantly, the client does not send a CLOSE_CLIENT_CID capsule because that would also remove the registration of the CID for the purpose of port sharing, potentially breaking the tunnelled path.¶

4.6. Handling Server Preferred Addresses

QUIC allows servers to tell clients about a preferred address the server would like to use (Section 9.6 of [QUIC]). When this happens, the client can migrate to the preferred address.¶

When a client using a proxy wants to migrate to the preferred address of the target server, it needs to create a new UDP proxying request to the proxy (using the method defined in [CONNECT-UDP]) and using the preferred IP address of the target as the host to which to connect. This is the behavior clients using a proxy will have regardless of using the QUIC-aware mechanisms defined in this document. From the proxy's perspective, the migrating request is separate from the original request.¶

Note that clients can be aware of the target address being used for the original proxy request by looking at next-hop parameter on a Proxy-Status header sent by the proxy in its response (Section 2.1.2 of [RFC9209]). This allows a client to know if it ought to migrate to the preferred address, or is already connected to the preferred address. To support the ability of clients to do this, proxies implementing this specification SHOULD send the Proxy-Status header in responses and include the next-hop parameter.¶

4.7. Handling ECN Marking

Explicit Congestion Notification marking [ECN] 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 [ECN-TUNNEL] 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.¶

Forwarding ECN markings introduces certain active attacks. See Section 8.2 for more detail.¶

4.8. Stateless Resets for Forwarded Mode QUIC Packets

While the lifecycle of forwarding rules are bound to the lifecycle of the client-to-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-to-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-to-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-to-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 target VCID, it can send a stateless reset packet to quickly notify the client that the client-to-target connection is broken.¶

8.1. 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 Section 4.3.1), 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 Section 4.3.2).¶

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.¶

8.2. 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.¶

Forwarded mode is vulnerable to some active attacks that tunneled mode is not. For example, 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 [RFC9001]). 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. Similarly, Forwarded mode packets are vulnerable to active attacks when [ECN] markings are forwarded. Specifically, an attacker could embed a signal over a series of packets by clearing or setting ECN bits. This attack is possible without injecting, dropping, or modifying the QUIC packet, but instead modifying the packet's IP header.¶

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.¶

8.3. Connection ID Registration Attacks

Chaining of proxies using forwarded mode introduces the risk of forwarding loop attacks. Proxies are known to be vulnerable to one such forwarding loop attack when sharing an IP address and port with other proxies, supporting target-facing port sharing, and proxying to other proxies. Preventing client VCID conflicts across proxies sharing an IP address and port mitigates one such forwarding loop attack. Conflicts can be avoided by partitioning the client VCID space across proxies, using sufficiently long and random values, or by other means.¶

9.1. HTTP Header Fields

This document registers the "Proxy-QUIC-Forwarding" and "Proxy-QUIC-Port-Sharing" header fields in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" <https://www.iana.org/assignments/http-fields>.¶

    +-------------------------+-----------+-----------------+---------------+----------+
    |      Field Name         |  Status   | Structured Type |   Reference   | Comments |
    +-------------------------+-----------+-----------------+---------------+----------+
    | Proxy-QUIC-Forwarding   | permanent |      Item       | This document |   None   |
    | Proxy-QUIC-Port-Sharing | permanent |      Item       | This document |   None   |
    +-------------------------+-----------+-----------------+---------------+----------+

9.2. 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 <https://www.iana.org/assignments/masque/masque.xhtml>. Registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [IANA-POLICY]).¶

    +-----------------------+-------------------------------------+---------------+--------------------------------+
    | Parameter Name        | Description                         | Reference     | Notes                          |
    +-----------------------+-------------------------------------+---------------+--------------------------------+
    | accept-transform      | contains supported transforms       | This document | Section {{forwarding-header}}  |
    +-----------------------+-------------------------------------+---------------+--------------------------------+
    | transform             | indicates selected transforms       | This document | Section {{forwarding-header}}  |
    +-----------------------+-------------------------------------+---------------+--------------------------------+
    | scramble-key          | contains key for scramble transform | This document | Section {{scramble-transform}} |
    +-----------------------+-------------------------------------+---------------+--------------------------------+

9.3. Packet Transform Names

This document establishes a new registry for packet transform names in <https://www.iana.org/assignments/masque/masque.xhtml> 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]).¶

9.4. CID Capsule Reason Codes

This document establishes a new registry, "CID Capsule Reason Codes", for reason codes used in REGISTER_CLIENT_CID, REGISTER_TARGET_CID, REJECT_CLIENT_CID, REJECT_TARGET_CID, CLOSE_CLIENT_CID, and CLOSE_TARGET_CID capsules, in <https://www.iana.org/assignments/masque/masque.xhtml>. This registry governs a 62-bit space and operates under the QUIC registration policy documented in Section 22.1 of [QUIC]. This new registry includes the common set of fields listed in Section 22.1.1 of [QUIC]. In addition to those common fields, all registrations in this registry MUST include a "Name" field that contains a short name or label for the Reason.¶

Permanent registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [IANA-POLICY]), except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Sections 4.9 and 4.10 of [IANA-POLICY].¶

9.5. Capsule Types

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

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¶

10.1. Normative References

[CONNECT-UDP]

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

[ECN]

Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, <https://www.rfc-editor.org/rfc/rfc3168>.

[HTTP3]

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

[HTTP-CAPSULES]

Schinazi, D. and L. Pardue, "HTTP Datagrams and the Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August 2022, <https://www.rfc-editor.org/rfc/rfc9297>.

[IANA-POLICY]

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

[INVARIANTS]

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

[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>.

[QUIC-RETRANSMISSION]

Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.

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

[SP800-38A]

Dworkin, M., "Recommendation for Block Cipher Modes of Operation: Methods and Techniques", 1 December 2001, <https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication800-38a.pdf>.

10.2. Informative References

[ECN-TUNNEL]

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

[QUIC-GREASE]

Schinazi, D. and L. Pardue, "HTTP Datagrams and the Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August 2022, <https://www.rfc-editor.org/rfc/rfc9297>.

[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-21, 27 August 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-quic-load-balancers-21>.

[RFC8999]

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

[RFC9001]

Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, <https://www.rfc-editor.org/rfc/rfc9001>.

[RFC9209]

Nottingham, M. and P. Sikora, "The Proxy-Status HTTP Response Header Field", RFC 9209, DOI 10.17487/RFC9209, June 2022, <https://www.rfc-editor.org/rfc/rfc9209>.