3.1. Provide DNS Privacy

DoT [RFC7858] defines how to mitigate some of the issues described in "DNS Privacy Considerations" [RFC9076] by specifying how to transmit DNS messages over TLS. The "Usage Profiles for DNS over TLS and DNS over DTLS" [RFC8310] specify Strict and Opportunistic usage profiles for DoT including how stub resolvers can authenticate recursive resolvers.¶

QUIC connection setup includes the negotiation of security parameters using TLS, as specified in "Using TLS to Secure QUIC" [RFC9001], enabling encryption of the QUIC transport. Transmitting DNS messages over QUIC will provide essentially the same privacy protections as DoT [RFC7858] including Strict and Opportunistic usage profiles [RFC8310]. Further discussion on this is provided in Section 7.¶

3.2. Design for Minimum Latency

QUIC is specifically designed to reduce protocol-induced delays, with features such as:¶

1. Support for 0-RTT data during session resumption.¶
2. Support for advanced packet-loss recovery procedures as specified in "QUIC Loss Detection and Congestion Control" [RFC9002].¶
3. Mitigation of head-of-line blocking by allowing parallel delivery of data on multiple streams.¶

This mapping of DNS to QUIC will take advantage of these features in three ways:¶

1. Optional support for sending 0-RTT data during session resumption (the security and privacy implications of this are discussed in later sections).¶
2. Long-lived QUIC connections over which multiple DNS transactions are performed, generating the sustained traffic required to benefit from advanced recovery features.¶
3. Mapping of each DNS Query/Response transaction to a separate stream, to mitigate head-of-line blocking. This enables servers to respond to queries "out of order". It also enables clients to process responses as soon as they arrive, without having to wait for in-order delivery of responses previously posted by the server.¶

These considerations are reflected in the mapping of DNS traffic to QUIC streams in Section 4.2.¶

3.3. Middlebox Considerations

Using QUIC might allow a protocol to disguise its purpose from devices on the network path using encryption and traffic analysis resistance techniques like padding, traffic pacing, and traffic shaping. This specification does not include any measures that are designed to avoid such classification; the padding mechanisms defined in Section 5.4 are intended to obfuscate the specific records contained in DNS queries and responses, but not the fact that this is DNS traffic. Consequently, firewalls and other middleboxes might be able to distinguish DoQ from other protocols that use QUIC, like HTTP, and apply different treatment.¶

The lack of measures in this specification to avoid protocol classification is not an endorsement of such practices.¶

3.4. No Server-Initiated Transactions

As stated in Section 1, this document does not specify support for server-initiated transactions within established DoQ connections. That is, only the initiator of the DoQ connection may send queries over the connection.¶

DSO does support server-initiated transactions within existing connections. However, DoQ as defined here does not meet the criteria for an applicable transport for DSO because it does not guarantee in-order delivery of messages; see Section 4.2 of [RFC8490].¶

4.1. Connection Establishment

DoQ connections are established as described in the QUIC transport specification [RFC9000]. During connection establishment, DoQ support is indicated by selecting the Application-Layer Protocol Negotiation (ALPN) token "doq" in the crypto handshake.¶

4.2. Stream Mapping and Usage

The mapping of DNS traffic over QUIC streams takes advantage of the QUIC stream features detailed in Section 2 of [RFC9000], the QUIC transport specification.¶

DNS query/response traffic [RFC1034] [RFC1035] follows a simple pattern in which the client sends a query, and the server provides one or more responses (multiple responses can occur in zone transfers).¶

The mapping specified here requires that the client select a separate QUIC stream for each query. The server then uses the same stream to provide all the response messages for that query. In order for multiple responses to be parsed, a 2-octet length field is used in exactly the same way as the 2-octet length field defined for DNS over TCP [RFC1035]. The practical result of this is that the content of each QUIC stream is exactly the same as the content of a TCP connection that would manage exactly one query.¶

All DNS messages (queries and responses) sent over DoQ connections MUST be encoded as a 2-octet length field followed by the message content as specified in [RFC1035].¶

The client MUST select the next available client-initiated bidirectional stream for each subsequent query on a QUIC connection, in conformance with the QUIC transport specification [RFC9000]. Packet losses and other network events might cause queries to arrive in a different order. Servers SHOULD process queries as they arrive, as not doing so would cause unnecessary delays.¶

The client MUST send the DNS query over the selected stream and MUST indicate through the STREAM FIN mechanism that no further data will be sent on that stream.¶

The server MUST send the response(s) on the same stream and MUST indicate, after the last response, through the STREAM FIN mechanism that no further data will be sent on that stream.¶

Therefore, a single DNS transaction consumes a single bidirectional client-initiated stream. This means that the client's first query occurs on QUIC stream 0, the second on 4, and so on (see Section 2.1 of [RFC9000]).¶

Servers MAY defer processing of a query until the STREAM FIN has been indicated on the stream selected by the client.¶

Servers and clients MAY monitor the number of "dangling" streams. These are open streams where the following events have not occurred after implementation-defined timeouts:¶

• the expected queries or responses have not been received or,¶
• the expected queries or responses have been received but not the STREAM FIN¶

Implementations MAY impose a limit on the number of such dangling streams. If limits are encountered, implementations MAY close the connection.¶

4.3. DoQ Error Codes

The following error codes are defined for use when abruptly terminating streams, for use as application protocol error codes when aborting reading of streams, or for immediately closing connections:¶

DOQ_NO_ERROR (0x0):

No error. This is used when the connection or stream needs to be closed, but there is no error to signal.¶

DOQ_INTERNAL_ERROR (0x1):

The DoQ implementation encountered an internal error and is incapable of pursuing the transaction or the connection.¶

DOQ_PROTOCOL_ERROR (0x2):

The DoQ implementation encountered a protocol error and is forcibly aborting the connection.¶

DOQ_REQUEST_CANCELLED (0x3):

A DoQ client uses this to signal that it wants to cancel an outstanding transaction.¶

DOQ_EXCESSIVE_LOAD (0x4):

A DoQ implementation uses this to signal when closing a connection due to excessive load.¶

DOQ_UNSPECIFIED_ERROR (0x5):

A DoQ implementation uses this in the absence of a more specific error code.¶

DOQ_ERROR_RESERVED (0xd098ea5e):

An alternative error code used for tests.¶

See Section 8.4 for details on registering new error codes.¶

4.4. Connection Management

Section 10 of [RFC9000], the QUIC transport specification, specifies that connections can be closed in three ways:¶

• idle timeout¶
• immediate close¶
• stateless reset¶

Clients and servers implementing DoQ SHOULD negotiate use of the idle timeout. Closing on idle timeout is done without any packet exchange, which minimizes protocol overhead. Per Section 10.1 of [RFC9000], the QUIC transport specification, the effective value of the idle timeout is computed as the minimum of the values advertised by the two endpoints. Practical considerations on setting the idle timeout are discussed in Section 5.5.2.¶

Clients SHOULD monitor the idle time incurred on their connection to the server, defined by the time spent since the last packet from the server has been received. When a client prepares to send a new DNS query to the server, it SHOULD check whether the idle time is sufficiently lower than the idle timer. If it is, the client SHOULD send the DNS query over the existing connection. If not, the client SHOULD establish a new connection and send the query over that connection.¶

Clients MAY discard their connections to the server before the idle timeout expires. A client that has outstanding queries SHOULD close the connection explicitly using QUIC's CONNECTION_CLOSE mechanism and the DoQ error code DOQ_NO_ERROR.¶

Clients and servers MAY close the connection for a variety of other reasons, indicated using QUIC's CONNECTION_CLOSE. Client and servers that send packets over a connection discarded by their peer might receive a stateless reset indication. If a connection fails, all the in-progress transactions on that connection MUST be abandoned.¶

4.5. Session Resumption and 0-RTT

A client MAY take advantage of the session resumption and 0-RTT mechanisms supported by QUIC transport [RFC9000] and QUIC TLS [RFC9001] if the server supports them. Clients SHOULD consider potential privacy issues associated with session resumption before deciding to use this mechanism and specifically evaluate the trade-offs presented in the various sections of this document. The privacy issues are detailed in Sections 7.1 and 7.2, and the implementation considerations are discussed in Section 5.5.3.¶

The 0-RTT mechanism MUST NOT be used to send DNS requests that are not "replayable" transactions. In this specification, only transactions that have an OPCODE of QUERY or NOTIFY are considered replayable; therefore, other OPCODES MUST NOT be sent in 0-RTT data. See Appendix A for a detailed discussion of why NOTIFY is included here.¶

Servers MAY support session resumption, and MAY do that with or without supporting 0-RTT, using the mechanisms described in Section 4.6.1 of [RFC9001]. Servers supporting 0-RTT MUST NOT immediately process non-replayable transactions received in 0-RTT data but instead MUST adopt one of the following behaviors:¶

• Queue the offending transaction and only process it after the QUIC handshake has been completed, as defined in Section 4.1.1 of [RFC9001].¶
• Reply to the offending transaction with a response code REFUSED and an Extended DNS Error Code (EDE) "Too Early" using the extended RCODE mechanisms defined in [RFC6891] and the extended DNS errors defined in [RFC8914]; see Section 8.3.¶
• Close the connection with the error code DOQ_PROTOCOL_ERROR.¶

4.6. Message Sizes

DoQ queries and responses are sent on QUIC streams, which in theory can carry up to 2⁶² bytes. However, DNS messages are restricted in practice to a maximum size of 65535 bytes. This maximum size is enforced by the use of a 2-octet message length field in DNS over TCP [RFC1035] and DoT [RFC7858], and by the definition of the "application/dns-message" for DoH [RFC8484]. DoQ enforces the same restriction.¶

The Extension Mechanisms for DNS (EDNS(0)) [RFC6891] allow peers to specify the UDP message size. This parameter is ignored by DoQ. DoQ implementations always assume that the maximum message size is 65535 bytes.¶

5.1. Authentication

For the stub to recursive scenario, the authentication requirements are the same as described in DoT [RFC7858] and "Usage Profiles for DNS over TLS and DNS over DTLS" [RFC8310]. [RFC8932] states that DNS privacy services SHOULD provide credentials that clients can use to authenticate the server. Given this, and to align with the authentication model for DoH, DoQ stubs SHOULD use a Strict usage profile. Client authentication for the encrypted stub to recursive scenario is not described in any DNS RFC.¶

For zone transfer, the authentication requirements are the same as described in [RFC9103].¶

For the recursive to authoritative scenario, authentication requirements are unspecified at the time of writing and are the subject of ongoing work in the DPRIVE WG.¶

5.2. Fallback to Other Protocols on Connection Failure

If the establishment of the DoQ connection fails, clients MAY attempt to fall back to DoT and then potentially cleartext, as specified in DoT [RFC7858] and "Usage Profiles for DNS over TLS and DNS over DTLS" [RFC8310], depending on their usage profile.¶

DNS clients SHOULD remember server IP addresses that don't support DoQ. Mobile clients might also remember the lack of DoQ support by given IP addresses on a per-context basis (e.g., per network or provisioning domain).¶

Timeouts, connection refusals, and QUIC handshake failures are indicators that a server does not support DoQ. Clients SHOULD NOT attempt DoQ queries to a server that does not support DoQ for a reasonable period (such as one hour per server). DNS clients following an out-of-band key-pinned usage profile [RFC7858] MAY be more aggressive about retrying after DoQ connection failures.¶

5.3. Address Validation

Section 8 of [RFC9000], the QUIC transport specification, defines Address Validation procedures to avoid servers being used in address amplification attacks. DoQ implementations MUST conform to this specification, which limits the worst-case amplification to a factor 3.¶

DoQ implementations SHOULD consider configuring servers to use the Address Validation using Retry Packets procedure defined in Section 8.1.2 of [RFC9000], the QUIC transport specification. This procedure imposes a 1-RTT delay for verifying the return routability of the source address of a client, similar to the DNS Cookies mechanism [RFC7873].¶

DoQ implementations that configure Address Validation using Retry Packets SHOULD implement the Address Validation for Future Connections procedure defined in Section 8.1.3 of [RFC9000], the QUIC transport specification. This defines how servers can send NEW_TOKEN frames to clients after the client address is validated in order to avoid the 1-RTT penalty during subsequent connections by the client from the same address.¶

5.4. Padding

Implementations MUST protect against the traffic analysis attacks described in Section 7.5 by the judicious injection of padding. This could be done either by padding individual DNS messages using the EDNS(0) Padding Option [RFC7830] or by padding QUIC packets (see Section 19.1 of [RFC9000]).¶

In theory, padding at the QUIC packet level could result in better performance for the equivalent protection, because the amount of padding can take into account non-DNS frames such as acknowledgements or flow control updates, and also because QUIC packets can carry multiple DNS messages. However, applications can only control the amount of padding in QUIC packets if the implementation of QUIC exposes adequate APIs. This leads to the following recommendations:¶

• If the implementation of QUIC exposes APIs to set a padding policy, DoQ SHOULD use that API to align the packet length to a small set of fixed sizes.¶
• If padding at the QUIC packet level is not available or not used, DoQ MUST ensure that all DNS queries and responses are padded to a small set of fixed sizes, using the EDNS(0) padding extension as specified in [RFC7830].¶

Implementations might choose not to use a QUIC API for padding if it is significantly simpler to reuse existing DNS message padding logic that is applied to other encrypted transports.¶

In the absence of a standard policy for padding sizes, implementations SHOULD follow the recommendations of the Experimental status "Padding Policies for Extension Mechanisms for DNS (EDNS(0))" [RFC8467]. While Experimental, these recommendations are referenced because they are implemented and deployed for DoT and provide a way for implementations to be fully compliant with this specification.¶

5.5. Connection Handling

"DNS Transport over TCP - Implementation Requirements" [RFC7766] provides updated guidance on DNS over TCP, some of which is applicable to DoQ. This section provides similar advice on connection handling for DoQ.¶

5.6. Processing Queries in Parallel

As specified in Section 7 of [RFC7766] "DNS Transport over TCP - Implementation Requirements", resolvers are RECOMMENDED to support the preparing of responses in parallel and sending them out of order. In DoQ, they do that by sending responses on their specific stream as soon as possible, without waiting for availability of responses for previously opened streams.¶

5.7. Zone Transfer

[RFC9103] specifies zone transfer over TLS (XoT) and includes updates to [RFC1995] (IXFR), [RFC5936] (AXFR), and [RFC7766]. Considerations relating to the reuse of XoT connections described there apply analogously to zone transfers performed using DoQ connections. One reason for reiterating such specific guidance is the lack of effective connection reuse in existing TCP/TLS zone transfer implementations today. The following recommendations apply:¶

• DoQ servers MUST be able to handle multiple concurrent IXFR requests on a single QUIC connection.¶
• DoQ servers MUST be able to handle multiple concurrent AXFR requests on a single QUIC connection.¶

• DoQ implementations SHOULD¶

  • use the same QUIC connection for both AXFR and IXFR requests to the same primary¶
  • send those requests in parallel as soon as they are queued, i.e., do not wait for a response before sending the next query on the connection (this is analogous to pipelining requests on a TCP/TLS connection)¶
  • send the response(s) for each request as soon as they are available, i.e., response streams MAY be sent intermingled¶

5.8. Flow Control Mechanisms

Servers and clients manage flow control using the mechanisms defined in Section 4 of [RFC9000]. These mechanisms allow clients and servers to specify how many streams can be created, how much data can be sent on a stream, and how much data can be sent on the union of all streams. For DoQ, controlling how many streams are created allows servers to control how many new requests the client can send on a given connection.¶

Flow control exists to protect endpoint resources. For servers, global and per-stream flow control limits control how much data can be sent by clients. The same mechanisms allow clients to control how much data can be sent by servers. Values that are too small will unnecessarily limit performance. Values that are too large might expose endpoints to overload or memory exhaustion. Implementations or deployments will need to adjust flow control limits to balance these concerns. In particular, zone transfer implementations will need to control these limits carefully to ensure both large and concurrent zone transfers are well managed.¶

Initial values of parameters control how many requests and how much data can be sent by clients and servers at the beginning of the connection. These values are specified in transport parameters exchanged during the connection handshake. The parameter values received in the initial connection also control how many requests and how much data can be sent by clients using 0-RTT data in a resumed connection. Using too small values of these initial parameters would restrict the usefulness of allowing 0-RTT data.¶

7.1. Privacy Issues with 0-RTT data

The 0-RTT data can be replayed by adversaries. That data may trigger queries by a recursive resolver to authoritative resolvers. Adversaries may be able to pick a time at which the recursive resolver outgoing traffic is observable and thus find out what name was queried for in the 0-RTT data.¶

This risk is in fact a subset of the general problem of observing the behavior of the recursive resolver discussed in "DNS Privacy Considerations" [RFC9076]. The attack is partially mitigated by reducing the observability of this traffic. The mandatory replay protection mechanisms in TLS 1.3 [RFC8446] limit but do not eliminate the risk of replay. 0-RTT packets can only be replayed within a narrow window, which is only wide enough to account for variations in clock skew and network transmission.¶

The recommendation for TLS 1.3 [RFC8446] is that the capability to use 0-RTT data should be turned off by default and only enabled if the user clearly understands the associated risks. In the case of DoQ, allowing 0-RTT data provides significant performance gains, and there is a concern that a recommendation to not use it would simply be ignored. Instead, a set of practical recommendations is provided in Sections 4.5 and 5.5.3.¶

The specifications in Section 4.5 block the most obvious risks of replay attacks, as they only allow for transactions that will not change the long-term state of the server.¶

The attacks described above apply to the stub resolver to recursive resolver scenario, but similar attacks might be envisaged in the recursive resolver to authoritative resolver scenario, and the same mitigations apply.¶

7.2. Privacy Issues with Session Resumption

The QUIC session resumption mechanism reduces the cost of re-establishing sessions and enables 0-RTT data. There is a linkability issue associated with session resumption, if the same resumption token is used several times. Attackers on path between client and server could observe repeated usage of the token and use that to track the client over time or over multiple locations.¶

The session resumption mechanism allows servers to correlate the resumed sessions with the initial sessions and thus to track the client. This creates a virtual long duration session. The series of queries in that session can be used by the server to identify the client. Servers can most probably do that already if the client address remains constant, but session resumption tickets also enable tracking after changes of the client's address.¶

The recommendations in Section 5.5.3 are designed to mitigate these risks. Using session tickets only once mitigates the risk of tracking by third parties. Refusing to resume a session if addresses change mitigates the incremental risk of tracking by the server (but the risk of tracking by IP address remains).¶

The privacy trade-offs here may be context specific. Stub resolvers will have a strong motivation to prefer privacy over latency since they often change location. However, recursive resolvers that use a small set of static IP addresses are more likely to prefer the reduced latency provided by session resumption and may consider this a valid reason to use resumption tickets even if the IP address changed between sessions.¶

Encrypted zone transfer ([RFC9103]) explicitly does not attempt to hide the identity of the parties involved in the transfer; at the same time, such transfers are not particularly latency sensitive. This means that applications supporting zone transfers may decide to apply the same protections as stub to recursive applications.¶

7.3. Privacy Issues with Address Validation Tokens

QUIC specifies address validation mechanisms in Section 8 of [RFC9000]. Use of an address validation token allows QUIC servers to avoid an extra RTT for new connections. Address validation tokens are typically tied to an IP address. QUIC clients normally only use these tokens when setting up a new connection from a previously used address. However, clients are not always aware that they are using a new address. This could be due to NAT, or because the client does not have an API available to check if the IP address has changed (which can be quite often for IPv6). There is a linkability risk if clients mistakenly use address validation tokens after unknowingly moving to a new location.¶

The recommendations in Section 5.5.3 mitigates this risk by tying the usage of the NEW_TOKEN to that of session resumption, though this recommendation does not cover the case where the client is unaware of the address change.¶

7.4. Privacy Issues with Long Duration Sessions

A potential alternative to session resumption is the use of long duration sessions: if a session remains open for a long time, new queries can be sent without incurring connection establishment delays. It is worth pointing out that the two solutions have similar privacy characteristics. Session resumption may allow servers to keep track of the IP addresses of clients, but long duration sessions have the same effect.¶

In particular, a DoQ implementation might take advantage of the connection migration features of QUIC to maintain a session even if the client's connectivity changes, for example, if the client migrates from a Wi-Fi connection to a cellular network connection and then to another Wi-Fi connection. The server would be able to track the client location by monitoring the succession of IP addresses used by the long duration connection.¶

The recommendation in Section 5.5.4 mitigates the privacy concerns related to long duration sessions using multiple client addresses.¶

7.5. Traffic Analysis

Even though QUIC packets are encrypted, adversaries can gain information from observing packet lengths, in both queries and responses, as well as packet timing. Many DNS requests are emitted by web browsers. Loading a specific web page may require resolving dozens of DNS names. If an application adopts a simple mapping of one query or response per packet, or "one QUIC STREAM frame per packet", then the succession of packet lengths may provide enough information to identify the requested site.¶

Implementations SHOULD use the mechanisms defined in Section 5.4 to mitigate this attack.¶

8.1. Registration of a DoQ Identification String

This document creates a new registration for the identification of DoQ in the "TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry [RFC7301].¶

The "doq" string identifies DoQ:¶

Protocol:

DoQ¶

Identification Sequence:

0x64 0x6F 0x71 ("doq")¶

Specification:

This document¶

8.2. Reservation of a Dedicated Port

For both TCP and UDP, port 853 is currently reserved for "DNS query-response protocol run over TLS/DTLS" [RFC7858].¶

However, the specification for DNS over DTLS (DoD) [RFC8094] is experimental, limited to stub to resolver, and no implementations or deployments currently exist to the authors' knowledge (even though several years have passed since the specification was published).¶

This specification additionally reserves the use of UDP port 853 for DoQ. QUIC version 1 was designed to be able to coexist with other protocols on the same port, including DTLS; see Section 17.2 of [RFC9000]. This means that deployments that serve DoD and DoQ (QUIC version 1) on the same port will be able to demultiplex the two due to the second most significant bit in each UDP payload. Such deployments ought to check the signatures of future versions or extensions (e.g., [GREASING-QUIC]) of QUIC and DTLS before deploying them to serve DNS on the same port.¶

IANA has updated the following value in the "Service Name and Transport Protocol Port Number Registry" in the System range. The registry for that range requires IETF Review or IESG Approval [RFC6335].¶

Service Name:

domain-s¶

Port Number:

853¶

Transport Protocol(s):

UDP¶

Assignee:

IESG¶

Contact:

IETF Chair¶

Description:

DNS query-response protocol run over DTLS or QUIC¶

Reference:

[RFC7858][RFC8094] This document¶

Additionally, IANA has updated the Description field for the corresponding TCP port 853 allocation to be "DNS query-response protocol run over TLS" and removed [RFC8094] from the TCP allocation's Reference field for consistency and clarity.¶

8.3. Reservation of an Extended DNS Error Code: Too Early

IANA has registered the following value in the "Extended DNS Error Codes" registry [RFC8914]:¶

INFO-CODE:

26¶

Purpose:

Too Early¶

Reference:

This document¶

8.4. DNS-over-QUIC Error Codes Registry

IANA has added a registry for "DNS-over-QUIC Error Codes" on the "Domain Name System (DNS) Parameters" web page.¶

The "DNS-over-QUIC Error Codes" registry governs a 62-bit space. This space is split into three regions that are governed by different policies:¶

• Permanent registrations 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 [RFC8126]¶
• Permanent registrations for values larger than 0x3f, which are assigned using the Specification Required policy ([RFC8126])¶
• Provisional registrations for values larger than 0x3f, which require Expert Review, as defined in Section 4.5 of [RFC8126].¶

Provisional reservations share the range of values larger than 0x3f with some permanent registrations. This is by design to enable conversion of provisional registrations into permanent registrations without requiring changes in deployed systems. (This design is aligned with the principles set in Section 22 of [RFC9000].)¶

Registrations in this registry MUST include the following fields:¶

Value:

The assigned codepoint¶

Status:

"Permanent" or "Provisional"¶

Contact:

Contact details for the registrant¶

In addition, permanent registrations MUST include:¶

Error:

A short mnemonic for the parameter¶

Specification:

A reference to a publicly available specification for the value (optional for provisional registrations)¶

Description:

A brief description of the error code semantics, which MAY be a summary if a specification reference is provided¶

Provisional registrations of codepoints are intended to allow for private use and experimentation with extensions to DoQ. However, provisional registrations could be reclaimed and reassigned for other purposes. In addition to the parameters listed above, provisional registrations MUST include:¶

Date:

The date of last update to the registration¶

A request to update the date on any provisional registration can be made without review from the designated expert(s).¶

The initial content of this registry is shown in Table 1 and all entries share the following fields:¶

Status:

Permanent¶

Contact:

DPRIVE WG¶

Specification:

Section 4.3¶