HTTP Datagrams and the Capsule Protocol

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.¶

This document uses terminology from [QUIC].¶

Where this document defines protocol types, the definition format uses the notation from Section 1.3 of [QUIC]. Where fields within types are integers, they are encoded using the variable-length integer encoding from Section 16 of [QUIC]. Integer values do not need to be encoded on the minimum number of bytes necessary.¶

2.1. HTTP/3 Datagrams

When used with HTTP/3, the Datagram Data field of QUIC DATAGRAM frames uses the following format:¶

HTTP/3 Datagram {
  Quarter Stream ID (i),
  HTTP Datagram Payload (..),
}

Quarter Stream ID:

A variable-length integer that contains the value of the client-initiated bidirectional stream that this datagram is associated with, divided by four (the division by four stems from the fact that HTTP requests are sent on client-initiated bidirectional streams, and those have stream IDs that are divisible by four). The largest legal QUIC stream ID value is 2⁶²-1, so the largest legal value of Quarter Stream ID is 2⁶⁰-1. Receipt of an HTTP/3 Datagram that includes a larger value MUST be treated as an HTTP/3 connection error of type H3_DATAGRAM_ERROR (0x33).¶

HTTP Datagram Payload:

The payload of the datagram, whose semantics are defined by the extension that is using HTTP Datagrams. Note that this field can be empty.¶

Receipt of a QUIC DATAGRAM frame whose payload is too short to allow parsing the Quarter Stream ID field MUST be treated as an HTTP/3 connection error of type H3_DATAGRAM_ERROR (0x33).¶

HTTP/3 Datagrams MUST NOT be sent unless the corresponding stream's send side is open. If a datagram is received after the corresponding stream's receive side is closed, the received datagrams MUST be silently dropped.¶

If an HTTP/3 datagram is received and its Quarter Stream ID maps to a stream that has not yet been created, the receiver SHALL either drop that datagram silently or buffer it temporarily (on the order of a round trip) while awaiting the creation of the corresponding stream.¶

If an HTTP/3 datagram is received and its Quarter Stream ID maps to a stream that cannot be created due to client-initiated bidirectional stream limits, it SHOULD be treated as an HTTP/3 connection error of type H3_ID_ERROR. Generating an error is not mandatory in this case because HTTP/3 implementations might have practical barriers to determining the active stream concurrency limit that is applied by the QUIC layer.¶

Prioritization of HTTP/3 datagrams is not defined in this document. Future extensions MAY define how to prioritize datagrams, and MAY define signaling to allow communicating prioritization preferences.¶

2.2. HTTP Datagrams using Capsules

When HTTP/3 Datagrams are unavailable or undesirable, HTTP Datagrams can be sent using the Capsule Protocol, see Section 3.5.¶

3.1. HTTP Data Streams

This specification defines the "data stream" of an HTTP request as the bidirectional stream of bytes that follows the header section of the request message and the final, successful (i.e., 2xx) response message.¶

In HTTP/1.x, the data stream consists of all bytes on the connection that follow the blank line that concludes either the request header section, or the response header section. As a result, only the last HTTP request on an HTTP/1.x connection can start the capsule protocol.¶

In HTTP/2 and HTTP/3, the data stream of a given HTTP request consists of all bytes sent in DATA frames with the corresponding stream ID.¶

The concept of a data stream is particularly relevant for methods such as CONNECT where there is no HTTP message content after the headers.¶

Data streams can be prioritized using any means suited to stream or request prioritization. For example, see Section 11 of [PRIORITY].¶

Data streams are subject to the flow control mechanisms of the underlying layers (for example, HTTP/2 stream flow control, HTTP/2 connection flow control, and TCP flow control).¶

3.2. The Capsule Protocol

Definitions of new HTTP Upgrade Tokens can state that their associated request's data stream uses the Capsule Protocol. If they do so, that means that the contents of the associated request's data stream uses the following format:¶

Capsule Protocol {
  Capsule (..) ...,
}

Capsule {
  Capsule Type (i),
  Capsule Length (i),
  Capsule Value (..),
}

Capsule Type:

A variable-length integer indicating the Type of the capsule.¶

Capsule Length:

The length in bytes of the Capsule Value field following this field, encoded as a variable-length integer. Note that this field can have a value of zero.¶

Capsule Value:

The payload of this capsule. Its semantics are determined by the value of the Capsule Type field.¶

An intermediary can identify the use of the capsule protocol either through the presence of the Capsule-Protocol header field (Section 3.4) or by understanding the chosen HTTP Upgrade token.¶

Because new protocols or extensions might define new capsule types, intermediaries that wish to allow for future extensibility SHOULD forward capsules without modification, unless the definition of the Capsule Type in use specifies additional intermediary processing. One such Capsule Type is the DATAGRAM capsule; see Section 3.5. In particular, intermediaries SHOULD forward Capsules with an unknown Capsule Type without modification.¶

Endpoints which receive a Capsule with an unknown Capsule Type MUST silently drop that Capsule and skip over it to parse the next Capsule.¶

By virtue of the definition of the data stream:¶

• The Capsule Protocol is not in use unless the response includes a 2xx (Successful) status code.¶
• When the Capsule Protocol is in use, the associated HTTP request and response do not carry HTTP content. A future extension MAY define a new capsule type to carry HTTP content.¶

Since the Capsule Protocol only applies to definitions of new HTTP Upgrade Tokens, in HTTP/2 and HTTP/3 it can only be used with the CONNECT method. Therefore, once both endpoints agree to use the Capsule Protocol, the frame usage requirements of the stream change as specified in Section 8.5 of [HTTP/2] and Section 4.2 of [HTTP/3].¶

The Capsule Protocol MUST NOT be used with messages that contain Content-Length, Content-Type, or Transfer-Encoding header fields. Additionally, HTTP status codes 204 (No Content), 205 (Reset Content), and 206 (Partial Content) MUST NOT be sent on responses that use the Capsule Protocol. A receiver that observes a violation of these requirements MUST treat the HTTP message as malformed.¶

When processing capsules, a receiver might be tempted to accumulate the full length of the capsule value in the data stream before handling it. This approach can consume flow control in underlying layers, which might lead to deadlocks if the capsule data exhausts the flow control window.¶

3.3. Error Handling

When a receiver encounters an error processing the Capsule Protocol, the receiver MUST treat it as if it had received a malformed or incomplete HTTP message. For HTTP/3, the handling of malformed messages is described in Section 4.1.3 of [HTTP/3]. For HTTP/2, the handling of malformed messages is described in Section 8.1.1 of [HTTP/2]. For HTTP/1.1, the handling of incomplete messages is described in Section 8 of [HTTP/1.1].¶

Each capsule's payload MUST contain exactly the fields identified in its description. A capsule payload that contains additional bytes after the identified fields or a capsule payload that terminates before the end of the identified fields MUST be treated as it if were a malformed or incomplete message. In particular, redundant length encodings MUST be verified to be self-consistent.¶

If the receive side of a stream carrying capsules is terminated cleanly (for example, in HTTP/3 this is defined as receiving a QUIC STREAM frame with the FIN bit set) and the last capsule on the stream was truncated, this MUST be treated as if it were a malformed or incomplete message.¶

3.4. The Capsule-Protocol Header Field

The "Capsule-Protocol" header field is an Item Structured Field, see Section 3.3 of [STRUCT-FIELD]; its value MUST be a Boolean; any other value type MUST be handled as if the field were not present by recipients (for example, if this field is included multiple times, its type will become a List and the field will therefore be ignored). This document does not define any parameters for the Capsule-Protocol header field value, but future documents might define parameters. Receivers MUST ignore unknown parameters.¶

Endpoints indicate that the Capsule Protocol is in use on a data stream by sending a Capsule-Protocol header field with a true value. A Capsule-Protocol header field with a false value has the same semantics as when the header is not present.¶

Intermediaries MAY use this header field to allow processing of HTTP Datagrams for unknown HTTP Upgrade Tokens; note that this is only possible for HTTP Upgrade or Extended CONNECT.¶

The Capsule-Protocol header field MUST NOT be used on HTTP responses with a status code outside the 2xx range.¶

When using the Capsule Protocol, HTTP endpoints SHOULD send the Capsule-Protocol header field to simplify intermediary processing. Definitions of new HTTP Upgrade Tokens that use the Capsule Protocol MAY alter this recommendation.¶

3.5. The DATAGRAM Capsule

This document defines the DATAGRAM (0x00) capsule type. This capsule allows HTTP Datagrams to be sent on a stream using the Capsule Protocol. This is particularly useful when HTTP is running over a transport that does not support the QUIC DATAGRAM frame.¶

Datagram Capsule {
  Type (i) = 0x00,
  Length (i),
  HTTP Datagram Payload (..),
}

HTTP Datagram Payload:

The payload of the datagram, whose semantics are defined by the extension that is using HTTP Datagrams. Note that this field can be empty.¶

HTTP Datagrams sent using the DATAGRAM capsule have the same semantics as those sent in QUIC DATAGRAM frames. In particular, the restrictions on when it is allowed to send an HTTP Datagram and how to process them from Section 2.1 also apply to HTTP Datagrams sent and received using the DATAGRAM capsule.¶

An intermediary can reencode HTTP Datagrams as it forwards them. In other words, an intermediary MAY send a DATAGRAM capsule to forward an HTTP Datagram that was received in a QUIC DATAGRAM frame, and vice versa. Intermediaries MUST NOT perform this reencoding unless they have identified the use of the Capsule Protocol on the corresponding request stream; see Section 3.2.¶

Note that while DATAGRAM capsules that are sent on a stream are reliably delivered in order, intermediaries can reencode DATAGRAM capsules into QUIC DATAGRAM frames when forwarding messages, which could result in loss or reordering.¶

If an intermediary receives an HTTP Datagram in a QUIC DATAGRAM frame and is forwarding it on a connection that supports QUIC DATAGRAM frames, the intermediary SHOULD NOT convert that HTTP Datagram to a DATAGRAM capsule. If the HTTP Datagram is too large to fit in a DATAGRAM frame (for example because the path MTU of that QUIC connection is too low or if the maximum UDP payload size advertised on that connection is too low), the intermediary SHOULD drop the HTTP Datagram instead of converting it to a DATAGRAM capsule. This preserves the end-to-end unreliability characteristic that methods such as Datagram Packetization Layer Path MTU Discovery (DPLPMTUD) depend on [DPLPMTUD]. An intermediary that converts QUIC DATAGRAM frames to DATAGRAM capsules allows HTTP Datagrams to be arbitrarily large without suffering any loss; this can misrepresent the true path properties, defeating methods such as DPLPMTUD.¶

While DATAGRAM capsules can theoretically carry a payload of length 2⁶²-1, most HTTP extensions that use HTTP Datagrams will have their own limits on what datagram payload sizes are practical. Implementations SHOULD take those limits into account when parsing DATAGRAM capsules: if an incoming DATAGRAM capsule has a length that is known to be so large as to not be usable, the implementation SHOULD discard the capsule without buffering its contents into memory.¶

Since QUIC DATAGRAM frames are required to fit within a QUIC packet, implementations that reencode DATAGRAM capsules into QUIC DATAGRAM frames might be tempted to accumulate the entire capsule before reencoding it. This can cause flow control problems; see Section 3.2.¶

Note that it is possible for an HTTP extension to use HTTP Datagrams without using the Capsule Protocol. For example, if an HTTP extension that uses HTTP Datagrams is only defined over transports that support QUIC DATAGRAM frames, it might not need a stream encoding. Additionally, HTTP extensions can use HTTP Datagrams with their own data stream protocol. However, new HTTP extensions that wish to use HTTP Datagrams SHOULD use the Capsule Protocol as failing to do so will make it harder for the HTTP extension to support versions of HTTP other than HTTP/3 and will prevent interoperability with intermediaries that only support the Capsule Protocol.¶

5.1. HTTP/3 Setting

This document will request IANA to register the following entry in the "HTTP/3 Settings" registry:¶

Value:

0x33¶

Setting Name:

SETTINGS_H3_DATAGRAM¶

Default:

0¶

Status:

provisional (permanent if this document is approved)¶

Specification:

This Document¶

Change Controller:

IETF¶

Contact:

HTTP_WG; HTTP working group; ietf-http-wg@w3.org¶

5.2. HTTP/3 Error Code

This document will request IANA to register the following entry in the "HTTP/3 Error Codes" registry:¶

Value:

0x33¶

Name:

H3_DATAGRAM_ERROR¶

Description:

Datagram or capsule protocol parse error¶

Status:

provisional (permanent if this document is approved)¶

Specification:

This Document¶

Change Controller:

IETF¶

Contact:

HTTP_WG; HTTP working group; ietf-http-wg@w3.org¶

5.3. HTTP Header Field Name

This document will request IANA to register the following entry in the "HTTP Field Name" registry:¶

Field Name:

Capsule-Protocol¶

Template:

None¶

Status:

provisional (permanent if this document is approved)¶

Reference:

This document¶

Comments:

None¶

5.4. Capsule Types

This document establishes a registry for HTTP capsule type codes. The "HTTP Capsule Types" 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 "Capsule Type" field which contains a short name or label for the capsule type.¶

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].¶

Capsule types with a value of the form 0x29 * N + 0x17 for integer values of N are reserved to exercise the requirement that unknown capsule types be ignored. These capsules have no semantics and can carry arbitrary values. These values MUST NOT be assigned by IANA and MUST NOT appear in the listing of assigned values.¶

This registry initially contains the following entry:¶

Value:

0x00¶

Capsule Type:

DATAGRAM¶

Status:

permanent¶

Specification:

This document¶

Change Controller:

IETF¶

Contact:

MASQUE Working Group masque@ietf.org¶

Notes:

None¶