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.¶

2.1. HTTP/3 Datagrams

When used with HTTP/3, the Datagram Data field of QUIC DATAGRAM frames uses the following format (using the notation from the "Notational Conventions" section of [QUIC]):¶

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.¶

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.¶

HTTP/3 Datagrams MUST NOT be sent unless the corresponding stream's send side is open. Once the receive side of a stream is closed, incoming datagrams for this stream are no longer expected so related state can be released. State MAY be kept for a short time to account for reordering. Once the state is released, the received associated 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 a single HTTP request starting the capsule protocol can be sent on HTTP/1.x connections.¶

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].¶

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 (using the notation from the "Notational Conventions" section of [QUIC]):¶

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 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 on responses unless the response includes a 2xx (Successful) status code.¶

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.¶

3.3. Error Handling

When an error occurs in processing the Capsule Protocol, the receiver MUST treat the message as malformed or incomplete, according to the underlying transport protocol. For HTTP/3, the handling of malformed messages is described in Section 4.1.3 of [H3]. For HTTP/2, the handling of malformed messages is described in Section 8.1.1 of [H2]. For HTTP/1.1, the handling of incomplete messages is described in Section 8 of [H1].¶

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 a malformed or incomplete message. In particular, redundant length encodings MUST be verified to be self-consistent.¶

When a stream carrying capsules terminates cleanly, if the last capsule on the stream was truncated, this MUST be treated as 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 capsule type (see Section 5.4 for the value of the 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) = DATAGRAM,
  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 which was received in a QUIC DATAGRAM frame, and vice versa.¶

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.¶

Note that use of the Capsule Protocol is not required to use HTTP Datagrams. 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 unless they have a good reason not to.¶

5.1. HTTP/3 SETTINGS Parameter

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

Value:

0xffd277 (note that this will switch to a lower value before publication)¶

Setting Name:

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:

0x4A1268 (note that this will switch to a lower value before publication)¶

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. Registrations in this registry MUST include the following fields:¶

Type:

A name or label for the capsule type.¶

Value:

The value of the Capsule Type field (see Section 3.2) is a 62-bit integer.¶

Reference:

An optional reference to a specification for the type. This field MAY be empty.¶

Registrations follow the "First Come First Served" policy (see Section 4.4 of [IANA-POLICY]) where two registrations MUST NOT have the same Type.¶

This registry initially contains the following entry:¶

Capsule Type:

DATAGRAM¶

Value:

0xff37a5 (note that this will switch to a lower value before publication)¶

Reference:

This document¶

Capsule types with a value of the form 41 * N + 23 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.¶