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MASQUE Internet-Draft This document describes HTTP Datagrams, a convention for conveying multiplexed, potentially unreliable datagrams inside an HTTP connection. In HTTP/3, HTTP Datagrams can be conveyed natively using the QUIC DATAGRAM extension. When the QUIC DATAGRAM frame is unavailable or undesirable, they can be sent using the Capsule Protocol, a more general convention for conveying data in HTTP connections. Both are intended for use by HTTP extensions, not applications. Discussion Venues Discussion of this document takes place on the MASQUE WG mailing list (masque@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction HTTP extensions sometimes need to access underlying transport protocol features such as unreliable delivery (as offered by ) to enable desirable features like an unreliable version of the CONNECT method, and unreliable delivery in WebSockets (or its successors). In , this document describes HTTP Datagrams, a convention that supports the bidirectional and possibly multiplexed exchange of data inside an HTTP connection. While HTTP datagrams are associated with HTTP requests, they are not part of message content; instead, they are intended for use by HTTP extensions (such as the CONNECT method), and are compatible with all versions of HTTP. When the underlying transport protocol supports unreliable delivery (such as when the QUIC DATAGRAM extension is available in HTTP/3), they can use that capability. This document also describes the HTTP Capsule Protocol in , to allow conveyance of HTTP Datagrams when the QUIC DATAGRAM frame is unavailable or undesirable, such as when earlier versions of HTTP are in use.

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

HTTP Datagrams HTTP Datagrams are a convention for conveying bidirectional and potentially unreliable datagrams inside an HTTP connection, with multiplexing when possible. All HTTP Datagrams are associated with an HTTP request. When HTTP Datagrams are conveyed on an HTTP/3 connection, the QUIC DATAGRAM frame can be used to achieve these goals, including unreliable delivery; see . Negotiation is achieved using a setting; see . When running over HTTP/2, demultiplexing is provided by the HTTP/2 framing layer, but unreliable delivery is unavailable. HTTP Datagrams are negotiated and conveyed using the Capsule Protocol; see . When running over HTTP/1, requests are strictly serialized in the connection, and therefore demultiplexing is not available. Unreliable delivery is likewise not available. HTTP Datagrams are negotiated and conveyed using the Capsule Protocol; see . HTTP Datagrams MUST only be sent with an association to a stream whose HTTP semantics explicitly supports HTTP Datagrams. For example, existing HTTP methods GET and POST do not define semantics for associated HTTP Datagrams; therefore, HTTP Datagrams cannot be sent associated with GET or POST request streams. If an HTTP Datagram associated with a method that has no known semantics for HTTP Datagrams is received, the receiver MUST abort the corresponding stream; if HTTP/3 is in use, the stream MUST be aborted with H3_DATAGRAM_ERROR. HTTP extensions can override these requirements by defining a negotiation mechanism and semantics for HTTP Datagrams.

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 ):

HTTP/3 Datagram Format

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.

The H3_DATAGRAM HTTP/3 SETTINGS Parameter Implementations of HTTP/3 that support HTTP Datagrams can indicate that to their peer by sending the H3_DATAGRAM SETTINGS parameter with a value of 1. The value of the H3_DATAGRAM SETTINGS parameter MUST be either 0 or 1. A value of 0 indicates that HTTP Datagrams are not supported. If the H3_DATAGRAM SETTINGS parameter is received with a value that is neither 0 or 1, the receiver MUST terminate the connection with error H3_SETTINGS_ERROR. QUIC DATAGRAM frames MUST NOT be sent until the H3_DATAGRAM SETTINGS parameter has been both sent and received with a value of 1. When clients use 0-RTT, they MAY store the value of the server's H3_DATAGRAM SETTINGS parameter. Doing so allows the client to send QUIC DATAGRAM frames in 0-RTT packets. When servers decide to accept 0-RTT data, they MUST send a H3_DATAGRAM SETTINGS parameter greater than or equal to the value they sent to the client in the connection where they sent them the NewSessionTicket message. If a client stores the value of the H3_DATAGRAM SETTINGS parameter with their 0-RTT state, they MUST validate that the new value of the H3_DATAGRAM SETTINGS parameter sent by the server in the handshake is greater than or equal to the stored value; if not, the client MUST terminate the connection with error H3_SETTINGS_ERROR. In all cases, the maximum permitted value of the H3_DATAGRAM SETTINGS parameter is 1. It is RECOMMENDED that implementations that support receiving HTTP Datagrams using QUIC always send the H3_DATAGRAM SETTINGS parameter with a value of 1, even if the application does not intend to use HTTP Datagrams. This helps to avoid "sticking out"; see .

Note About Draft Versions [[RFC editor: please remove this section before publication.]] Some revisions of this draft specification use a different value (the Identifier field of a Setting in the HTTP/3 SETTINGS frame) for the H3_DATAGRAM Settings Parameter. This allows new draft revisions to make incompatible changes. Multiple draft versions MAY be supported by sending multiple values for H3_DATAGRAM. Once SETTINGS have been sent and received, an implementation that supports multiple drafts MUST compute the intersection of the values it has sent and received, and then it MUST select and use the most recent draft version from the intersection set. This ensures that both peers negotiate the same draft version.

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

Capsules One mechanism to extend HTTP is to introduce new HTTP Upgrade Tokens (see ). In HTTP/1.x, these tokens are used via the Upgrade mechanism (see ). In HTTP/2 and HTTP/3, these tokens are used via the Extended CONNECT mechanism (see and ). This specification introduces the Capsule Protocol. The Capsule Protocol is a sequence of type-length-value tuples that definitions of new HTTP Upgrade Tokens can choose to use. It allows endpoints to reliably communicate request-related information end-to-end on HTTP request streams, even in the presence of HTTP intermediaries. The Capsule Protocol can be used to exchange HTTP Datagrams, which is necessary when HTTP is running over a transport that does not support the QUIC DATAGRAM frame.

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 .

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 ):

Capsule Protocol Stream Format

Capsule Format

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 () 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 . 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.

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 . For HTTP/2, the handling of malformed messages is described in . For HTTP/1.1, the handling of incomplete messages is described in . 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.

The Capsule-Protocol Header Field The "Capsule-Protocol" header field is an Item Structured Field, see ; 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.

The DATAGRAM Capsule This document defines the DATAGRAM capsule type (see 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 Format

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 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 . 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.

Security Considerations Since transmitting HTTP Datagrams using QUIC DATAGRAM frames requires sending an HTTP/3 Settings parameter, it "sticks out". In other words, probing clients can learn whether a server supports HTTP Datagrams over QUIC DATAGRAM frames. As some servers might wish to obfuscate the fact that they offer application services that use HTTP datagrams, it's best for all implementations that support this feature to always send this Settings parameter, see . Since use of the Capsule Protocol is restricted to new HTTP Upgrade Tokens, it is not accessible from Web Platform APIs (such as those commonly accessed via JavaScript in web browsers).

IANA Considerations

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

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

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

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 ) 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 ) 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.

References Normative References Apple Inc. Apple Inc. Google LLC This document defines an extension to the QUIC transport protocol to add support for sending and receiving unreliable datagrams over a QUIC connection. Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the QUIC Working Group mailing list (mailto:quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/quic/. Source for this draft and an issue tracker can be found at https://github.com/quicwg/datagram. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230. Akamai The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3. DO NOT DEPLOY THIS VERSION OF HTTP DO NOT DEPLOY THIS VERSION OF HTTP/3 UNTIL IT IS IN AN RFC. This version is still a work in progress. For trial deployments, please use earlier versions. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic. Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-http. Mozilla Apple Inc. This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced latency by introducing field compression and allowing multiple concurrent exchanges on the same connection. This document obsoletes RFC 7540 and RFC 8740. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Informative References The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code. The security model used for this is the origin-based security model commonly used by web browsers. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications that need two-way communication with servers that does not rely on opening multiple HTTP connections (e.g., using XMLHttpRequest or <iframe>s and long polling). [STANDARDS-TRACK] This document defines a mechanism for running the WebSocket Protocol (RFC 6455) over a single stream of an HTTP/2 connection. Google The mechanism for running the WebSocket Protocol over a single stream of an HTTP/2 connection is equally applicable to HTTP/3, but the HTTP version-specific details need to be specified. This document describes how the mechanism is adapted for HTTP/3. Fastly Cloudflare This document describes a scheme that allows an HTTP client to communicate its preferences for how the upstream server prioritizes responses to its requests, and also allows a server to hint to a downstream intermediary how its responses should be prioritized when they are forwarded. This document defines the Priority header field for communicating the initial priority in an HTTP version-independent manner, as well as HTTP/2 and HTTP/3 frames for reprioritizing responses. These share a common format structure that is designed to provide future extensibility. This document specifies Datagram Packetization Layer Path MTU Discovery (DPLPMTUD). This is a robust method for Path MTU Discovery (PMTUD) for datagram Packetization Layers (PLs). It allows a PL, or a datagram application that uses a PL, to discover whether a network path can support the current size of datagram. This can be used to detect and reduce the message size when a sender encounters a packet black hole. It can also probe a network path to discover whether the maximum packet size can be increased. This provides functionality for datagram transports that is equivalent to the PLPMTUD specification for TCP, specified in RFC 4821, which it updates. It also updates the UDP Usage Guidelines to refer to this method for use with UDP datagrams and updates SCTP. The document provides implementation notes for incorporating Datagram PMTUD into IETF datagram transports or applications that use datagram transports. This specification updates RFC 4960, RFC 4821, RFC 6951, RFC 8085, and RFC 8261.

Acknowledgments Portions of this document were previously part of the QUIC DATAGRAM frame definition itself, the authors would like to acknowledge the authors of that document and the members of the IETF MASQUE working group for their suggestions. Additionally, the authors would like to thank Martin Thomson for suggesting the use of an HTTP/3 SETTINGS parameter. Furthermore, the authors would like to thank Ben Schwartz for writing the first proposal that used two layers of indirection. The final design in this document came out of the HTTP Datagrams Design Team, whose members were Alan Frindell, Alex Chernyakhovsky, Ben Schwartz, Eric Rescorla, Marcus Ihlar, Martin Thomson, Mike Bishop, Tommy Pauly, Victor Vasiliev, and the authors of this document. The authors thank Mark Nottingham and Philipp Tiesel for their helpful comments.