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rfc:rfc9297



Internet Engineering Task Force (IETF) D. Schinazi Request for Comments: 9297 Google LLC Category: Standards Track L. Pardue ISSN: 2070-1721 Cloudflare

                                                           August 2022
              HTTP Datagrams and the Capsule Protocol

Abstract

 This document describes HTTP Datagrams, a convention for conveying
 multiplexed, potentially unreliable datagrams inside an HTTP
 connection.
 In HTTP/3, HTTP Datagrams can be sent unreliably using the QUIC
 DATAGRAM extension.  When the QUIC DATAGRAM frame is unavailable or
 undesirable, HTTP Datagrams can be sent using the Capsule Protocol,
 which is a more general convention for conveying data in HTTP
 connections.
 HTTP Datagrams and the Capsule Protocol are intended for use by HTTP
 extensions, not applications.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9297.

Copyright Notice

 Copyright (c) 2022 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1.  Introduction
   1.1.  Conventions and Definitions
 2.  HTTP Datagrams
   2.1.  HTTP/3 Datagrams
     2.1.1.  The SETTINGS_H3_DATAGRAM HTTP/3 Setting
   2.2.  HTTP Datagrams Using Capsules
 3.  Capsules
   3.1.  HTTP Data Streams
   3.2.  The Capsule Protocol
   3.3.  Error Handling
   3.4.  The Capsule-Protocol Header Field
   3.5.  The DATAGRAM Capsule
 4.  Security Considerations
 5.  IANA Considerations
   5.1.  HTTP/3 Setting
   5.2.  HTTP/3 Error Code
   5.3.  HTTP Header Field Name
   5.4.  Capsule Types
 6.  References
   6.1.  Normative References
   6.2.  Informative References
 Acknowledgments
 Authors' Addresses

1. Introduction

 HTTP extensions (as defined in Section 16 of [HTTP]) sometimes need
 to access underlying transport protocol features such as unreliable
 delivery (as offered by [QUIC-DGRAM]) to enable desirable features.
 For example, this could allow for the introduction of an unreliable
 version of the CONNECT method and the addition of unreliable delivery
 to WebSockets [WEBSOCKET].
 In Section 2, this document describes HTTP Datagrams, a convention
 for conveying bidirectional and potentially unreliable datagrams
 inside an HTTP connection, with multiplexing when possible.  While
 HTTP Datagrams are associated with HTTP requests, they are not a 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 HTTP is running over a transport protocol that supports
 unreliable delivery (such as when the QUIC DATAGRAM extension
 [QUIC-DGRAM] is available to HTTP/3 [HTTP/3]), HTTP Datagrams can use
 that capability.
 In Section 3, this document describes the HTTP Capsule Protocol,
 which allows the conveyance of HTTP Datagrams using reliable
 delivery.  This addresses HTTP/3 cases where use of the QUIC DATAGRAM
 frame is unavailable or undesirable or where the transport protocol
 only provides reliable delivery, such as with HTTP/1.1 [HTTP/1.1] or
 HTTP/2 [HTTP/2] over TCP [TCP].

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.
 In this document, the term "intermediary" refers to an HTTP
 intermediary as defined in Section 3.7 of [HTTP].

2. 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 provide demultiplexing and unreliable
 delivery; see Section 2.1.  Negotiating the use of QUIC DATAGRAM
 frames for HTTP Datagrams is achieved via the exchange of HTTP/3
 settings; see Section 2.1.1.
 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
 Section 3.5.
 When running over HTTP/1.x, requests are strictly serialized in the
 connection; therefore, demultiplexing is not available.  Unreliable
 delivery is likewise not available.  HTTP Datagrams are negotiated
 and conveyed using the Capsule Protocol; see Section 3.5.
 HTTP Datagrams MUST only be sent with an association to an HTTP
 request that explicitly supports them.  For example, existing HTTP
 methods GET and POST do not define semantics for associated HTTP
 Datagrams; therefore, HTTP Datagrams associated with GET or POST
 request streams cannot be sent.
 If an HTTP Datagram is received and it is associated with a request
 that has no known semantics for HTTP Datagrams, the receiver MUST
 terminate the request.  If HTTP/3 is in use, the request stream MUST
 be aborted with H3_DATAGRAM_ERROR (0x33).  HTTP extensions MAY
 override these requirements by defining a negotiation mechanism and
 semantics for HTTP Datagrams.

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 (..),
 }
                    Figure 1: 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, which have stream IDs that are divisible by
    four).  The largest legal QUIC stream ID value is 2^62-1, so the
    largest legal value of the Quarter Stream ID field is 2^60-1.
    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 field
 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 field
 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 because the QUIC stream limit might be unknown to the
 HTTP/3 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.

2.1.1. The SETTINGS_H3_DATAGRAM HTTP/3 Setting

 An endpoint can indicate to its peer that it is willing to receive
 HTTP/3 Datagrams by sending the SETTINGS_H3_DATAGRAM (0x33) setting
 with a value of 1.
 The value of the SETTINGS_H3_DATAGRAM setting MUST be either 0 or 1.
 A value of 0 indicates that the implementation is not willing to
 receive HTTP Datagrams.  If the SETTINGS_H3_DATAGRAM setting is
 received with a value that is neither 0 nor 1, the receiver MUST
 terminate the connection with error H3_SETTINGS_ERROR.
 QUIC DATAGRAM frames MUST NOT be sent until the SETTINGS_H3_DATAGRAM
 setting 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
 SETTINGS_H3_DATAGRAM setting.  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 SETTINGS_H3_DATAGRAM setting 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 SETTINGS_H3_DATAGRAM setting with their 0-RTT
 state, they MUST validate that the new value of the
 SETTINGS_H3_DATAGRAM setting 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 SETTINGS_H3_DATAGRAM setting
 parameter is 1.
 It is RECOMMENDED that implementations that support receiving HTTP/3
 Datagrams always send the SETTINGS_H3_DATAGRAM setting with a value
 of 1, even if the application does not intend to use HTTP/3
 Datagrams.  This helps to avoid "sticking out"; see Section 4.

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. Capsules

 One mechanism to extend HTTP is to introduce new HTTP upgrade tokens;
 see Section 16.7 of [HTTP].  In HTTP/1.x, these tokens are used via
 the Upgrade mechanism; see Section 7.8 of [HTTP].  In HTTP/2 and
 HTTP/3, these tokens are used via the Extended CONNECT mechanism; see
 [EXT-CONNECT2] and [EXT-CONNECT3].
 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.  The Capsule Protocol can also be used to
 communicate reliable and bidirectional control messages associated
 with a datagram-based protocol even when HTTP/3 Datagrams are in use.

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 response message that is either
 successful (i.e., 2xx) or upgraded (i.e., 101).
 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 final 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; examples include 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, the contents of the associated request's data stream uses the
 following format:
 Capsule Protocol {
   Capsule (..) ...,
 }
                Figure 2: Capsule Protocol Stream Format
 Capsule {
   Capsule Type (i),
   Capsule Length (i),
   Capsule Value (..),
 }
                        Figure 3: Capsule Format
 Capsule Type:  A variable-length integer indicating the type of the
    capsule.  An IANA registry is used to manage the assignment of
    Capsule Types; see Section 5.4.
 Capsule Length:  The length, in bytes, of the Capsule Value field,
    which follows 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 that 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) or 101 (Switching Protocols) 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.
 The Capsule Protocol only applies to definitions of new HTTP upgrade
 tokens; thus, 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 4.4 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 field in the data stream before
 handling it.  This approach SHOULD be avoided because it can consume
 flow control in underlying layers, and that 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 4.1.2 of [HTTP/3].  For HTTP/2, the
 handling of malformed messages is described in Section 8.1.1 of
 [HTTP/2].  For HTTP/1.x, 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 [STRUCTURED-FIELDS].  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 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 that is both different from 101 (Switching
 Protocols) and outside the 2xx (Successful) 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 (..),
 }
                   Figure 4: 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 Section 2.1) also apply to HTTP Datagrams sent
 and received using the DATAGRAM Capsule.
 An intermediary can re-encode 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 re-encoding unless they
 have identified the use of the Capsule Protocol on the corresponding
 request stream; see Section 3.2.
 Note that while DATAGRAM Capsules, which are sent on a stream, are
 reliably delivered in order, intermediaries can re-encode 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 (PMTU) 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 PMTU 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
 2^62-1, 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 re-encode DATAGRAM Capsules into QUIC DATAGRAM
 frames might be tempted to accumulate the entire Capsule in the
 stream before re-encoding it.  This SHOULD be avoided, because it 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.

4. Security Considerations

 Since transmitting HTTP Datagrams using QUIC DATAGRAM frames requires
 sending the HTTP/3 SETTINGS_H3_DATAGRAM setting, 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 setting; see Section 2.1.1.
 Since use of the Capsule Protocol is restricted to new HTTP upgrade
 tokens, it is not directly accessible from Web Platform APIs (such as
 those commonly accessed via JavaScript in web browsers).
 Definitions of new HTTP upgrade tokens that use the Capsule Protocol
 need to include a security analysis that considers the impact of HTTP
 Datagrams and Capsules in the context of their protocol.

5. IANA Considerations

5.1. HTTP/3 Setting

 IANA has registered the following entry in the "HTTP/3 Settings"
 registry maintained at <https://www.iana.org/assignments/
 http3-parameters>:
 Value:  0x33
 Setting Name:  SETTINGS_H3_DATAGRAM
 Default:  0
 Status:  permanent
 Reference:  RFC 9297
 Change Controller:  IETF
 Contact:  HTTP_WG; HTTP working group; ietf-http-wg@w3.org
 Notes:  None

5.2. HTTP/3 Error Code

 IANA has registered the following entry in the "HTTP/3 Error Codes"
 registry maintained at <https://www.iana.org/assignments/
 http3-parameters>:
 Value:  0x33
 Name:  H3_DATAGRAM_ERROR
 Description:  Datagram or Capsule Protocol parse error
 Status:  permanent
 Reference:  RFC 9297
 Change Controller:  IETF
 Contact:  HTTP_WG; HTTP working group; ietf-http-wg@w3.org
 Notes:  None

5.3. HTTP Header Field Name

 IANA has registered the following entry in the "Hypertext Transfer
 Protocol (HTTP) Field Name Registry" maintained at
 <https://www.iana.org/assignments/http-fields>:
 Field Name:  Capsule-Protocol
 Template:  None
 Status:  permanent
 Reference:  RFC 9297
 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
 that 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
 Reference:  RFC 9297
 Change Controller:  IETF
 Contact:  MASQUE Working Group masque@ietf.org
    (mailto:masque@ietf.org)
 Notes:  None

6. References

6.1. Normative References

 [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP Semantics", STD 97, RFC 9110,
            DOI 10.17487/RFC9110, June 2022,
            <https://www.rfc-editor.org/info/rfc9110>.
 [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
            June 2022, <https://www.rfc-editor.org/info/rfc9112>.
 [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
            DOI 10.17487/RFC9113, June 2022,
            <https://www.rfc-editor.org/info/rfc9113>.
 [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
            June 2022, <https://www.rfc-editor.org/info/rfc9114>.
 [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/info/rfc8126>.
 [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/info/rfc9000>.
 [QUIC-DGRAM]
            Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
            Datagram Extension to QUIC", RFC 9221,
            DOI 10.17487/RFC9221, March 2022,
            <https://www.rfc-editor.org/info/rfc9221>.
 [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/info/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/info/rfc8174>.
 [STRUCTURED-FIELDS]
            Nottingham, M. and P-H. Kamp, "Structured Field Values for
            HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
            <https://www.rfc-editor.org/info/rfc8941>.
 [TCP]      Eddy, W., Ed., "Transmission Control Protocol (TCP)",
            STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
            <https://www.rfc-editor.org/info/rfc9293>.

6.2. Informative References

 [DPLPMTUD] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
            Völker, "Packetization Layer Path MTU Discovery for
            Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
            September 2020, <https://www.rfc-editor.org/info/rfc8899>.
 [EXT-CONNECT2]
            McManus, P., "Bootstrapping WebSockets with HTTP/2",
            RFC 8441, DOI 10.17487/RFC8441, September 2018,
            <https://www.rfc-editor.org/info/rfc8441>.
 [EXT-CONNECT3]
            Hamilton, R., "Bootstrapping WebSockets with HTTP/3",
            RFC 9220, DOI 10.17487/RFC9220, June 2022,
            <https://www.rfc-editor.org/info/rfc9220>.
 [PRIORITY] Oku, K. and L. Pardue, "Extensible Prioritization Scheme
            for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,
            <https://www.rfc-editor.org/info/rfc9218>.
 [WEBSOCKET]
            Fette, I. and A. Melnikov, "The WebSocket Protocol",
            RFC 6455, DOI 10.17487/RFC6455, December 2011,
            <https://www.rfc-editor.org/info/rfc6455>.

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 setting.
 Furthermore, the authors would like to thank Ben Schwartz for
 substantive input.  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.

Authors' Addresses

 David Schinazi
 Google LLC
 1600 Amphitheatre Parkway
 Mountain View, CA 94043
 United States of America
 Email: dschinazi.ietf@gmail.com
 Lucas Pardue
 Cloudflare
 Email: lucaspardue.24.7@gmail.com
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