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Internet Engineering Task Force (IETF) T. Pauly Request for Comments: 9221 E. Kinnear Category: Standards Track Apple Inc. ISSN: 2070-1721 D. Schinazi 

                                                            Google LLC
                                                            March 2022
              An Unreliable Datagram Extension to QUIC

Abstract 

 This document defines an extension to the QUIC transport protocol to
 add support for sending and receiving unreliable datagrams over a
 QUIC connection.

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/rfc9221.

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.  Specification of Requirements
 2.  Motivation
 3.  Transport Parameter
 4.  Datagram Frame Types
 5.  Behavior and Usage
   5.1.  Multiplexing Datagrams
   5.2.  Acknowledgement Handling
   5.3.  Flow Control
   5.4.  Congestion Control
 6.  Security Considerations
 7.  IANA Considerations
   7.1.  QUIC Transport Parameter
   7.2.  QUIC Frame Types
 8.  References
   8.1.  Normative References
   8.2.  Informative References
 Acknowledgments
 Authors' Addresses

1. Introduction 

 The QUIC transport protocol [RFC9000] provides a secure, multiplexed
 connection for transmitting reliable streams of application data.
 QUIC uses various frame types to transmit data within packets, and
 each frame type defines whether the data it contains will be
 retransmitted.  Streams of reliable application data are sent using
 STREAM frames.
 Some applications, particularly those that need to transmit real-time
 data, prefer to transmit data unreliably.  In the past, these
 applications have built directly upon UDP [RFC0768] as a transport
 and have often added security with DTLS [RFC6347].  Extending QUIC to
 support transmitting unreliable application data provides another
 option for secure datagrams with the added benefit of sharing the
 cryptographic and authentication context used for reliable streams.
 This document defines two new DATAGRAM QUIC frame types that carry
 application data without requiring retransmissions.

1.1. Specification of Requirements 

 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. Motivation 

 Transmitting unreliable data over QUIC provides benefits over
 existing solutions:
  • Applications that want to use both a reliable stream and an

unreliable flow to the same peer can benefit by sharing a single 

    handshake and authentication context between a reliable QUIC
    stream and a flow of unreliable QUIC datagrams.  This can reduce
    the latency required for handshakes compared to opening both a TLS
    connection and a DTLS connection.
  • QUIC uses a more nuanced loss recovery mechanism than the DTLS

handshake. This can allow loss recovery to occur more quickly for 

    QUIC data.
  • QUIC datagrams are subject to QUIC congestion control. Providing

a single congestion control for both reliable and unreliable data 

    can be more effective and efficient.
 These features can be useful for optimizing audio/video streaming
 applications, gaming applications, and other real-time network
 applications.
 Unreliable QUIC datagrams can also be used to implement an IP packet
 tunnel over QUIC, such as for a Virtual Private Network (VPN).
 Internet-layer tunneling protocols generally require a reliable and
 authenticated handshake followed by unreliable secure transmission of
 IP packets.  This can, for example, require a TLS connection for the
 control data and DTLS for tunneling IP packets.  A single QUIC
 connection could support both parts with the use of unreliable
 datagrams in addition to reliable streams.

3. Transport Parameter 

 Support for receiving the DATAGRAM frame types is advertised by means
 of a QUIC transport parameter (name=max_datagram_frame_size,
 value=0x20).  The max_datagram_frame_size transport parameter is an
 integer value (represented as a variable-length integer) that
 represents the maximum size of a DATAGRAM frame (including the frame
 type, length, and payload) the endpoint is willing to receive, in
 bytes.
 The default for this parameter is 0, which indicates that the
 endpoint does not support DATAGRAM frames.  A value greater than 0
 indicates that the endpoint supports the DATAGRAM frame types and is
 willing to receive such frames on this connection.
 An endpoint MUST NOT send DATAGRAM frames until it has received the
 max_datagram_frame_size transport parameter with a non-zero value
 during the handshake (or during a previous handshake if 0-RTT is
 used).  An endpoint MUST NOT send DATAGRAM frames that are larger
 than the max_datagram_frame_size value it has received from its peer.
 An endpoint that receives a DATAGRAM frame when it has not indicated
 support via the transport parameter MUST terminate the connection
 with an error of type PROTOCOL_VIOLATION.  Similarly, an endpoint
 that receives a DATAGRAM frame that is larger than the value it sent
 in its max_datagram_frame_size transport parameter MUST terminate the
 connection with an error of type PROTOCOL_VIOLATION.
 For most uses of DATAGRAM frames, it is RECOMMENDED to send a value
 of 65535 in the max_datagram_frame_size transport parameter to
 indicate that this endpoint will accept any DATAGRAM frame that fits
 inside a QUIC packet.
 The max_datagram_frame_size transport parameter is a unidirectional
 limit and indication of support of DATAGRAM frames.  Application
 protocols that use DATAGRAM frames MAY choose to only negotiate and
 use them in a single direction.
 When clients use 0-RTT, they MAY store the value of the server's
 max_datagram_frame_size transport parameter.  Doing so allows the
 client to send DATAGRAM frames in 0-RTT packets.  When servers decide
 to accept 0-RTT data, they MUST send a max_datagram_frame_size
 transport 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
 max_datagram_frame_size transport parameter with their 0-RTT state,
 they MUST validate that the new value of the max_datagram_frame_size
 transport 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 PROTOCOL_VIOLATION.
 Application protocols that use datagrams MUST define how they react
 to the absence of the max_datagram_frame_size transport parameter.
 If datagram support is integral to the application, the application
 protocol can fail the handshake if the max_datagram_frame_size
 transport parameter is not present.

4. Datagram Frame Types 

 DATAGRAM frames are used to transmit application data in an
 unreliable manner.  The Type field in the DATAGRAM frame takes the
 form 0b0011000X (or the values 0x30 and 0x31).  The least significant
 bit of the Type field in the DATAGRAM frame is the LEN bit (0x01),
 which indicates whether there is a Length field present: if this bit
 is set to 0, the Length field is absent and the Datagram Data field
 extends to the end of the packet; if this bit is set to 1, the Length
 field is present.
 DATAGRAM frames are structured as follows:
 DATAGRAM Frame {
   Type (i) = 0x30..0x31,
   [Length (i)],
   Datagram Data (..),
 }
                    Figure 1: DATAGRAM Frame Format
 DATAGRAM frames contain the following fields:
 Length:  A variable-length integer specifying the length of the
    Datagram Data field in bytes.  This field is present only when the
    LEN bit is set to 1.  When the LEN bit is set to 0, the Datagram
    Data field extends to the end of the QUIC packet.  Note that empty
    (i.e., zero-length) datagrams are allowed.
 Datagram Data:  The bytes of the datagram to be delivered.

5. Behavior and Usage 

 When an application sends a datagram over a QUIC connection, QUIC
 will generate a new DATAGRAM frame and send it in the first available
 packet.  This frame SHOULD be sent as soon as possible (as determined
 by factors like congestion control, described below) and MAY be
 coalesced with other frames.
 When a QUIC endpoint receives a valid DATAGRAM frame, it SHOULD
 deliver the data to the application immediately, as long as it is
 able to process the frame and can store the contents in memory.
 Like STREAM frames, DATAGRAM frames contain application data and MUST
 be protected with either 0-RTT or 1-RTT keys.
 Note that while the max_datagram_frame_size transport parameter
 places a limit on the maximum size of DATAGRAM frames, that limit can
 be further reduced by the max_udp_payload_size transport parameter
 and the Maximum Transmission Unit (MTU) of the path between
 endpoints.  DATAGRAM frames cannot be fragmented; therefore,
 application protocols need to handle cases where the maximum datagram
 size is limited by other factors.

5.1. Multiplexing Datagrams 

 DATAGRAM frames belong to a QUIC connection as a whole and are not
 associated with any stream ID at the QUIC layer.  However, it is
 expected that applications will want to differentiate between
 specific DATAGRAM frames by using identifiers, such as for logical
 flows of datagrams or to distinguish between different kinds of
 datagrams.
 Defining the identifiers used to multiplex different kinds of
 datagrams or flows of datagrams is the responsibility of the
 application protocol running over QUIC.  The application defines the
 semantics of the Datagram Data field and how it is parsed.
 If the application needs to support the coexistence of multiple flows
 of datagrams, one recommended pattern is to use a variable-length
 integer at the beginning of the Datagram Data field.  This is a
 simple approach that allows a large number of flows to be encoded
 using minimal space.
 QUIC implementations SHOULD present an API to applications to assign
 relative priorities to DATAGRAM frames with respect to each other and
 to QUIC streams.

5.2. Acknowledgement Handling 

 Although DATAGRAM frames are not retransmitted upon loss detection,
 they are ack-eliciting ([RFC9002]).  Receivers SHOULD support
 delaying ACK frames (within the limits specified by max_ack_delay) in
 response to receiving packets that only contain DATAGRAM frames,
 since the sender takes no action if these packets are temporarily
 unacknowledged.  Receivers will continue to send ACK frames when
 conditions indicate a packet might be lost, since the packet's
 payload is unknown to the receiver, and when dictated by
 max_ack_delay or other protocol components.
 As with any ack-eliciting frame, when a sender suspects that a packet
 containing only DATAGRAM frames has been lost, it sends probe packets
 to elicit a faster acknowledgement as described in Section 6.2.4 of
 [RFC9002].
 If a sender detects that a packet containing a specific DATAGRAM
 frame might have been lost, the implementation MAY notify the
 application that it believes the datagram was lost.
 Similarly, if a packet containing a DATAGRAM frame is acknowledged,
 the implementation MAY notify the sender application that the
 datagram was successfully transmitted and received.  Due to
 reordering, this can include a DATAGRAM frame that was thought to be
 lost but, at a later point, was received and acknowledged.  It is
 important to note that acknowledgement of a DATAGRAM frame only
 indicates that the transport-layer handling on the receiver processed
 the frame and does not guarantee that the application on the receiver
 successfully processed the data.  Thus, this signal cannot replace
 application-layer signals that indicate successful processing.

5.3. Flow Control 

 DATAGRAM frames do not provide any explicit flow control signaling
 and do not contribute to any per-flow or connection-wide data limit.
 The risk associated with not providing flow control for DATAGRAM
 frames is that a receiver might not be able to commit the necessary
 resources to process the frames.  For example, it might not be able
 to store the frame contents in memory.  However, since DATAGRAM
 frames are inherently unreliable, they MAY be dropped by the receiver
 if the receiver cannot process them.

5.4. Congestion Control 

 DATAGRAM frames employ the QUIC connection's congestion controller.
 As a result, a connection might be unable to send a DATAGRAM frame
 generated by the application until the congestion controller allows
 it [RFC9002].  The sender MUST either delay sending the frame until
 the controller allows it or drop the frame without sending it (at
 which point it MAY notify the application).  Implementations that use
 packet pacing (Section 7.7 of [RFC9002]) can also delay the sending
 of DATAGRAM frames to maintain consistent packet pacing.
 Implementations can optionally support allowing the application to
 specify a sending expiration time beyond which a congestion-
 controlled DATAGRAM frame ought to be dropped without transmission.

6. Security Considerations 

 The DATAGRAM frame shares the same security properties as the rest of
 the data transmitted within a QUIC connection, and the security
 considerations of [RFC9000] apply accordingly.  All application data
 transmitted with the DATAGRAM frame, like the STREAM frame, MUST be
 protected either by 0-RTT or 1-RTT keys.
 Application protocols that allow DATAGRAM frames to be sent in 0-RTT
 require a profile that defines acceptable use of 0-RTT; see
 Section 5.6 of [RFC9001].
 The use of DATAGRAM frames might be detectable by an adversary on
 path that is capable of dropping packets.  Since DATAGRAM frames do
 not use transport-level retransmission, connections that use DATAGRAM
 frames might be distinguished from other connections due to their
 different response to packet loss.

7. IANA Considerations

7.1. QUIC Transport Parameter 

 This document registers a new value in the "QUIC Transport
 Parameters" registry maintained at <https://www.iana.org/assignments/
 quic>.
 Value:  0x20
 Parameter Name:  max_datagram_frame_size
 Status:  permanent
 Specification:  RFC 9221

7.2. QUIC Frame Types 

 This document registers two new values in the "QUIC Frame Types"
 registry maintained at <https://www.iana.org/assignments/quic>.
 Value:  0x30-0x31
 Frame Name:  DATAGRAM
 Status:  permanent
 Specification:  RFC 9221

8. References

8.1. Normative References 

 [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>.
 [RFC9000]  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>.
 [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
            QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
            <https://www.rfc-editor.org/info/rfc9001>.
 [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
            and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
            May 2021, <https://www.rfc-editor.org/info/rfc9002>.

8.2. Informative References 

 [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            DOI 10.17487/RFC0768, August 1980,
            <https://www.rfc-editor.org/info/rfc768>.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
            January 2012, <https://www.rfc-editor.org/info/rfc6347>.

Acknowledgments 

 The original proposal for this work came from Ian Swett.
 This document had reviews and input from many contributors in the
 IETF QUIC Working Group, with substantive input from Nick Banks,
 Lucas Pardue, Rui Paulo, Martin Thomson, Victor Vasiliev, and Chris
 Wood.

Authors' Addresses 

 Tommy Pauly
 Apple Inc.
 One Apple Park Way
 Cupertino, CA 95014
 United States of America
 Email: tpauly@apple.com
 Eric Kinnear
 Apple Inc.
 One Apple Park Way
 Cupertino, CA 95014
 United States of America
 Email: ekinnear@apple.com
 David Schinazi
 Google LLC
 1600 Amphitheatre Parkway
 Mountain View, CA 94043
 United States of America
 Email: dschinazi.ietf@gmail.com
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