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Network Working Group                                           T. Pauly
Internet-Draft                                                E. Kinnear
Intended status: Standards Track                              Apple Inc.
Expires: January 7, 2020                                     D. Schinazi
                                                              Google LLC
                                                           July 06, 2019

                An Unreliable Datagram Extension to QUIC


   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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Specification of Requirements . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Transport Parameter . . . . . . . . . . . . . . . . . . . . .   3
   4.  Datagram Frame Type . . . . . . . . . . . . . . . . . . . . .   4
   5.  Behavior and Usage  . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Flow Identifiers  . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Acknowledgement Handling  . . . . . . . . . . . . . . . .   5
     5.3.  Flow Control  . . . . . . . . . . . . . . . . . . . . . .   6
     5.4.  Congestion Control  . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The QUIC Transport Protocol [I-D.ietf-quic-transport] provides a
   secure, multiplexed connection for transmitting reliable streams of
   application data.  Reliability within QUIC is performed on a per-
   stream basis, so some frame types are not eligible for

   Some applications, particularly those that need to transmit real-time
   data, prefer to transmit data unreliably.  These applications can
   build directly upon UDP [RFC0768] as a transport, and can add
   security with DTLS [RFC6347].  Extending QUIC to support transmitting
   unreliable application data would provide another option for secure
   datagrams, with the added benefit of sharing a cryptographic and
   authentication context used for reliable streams.

   This document defines four new DATAGRAM QUIC frame types, which carry
   application data without requiring retransmissions.

1.1.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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2.  Motivation

   Transmitting unreliable data over QUIC provides benefits over
   existing solutions:

   o  Applications that open both a reliable TLS stream and an
      unreliable DTLS flow to the same peer can benefit by sharing a
      single handshake and authentication context between a reliable
      QUIC stream and flow of unreliable QUIC datagrams.  This can
      reduce the latency required for handshakes.

   o  QUIC uses a more nuanced loss recovery mechanism than the DTLS
      handshake, which has a basic packet loss retransmission timer.
      This may allow loss recovery to occur more quickly for QUIC data.

   o  QUIC datagrams, while unreliable, can support acknowledgements,
      allowing applications to be aware of whether a datagram was
      successfully received.

   These reductions in connection latency, and application insight into
   the delivery of datagrams, 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

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=0x0020).  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, flow identifier, length and payload) the endpoint is willing to
   receive, in bytes.  An endpoint that includes this parameter supports
   the DATAGRAM frame types and is willing to receive such frames on
   this connection.  Endpoints MUST NOT send DATAGRAM frames until they
   have sent and received the max_datagram_frame_size transport
   parameter.  Endpoints MUST NOT send DATAGRAM frames of size strictly
   larger than the value of max_datagram_frame_size the endpoint has
   received from its peer.  An endpoint that receives a DATAGRAM frame

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   when it has not sent the max_datagram_frame_size transport parameter
   MUST terminate the connection with error PROTOCOL_VIOLATION.  An
   endpoint that receives a DATAGRAM frame that is strictly larger than
   the value it sent in its max_datagram_frame_size transport parameter
   MUST terminate the connection with error PROTOCOL_VIOLATION.

4.  Datagram Frame Type

   DATAGRAM frames are used to transmit application data in an
   unreliable manner.  The DATAGRAM frame type takes the form 0b001100XX
   (or the set of values from 0x30 to 0x33).  The least significant bit
   of the DATAGRAM frame type is the LEN bit (0x01).  It indicates that
   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.  The
   second least significant bit of the DATAGRAM frame type is the
   FLOW_ID bit (0x02).  It indicates that there is a Flow ID field
   present.  If this bit is set to 0, the Flow ID field is absent and
   the Flow ID is assumed to be zero.  If this bit is set to 1, the Flow
   ID field is present.

   The DATAGRAM frame is structured as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                        [Flow ID (i)]                        ...
   |                        [Length (i)]                         ...
   |                      Datagram Data (*)                      ...

                      Figure 1: DATAGRAM Frame Format

   DATAGRAM frames contain the following fields:

   Flow ID:  A variable-length integer indicating the Flow ID of the
      datagram (see Section 5.1).  This field is present when the
      FLOW_ID bit is set, and is assumed to be zero otherwise.

   Length:  A variable-length integer specifying the length of the
      datagram in bytes.  This field is present only when the LEN bit is
      set.  If the LEN bit is not set, the datagram data extends to the
      end of the QUIC packet.

   Datagram Data:  The bytes of the datagram to be delivered.

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5.  Behavior and Usage

   When an application sends an unreliable 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, 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.

   DATAGRAM frames MUST be protected with either 0-RTT or 1-RTT keys.

5.1.  Flow Identifiers

   Flow identifiers represent bidirectional flows of datagrams within a
   single QUIC connection.  These are effectively equivalent to UDP
   ports, that allow basic demultiplexing of application data.  Whenever
   one side of a connection receives a frame with a Flow ID was was not
   previously known, it MAY represent this to the application as a new
   flow of datagrams.

   The primary role of the QUIC transport towards the flow identifier is
   to provide a standard mechanism for demultiplexing application data
   flows, which may be destined for different processing threads in the
   application, akin to UDP sockets.

   Beyond this, a sender SHOULD ensure that DATAGRAM frames within a
   single flow are transmitted in order relative to one another.  If
   multiple DATAGRAM frames can packed into a single packet, the sender
   SHOULD group them by Flow ID to promote fate-sharing within a
   specific flow and improve the ability to process batches of datagram
   messages efficiently on the receiver.

   Applications that do not have a need for the Flow ID can use the
   value zero on their DATAGRAM frames and clear the FLOW_ID bit to omit
   sending the identifier over the wire.  If an application uses a
   mixture of DATAGRAM frames with and without the FLOW_ID bit set, the
   frames without it are assumed to be part of the application-level
   flow with Flow ID zero.

5.2.  Acknowledgement Handling

   Although DATAGRAM frames are not retransmitted upon loss detection,
   they are ack-eliciting ([I-D.ietf-quic-recovery]).  Receivers SHOULD
   support delaying ACK frames (within the limits specified by
   max_ack_delay) in reponse to receiving packets that only contain

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   DATAGRAM frames, since the timing of these acknowledgements is not
   used for loss recovery.

   If a sender detects that a packet containing a specific DATAGRAM
   frame has been lost, the implementation MAY notify the application
   that the datagram was lost.  Similarly, if a packet containing a
   DATAGRAM frame is acknowledged, the implementation MAY notify the
   application that the datagram was successfully transmitted and

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 may not be able to commit the necessary
   resources to process the frames.  For example, it may 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 are subject to a QUIC connection's congestion
   control.  Specifically, if a DATAGRAM frame is enqueued to be sent by
   the application, but sending a packet with this frame is not allowed
   by the congestion control window as specified in
   [I-D.ietf-quic-recovery], the packet cannot be sent.  The sender
   implementation MUST either drop the frame without sending it (at
   which point it MAY notify the application) or else delay sending the
   frame until the window opens.

   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.  All application data
   transmitted with the DATAGRAM frame, like the STREAM frame, MUST be
   protected either by 0-RTT or 1-RTT keys.

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7.  IANA Considerations

   This document registers a new value in the QUIC Transport Parameter

   Value:  0x0020 (if this document is approved)

   Parameter Name:  max_datagram_frame_size

   Specification:  Indicates that the connection should enable support
      for unreliable DATAGRAM frames.  An endpoint that advertises this
      transport parameter can receive datagrams frames from the other
      endpoint, up to and including the length in bytes provided in the
      transport parameter.

   This document also registers a new value in the QUIC Frame Type

   Value:  0x30 - 0x33 (if this document is approved)

   Frame Name:  DATAGRAM

   Specification:  Unreliable application data

8.  Acknowledgments

   Thanks to Ian Swett, who inspired this proposal.

9.  References

9.1.  Normative References

              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-20 (work in
              progress), April 2019.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), April 2019.

9.2.  Informative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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>.

   [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>.

Authors' Addresses

   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: tpauly@apple.com

   Eric Kinnear
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014
   United States of America

   Email: ekinnear@apple.com

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America

   Email: dschinazi.ietf@gmail.com

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