An Unreliable Datagram Extension to QUIC

Introduction The QUIC Transport Protocol 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 retransmission. Some applications, particularly those that need to transmit real-time data, prefer to transmit data unreliably. These applications can build directly upon UDP as a transport, and can add security with DTLS . 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 two new DATAGRAM QUIC frame types, which carry application data without requiring retransmissions. Discussion of this work is encouraged to happen on the QUIC IETF mailing list quic@ietf.org or on the GitHub repository which contains the draft: https://github.com/quicwg/datagram.

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 when, and only when, they appear in all capitals, as shown here.

Motivation Transmitting unreliable data over QUIC provides benefits over existing solutions:

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.
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.
QUIC datagrams, while unreliable, can support acknowledgements, allowing applications to be aware of whether a datagram was successfully received.
QUIC datagrams are subject to QUIC congestion control, allowing applications to avoid implementing their own.

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 datagrams.

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, 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 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. Endpoints that wish to use DATAGRAM frames need to ensure they send a max_datagram_frame_size value sufficient to allow their peer to use them. It is RECOMMENDED to send the value 65536 in the max_datagram_frame_size transport parameter as that indicates to the peer that this endpoint will accept any DATAGRAM frame that fits inside a QUIC packet. 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 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 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 max_datagram_frame_size transport parameter being missing. 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.

Datagram Frame Type DATAGRAM frames are used to transmit application data in an unreliable manner. The DATAGRAM frame type takes the form 0b0011000X (or the values 0x30 and 0x31). 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 DATAGRAM frame is structured as follows:

DATAGRAM Frame Format DATAGRAM frames contain the following fields:

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. Note that empty (i.e., zero-length) datagrams are allowed.
Datagram Data:: The bytes of the datagram to be delivered.

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. Application protocols using datagrams are responsible for defining the semantics of the Datagram Data field, and how it is parsed. If the application protocol supports the coexistence of multiple entities using datagrams inside a single QUIC connection, it may need a mechanism to allow demultiplexing between them. For example, using datagrams with HTTP/3 involves prepending a flow identifier to all datagrams, see . 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_packet_size transport parameter, and by 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.

Acknowledgement Handling Although DATAGRAM frames are not retransmitted upon loss detection, they are ack-eliciting (). Receivers SHOULD support delaying ACK frames (within the limits specified by max_ack_delay) in reponse to receiving packets that only contain 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 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 application that the datagram was successfully transmitted and received. Note that, due to reordering, a DATAGRAM frame that was thought to be lost could at a later point be received and acknowledged.

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.

Congestion Control DATAGRAM frames employ the QUIC connection's congestion controller. As a result, a connection may be unable to send a DATAGRAM frame generated by the application until the congestion controller allows it . The sender implementation 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 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.

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.

IANA Considerations This document registers a new value in the QUIC Transport Parameter Registry:

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 registry:

Value:: 0x30 and 0x31 (if this document is approved)
Frame Name:: DATAGRAM
Specification:: Unreliable application data

Acknowledgments Thanks to Ian Swett, who inspired this proposal.