An Unreliable Datagram Extension to QUIC

Introduction The QUIC Transport Protocol 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 or not 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 as a transport, and have often added security with DTLS . 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, which carry application data without requiring retransmissions.

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 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.

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. 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. 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 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 Types 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. DATAGRAM frames are structured as follows:

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.

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, 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_packet_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.

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. Identifiers used to multiplex different kinds of datagrams, or flows of datagrams, are the responsibility of the application protocol running over QUIC to define. 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.

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 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 . 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 which 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.

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.

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 . 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 that use packet pacing () 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.

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. Application protocols that allow DATAGRAM frames to be sent in 0-RTT require a profile that defines acceptable use of 0-RTT; see . 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 frames using the different response to loss.

IANA Considerations

QUIC Transport Parameter This document registers a new value in the QUIC Transport Parameter Registry maintained at .

Value:: 0x20 (if this document is approved)
Parameter Name:: max_datagram_frame_size
Status:: permanent
Specification:: This document

QUIC Frame Types This document registers two new values in the QUIC Frame Type registry maintained at .

Value:: 0x30 and 0x31 (if this document is approved)
Frame Name:: DATAGRAM
Status:: permanent
Specification:: This document

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.