An Unreliable Datagram Extension to QUIC

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Table of Contents

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 retransmission.¶

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 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.¶

2. 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.¶

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, 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 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 65535 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.¶

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.¶

4. 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:¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        [Length (i)]                         ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Datagram Data (*)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.¶

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.¶

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 [I-D.schinazi-quic-h3-datagram].¶

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.¶

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.¶

7. 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¶

8. Acknowledgments

Thanks to Ian Swett, who inspired this proposal.¶

Authors' Addresses