DTN QUIC Bundle Protocol Convergence Layer (qubicle)

Introduction Bundle Protocol version 7 (BPv7) requires Convergence Layer Adapters (CLAs) to transfer bundles between nodes. This document specifies the QUIC Bundle Protocol Convergence Layer (QBCL or "qubicle"), a minimal CLA using QUIC that embraces QUIC's native capabilities rather than layering additional protocol machinery. The design philosophy is simple: QUIC already provides reliable streams, multiplexing, flow control, congestion control, and integrated security. This specification adds only what is strictly necessary to transfer bundles. The protocol provides two services:

Reliable Service:: Bundles are transferred on QUIC streams with guaranteed delivery.
Unreliable Service:: Bundles are transferred via QUIC datagrams using framing.

Conventions and Definitions 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.

Client:: The Qubicle peer that initiates the QUIC connection. This is a connection-level role and does not imply any restriction on bundle transfer direction.
Server:: The Qubicle peer that accepts the QUIC connection. This is a connection-level role and does not imply any restriction on bundle transfer direction.
Qubicle Session:: The period during which a QUIC connection is established between two Qubicle peers. A session begins when the QUIC handshake completes and ends when the QUIC connection closes. Both client and server are equal peers for the purpose of bundle transfer.

Applicability Statement QBCL SHOULD NOT be used in deployments where the QUIC transport may not perform well. It is primarily targeted to deployments where round-trip times remain under a few seconds, making QUIC's 1-RTT handshake overhead negligible relative to data transfer time. For extremely high-delay or distrupted environments such as deep space communications (e.g., Earth-Mars links with multi-minute RTTs), any handshake may represent significant absolute delay, and specialized protocols like LTP ({?RFC5326}) may be more appropriate. Similarly, the SVCB-based DNS service discovery mechanism ({<dns-example}) SHOULD NOT be used in environments where DNS itself might not perform well. DNS-based discovery is NOT RECOMMENDED for use in DTN environments where DNS infrastructure is unavailable, network disruptions cause failed lookups or stale cached records, DNSSEC validation fails due to a mismatch between query RTT and valid signature lifetimes, or DNS query overhead is significant relative to available bandwidth. For such environments, implementations SHOULD support alternative CL provisioning mechanisms including manual configuration with pre-planned contact schedules, contact graph routing protocols that maintain topology independently of DNS, or out-of-band metadata distribution through mission management plane channels. A hybrid approach is RECOMMENDED for nodes bridging Internet and deep-space networks: use QBCL with DNS discovery for Internet-side connections, and use alternate mission management planes for space-side connections.

Protocol Overview

Connection Establishment A Qubicle session is established by initiating a QUIC connection to a peer. The QUIC handshake provides mutual authentication via TLS 1.3 . The ALPN identifier for Qubicle is qbcl.

Reliable Bundle Transfer For reliable transfer, each bundle is sent on a dedicated QUIC unidirectional stream:

The sender creates a new unidirectional stream.
The sender writes the complete bundle (CBOR-encoded per ) to the stream.
The sender closes the stream by sending a STREAM frame with the FIN bit set.

The bundle is implicitly framed by the stream boundaries. No length prefix or application-layer framing is required. The receiver reads data from the stream until FIN is received, then delivers the complete bundle to the BPA. Receipt of more than one Bundle on a given stream is a protocol error, and the receiver MUST abort the connection with QBCL_PROTOCOL_ERROR. QUIC guarantees reliable, in-order delivery of stream data. No application-layer acknowledgment is required; the sender can consider the transfer complete when QUIC confirms the stream data has been acknowledged by the peer. Completed transfer at the convergence layer does not guarantee successful receipt at the receiving Bundle Protocol Agent, so this signal alone does not suffice to indicate when a Bundle can be deleted from the sender. Additional information at the Bundle Protocol layer is required to confirm successful transfer.

Bidirectional Bundle Flow Both peers can send bundles simultaneously. Each peer creates unidirectional streams to send its bundles. QUIC stream IDs inherently separate client-initiated streams (IDs 2, 6, 10...) from server-initiated streams (IDs 3, 7, 11...), ensuring no collision between the two directions of bundle flow.

Stream Selection and Priority Senders MAY use QUIC stream priorities to expedite higher-priority bundles. The mapping of bundle priority to QUIC stream priority is an implementation matter.

Stream Exhaustion QUIC stream identifiers are 62-bit values, providing an effectively unlimited number of streams per connection. The MAX_STREAMS transport parameter limits concurrent streams, not the total number of streams over a connection's lifetime. If an implementation reaches practical limits on stream creation, it SHOULD close the connection and establish a new one.

Unreliable Bundle Transfer For unreliable transfer, bundles are sent using QUIC datagrams with framing. Each QUIC datagram contains one or more messages. The specification defines segmentation, reassembly, transfer identification, and optional repetition for probabilistic reliability. Implementations MUST negotiate the QUIC max_datagram_frame_size transport parameter to enable datagram support. The mapping of bundle priority to transfer interleaving is an implementation matter.

Connection Termination To terminate a session, a peer closes the QUIC connection using CONNECTION_CLOSE. Application-specific error codes are defined in . A peer MAY close the connection at any time. In-flight reliable transfers on incomplete streams will fail; the BPA is notified of the failure.

Transfer Cancellation TODO(ek): describe how a receiver can cancel a transfer via STOP_SENDING.

Keepalive Qubicle relies on QUIC's native idle timeout mechanism. Peers negotiate the max_idle_timeout transport parameter during connection establishment. If application-layer liveness detection is required, implementations MAY send QUIC PING frames.

Error Codes The following application error codes are defined for use with QUIC CONNECTION_CLOSE: Qubicle Error Codes

Code	Name	Description
0x00	QBCL_NO_ERROR	Graceful closure, no error
0x01	QBCL_PROTOCOL_ERROR	Qubicle protocol error encountered

Security Considerations

Transport Security QUIC mandates TLS 1.3 for all connections, providing confidentiality, integrity, and authentication. Qubicle inherits these security properties. Implementations SHOULD require peer certificate authentication. The Node ID in the transport parameter SHOULD match an identity in the peer's certificate. The BundleEID OtherName form defined in provides a standard mechanism for embedding DTN Node IDs in X.509 certificates. Automated certificate provisioning is available via the ACME extensions defined in .

Bundle Security Transport security protects bundles in transit between adjacent nodes. For end-to-end bundle security, implementations SHOULD use BPSec .

Denial of Service QUIC provides built-in protection against many denial-of-service attacks, including address validation and amplification prevention. Implementations SHOULD apply rate limiting on bundle reception to prevent resource exhaustion.

0-RTT Considerations QUIC 0-RTT data is subject to replay attacks. Implementations that enable 0-RTT SHOULD only send bundles that are safe to replay (e.g., bundles with replay protection at the bundle layer).

Operational Considerations

Version Negotiation Qubicle endpoints wishing to combat various ossification vectors are RECOMMENDED to support version negotiation and the same Bundle transfer operations described in this memo over QUIC v2 .

Convergence Layer Fallback As noted in , some networks block UDP traffic such that Qubicle connections cannot be established. Bundle Protocol Agents that employ Qubicle are RECOMMENDED to support additional Convergence Layers, e.g. TCPCLv4 .

Coexistence With Other UDP-based Convergence Layers It is RECOMMENDED that Qubicle implementations use a dedicated UDP port for operational simplicity. Bundle Protocol Agents that employ Qubicle and other UDP-based Convergence Layers on the same UDP port MUST be able to disambiguate received datagrams in order to route them to the correct CLA. For UDP CLs that use DTLS, provides the required guidance to disambiguate QUIC traffic from DTLS-encapsulated CL traffic.

Finding a Qubicle Endpoint Via DNS {#dns-example} Qubicle senders may be manually provisioned with a hostname (or IP addresses) and UDP port corresponding to the listening Qubicle endpoint for a peer Bundle Protocol Agent. If only a hostname is known but a port is not, SVCB Resource Records may be looked up to find a listening UDP port and confirm expected ALPN configuration. Consider this zone file for example.: A BPA supporting both may attempt to resolve an SRV record for the _dtn-bundle._tcp prefixed hostname. A BPA that support Qubicle might also issue DNS SVCB queries for the prefix "_qbcl". The sample above indicates that mars-orbiter.example. has an SVCB record in AliasMode referring to cloud-agent.example. The SVCB record associated with cloud-agent.example. contains all required QUIC transport rendezvous information.

IANA Considerations

ALPN Identifier IANA is requested to register the following ALPN identifier in the "TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry: ALPN Registration

Protocol	Identification Sequence	Reference
Qubicle	0x71 0x62 0x63 0x6C ("qbcl")	This document

AttrLeaf Node Name Per , IANA is request to add the following entry to the DNS "Underscored and Globally Scoped DNS Node Names" registry: AttrLeaf Registration

RR Type	_NODE NAME	Reference
SVCB	_qbcl	this document

Application Error Codes IANA is requested to create a new registry "Qubicle Error Codes" with the following initial values: Error Code Registry

Code	Name	Reference
0x00	QBCL_NO_ERROR	This document
0x01	QBCL_PROTOCOL_ERROR	This document
0x02-0xEF	Unassigned
0xF0-0xFF	Reserved for Private Use	This document