Multi-stream Transport Specification

This specification defines how the roam protocol operates over multi-stream transports like QUIC and WebTransport. These transports provide multiple independent streams, which can eliminate head-of-line blocking.

Status

This specification is not yet implemented. It will be implemented when QUIC/WebTransport support is added to roam.

Control Stream

multistream.control

Implementations MUST designate a control stream for control messages (Hello, Goodbye, Request, Response, Cancel, Credit). The initiator opens this stream first; the acceptor's first received stream is the control stream. Control messages are COBS-framed POSTCARD-encoded Message values.

Stream Mapping

multistream.streams

Implementations MUST map each roam stream to a dedicated unidirectional transport stream. roam streams are unidirectional (see r[core.stream] in the main roam spec).

multistream.stream-id-header

Each dedicated transport stream MUST begin with a 8-byte header containing the roam stream_id (little-endian u64). This allows the receiver to associate the transport stream with the stream ID from the Request/Response payload.

multistream.stream-id-mapping

The data sender opens a transport stream, writes the stream ID header, then sends data. For Push<T> the caller opens it; for Pull<T> the callee opens it. The receiver reads the header to determine which roam stream this transport stream carries.

Note: Transport stream IDs (e.g., QUIC stream IDs) are transport-specific and not visible to roam. The roam stream_id is allocated by the caller according to the binding's scheme (e.g., r[streaming.id.parity] for peer-to-peer).

Stream Data

multistream.stream-data

After the stream ID header, data is sent as COBS-framed POSTCARD- encoded values of the stream's element type T. No Message wrapper is needed — the stream identity was established by the header.

Stream Lifecycle

multistream.stream-close

Closing a roam stream is signaled by closing the transport stream (e.g., QUIC FIN). The Close message is not used on multi-stream transports.

multistream.stream-reset

Resetting a roam stream is signaled by resetting the transport stream (e.g., QUIC RESET_STREAM). The Reset message is not used on multi-stream transports.

Why COBS on Control Streams?

QUIC streams are byte streams, not message streams. We need framing. COBS provides:

Guaranteed message boundaries (0x00 delimiter)
Low overhead (≈1 byte per 254 bytes)
No escape sequences that could cause ambiguity