RUDPClient/openspec/specs/kcp-transport/spec.md

kcp-transport Specification

Purpose

TBD - created by archiving change introduce-kcp-transport. Update Purpose after archive.

Requirements

Requirement: Client mode uses a default KCP session

The transport SHALL support a client mode that is constructed with a default remote endpoint and creates exactly one default KCP session after StartAsync. Calls to Send(byte[] data) in client mode MUST encode the payload through that session and emit the resulting UDP datagrams to the configured remote endpoint.

Scenario: Client sends through the default remote session

• WHEN the application starts a client-mode transport and calls Send with a business payload
• THEN the transport routes the payload through the default KCP session
• THEN the UDP socket sends the encoded datagrams to the configured remote endpoint

Requirement: Server mode isolates KCP session state per remote endpoint

The transport SHALL support a server mode that receives UDP datagrams from multiple remote endpoints and maintains independent KCP session state for each active remote endpoint. A datagram received from one endpoint MUST only be applied to that endpoint's session, and SendToAll(byte[] data) MUST encode and enqueue the payload once per active session.

Scenario: Two remotes do not share KCP state

• WHEN a server-mode transport receives KCP traffic from two different remote endpoints
• THEN the transport creates or reuses separate KCP session state for each endpoint
• THEN payloads reconstructed for one endpoint are delivered with that endpoint as the sender

Scenario: Broadcast writes through each active session

• WHEN the application calls SendToAll while the server transport has multiple active sessions
• THEN the transport encodes the payload for each active KCP session
• THEN the UDP socket sends datagrams to every active remote endpoint without collapsing them into a shared session

Requirement: OnReceive only dispatches complete KCP payloads

The transport SHALL invoke OnReceive only after a complete application payload has been reconstructed from Kcp.Recv. Raw UDP packets, partial KCP fragments, and transport-level acknowledgements MUST NOT be surfaced to the message layer.

Scenario: Fragmented payload is withheld until complete

• WHEN a business message spans multiple UDP datagrams and only a subset of those datagrams has been processed
• THEN the transport does not invoke OnReceive

Scenario: Reassembled payload is forwarded to the message layer

• WHEN the remaining datagrams for a fragmented KCP message are processed and Kcp.Recv yields a complete payload
• THEN the transport invokes OnReceive exactly once with the reconstructed payload and the originating remote endpoint

Requirement: Active sessions are driven until stop and cleaned up on shutdown

The transport SHALL continue driving KCP timers for every active session while it is running, so retransmissions, acknowledgements, and flushes can progress even when no new UDP datagrams arrive. Calling Stop() MUST stop the receive and update loops, release the UDP socket, and clear active session state.

Scenario: Idle sessions still receive KCP timer updates

• WHEN the transport has an active session with pending KCP work but no new incoming UDP datagrams
• THEN the transport continues calling the KCP update path according to its internal schedule

Scenario: Stop releases transport resources

• WHEN the application calls Stop() on a running transport
• THEN the transport stops receiving new UDP datagrams
• THEN the transport clears its active KCP session state before shutdown completes