WTP State Machine

Status: ACTIVE Date: 2026-03-04

Defines the transaction-layer contract for implementation and test.

Source grounding:

• spec-processing/source-material/WAP-224-WTP-20010710-a.pdf
• spec-processing/source-material/OMA-WAP-224_002-WTP-SIN-20020827-a.PDF

1) Transaction classes

Class	Reliability model	MVP scope	Key protocol obligations
0	Unreliable invoke	Deferred (parsing support first)	no retransmission
1	Reliable one-way	Implemented	Invoke + ACK path
2	Reliable request/response	Primary	Invoke -> ACK -> RESULT

2) Core state model

• Idle
• InvokeSent
• AwaitingAck
• AwaitingResult
• Completed
• Aborted
• TimedOut
• Closed

3) Class-specific transitions

Class 0 (unreliable invoke)

From	Event	To	Side effects
Idle	send_invoke	Completed	emit invoke; no ack/result handling

Class 1 (reliable one-way)

From	Event	Guard	To	Side effects
Idle	send_invoke	valid TID, encoding complete	InvokeSent	send invoke; start retransmission
InvokeSent	rx_ack	tid/session match	Completed	stop retransmission; notify success
InvokeSent	rx_nack	transport accepted negative ack	Completed	surface protocol fault
InvokeSent	timer_expired	retries remaining	InvokeSent	resend invoke
InvokeSent	timer_expired	no retries	TimedOut	timeout event and user-facing error

Class 2 (reliable request/response)

From	Event	Guard	To	Side effects
Idle	send_invoke	valid TID, encoding complete	InvokeSent	send invoke; start retransmission
InvokeSent	tx_fragment	transport layer accepts	AwaitingAck	transaction remains queued
AwaitingAck	rx_ack	tid/session match	AwaitingResult	stop invoke retry timers
AwaitingAck	rx_result	no reliable ordering requirement in this layer	AwaitingResult	allow optimistic result handling
AwaitingResult	rx_result	tid/session match	Completed	emit response payload
AwaitingResult	timer_expired	result timeout policy met	TimedOut	timeout and finalize
AwaitingAck/AwaitingResult	abort	user cancel or peer abort primitive	Aborted	generate abort result

4) Duplicate handling and idempotency

• Track (peer, tid, opcode) window.
• If duplicate invoke is detected and transaction is already pending, emit idempotent acknowledgement when required.
• If completed/closed duplicate carries a terminal side effect, return/replay terminal outcome only.
• If duplicate is observed from unknown opcode, emit PROTOERR path per profile.

5) Deterministic contract rules

For every transition:

• previous state
• triggering event
• preconditions
• next state
• event record written to trace log

No state mutation in parser functions. Parser functions return a ParsedPdu only.

6) Retransmission and timer policy

• Use deterministic timer profile per bearer (per spec-defined defaults in WTP conformance text).
• Retry loop is bounded by a configured max attempts and explicit stop-on-success.
• Use a deterministic clock in tests for reproducibility.

Implement in RetransmissionPolicy with a simple stateful policy object:

pub struct RetransmissionPolicy {
    pub initial_ms: u64,
    pub max_retries: u8,
    pub backoff: BackoffKind,
    pub max_timeout_ms: u64,
}

7) TID policy and replay windows

• Maintain separate pools for client and server roles.
• Enforce valid incoming TID ranges and window policy.
• TID allocation is deterministic and supports wrap behavior.
• responder side should detect stale/duplicate TIDs and reply according to the profile-defined policy (including retransmitting cached terminal outcomes).

8) SAR/ESAR scope (optional)

Selective acknowledgment and retransmission can be enabled on top of class-2 where profile requires:

• packet grouping,
• per-group NACK and hold-off behavior,
• selective replay of missed segments.

Current profile defaults to off unless explicitly enabled by transport configuration.