2-Phase HTTP Protocol for Distributed Transaction Tracking (2PHP-DTT)

Introduction Microservices architectures built on HTTP REST APIs are widely deployed for distributed systems communication. Mutation requests--those using HTTP methods POST, PUT, PATCH, and DELETE--are treated as single atomic operations from the client's perspective. In practice, however, no standard protocol mechanism exists to confirm that both the client and server have durably registered the intent of a request before processing begins. Existing approaches--including the Saga pattern , Transactional Outbox , and application-layer idempotency keys--address failure recovery after the fact or are implemented ad hoc without a standard contract. Distributed tracing systems such as OpenTelemetry provide execution telemetry but cannot answer whether both parties registered intent before processing commenced. This document specifies the 2-Phase HTTP Protocol for Distributed Transaction Tracking (2PHP-DTT), an opt-in, backward-compatible extension to HTTP that introduces a lightweight two-phase handshake at each service boundary. 2PHP ensures bilateral durable registration of request intent before any business logic is executed. It further defines a standardized Intent Ledger model that enables cross-service correlation of request intent across distributed call trees. The acronym DTT expands to Distributed Transaction Tracking. 2PHP is not Two-Phase Commit (2PC). 2PC requires cross-service locking and blocking coordination that microservices architectures deliberately avoid. 2PHP introduces no distributed transaction coordinator, no cross-service locks, and no thread-blocking. Each service boundary owns its own handshake independently; the Intent Ledger is a passive, queryable record of that handshake, not a coordinator. What is standardized here is the tracking pattern that implementations currently build ad hoc: a common wire contract and a common ledger contract, rather than a per-team reinvention. A primary motivator for this work is the growing use of agent-driven HTTP invocations, including Model Context Protocol (MCP) tool calls and agent-to-agent (A2A) integrations. Agents default to retrying when a response is not received within an expected window. Without a queryable record of prior intent, successive retries--often from multiple relays or peer agents--pile onto services that may already be overwhelmed. The result is server-side deadlocks, resource exhaustion, and cascading outages. 2PHP's Phase1 ledger record enables the client, upstream service, or orchestrating agent to inspect whether the prior intent was durably registered before issuing a retry, breaking the retry-storm feedback loop at the protocol layer. This inspection step is the first line of defense in agent-driven integration topologies where retry pressure is otherwise unbounded. This gap is specific to synchronous HTTP invocations. Asynchronous integration patterns--202 Accepted responses with Location-based status polling (), Prefer: respond-async negotiation (), webhooks, and message-broker delivery--already provide a durable acknowledgement of intent as part of their wire contract: the client receives a handle or ack the moment the server accepts the request, and inspecting prior state before retrying is built in. 2PHP-DTT does not attempt to duplicate those mechanisms. It addresses the boundary they leave open: synchronous request-response calls, which remain the dominant integration style for microservices and agent-driven tool invocations. The value proposition of this specification is standardization, not novel mechanism. Intent tracking, idempotent retries, and correlation-based inspection are all patterns implemented today across microservices deployments, typically ad hoc and per-team. This document does not claim to be the only solution to any of these problems, nor does it claim novelty over existing patterns. Its aim is a consistent, standardized wire contract for intent registration and Distributed Transaction Tracking: client-supplied correlation identifiers, an explicit phase state contract carried in HTTP header fields, and a queryable Intent Ledger schema, specified together so that a common implementation--packaged as a library in Spring Boot, Express.js, an API gateway policy, or an agent framework middleware--can serve heterogeneous deployments without per-vendor reinvention. Where deployments already use client-supplied identifiers for retry safety, the correlation identifier defined here MAY carry the same value; nothing in this document precludes composing 2PHP-DTT with existing idempotency, retry, or tracing conventions. 2PHP is designed to be:

Backward compatible--servers and clients that do not implement 2PHP are unaffected.
Lightweight--two HTTP round trips; no new transport layer.
Composable--each service boundary operates independently; no distributed coordination is required.
Standardized--a consistent header contract enabling uniform tooling, observability, and governance.

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.

2PHP:: The 2-Phase HTTP Protocol, as defined in this document.
Phase 1:: The Intent Registration phase, in which the client sends a mutation request and the server durably persists the request payload and an initial ledger record before responding.
Phase 2:: The Client Confirmation phase, in which the client confirms receipt of the Phase1 acknowledgement, triggering server-side business logic execution.
PONR (Point of No Return):: The moment at which the server begins executing business logic after receiving Phase2 confirmation. Processing cannot be rolled back after this point.
TTL (Time to Live):: The server-defined duration, in milliseconds, within which the client MUST send Phase2 confirmation.
Delta (delta):: A server-internal grace window applied after TTL expiry to handle race conditions at the PONR boundary. Not exposed to the client.
Intent Ledger:: A durable, queryable log of 2PHP transactions, distinct from execution telemetry.
Local Ledger:: A per-service, in-process ledger authoritative for that service's phase states.
Central Ledger:: A read-optimized aggregate view of all local ledgers, populated via background batch sync.
Client Correlation ID:: A client-assigned identifier for a specific request, included in all protocol headers and ledger entries for that interaction.
Server Correlation ID:: A server-assigned identifier for a specific intent registration, returned in the Phase1 response.
Parent Reference ID:: The correlation ID that the immediate upstream caller provided when initiating the call to a downstream service. Used to reconstruct distributed call trees.

Problem Statement

The Request Boundary Gap Current HTTP REST semantics treat a mutation request as a single atomic operation from the client's perspective. In reality, a server may:

Receive and process the request without the client ever confirming receipt of the response.
Fail mid-processing with no durable record of the original intent.
Return a success response that is lost in transit.

No standard HTTP mechanism exists to confirm that both the client and the server have registered the intent of a request before processing commences.

Limitations of Existing Telemetry Modern distributed tracing systems provide visibility into what executed across services, how long operations took, and where failures occurred. However, they cannot answer:

Did both client and server register the intent before processing?
Was a request abandoned or confirmed?
At what phase did a distributed transaction stall?

Limitations of Existing Patterns

Pattern	Scope	Client Involved?	Protocol Level?	Intent Ledger?
Two-Phase Commit (2PC)	Database	No	No	No
Saga Pattern	Application	No	No	No
Transactional Outbox	Server-side	No	No	No
Idempotency Keys	Application	Partial	No	No
HTTP 100 Continue	Protocol	Yes	Yes	No
2PHP (this document)	Protocol	Yes	Yes	Yes

Protocol Overview 2PHP extends HTTP mutation semantics with an opt-in two-phase handshake. Each service boundary owns its own independent handshake. Cross-service traceability is the responsibility of the centralized Intent Ledger, not the protocol itself.

2PHP Protocol Flow (Synchronous Mode) | | | Persist payload + | | ledger record atomically | <-- 200 OK + Server-Corr-ID ------- | | | | (Client writes local ledger entry) | | | |-- Phase 2: POST confirm ----------> | | | PONR crossed; | | business logic executes | <-- 200 OK + Resource-ID ---------- | | | ]]> The server MUST atomically persist the intent payload and the initial ledger record before returning the Phase1 response. The resource is created only after Phase2 confirmation is received and business logic has successfully executed.

Protocol Specification

Opt-In Header Clients signal 2PHP intent by including the following header on any mutation request: Servers that do not recognize this header MUST ignore it and process the request normally, preserving backward compatibility.

Phase 1 -- Intent Registration

Client Request DTT-2PHP-Client-Correlation-ID: Content-Type: application/json { ...payload... } ]]> The client MUST include DTT-2PHP-Client-Correlation-ID with a unique identifier for this request. This identifier MUST be used in all subsequent protocol exchanges and ledger entries for this interaction. The server MUST NOT substitute a server-generated proxy for this identifier: without a client-supplied identifier the client cannot deterministically query the ledger for prior intent, which is the retry-inspection guarantee that motivates this specification. If a request carries DTT-2PHP-Enabled: true but omits DTT-2PHP-Client-Correlation-ID, the server MUST reject the request with 400 Bad Request and populate DTT-2PHP-Message to identify the missing header. The server MUST NOT synthesize a correlation identifier on the client's behalf. This requirement applies to both Standard 2PHP () and Auto-Confirm mode (). The client MAY include DTT-2PHP-Requested-TTL (in milliseconds) to request that the server override its default TTL setting.

Server Response The server MUST atomically persist the intent payload and an initial ledger record with phase state WAITING_CONFIRM before returning the response. DTT-2PHP-Phase-State: WAITING_CONFIRM DTT-2PHP-TTL: DTT-2PHP-PONR-Deadline: ]]> Response header semantics:

DTT-2PHP-Server-Correlation-ID:: Server-assigned identifier for this intent registration. The client MUST echo this value in the Phase2 confirmation.
DTT-2PHP-TTL:: Milliseconds the server will wait for Phase2 confirmation. Server-defined. The client MUST send Phase2 before this duration elapses. If the client includes DTT-2PHP-Requested-TTL, the server MAY accommodate by increasing the TTL.
DTT-2PHP-PONR-Deadline:: Server-computed absolute deadline in ISO8601 format. The client MUST send Phase2 before this timestamp.
DTT-2PHP-Phase-State:: Current phase state. Value is WAITING_CONFIRM on Phase1 response.

No DTT-2PHP-Resource-ID is returned in Phase1. The resource does not yet exist; it is created only after Phase2 confirmation is received and business logic has successfully executed.

Client Ledger Write After receiving the Phase1 response, the client MUST write a ledger entry to its local ledger. The client writes after the Phase1 response so that both client_correlation_id and server_correlation_id are available at write time. The client's parent_reference_id MUST be set to its own client_correlation_id as the root anchor.

Phase 2 -- Client Confirmation

Client Request The client confirms receipt of the Phase1 acknowledgement: DTT-2PHP-Server-Correlation-ID: ]]> The client MUST send Phase2 before the DTT-2PHP-PONR-Deadline timestamp received in the Phase1 response. A 2PHP-capable client MUST NOT send a Phase2 confirmation if the Phase1 response omitted the DTT-2PHP-Server-Correlation-ID header. The absence of that header indicates the server did not process the request under 2PHP semantics--typically because the server does not implement this specification. In that case the response MUST be treated as a conventional terminal HTTP response and the client MUST NOT issue a Phase2 request against any derived path. This preserves backward compatibility with servers that ignore the DTT-2PHP-Enabled opt-in header (see ).

Server Response This revision specifies synchronous operation only. On receipt of a valid Phase2 confirmation, the server crosses the PONR, executes business logic inline, and responds once processing is complete: ]]> The server MUST return 200 OK only after business logic has completed and the resource is fully created. The DTT-2PHP-Resource-ID is authoritative--the resource exists at the point this response is returned. If business logic fails after PONR is crossed, the server MUST return 500 Internal Server Error. If the request is invalid, the server MUST return the appropriate 4xx response. In both cases the client receives a definitive outcome in the Phase2 response--no polling is required. Asynchronous operation is out of scope for this specification by design, not deferred. Asynchronous HTTP already provides a durable acknowledgement primitive: 202 Accepted with a Location handle for status polling as defined in , optionally negotiated with Prefer: respond-async as defined in . Webhook and message-broker delivery models similarly bake acknowledgement into their wire contract. In all of these, the client learns whether the server registered its intent the moment the request is accepted, which is precisely the gap 2PHP-DTT addresses for synchronous calls. Servers requiring asynchronous execution semantics SHOULD use those established mechanisms rather than extending 2PHP-DTT.

TTL Expiry Any Phase2 confirmation received after TTL expiry MUST be rejected with: The server MAY apply an internal grace window (delta) after TTL expiry to handle race conditions at the PONR boundary. This grace window is server-internal and MUST NOT be exposed in protocol headers or communicated to the client.

Auto-Confirm Mode

Overview Auto-Confirm mode is an opt-in server-side mode that eliminates the explicit Phase2 client confirmation round trip. When Auto-Confirm is enabled, the server writes the ledger record and crosses the PONR immediately after Phase1, then executes business logic inline on the original request channel. The final result (200 OK + DTT-2PHP-Resource-ID) is returned synchronously on that same channel-- behaviorally identical to a standard HTTP request from the client's perspective. Auto-Confirm mode does not include an out-of-band callback. Earlier revisions of this specification described an asynchronous server-to-client callback to deliver the intent acknowledgement independently of the original response channel. That mechanism has been removed because it introduced failure modes (callback delivery loss, client callback endpoint availability, and server-side SSRF exposure via a client-supplied URL) that outweighed its observability benefit. Clients that require an out-of-band record of registered intent SHOULD use Standard 2PHP (), which records client-side intent by construction. The client MUST include DTT-2PHP-Client-Correlation-ID on the Phase1 request in Auto-Confirm mode, as in Standard 2PHP (). The server MUST NOT substitute a server-generated proxy for this identifier: without a client-supplied correlation identifier the client cannot deterministically query the ledger for prior intent, which defeats the retry-inspection guarantee that motivates this specification. Requests missing this header MUST be rejected with 400 Bad Request and DTT-2PHP-Message indicating the missing header. The server MUST return DTT-2PHP-Server-Correlation-ID in the response so the client can correlate the ledger record with its own request. Auto-Confirm mode is valuable for:

Low-latency critical paths--the Phase2 round trip cost is eliminated while preserving bilateral ledger participation.
Server-side opt-in--platform teams can enable ledger tracking without requiring clients to implement the Phase2 confirmation step, provided the client is willing to attach a correlation identifier.
Idempotent retries by inspection-- because the client supplies the correlation identifier, a client that does not receive the final response can query the ledger by DTT-2PHP-Client-Correlation-ID to determine whether the intent was registered and, if so, its terminal outcome, before issuing any retry.

Activation Header

DTT-2PHP-Auto-Confirm:: When true, the server does not wait for a Phase2 confirmation before crossing the PONR and executing business logic. The final outcome is returned synchronously on the original request channel.

Protocol Flow

State Machine -- Auto-Confirm Mode

Auto-Confirm Phase State Machine ( COMMITTED ) <-- 200 OK + Resource-ID | on original channel | +-- Failure --> ( FAILED ) <-- 500 / 4xx on original channel ]]> In Auto-Confirm mode, WAITING_CONFIRM is a transient internal state written and immediately superseded by PROCESSING within the same atomic ledger operation. It is never visible to the client on the original channel.

Mode Comparison

Mode	Phase 2 Required	Latency	Client 2PHP Support	Ledger Participation
Standard 2PHP	Yes	2 round trips	Full (correlation ID + Phase2)	Both sides
Auto-Confirm	No	1 round trip	Correlation ID only	Both sides

State Machine

Phase States

State	Description
`WAITING_CONFIRM`	Phase1 received and persisted. Awaiting client Phase2 confirmation.
`PROCESSING`	Phase2 confirmed. PONR crossed. Business logic executing (synchronous mode).
`COMMITTED`	Processing complete. Resource created. Terminal success state.
`FAILED`	Processing failed after PONR. Server returned 500 or 4xx. Terminal failure state.
`TTL_EXPIRED`	Client did not confirm within TTL. Pending cleanup.
`ABANDONED`	Cleaned up after TTL + delta grace window. Terminal state.

Idempotency and Replay Behavior

Replay Before TTL If a client replays Phase2 before TTL expiry, the server MUST return the same response associated with the original DTT-2PHP-Server-Correlation-ID. No duplicate processing occurs.

Replay After TTL Any Phase2 confirmation received after TTL expiry MUST be rejected with 408 Request Timeout and DTT-2PHP-Phase-State: TTL_EXPIRED, consistent with . The client MUST re-submit as a new Phase1 request with a new DTT-2PHP-Client-Correlation-ID. A late Phase2 confirmation carrying the same correlation IDs MUST NOT be reused to trigger execution of the persisted intent. This revision defines a single replay behavior. Earlier revisions described a per-endpoint REUSE policy that allowed acceptance of a late Phase2 within the server-internal delta grace window. That policy has been removed because it created an ambiguous boundary with and expanded the replay-attack surface without a durable operational benefit.

Intent Ledger

Overview The Intent Ledger is a durable, queryable log of all 2PHP transactions. It is distinct from execution telemetry--it is an intent registry, not a trace of execution. Its purpose is to provide provable bilateral registration of every mutation intent across all participating services.

Relationship to Distributed Tracing The Intent Ledger is complementary to distributed tracing systems such as OpenTelemetry , not a replacement. Distributed tracing records what executed: spans, timings, causal links, and error paths, typically aggregated in a downstream store with some ingestion lag. It is the appropriate tool for post-hoc analysis of execution behavior. The Intent Ledger records what was registered as intent before execution began, and makes that record queryable in real time at the point of registration. This serves a different question than tracing: not "what happened?" but "did the server acknowledge my intent, and should I retry?" A client that does not receive the expected response can query the ledger by DTT-2PHP-Client-Correlation-ID to determine whether the prior intent was durably registered before issuing a retry. This inspection step is only useful if the answer is available synchronously and locally to the participating service, which is why the ledger is normatively local-first () rather than a downstream aggregation. Deployments SHOULD run both. Tracing answers questions about execution across services; the Intent Ledger answers questions about registered intent at each service boundary. Neither subsumes the other.

Normative Role At each service boundary, the Intent Ledger MUST record a durable entry for every Phase1 receipt and every subsequent phase transition, and that entry MUST be queryable by DTT-2PHP-Client-Correlation-ID and by DTT-2PHP-Server-Correlation-ID before Phase2 is processed. The ledger write and the intent-payload persistence MUST be committed as a single unit of work (see ). 2PHP-DTT is not a Distributed Transaction Coordinator: no cross-service locking, thread blocking, or two-phase commit is defined or required. Each service boundary owns its own handshake and its own ledger authoritative record. How the ledger is stored, tiered, replicated, or exposed for aggregate query is an implementation matter and is discussed in .

Ledger Schema Each ledger entry MUST include the following fields:

Field	Description
`client_correlation_id`	Client-assigned request identifier.
`server_correlation_id`	Server-assigned intent identifier.
`service_ledger_id`	GUID issued to the service at ledger registration.
`service_endpoint`	Target service and resource path.
`actor`	Role of the entry writer: `client` or `server`.
`source`	Identity of the participant writing this entry (e.g., `client`, `serviceA`, `serviceB1`).
`target`	The downstream service being called. Populated only when `actor = client`. MUST be null for `actor = server` entries.
`parent_reference_id`	The correlation ID that the immediate upstream caller provided when calling this service. Used to reconstruct the full distributed call tree.
`phase`	Current or terminal phase state.
`phase_1_timestamp`	When Phase1 was received (ISO8601).
`phase_2_timestamp`	When Phase2 confirmation was received, if applicable (ISO8601).
`ttl_ms`	Configured TTL for this request, in milliseconds.
`outcome`	Terminal outcome: `COMMITTED`, `ABANDONED`, or `TTL_EXPIRED`.
`payload_ref`	Reference to the persisted payload. Not the payload itself.
`sync_timestamp`	When this entry was synced to the Central Ledger (ISO8601).
`transaction_reference`	Reference to transaction ID (optional).

Cross-Service Correlation

Overview When a service fans out to multiple downstream services, the Intent Ledger captures every participant's view of every call--both the caller (actor = client) and the callee (actor = server). This produces a unified, traversable ledger that can reconstruct the full distributed call tree without any distributed coordination at the protocol level.

Propagation Rules

Every service MUST write its own ledger entry after each phase transition.
The client MUST write its ledger entry after receiving the Phase1 response, so that both client_correlation_id and server_correlation_id are available at write time.
parent_reference_id MUST always record what the immediate upstream caller provided as its correlation ID when calling this service-- enabling upward tree traversal.
Each call MUST produce exactly two matching ledger rows: one actor = client and one actor = server, linked by the same client_correlation_id + server_correlation_id pair. This confirms dual-side registration--the core guarantee of 2PHP.
target MUST be populated only on actor = client rows. It MUST be null on actor = server rows.

Example Call Chain

Fan-Out Call Chain Service A --> Service B1 --> Service C1 --> Service B2 --> Service D1 --> Service B3 --> Service E1 ]]> The complete ledger for this call chain contains 14 entries--two rows (one actor = client, one actor = server) for each of the seven service-boundary interactions. The root client entry uses client-corr-id1 as both its client_correlation_id and its parent_reference_id, establishing the tree anchor. Each subsequent tier sets parent_reference_id to the correlation ID received from its immediate upstream caller.

Tree Reconstruction Query Patterns

Full tree from root: Query parent_reference_id = client-corr-id1 recursively--walks the entire fan-out tree.
Everything a service initiated: Query source = serviceA AND actor = client.
Confirm dual registration: Match rows by client_correlation_id + server_correlation_id--one actor = client, one actor = server.
Find stalled transactions: Query phase = WAITING_CONFIRM or phase = TTL_EXPIRED across any source.

Backward Compatibility 2PHP is designed to be fully backward compatible:

Clients that do not include DTT-2PHP-Enabled: true MUST experience no change in behavior.
Servers that do not implement 2PHP MUST ignore the header and process normally.
A 2PHP-capable client MUST NOT send a Phase2 confirmation against a server that did not return DTT-2PHP-Server-Correlation-ID in the response to the initial mutation request (see ). The absence of that header identifies the response as originating from a non-2PHP server; the response MUST be treated as a conventional terminal HTTP response. This prevents a 2PHP client from issuing a spurious Phase2 request to a legacy server that has already committed the mutation.
2PHP MAY be adopted incrementally--per endpoint, per service, or organization-wide.
2PHP makes no changes to existing HTTP methods (POST, PUT, PATCH, DELETE). It is purely additive.
No new transport layer or connection upgrade is required.

Implementation Considerations

Reference Implementation Targets

Spring Boot-- OncePerRequestFilter interceptor
Express.js--middleware
Azure API Management--inbound policy
MuleSoft--HTTP request/response interceptor policy

Durable Storage for Phase 1 Persistence Implementations SHOULD use one of the following for durable storage of intent payload and ledger records:

Relational database with TTL-aware cleanup job.
Redis with TTL-based key expiry (plus delta buffer).
Distributed queue (e.g., Azure Service Bus, AWS SQS) with visibility timeout.

TTL Defaults The following are recommended starting values. Implementations MAY adjust these per endpoint.

Scenario	TTL	delta (server-internal)
Low-latency APIs	5,000 ms	1,000 ms
Standard APIs	30,000 ms	5,000 ms
Long-running operations	120,000 ms	10,000 ms

delta values are server implementation defaults and MUST NOT be exposed in protocol headers.

Service Ledger Registration Each service SHOULD register with the Intent Ledger at onboarding and be issued a GUID (service_ledger_id) that is included in all ledger entries for that service. This enables cross-service querying while maintaining service-level isolation.

Recommended Ledger Architecture The normative requirement () is that each service maintains a durable, locally queryable Intent Ledger. This section describes a recommended local-first, batch-sync architecture that satisfies that requirement without placing the ledger on the critical path of request processing. Implementations MAY choose a different architecture provided the normative requirements are met.

Tier1 -- Local Ledger:: Per-service, in-process write. Synchronous but local--no network hop. Authoritative source of truth for that service's phase states.
Tier2 -- Batch Sync:: Scheduled background job syncing local ledger entries to the Central Ledger. Frequency configurable per service. Not on the critical path.
Tier3 -- Central Ledger:: Read-optimized aggregate view across all services. Used for audit, incident investigation, and SLA reporting. Does not require the same availability or latency guarantees as the Local Ledger.

For highly critical services, the intent payload and the ledger entry SHOULD be co-located in the same durable store so the atomic write required by can be satisfied without a distributed transaction. For less critical services, a temporary co-located copy is sufficient for Phase1 and Phase2 lifecycle management, with the ledger entry propagated to the Central Ledger via event publish.

Recommended Ledger Write Flow Central Ledger (eventually consistent) ]]>

On-Demand Pull from Central Ledger Where a Central Ledger is deployed, its management application MAY trigger an on-demand pull from any service's Local Ledger. This is intended for incident investigation, where the latest data is required immediately without waiting for the next scheduled batch sync. Access controls for on-demand pull are discussed in .

Operational Modes The following table summarizes typical operational modes for the recommended architecture:

Scenario	Ledger Used	Trigger
Normal operation	Local Ledger only	Per phase transition
Retry-inspection query	Local Ledger	Client query before retry
Routine audit	Central Ledger	Batch sync (scheduled)
Incident investigation	Central Ledger	On-demand pull from management app
Compliance reporting	Central Ledger	Batch sync (scheduled)

Security Considerations

Correlation ID Confidentiality DTT-2PHP-Client-Correlation-ID and DTT-2PHP-Server-Correlation-ID values MUST be treated as opaque identifiers. Implementations SHOULD use UUIDv4 or equivalent cryptographically random values to prevent enumeration or prediction of correlation IDs by unauthorized parties.

Phase 2 Spoofing A malicious actor in possession of both DTT-2PHP-Client-Correlation-ID and DTT-2PHP-Server-Correlation-ID could attempt to submit a spoofed Phase2 confirmation. Implementations MUST authenticate Phase2 confirmation requests using the same authentication mechanism applied to Phase1 requests. The server MUST verify that the authenticated identity of the Phase2 requester matches that of the Phase1 requester.

Replay Semantics Replay of a Phase2 confirmation is a normal, expected behavior in 2PHP-DTT and is not by itself an attack. Phase2 confirmations are idempotent within the TTL window () and are rejected after TTL expiry (). This design allows legitimate clients to safely re-send Phase2 in the face of transient network loss without duplicating server-side effects, and bounds the window in which any captured Phase2 request remains useful to a party that possesses both correlation identifiers. Because a re-sent Phase2 within TTL is honored, an unauthenticated party in possession of both correlation identifiers could re-drive the server's idempotent response. This is a confidentiality-of-identifiers concern rather than a duplication concern. Implementations MUST authenticate Phase2 requests under the same authentication mechanism used for Phase1 (see ) and SHOULD pair Phase2 requests with short-lived credentials or request signing where the deployment threat model warrants it.

Intent Payload Confidentiality The intent payload persisted during Phase1 may contain sensitive data. Implementations MUST ensure that persisted payloads are protected at rest using appropriate encryption mechanisms consistent with the sensitivity of the data.

Ledger Access Control The Central Ledger aggregates intent records across all services. Access to the Central Ledger MUST be restricted to authorized audit, operations, and compliance consumers. The on-demand pull mechanism () MUST be protected by appropriate authentication and authorization controls.

Transport Security All 2PHP exchanges MUST be conducted over TLS (HTTPS). Use of 2PHP over unencrypted HTTP is NOT RECOMMENDED.

IANA Considerations

HTTP Field Name Registrations This document requests registration of the following HTTP field names in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" defined in , Section16.3.1:

Field Name	Status	Reference
`DTT-2PHP-Enabled`	Experimental	This document
`DTT-2PHP-Client-Correlation-ID`	Experimental	This document
`DTT-2PHP-Auto-Confirm`	Experimental	This document
`DTT-2PHP-Requested-TTL`	Experimental	This document
`DTT-2PHP-Server-Correlation-ID`	Experimental	This document
`DTT-2PHP-Phase-State`	Experimental	This document
`DTT-2PHP-TTL`	Experimental	This document
`DTT-2PHP-PONR-Deadline`	Experimental	This document
`DTT-2PHP-Resource-ID`	Experimental	This document
`DTT-2PHP-Message`	Experimental	This document