Independent Submission Douglas Dohmeyer Internet-Draft Independent Researcher Intended status: Informational 2 December 2025 Expires: 5 June 2026 ChainSync: A Synchronization Protocol for Strict Sequential Execution in Linear Distributed Pipelines draft-dohmeyer-chainsync-00 Abstract ChainSync is a lightweight application-layer protocol that runs over reliable TCP connections to synchronize a fixed linear chain of distributed processes (labeled A, B, C, ..., N) such that they execute their local tasks in strict sequential order (A -> B -> C -> ... -> N) *and only after every process in the chain has confirmed it is ready*. The protocol uses three distinct phases: 1. Forward "readiness" wave (SYNC -> READY propagation from head to tail) 2. Backward "start" wave with deferred execution and watching (START propagation from tail to head) 3. Forward "completion" wave that triggers execution in the required order and provides clean backward-propagating exit The design guarantees strict ordering even when nodes become ready at very different times and requires only point-to-point TCP connections along the chain -- no central coordinator is needed. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." Douglas Dohmeyer Expires 5 June 2026 [Page 1] Internet-Draft ChainSync December 2025 This Internet-Draft will expire on 5 June 2026. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Topology and Configuration . . . . . . . . . . . . . . . 3 1.3. States . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.4. Message Types . . . . . . . . . . . . . . . . . . . . . . 4 1.5. Protocol Operation . . . . . . . . . . . . . . . . . . . 4 1.5.1. Phase 1 -- Readiness Collection (Forward Wave) . . . 4 1.5.2. Phase 2 -- Start Trigger Propagation (Backward Wave) . . . . . . . . . . . . . . . . . . . . . . . . 4 1.5.3. Phase 3 -- Ordered Execution and Completion (Forward Wave) . . . . . . . . . . . . . . . . . . . . . . . . 5 1.6. Waiting in WATCH State . . . . . . . . . . . . . . . . . 5 1.7. Example Message Flow (A-B-C-D) . . . . . . . . . . . . . 5 2. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 3. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4. Normative References . . . . . . . . . . . . . . . . . . . . 6 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction Many distributed workflows (pipeline parallelism in machine-learning training, staged data processing, multi-organization business processes, ordered multi-phase computation, etc.) require that tasks execute in a fixed order across different machines, yet must not begin until every participant is ready. Standard barriers do not enforce execution order. Token-passing or leader-based schemes introduce complexity and single points of failure. Douglas Dohmeyer Expires 5 June 2026 [Page 2] Internet-Draft ChainSync December 2025 ChainSync solves this with a simple, fully decentralized three-wave algorithm on a line topology that guarantees: * No process starts until the entire chain is ready. * Execution order is strictly A -> B -> ... -> N. * Clean backward-propagating exit after N finishes. 1.1. Requirements Language 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 1.2. Topology and Configuration The processes form a static logical chain: (Head) A <-> B <-> C <-> ... <-> N (Tail) Each process knows: * IP address and port of its predecessor (Head has none) * IP address and port of its successor (Tail has none) * Whether it is head, tail, or intermediate (inferable from presence/absence of predecessor/successor) Each adjacent pair maintains a single persistent bidirectional TCP connection. 1.3. States +==========+=======================================+ | State | Meaning | +==========+=======================================+ | SYNC | Initial state; waiting for READY from | | | predecessor (Head starts here but | | | moves to READY when locally ready) | +----------+---------------------------------------+ | READY | Chain segment to the left is ready; | | | has sent READY to successor | +----------+---------------------------------------+ | WATCH | Has propagated START leftward; | Douglas Dohmeyer Expires 5 June 2026 [Page 3] Internet-Draft ChainSync December 2025 | | waiting for COMPLETE from predecessor | +----------+---------------------------------------+ | START | Currently executing its local task | +----------+---------------------------------------+ | COMPLETE | Local task finished; has sent | | | COMPLETE to both directions as | | | required | +----------+---------------------------------------+ Table 1 1.4. Message Types Messages are simple ASCII text lines terminated by LF. Recommended format: [:]\n Defined commands: * READY[:] * START[:] * COMPLETE[:] is optional but RECOMMENDED (e.g., UUID) to support multiple concurrent rounds on the same connection. Implementations running only one round at a time MAY omit it. 1.5. Protocol Operation 1.5.1. Phase 1 -- Readiness Collection (Forward Wave) * Head (A), when locally ready, moves SYNC -> READY and sends READY to successor. * Every other node starts in SYNC. When it receives READY from predecessor *and* becomes locally ready, it moves SYNC -> READY and sends READY to successor. * When tail (N) enters READY, Phase 2 begins automatically. 1.5.2. Phase 2 -- Start Trigger Propagation (Backward Wave) * Tail, upon entering READY, sends START to predecessor and moves to WATCH. Douglas Dohmeyer Expires 5 June 2026 [Page 4] Internet-Draft ChainSync December 2025 * Intermediate node, upon receiving START from successor: 1. Sends START to its predecessor (if any) 2. Moves to WATCH and waits for COMPLETE from predecessor * Head, upon receiving START, has no predecessor and therefore moves directly to START and begins execution. This phase completes in O(n) messages and guarantees every node knows the entire chain is ready before any node starts. 1.5.3. Phase 3 -- Ordered Execution and Completion (Forward Wave) * A node in WATCH that receives COMPLETE from its predecessor moves to START and begins its local task. * When a node finishes its task, it moves START -> COMPLETE and: - Sends COMPLETE to successor (triggers successor to start) - Sends COMPLETE to predecessor (allows predecessor to exit) * A node in COMPLETE that receives COMPLETE from its successor MAY terminate. Execution order is therefore strictly A -> B -> C -> ... -> N. 1.6. Waiting in WATCH State The RECOMMENDED approach is *push-based*: the node simply blocks on read() from the predecessor's TCP socket. When the predecessor finishes, it pushes COMPLETE. An alternative approach is to poll the predecessor's TCP socket. Both approaches are compliant. 1.7. Example Message Flow (A-B-C-D) RD: READY ST: START CM: COMPLETE A.....B.....C.....D |-RD->|.....|.....| Phase 1 |.....|-RD->|.....| |.....|.....|-RD->| |.....|.....|<-ST-| Phase 2 Douglas Dohmeyer Expires 5 June 2026 [Page 5] Internet-Draft ChainSync December 2025 |.....|<-ST-|.....| |<-ST-|.....|.....| Phase 3 |.....|.....|.....| A starts immediately |-CM->|.....|.....| A finishes and B starts |.....|-CM->|.....| B finishes and C starts |.....|.....|-CM->| C finishes and D starts |.....|.....|<-CM-| D finishes |.....|<-CM-|.....X D exits |<-CM-|.....X...... C exits |.....X............ B exits X.................. A exits 2. IANA Considerations This memo includes no request to IANA. 3. Security Considerations Connections SHOULD use TLS 1.3. Production deployments SHOULD use mutual TLS with certificate pinning or pre-shared keys to prevent node impersonation. 4. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Acknowledgements Author's Address Douglas Russell Dohmeyer Independent Researcher United States of America Email: douglas.dohmeyer@protonmail.com Douglas Dohmeyer Expires 5 June 2026 [Page 6]