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specs/standards/application/reliable-channel-api.md
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---
title: RELIABLE-CHANNEL-API
name: Reliable Channel API definition
category: Standards Track
status: raw
tags: [reliability, application, api, sds, segmentation]
editors:
- Igor Sirotin <sirotin@status.im>
- Jazz Turner-Baggs <jazz@status.im>
- Ivan Folgueira Bande <ivansete@status.im>
---
## Table of contents
<!-- TOC -->
* [Table of contents](#table-of-contents)
* [Abstract](#abstract)
* [Motivation](#motivation)
* [Syntax](#syntax)
* [API design](#api-design)
* [Architectural position](#architectural-position)
* [IDL](#idl)
* [Components](#components)
* [Segmentation](#segmentation)
* [Scalable Data Sync (SDS)](#scalable-data-sync-sds)
* [Rate Limit Manager](#rate-limit-manager)
* [Encryption Hook](#encryption-hook)
* [Procedures](#procedures)
* [Node initialization](#node-initialization)
* [Outgoing message processing](#outgoing-message-processing)
* [Incoming message processing](#incoming-message-processing)
* [Rate limiting](#rate-limiting)
* [Encryption](#encryption)
* [The Reliable Channel API](#the-reliable-channel-api)
* [Channel lifecycle](#channel-lifecycle)
* [Channel usage](#channel-usage)
* [Node configuration](#node-configuration)
* [Type definitions](#type-definitions)
* [Security/Privacy Considerations](#securityprivacy-considerations)
* [Copyright](#copyright)
<!-- TOC -->
## Abstract
This document specifies the **Reliable Channel API**,
an application-level interface that sits between the application layer and the [MESSAGING-API](/standards/application/messaging-api.md) plus [P2P-RELIABILITY](/standards/application/p2p-reliability.md), i.e., `application` <-> **reliable-channel-api** <-> `messaging-api/p2p-reliability`.
It bundles segmentation, end-to-end reliability via [Scalable Data Sync (SDS)](https://lip.logos.co/ift-ts/raw/sds.html), rate limit management, and a pluggable encryption hook
into a single interface for sending and receiving messages reliably.
## Motivation
The [MESSAGING-API](/standards/application/messaging-api.md) provides peer-to-peer reliability via [P2P-RELIABILITY](/standards/application/p2p-reliability.md),
but does not provide high end-to-end delivery guarantees from sender to recipient.
This API addresses that gap by introducing:
- **[SEGMENTATION](/standards/application/segmentation.md)** to handle large messages exceeding network size limits.
- **[SDS](https://lip.logos.co/ift-ts/raw/sds.html)** to provide causal-history-based end-to-end acknowledgement and retransmission.
- **Rate Limit Manager** to comply with [RLN](https://lip.logos.co/messaging/standards/core/17/rln-relay.html) constraints when sending segmented messages.
- **Encryption Hook** to allow upper layers to provide a pluggable encryption mechanism. This enables applications to provide Confidentiality and Integrity if desired.
The separation between Reliable Channels and encryption ensures the API remains agnostic to identity and key management concerns,
which are handled by higher layers.
## Syntax
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119](https://www.ietf.org/rfc/rfc2119.txt).
## API design
### Architectural position
The Reliable Channel API sits between the application layer and the Messaging API, as follows:
```
┌────────────────────────────────────────────────────────────┐
│ Application Layer │
└───────────────────────────┬────────────────────────────────┘
│
┌───────────────────────────▼────────────────────────────────┐
│ Reliable Channel API │
│ ┌──────────────┐ ┌─────┐ ┌───────────────┐ ┌──────────┐ │
│ │ Segmentation │ │ SDS │ │ Rate Limit Mgr│ │Encryption│ │
│ │ │ │ │ │ │ │ Hook │ │
│ └──────────────┘ └─────┘ └───────────────┘ └──────────┘ │
└───────────────────────────┬────────────────────────────────┘
│
┌───────────────────────────▼────────────────────────────────┐
│ Messaging API │
│ (P2P Reliability, Relay, Filter, Lightpush, Store) │
└────────────────────────────────────────────────────────────┘
```
### IDL
A custom Interface Definition Language (IDL) in YAML is used, consistent with [MESSAGING-API](/standards/application/messaging-api.md).
## Components
### Segmentation
A protocol that splits message payloads into smaller units during transmission and reassembles them upon reception. The component is instantiated by supplying the appropriate value to SegmentationConfig.
See [SEGMENTATION](./segmentation.md).
### Scalable Data Sync (SDS)
[SDS](https://lip.logos.co/ift-ts/raw/sds.html) provides end-to-end delivery guarantees using causal history tracking.
- Each new segment to be sent, requires the following data:
- `MessageId`: a keccak-256([message](https://lip.logos.co/ift-ts/raw/sds.html#message)'s content) hex string (e.g. `4e03657aea45a94fc7d47ba826c8d667c0d1e6e33a64a036ec44f58fa12d6c45`), generated by the Reliable Channel API.
- `ChannelId`: the `channelId` passed to `createReliableChannel`.
- `Retrieval hint`: the transport `MessageHash` of previous segments, exposed by the underlying [MESSAGING-API](/standards/application/messaging-api.md) upon ``MessageSendPropagatedEvent` reception.
The hint provider registered during [Node initialization](#node-initialization) performs this `MessageId → MessageHash` lookup. In turn, that mapping MUST be persisted by SDS using the `persistence` backend configured in `SdsConfig`.
- Each sent segment is added to an outgoing buffer.
- The recipient sends acknowledgements back to the sender upon receiving segments.
- The sender removes acknowledged segments from the outgoing buffer.
- Unacknowledged segments are retransmitted after `acknowledgementTimeoutMs`.
- SDS state MUST be persisted using the `persistence` backend configured in `SdsConfig`.
### Rate Limit Manager
The Rate Limit Manager ensures compliance with [RLN](https://lip.logos.co/messaging/standards/core/17/rln-relay.html) rate constraints.
- It tracks how many messages have been sent in the current epoch.
- When the limit is approached, segment dispatch MUST be delayed to the next epoch.
- The epoch size MUST match the `[epochPeriodSec](https://lip.logos.co/messaging/standards/core/17/rln-relay.html#epoch-length)` configured in `RateLimitConfig`.
### Encryption Hook
The Encryption Hook provides a pluggable interface for upper layers to inject encryption.
- The hook is optional; when not provided, messages are sent unencrypted.
- Encryption is applied per segment, after segmentation and SDS.
- Decryption is applied per segment before being processed by SDS.
- The `Encryption` interface MUST be implemented by the caller when the hook is provided.
- The Reliable Channel API MUST NOT impose any specific encryption scheme.
## Procedures
### Node initialization
When a node is created via `createNode` (defined in [MESSAGING-API](/standards/application/messaging-api.md)),
the implementation MUST perform the following setup before the node is used:
1. **Configure SDS persistence**: Supply the `Persistence` backend from `SdsConfig` to the SDS module so that
causal history and outgoing buffers survive restarts.
2. **Configure SDS hint provider**: Register a hint provider with the SDS module.
The hint provider converts an SDS `MessageId` into its corresponding `MessageHash`.
3. **Configure Segmentation persistence**: Supply the `Persistence` backend from `SegmentationConfig` to the
[Segmentation](./segmentation.md) module so that partially reassembled messages survive restarts.
4. **Fetch missed messages**: Retrieve messages missed while offline as described in
[MESSAGING-API — Fetching missed messages on startup](/standards/application/messaging-api.md#init-node-extended-definitions).
### Outgoing message processing
When `send` is called, the implementation MUST process `message` in the following order:
1. **Segment**: Split the payload into segments as defined in [SEGMENTATION](./segmentation.md). The maximum segment size MUST be reduced by the size of the SDS header (all that is not content, considering [SDS message](https://lip.logos.co/ift-ts/raw/sds.html#message)) added in step 2, so that each segment together with its SDS header stays within the network message size limit.
2. **Apply [SDS](https://lip.logos.co/ift-ts/raw/sds.html)**: add each sds message to the SDS outgoing buffer (see [SDS](#scalable-data-sync-sds) for parameter bindings).
3. **Encrypt**: If an `Encryption` implementation is provided, encrypt each segment before transmission.
4. **Rate Limit**: If `RateLimitConfig.enabled` is `true`, delay dispatch as needed to comply with [RLN](https://lip.logos.co/messaging/standards/core/17/rln-relay.html) epoch constraints.
5. **Dispatch**: Send each segment via the underlying [MESSAGING-API](/standards/application/messaging-api.md).
### Incoming message processing
When a segment is received from the network, the implementation MUST process it in the following order:
1. **Decrypt**: If an `Encryption` implementation is provided, decrypt the segment.
2. **Apply [SDS](https://lip.logos.co/ift-ts/raw/sds.html)**: Deliver the segment to the SDS layer, which emits acknowledgements and detects gaps.
- **Detect missing dependencies**: If SDS detects a message in the causal history which has not yet been received, it MUST make a best-effort attempt to retrieve the missing message, and MAY use the store protocol internally for this purpose. If the message cannot be retrieved, SDS MAY mark it as lost.
3. **Reassemble**: Once all segments for a message have been received, reassemble and emit a `reliable:message:received` event.
### Rate limiting
When `RateLimitConfig.enabled` is `true`, the implementation MUST space segment transmissions
to comply with the [RLN](https://lip.logos.co/messaging/standards/core/17/rln-relay.html) epoch constraints defined in `[epochPeriodSec](https://lip.logos.co/messaging/standards/core/17/rln-relay.html#epoch-length)`.
Segments MUST NOT be sent at a rate that would violate the RLN message rate limit for the active epoch.
### Encryption
The `encryption` parameter in `createReliableChannel` is intentionally optional.
The Reliable Channel API is agnostic to encryption mechanisms.
When an `Encryption` implementation is provided, it MUST be applied as described in [Outgoing message processing](#outgoing-message-processing) and [Incoming message processing](#incoming-message-processing).
## The Reliable Channel API
This API considers the types defined by [MESSAGING-API](/standards/application/messaging-api.md) plus the following.
### Channel lifecycle
This point assumes that a WakuNode instance is created beforehand. See `createNode` function
in [MESSAGING-API](/standards/application/messaging-api.md).
```yaml
functions:
createReliableChannel:
description: "Creates a reliable channel over the given content topic. Sets up the required SDS state,
segmentation, and encryption, and subscribes to `contentTopic`."
parameters:
- name: node
type: WakuNode
description: "The underlying messaging node, as defined in [MESSAGING-API](/standards/application/messaging-api.md).
Used to send segments and to subscribe/unsubscribe to the content topics."
- name: channelId
type: string
description: "Unique identifier for this channel. Represents the reliable (SDS), segmented, and optionally-encrypted session."
- name: contentTopic
type: string
description: "The topic this channel listens and sends on. This has routing and filtering connotations."
- name: senderId
type: string
description: "An identifier for this sender. SHOULD be unique and persisted between sessions."
- name: encryption
type: optional<Encryption>
default: none
description: "Optional pluggable encryption implementation. If none, messages are sent unencrypted."
returns:
type: result<ReliableChannel, error>
closeChannel:
description: "Closes a reliable channel, releases all associated resources and internal state,
and unsubscribes from its content topic via the underlying [MESSAGING-API](/standards/application/messaging-api.md)."
parameters:
- name: channel
type: ReliableChannel
description: "The channel handle returned by `createReliableChannel`."
returns:
type: result<void, error>
```
### Channel usage
```yaml
functions:
send:
description: "Send a message through a reliable channel. The message is always segmented,
SDS-tracked, rate-limited (optional), and encrypted (optional)."
parameters:
- name: channel
type: ReliableChannel
description: "The channel handle returned by `createReliableChannel`."
- name: message
type: array<byte>
description: "The raw message payload to send."
returns:
type: result<RequestId, error>
description: "Returns a `RequestId` that callers can use to correlate subsequent `MessageSentEvent` or `MessageSendErrorEvent` events."
```
### Node configuration
This spec extends `NodeConfig`, needed to create a node, which is
defined in [MESSAGING-API](/standards/application/messaging-api.md),
with `sds_config` and `rate_limit_config` fields.
```yaml
NodeConfig: # Extends NodeConfig defined in MESSAGING-API
fields:
sds_config:
type: SdsConfig
description: "SDS configuration. See SdsConfig defined in this spec."
```
### Type definitions
```yaml
types:
ReliableChannel:
type: object
description: "A handle representing an open reliable channel.
Returned by `createReliableChannel` and used to send messages and receive events.
Internal state (SDS, segmentation, encryption) is managed by the implementation."
events:
"reliable:message:received":
type: MessageReceivedEvent
"reliable:message:sent":
type: MessageSentEvent
"reliable:message:delivered":
type: MessageDeliveredEvent
"reliable:message:send-error":
type: MessageSendErrorEvent
"reliable:message:delivery-error":
type: MessageDeliveryErrorEvent
MessageReceivedEvent:
type: object
description: "Event emitted when a complete message has been received and reassembled."
fields:
message:
type: array<byte>
description: "The reassembled message payload."
MessageSentEvent:
type: object
description: "Event emitted when all segments of a message have been transmitted to the network.
This confirms network-level dispatch only; it does not guarantee the recipient has processed the message.
For end-to-end confirmation, listen for `MessageDeliveredEvent`."
fields:
requestId:
type: RequestId
description: "The identifier of the `send` operation whose segments have all been dispatched to the network."
MessageDeliveredEvent:
type: object
description: "Event emitted when the recipient has confirmed end-to-end receipt of a message via SDS acknowledgements.
This event is fired asynchronously after `MessageSentEvent`, once the SDS layer receives explicit acknowledgements from the recipient."
fields:
requestId:
type: RequestId
description: "The identifier of the `send` operation confirmed as delivered by the recipient."
MessageSendErrorEvent:
type: object
description: "Event emitted when one or more segments of a message could not be dispatched to the network.
This indicates a network-level failure; the message was never fully transmitted."
fields:
requestId:
type: RequestId
description: "The identifier of the `send` operation that failed to dispatch."
error:
type: string
description: "Human-readable description of the dispatch failure."
MessageDeliveryErrorEvent:
type: object
description: "Event emitted when end-to-end delivery could not be confirmed.
Fired after `maxRetransmissions` attempts have been exhausted without receiving an SDS acknowledgement from the recipient."
fields:
requestId:
type: RequestId
description: "The identifier of the `send` operation that was not acknowledged by the recipient."
error:
type: string
description: "Human-readable description of the delivery failure."
RequestId:
type: string
description: "Unique identifier for a single `send` operation on a reliable channel.
It groups all segments produced by segmenting one message, so callers can correlate
acknowledgement and error events back to the original send call.
Internally, each segment is dispatched as an independent [MESSAGING-API](/standards/application/messaging-api.md) call,
producing one `RequestId` (as defined in [MESSAGING-API](/standards/application/messaging-api.md)) per segment.
A single `RequestId` therefore maps to one or more underlying [MESSAGING-API](/standards/application/messaging-api.md)'s `RequestId` values,
one per segment sent.
For example, the `RequestId` `Req_a` yields these MESSAGING-API requests:
`Req_a:1`, `Req_a:2`, ..., `Req_a:N`, where `Req_a:k` represents the k-th
MESSAGING-API segment `RequestId`.
That is, `Req_a` is the `RequestId` from the RELIABLE-CHANNEL-API spec PoV,
whereas `Req_a:k` is the `RequestId` from the MESSAGING-API spec PoV.
"
SegmentationConfig:
type: object
fields:
enableReedSolomon:
type: bool
default: false
description: When enabled, the message sender adds parity (redundant) segments to allow recovery in case of data segment loss. See [SEGMENTATION](./segmentation.md).
segmentSizeBytes:
type: uint
default: 102400 # 100 KiB
description: "Maximum segment size in bytes.
Messages larger than this value are split before SDS processing."
persistence:
type: Persistence
description: "Backend for persisting partial reassembly state across restarts.
Implementations MUST use this backend to store received segments until all segments of a message have arrived and can be reassembled.
Refer to [SEGMENTATION](./segmentation.md) for the full definition of what state must be persisted."
SdsConfig:
type: object
description: Scalable Data Sync config items.
fields:
persistence:
type: Persistence
description: "Backend for persisting the SDS local history. Implementations MAY support custom backends."
acknowledgementTimeoutMs:
type: uint
default: 5000
description: "Time in milliseconds to wait for acknowledgement before retransmitting."
maxRetransmissions:
type: uint
default: 5
description: "Maximum number of retransmission attempts before considering delivery failed."
causalHistorySize:
type: uint
default: 2
description: "Number of message IDs to consider in the causal history. With longer value, a stronger correctness is guaranteed but it requires higher bandwidth and memory."
RateLimitConfig:
type: object
description: Rate limiting configuration, containing RLN-specific attributes.
fields:
enabled:
type: bool
default: false
description: "Whether rate limiting is enforced. SHOULD be true when RLN is active."
epochPeriodSec:
type: uint
default: 600 # 10 minutes
description: "The epoch size used by the RLN relay, in milliseconds."
Encryption:
type: object
description: "Interface for a pluggable encryption mechanism.
When provided as a parameter to `createReliableChannel`, the API consumer MUST implement both encrypt and decrypt operations.
Implementations MAY use different signatures than those described below, as long as each operation accepts a byte array and returns a byte array."
fields:
encrypt:
type: function
description: "Encrypts a byte payload. Returns the encrypted payload."
parameters:
- name: content
type: array<byte>
returns:
type: result<array<byte>, error>
decrypt:
type: function
description: "Decrypts a byte payload. Returns the decrypted payload."
parameters:
- name: payload
type: array<byte>
returns:
type: result<array<byte>, error>
Persistence:
type: object
description: "Interface for a pluggable SDS persistence backend.
Implementations MUST provide all functions required to save and retrieve SDS state per channel. Implementations MUST also provide the persistence method of interest, e.g., SQLite, custom encrypted storage, etc.
Refer to the [SDS spec](https://lip.logos.co/ift-ts/raw/sds.html) for the full definition of what state must be persisted."
```
## Security/Privacy Considerations
- This API does not provide confidentiality by default. An `Encryption` implementation MUST be supplied when confidentiality is required.
- Segment metadata (message ID, segment index, total segments) is visible to network observers unless encrypted by the hook.
- SDS acknowledgement messages are sent over the same content topic and are subject to the same confidentiality concerns.
- Rate limiting compliance is required to avoid exclusion from the network by RLN-enforcing relays.
## Copyright
Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
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