The GMP (General Message Passing) module is an IBC middleware that processes messages embedded in IBC transfer packet memo fields.

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Message Format

GMP messages are embedded in IBC transfer memo fields as JSON: 
{
  "source_chain": "string",
  "source_address": "string", 
  "payload": "base64-encoded-bytes",
  "type": "integer"
}

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Field Requirements

FieldTypeRequiredDescription
source_chainstringYesOriginating blockchain identifier (informational)
source_addressstringYesOriginal sender address (informational)
payloadstringYesBase64-encoded message payload (cannot be empty)
typeintegerYesMessage type: 1 (GeneralMessage) or 2 (GeneralMessageWithToken)

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Message Types

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TypeGeneralMessage (1)

Standard message passing without specific token coordination. Processing: Payload replaces memo, transfer proceeds normally.

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TypeGeneralMessageWithToken (2)

Message passing that coordinates with token transfers. Processing: Payload replaces memo, indicates atomic message-token processing.

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TypeUnrecognized (0)

Invalid message type. Processing: Returns error acknowledgment, packet fails.

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IBC Middleware Processing

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OnRecvPacket Flow

  1. Extract packet data from IBC transfer packet
  2. Parse GMP message from memo field (if valid JSON with non-empty payload)
  3. Process by type:
    • Types 1 & 2: Replace memo with payload, forward to transfer module
    • Other types: Return error acknowledgment
  4. Pass through if not a valid GMP message

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Error Handling

  • Invalid packet data: “cannot unmarshal ICS-20 transfer packet data”
  • Unrecognized message type: “unrecognized message type: X”
  • Packet remarshaling error: “cannot marshal ICS-20 post-processed transfer packet data”

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Channel Operations

All channel lifecycle methods pass through to underlying transfer module without modification.

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Key Concepts

  • Middleware: Sits between IBC core and transfer module
  • Memo Processing: Extracts and processes messages from transfer memos
  • Pass-through: Non-GMP transfers work normally
  • Error Recovery: Invalid GMP messages return error acknowledgments
  • Payload Replacement: GMP payload becomes the new memo for downstream processing