cosmos-sdk/docs/spec/ibc/appendix-d.md

Appendix D: Universal IBC Packets

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The structures above can be used to define standard encodings for the basic IBC transactions that must be exposed by a blockchain: IBCreceive, IBCreceipt,_ IBCtimeout_, and IBCcleanup. As mentioned above, these are not complete transactions to be posted as is to a blockchain, but rather the "data" content of a transaction, which must also contain fees, nonce, and signatures. The other IBC transaction types IBCregisterChain, IBCupdateHeader, and IBCchangeValidators are specific to the consensus engine and use unique encodings. We define the tendermint-specific format in the next section.

 // IBCPacket sends a proven key/value pair from an IBCQueue.
 // Depending on the type of message, we require a certain type
 // of key (MessageKey at a given height, or StateKey).
 //
 // Includes src_chain and src_height to look up the proper
 // header to verify the merkle proof.
 message IBCPacket {
  // chain id it is coming from
  string src_chain = 1;
  // height for the header the proof belongs to
  uint64 src_height = 2;
  // the message type, which determines what key/value mean
  enum MsgType {
          RECEIVE = 0;
          RECEIPT = 1;
          TIMEOUT = 2;
          CLEANUP = 3;
  }
  MsgType msgType = 3;
  bytes key = 4;
  bytes value = 5;
  // the proof of the message
  MerkleProof proof = 6;
 }