packet-forward-middleware

Middleware for forwarding IBC packets.

Asynchronous acknowledgements are utilized for atomic multi-hop packet flows. The acknowledgement will only be written on the chain where the user initiated the packet flow after the forward/multi-hop sequence has completed (success or failure). This means that a user (i.e. an IBC application) only needs to monitor the chain where the initial transfer was sent for the response of the entire process.

About

The packet-forward-middleware is an IBC middleware module built for Cosmos blockchains utilizing the IBC protocol. A chain which incorporates the packet-forward-middleware is able to route incoming IBC packets from a source chain to a destination chain. As the Cosmos SDK/IBC become commonplace in the blockchain space more and more zones will come online, these new zones joining are noticing a problem: they need to maintain a large amount of infrastructure (archive nodes and relayers for each counterparty chain) to connect with all the chains in the ecosystem, a number that is continuing to increase quickly. Luckly this problem has been anticipated and IBC has been architected to accomodate multi-hop transactions. However, a packet forwarding/routing feature was not in the initial IBC release.

Sequence diagrams

Multi-hop A->B->C->D success

        channel-0 channel-1         channel-2 channel-3        channel-4 channel-5
┌───────┐       ibc        ┌───────┐        ibc       ┌───────┐        ibc       ┌───────┐
│Chain A│◄────────────────►│Chain B│◄────────────────►│Chain C│◄────────────────►│Chain D│
└───────┘                  └───────┘                  └───────┘                  └───────┘
     1. transfer 2. recv_packet  3. forward 4. recv_packet  5. forward 6. recv_packet
         ─────────────────► packet  ─────────────────► packet  ─────────────────►
     9. ack                 forward   8. ack           forward   7. ack  
         ◄───────────────── middleware◄─────────────── middleware◄───────────────

Multi-hop A->B->C->D, C->D recv_packet error, refund back to A

        channel-0 channel-1         channel-2 channel-3        channel-4 channel-5
┌───────┐       ibc        ┌───────┐        ibc       ┌───────┐        ibc       ┌───────┐
│Chain A│◄────────────────►│Chain B│◄────────────────►│Chain C│◄────────────────►│Chain D│
└───────┘                  └───────┘                  └───────┘                  └───────┘
     1. transfer 2. recv_packet  3. forward 4. recv_packet  5. forward 6. recv_packet ERR
         ─────────────────► packet  ─────────────────► packet  ─────────────────►
         9. ack ERR         forward   8. ack ERR       forward   7. ack ERR
         ◄───────────────── middleware◄─────────────── middleware◄───────────────

Forward A->B->C with 1 retry, max timeouts occurs, refund back to A

        channel-0 channel-1         channel-2 channel-3
┌───────┐       ibc        ┌───────┐        ibc       ┌───────┐
│Chain A│◄────────────────►│Chain B│◄────────────────►│Chain C│
└───────┘                  └───────┘                  └───────┘
     1. transfer 2. recv_packet     3. forward
         ─────────────────► packet  ─────────────────►
                            forward   4. timeout
                            middleware◄───────────────
                                    5. forward retry
                                    ─────────────────►
         7. ack ERR                 6. timeout
         ◄─────────────────         ◄─────────────────

Examples

Utilizing the packet memo field, instructions can be encoded as JSON for multi-hop sequences.

Minimal Example - Chain forward A->B->C

• The packet-forward-middleware integrated on Chain B.
• The packet memo is included in MsgTransfer by user on Chain A.

{
  "forward": {
    "receiver": "chain-c-bech32-address",
    "port": "transfer",
    "channel": "channel-123"
  }
}

Full Example - Chain forward A->B->C->D with retry on timeout

• The packet-forward-middleware integrated on Chain B and Chain C.
• The packet memo is included in MsgTransfer by user on Chain A.
• A packet timeout of 10 minutes and 2 retries is set for both forwards.

In the case of a timeout after 10 minutes for either forward, the packet would be retried up to 2 times, at which case an error ack would be written to issue a refund on the prior chain.

next is the memo to pass for the next transfer hop. Per memo intended usage of a JSON string, it should be either JSON which will be Marshaled retaining key order, or an escaped JSON string which will be passed directly.

next as JSON

{
  "forward": {
    "receiver": "chain-c-bech32-address",
    "port": "transfer",
    "channel": "channel-123",
    "timeout": "10m",
    "retries": 2,
    "next": {
      "forward": {
        "receiver": "chain-d-bech32-address",
        "port": "transfer",
        "channel":"channel-234",
        "timeout":"10m",
        "retries": 2
      }
    }
  }
}

next as escaped JSON string

{
  "forward": {
    "receiver": "chain-c-bech32-address",
    "port": "transfer",
    "channel": "channel-123",
    "timeout": "10m",
    "retries": 2,
    "next": "{\"forward\":{\"receiver\":\"chain-d-bech32-address\",\"port\":\"transfer\",\"channel\":\"channel-234\",\"timeout\":\"10m\",\"retries\":2}}"
  }
}

References

• https://www.mintscan.io/cosmos/proposals/56
• cosmos/ibc-go#373
• https://github.com/strangelove-ventures/governance/blob/master/proposals/2021-09-hub-ibc-router/README.md