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Blockchain Transmission Protocol

Introduction

Blockchain records lots of verifiable data. With this verifiable data, we may construct a network communicatingBTP Message verifiable messages between blockchains without direct connection.

The relay delivers messages between blockchains. Each messages are verified by smart contract, and also it sends messages for the reply.

Blockchain Transmission Protocol is used to deliver messages between blockchains. A Relay delivers BTP messages between blockchains and smart contracts will verify and decode the delivered messages according to the services. Complex and heavy functions of smart contracts can be shared if they are implemented according to standardized interfaces. Smart contracts are also more extensible staying compliant to the standards.

Motivation

BTP Messages from multiple services are delivered to multiple blockchains. BTP smart contracts to support multiple services and blockchains. BTP Message Center(BMC) is the center of smart contracts.

For each blockchain, BTP Message Verifier(BMV) verifies the relay message and decodes it into standardized messages (BTP Messages).

For each service, BTP Message Handler(BSH) handles received messages of the service and sends messages through the BTP Message Center(BMC).

Terminology

  • Network Address

    A string to identify blockchain network

  • BTP Address

    A string of URL for locating an account of the blockchain network

  • Relay Message

    A message that a relay sends to the blockchain.

  • BTP Message

    Standardized messages delivered between different blockchains

  • BTP Message Center(BMC)

    BMC accepts messages from a relay (Relay Messages). A Relay Message contains standardized messages(BTP Messages) and proof of existence for these messages. Corresponding BMV will verify and decode the Relay Message, then the BMC will process the BTP Messages.

    If the destination of the message isn't current BMC, then it's sent to the next BMC to reach its destination. If current BMC is the destination, then it's dispatched to the corresponding BSH. If the message cannot be processed, then it sends an error back to the source.

  • BTP Message Verifier(BMV)

    BMV verifies a Relay Message and decodes it into BTP Messages.

  • BTP Service Handler(BSH)

    BSH handles BTP Messages of the service. It also sends messages according to different service scenarios.

  • BTP Message Relay(BMR)

    The software relays Relay Message between blockchains

Network Address

A string to identify blockchain network

<NID>.<Network System>

Network System: Short name of the blockchain network system.

Name Description
icon Loopchain for ICON
iconee Loopchain Enterprise

NID: ID of the network in the blockchain network system.

Example

Network Address Description
0x1.icon ICON Network with nid="0x1" <- ICON Main-Net
0x1.iconee Loopchain Enterprise network with nid="0x1"

BTP Address

A string of URL for locating an account of the blockchain network

Example

btp://<Network Address>/<Account Identifier>

Account Identifier: Identifier of the account including smart contract. It should be composed of URL safe characters except "."(dot).

Example

btp://0x1.icon/hxc0007b426f8880f9afbab72fd8c7817f0d3fd5c0
btp://0x1.iconee/cx429731644462ebcfd22185df38727273f16f9b87

It could be expanded to other resources.

BTP Message

A message delivered across blockchains.

Name Type Description
src String BTP Address of source BMC
dst String BTP Address of destination BMC
svc String name of the service
sn Integer serial number of the message
msg Bytes serialized bytes of Service Message or Error Message

if sn is negative, msg should be Error Message. It would be serialized in RLP serialization.

Error Message

A message for delivering error information.

Name Type Description
code Integer error code
msg String error message

It would be serialized in RLP serialization.

RLP serialization

For encoding BTP Message and Error Message, it uses Recursive Length Prefix (RLP). RLP supports bytes and list naturally. Here are some descriptions about other types.

String

It uses UTF-8 encoded bytes of the string. There is no termination bytes.

Integer

It uses the shortest form of two's complemented bytes representations. If it's negative, the highest bit of the first byte should be 1.

Example

Value Encoded bytes
0 0x00
-1 0xff
-128 0x80

Relay Message

It's used to deliver BTP Messages along with other required contents. Normally, it contains the following.

  • BTP Messages along with their proof of existence
  • Trust information updates along with their proof of consensus

The relay gathers the information through APIs of a source blockchain system and its internal database. The actual content of the message is decided according to the blockchain system and BMV implementation.

Components

Following diagram shows an example, delivering a message from left blockchain to the other blockchain.

btp_components

In the diagram, there are a few components.

Message delivery.

  1. BSH sends an Service Message through BMC.

    • BSH calls BMC.sendMessage with followings.

      Name Type Description
      _to String Network Address of the destination blockchain
      _svc String Name of the service.
      _sn Integer Serial number of the message.
      _msg Bytes Service message to be delivered.

    • BMC lookup the destination BMC belonging to _to. If there is no known BMC to the network, then it fails.

    • BMC builds an BTP Message.

      Name Type Description
      src String BTP Address of current BMC
      dst String BTP Address of destination BMC in the network
      svc String Given service name
      sn Integer Given serial number
      msg Bytes Given service message

    • BMC decide the next BMC according to the destination. If there is no route to the destination BMC.

    • BMC generates an event with BTP Message.

      Name Type Description
      _next String BTP Address of the next BMC
      _seq Integer Sequence number of the msg to the next BMC
      _msg Bytes Serialized BTP Message

  2. The BTP Message Relay(BMR) detects events.

    • The relay detects BMC.Message through various ways.
    • The relay can confirm that it occurs and it's finalized.

  3. BMR gathers proofs

    • Relay gathers proofs of the event(POE)s
      • Proof for the new block
      • Proof for the event in the block
    • Relay builds Relay Message including followings.
      • Proof of the new events
      • New events including the BTP Message.
    • Relay calls BMC.handleRelayMessage with built Relay Message.
    Name Type Description
    _prev String BTP Address of the previous BMC
    _msg Bytes serialized Relay Message including BTP Messages

  4. BMC handles Relay Message

    • It finds BMV for the network address of the previous BMC.

    • It gets the sequence number of the next message from the source network.

    • BMC calls BMV.handleRelayMessage to decode Relay Message and get a list of BTP Messages.

      Name Type Description
      _bmc String BTP Address of the current BMC
      _prev String BTP Address of given previous BMC
      _seq Integer Next sequence number of the BTP Message from previous BMC
      _msg Bytes The Relay Message

  5. BMV decodes Relay Message

    • It verifies and decodes Relay Message, then returns a list of BTP Messages.
    • If it fails to verify the message, then it fails.
    • The events from the previous BMC to the current BMC will be processed.
    • The events should have proper sequence number, otherwise it fails.

  6. BSH handles Service Messages

    • BMC dispatches BTP Messages.

    • If the destination BMC isn't current one, then it locates the next BMC and generates the event.

    • If the destination BMC is the current one, then it locates BSH for the service of the BTP Message.

    • Calls BSH.handleBTPMessage if the message has a positive value as _sn.

      Name Type Description
      _from String Network Address of the source network
      _svc String Given service name
      _sn Integer Given serial number
      _msg Bytes Given service message

    • Otherwise, it calls BSH.handleBTPError.

      Name Type Description
      _src String BTP Address of the BMC that generated the error
      _svc String Given service name
      _sn Integer Given serial number
      _code Integer Given error code
      _msg String Given error message