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1 Communication

1.1 Introduction

Commmunication module exposes three main abstraction:

Transport Layer - implemented using gRPC
Node discovery - implemented using specic version of Kademlia.
RChain protocol - implemented in Connect.scala

...

Diagram below shows dependency usage between all three abstractions in the ideal world

+-------------------------+
|                         |
|  RChain Protocol        |
|                         |
+-------------------------+
       |           | uses
       |uses       v
       |
       |     +-----------+
       |     |           |
       |     | Node      |
       |     | Discovery |
       |     |           |
       |     +-----------+
       |            |
       v            v  uses

   +--------------------+
   |  Transport Layer   |
   +--------------------+

That's the dream. Reality is...

1.3 Reality

Reality is that Transport Layer, Node Discovery and RChain Protocol have circular dependencies. Some of those dependencies we could live with for now (just add overall complexity to the design), some are potential cause of weird bugs and security issues (and should be considered as high priority issues).

Diagram below illustrates those dependencies

              uses messages     +-----------+
         +----------------------+ Transport | <---------------+
         |                      | Layer     |                 |
         |                      +-----------+                 | use to send
         |     +----------------------^                       | messages
         |     |    uses to send messages                     |
         v     |                                              |
               |                                              |
+-----------------+          uses findPeers                   |
|                 | <----------------------------------------------------+
| Node Discovery  |                                       |   Rchain     |
|                 |                                       |   Protocol   |
+-----------------+                                       +--------------+
             +------------------------------------------------^
                            peers only added by rchan protocol

...

1.4 Detailed description of abstractions

Below detailed description of how the abstractions are currently implemented and what are the issues that have to be addressed.

1.4.1 Transport Layer

Transport Layer abstracts the communication pipeline, allowing two nodes sending messages between each other. Transport Layer is neither aware of Node Discovery nor RChain protocol. Current implementation TcpTransportLayer is missing integration tests, minimum set of missing tests are defined under CORE-607.

Transport Layer exposes following functions:

trait TransportLayer[F[_]] {
  def local: F[PeerNode]
  def roundTrip(peer: PeerNode, msg: Protocol, timeout: FiniteDuration): F[CommErr[Protocol]]
  def send(peer: PeerNode, msg: Protocol): F[Unit]
  def broadcast(peers: Seq[PeerNode], msg: Protocol): F[Unit]
  def receive(dispatch: Protocol => F[CommunicationResponse]): F[Unit]
  def disconnect(peer: PeerNode): F[Unit]
  def shutdown(msg: Protocol): F[Unit]
}

local
Method local returns PeerNode representing given running node. This method should be removed and PeerNode information should be provided via `ApplicativeAsk` abstraction (see CORE-738 and CORE739 for detail).
disconnect
Method disconnect should never be part of TransportLayer abstraction. It is TcpTransportLayer implementation detail. It will be removed from the abstraction (see CORE-835 for details).
shutdown
Shutdown method gracefully shuts down the Transport Layer. After being called, no message can be send (via roundTrip, send, broadcast) nor received.
roundTrip, send, broadcast
Methods roundTrip and send allow sending and receiving messages of type Protocol - object defined in routing.proto. Protocol message can be one of the following:
```
message Protocol {
    Header header                           = 1;

    oneof message {
        google.protobuf.Any upstream        = 2;

        Ping                ping            = 3;
        Pong                pong            = 4;
        Lookup              lookup          = 5;
        LookupResponse      lookup_response = 6;
        Disconnect          disconnect      = 7;
    }
}
```
1. Ping, Pong, Loookup, LookupResponse
  Ping+Pong and Lookup+LookupResponse are part of the Node Discovery abstraction, so there is clear coupeling between those two abstraction. More details in CORE-836.
2. Disconnect
  Disconnect is currently send by TcpTransportLayer when it shuts down (Disconnect is being broadcast). This method is NOT needed as nodes should realize that other peers disconnect regardlessly if they managed to shut down gracefully or not. This message will be removed from the Protocol object (see CORE-838 for details).
3. Any upstream
  This is a placeholder for ANY possible message that is build on top Protocol. Effectively this is where messages from RChain protocol will be placed. Originally was designed to decouple TransportLayer, NodeDiscovery and RChain protocol (not successfully). This coupeling should be removed, see CORE-839 for details.

1.4.2 Node Discovery

Below we describe how Node Discovery works currently

Relation to Transport Layer
Node discovery is using Tranport Layer to fullfil its work. It uses
```
+----------------+
|                |
| NODE DISCOVERY |
|                |
+----------------+
        |
        v uses

+----------------+
|   TRANSPORT    |
|   LAYER        |
+----------------+
```
This is almost true, as mentioned in previous section, TransportLayer sends messages of type Protocol (routing.proto) that are very Kademlia specific. See CORE-838 and CORE-839 to see the issue how to remove that cyclic dependency between Node Discovery and Transport Layer.

2. How current Kademlia implementation is different from the original paper

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Versions Compared

Old Version 6

New Version 7

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