PROTOCOL.md

@sofidev/ipc Protocol

The @sofidev/ipc protocol operates on a two-way long-polling TCP request. This document explains how this protocol works and how must be implemented.

Structure

The structure is divided in two key components: the Server and the Client, the server opens a IPC/TCP server and may close it eventually, while a Client can connect to and disconnect from them, whether @sofidev/ipc uses IPC or IPC is based on where the server is opened to, typically if a numeric port or an IP is passed, it will use TCP, and IPC when a path to a socket file is passed instead.

Furthermore each Client has a collection of sockets used to send messages to the servers it's connected to, named ClientSockets, and each Server has a collection of sockets used to send messages to the clients connected to it, named ServerSocket.

Assuming the server runs on TCP with the port 8002, the connection would be something like the following:

┌────────┐                              ┌────────┐
│ Server │         ClientSocket         │ Client │
├────────┤ ←--------------------------- ├────────┤
│  8002  │         ServerSocket         │  8002  │
└────────┘ ---------------------------→ └────────┘

ClientSocket allows Client to send messages to Server, and ServerSocket allows Server to send messages to Client.

However, multiple clients can connect to a server just fine.

┌────────┐      ┌────────┐      ┌────────┐
│ Client │  CS  │ Server │  CS  │ Client │
├────────┤ ---→ ├────────┤ ←--- ├────────┤
│  8002  │  SS  │  8002  │  SS  │  8002  │
└────────┘ ←--- └────────┘ ---→ └────────┘
                  ↑    |
               CS |    | SS
                  |    ↓
                ┌────────┐
                │ Client │
                ├────────┤
                │  8002  │
                └────────┘

CS and SS refer to ClientSocket and ServerSocket, respectively.

Communication

@sofidev/ipc uses messagepack-encoded messages to communicate between all sockets, to do so, it uses msgpackr to encode messages before sending them, and all the communication happens in ClientSocket and in ServerSocket exclusively, since they define a socket connection.

Each socket is a duplex connection, so they have both a message sender and a receiver. The composition of messages using the @sofidev/ipc protocol is the following:

ID	Receptive	Byte-Length	Bytes ...
6 bytes	1 byte	4 bytes	Byte-Length bytes

1. Implementation-wise there is no restriction about what ID may be, it can be a Cryptographic Nonce, it may be an incremental number, it may be anything, as long as two messages from the same sender do not conflict and is strictly 6 bytes long.
2. Receptive header defines whether the message is read-only or the server is awaiting its response, will always be 0x00 for non-receptive, or 0x01 for receptive.
3. Byte-Length is used to define how long the message is in bytes.
4. Bytes is the message encoded with msgpackr.

When a message is receptive, it must be replied with the same ID, this will help the sender identify the original message and resolve the value (like a HTTP GET would work, where you give the URL and you get the response from the request).

Notes

It is suggested to make a queue receiver to handle the messages, @sofidev/ipc takes advantage of TCP's buffering nature, which helps lowering the latency and the time it takes to send the messages, which can also mean that two or more messages may be received glued up; use the Byte-Length header to find the end of the message (and the start of the next), and it also may be received partial ─ most routers allow up to 65536 bytes in a segment, so when sending large messages (for example files), it is suggested to implement a buffer control to join all the segments once it is received full.

Connection

The connection between the Client and the Server is done over TCP but has an extra step: handshake.

The handshake has two purposes:

• Verify: This step helps identifying if the counterpart "understands" the same language. The slight decode error or type mismatch should end on a prompt disconnection.
• Identify: @sofidev/ipc nodes have a name for which they are identified as. For example if a Server is named master, all Servers connected to it may send messages to it using master as its name.

// Stablish a TCP connection by connecting Client
// to Server.
┌────────┐                              ┌────────┐
│ Server │                              │ Client │
├────────┤        TCP Connection        ├────────┤
│ master │ ←--------------------------→ │ socket │
│  8002  │                              │  8002  │
└────────┘                              └────────┘

// Once the connection has stablished, the server
// will send its name ('master') to the client.
┌────────┐                              ┌────────┐
│ Server │           "master"           │ Client │
├────────┤ ---------------------------→ ├────────┤
│ master │                              │ socket │
│  8002  │                              │  8002  │
├────────┤                              ├────────┤
│ ?????? │                              │ ?????? │
└────────┘                              └────────┘

// The client must reply with its name ('socket')
// to the server.
┌────────┐                              ┌────────┐
│ Server │                              │ Client │
├────────┤                              ├────────┤
│ master │                              │ socket │
│  8002  │           "socket"           │  8002  │
├────────┤ ←--------------------------- ├────────┤
│ socket │                              │ ?????? │
└────────┘                              └────────┘

// Now both nodes "know" each others names and the
// connection has successfully verified. Now you
// may send messages from the Server to the Client
// using "socket", or vice versa using "master".
┌────────┐                              ┌────────┐
│ Server │                              │ Client │
├────────┤                              ├────────┤
│ master │                              │ socket │
│  8002  │                              │  8002  │
├────────┤                              ├────────┤
│ socket │                              │ master │
└────────┘                              └────────┘