# P2P Multiplex Connection

# MConnection

MConnection is a multiplex connection that supports multiple independent streams with distinct quality of service guarantees atop a single TCP connection. Each stream is known as a Channel and each Channel has a globally unique byte id. Each Channel also has a relative priority that determines the quality of service of the Channel compared to other Channels. The byte id and the relative priorities of each Channel are configured upon initialization of the connection.

The MConnection supports three packet types:

  • Ping
  • Pong
  • Msg

# Ping and Pong

The ping and pong messages consist of writing a single byte to the connection; 0x1 and 0x2, respectively.

When we haven't received any messages on an MConnection in time pingTimeout, we send a ping message. When a ping is received on the MConnection, a pong is sent in response only if there are no other messages to send and the peer has not sent us too many pings (TODO).

If a pong or message is not received in sufficient time after a ping, the peer is disconnected from.

# Msg

Messages in channels are chopped into smaller msgPackets for multiplexing.

Copy type msgPacket struct { ChannelID byte EOF byte // 1 means message ends here. Bytes []byte }

The msgPacket is serialized using Proto3 (opens new window). The received Bytes of a sequential set of packets are appended together until a packet with EOF=1 is received, then the complete serialized message is returned for processing by the onReceive function of the corresponding channel.

# Multiplexing

Messages are sent from a single sendRoutine, which loops over a select statement and results in the sending of a ping, a pong, or a batch of data messages. The batch of data messages may include messages from multiple channels. Message bytes are queued for sending in their respective channel, with each channel holding one unsent message at a time. Messages are chosen for a batch one at a time from the channel with the lowest ratio of recently sent bytes to channel priority.

# Sending Messages

There are two methods for sending messages:

Copy func (m MConnection) Send(chID byte, msg interface{}) bool {} func (m MConnection) TrySend(chID byte, msg interface{}) bool {}

Send(chID, msg) is a blocking call that waits until msg is successfully queued for the channel with the given id byte chID. The message msg is serialized using the tendermint/go-amino submodule's WriteBinary() reflection routine.

TrySend(chID, msg) is a nonblocking call that queues the message msg in the channel with the given id byte chID if the queue is not full; otherwise it returns false immediately.

Send() and TrySend() are also exposed for each Peer.

# Peer

Each peer has one MConnection instance, and includes other information such as whether the connection was outbound, whether the connection should be recreated if it closes, various identity information about the node, and other higher level thread-safe data used by the reactors.

# Switch/Reactor

The Switch handles peer connections and exposes an API to receive incoming messages on Reactors. Each Reactor is responsible for handling incoming messages of one or more Channels. So while sending outgoing messages is typically performed on the peer, incoming messages are received on the reactor.

Copy // Declare a MyReactor reactor that handles messages on MyChannelID. type MyReactor struct{} func (reactor MyReactor) GetChannels() []*ChannelDescriptor { return []*ChannelDescriptor{ChannelDescriptor{ID:MyChannelID, Priority: 1}} } func (reactor MyReactor) Receive(chID byte, peer *Peer, msgBytes []byte) { r, n, err := bytes.NewBuffer(msgBytes), new(int64), new(error) msgString := ReadString(r, n, err) fmt.Println(msgString) } // Other Reactor methods omitted for brevity ... switch := NewSwitch([]Reactor{MyReactor{}}) ... // Send a random message to all outbound connections for _, peer := range switch.Peers().List() { if peer.IsOutbound() { peer.Send(MyChannelID, "Here's a random message") } }