Multiplexing

The multiplexing layer is a simple binary protocol which runs on top of the raw connection (TCP or local socket) and provides:

• Multiplexing of multiple mini-protocols¹ over a single connection
• Framing and segmentation of messages within a stream connection
• Timing information for latency measurement

This shows the arrangement for a typical node-to-node (N2N) connection:

graph LR
  subgraph n1 [Node 1]
    direction LR
    HS1(Handshake)
    CS1(ChainSync)
    BF1(BlockFetch)
    Mux1[Multiplexer]

    HS1 <--> Mux1
    CS1 <--> Mux1
    BF1 <--> Mux1
  end

  subgraph n2 [Node 2]
    direction LR
    Mux2[Multiplexer]
    HS2(Handshake)
    CS2(ChainSync)
    BF2(BlockFetch)

    Mux2 <--> HS2
    Mux2 <--> CS2
    Mux2 <--> BF2
  end

  Mux1 <==> Mux2

Packet format

A multiplexer packet consists of an 8-byte header followed by up to 65535 bytes of payload. Multiple payload segments can be combined to form a full message.

packet-beta
    0-31: "Transmission time"
    32: "M"
    33-47: "Mini-protocol ID"
    48-63: "Payload length N"
    64-95: "Payload (variable length N)"

All fields are network/big-endian byte order.

The mini-protocol ID is fixed for each one of the NTN protocols. The section that describes each mini-protocol starts specifying the mini-protocol number.

1. Although the multiplexer is only used with mini-protocols in Cardano, it’s actually completely agnostic as to data format. ↩