Relationship between Protocols

Prerequisites

This document assumes the reader is familiar with the following document:

Terminology

  • A node is a participant in an Interledger network. It may be a connector, a sender or a receiver.
  • A payment is sending value from one to another but it doesn't mean settled necessarily that is explained below. In the Interledger network, a payment could mean a node-to-node payment or an end-to-end payment.
    • A node-to-node payment means the intention of transferring value of a node (a connector, an endpoint sender, or an endpoint receiver) to another node which is directly connected to the sender node.
    • An end-to-end payment means the intention of transferring value of an endpoint sender to an endpoint receiver.
  • A settlement is ensuring that one has the paid value. For instance, executing a transaction on a certain ledger that the payee has an account on.
    • A node-to-node payment is settled by means that the two nodes agreed upon. It is usually a ledger that tracks the amount that both nodes have.
    • An end-to-end payment is settled through a chain of settlements of intermediaries above, because the end-to-end payment is a chain of node-to-node payments.
    • Payments on the Interledger network could be bidirectional, thus settlements could be as well.
    • Refer to Interledger Architecture and Perspective.

Motivation

Because the Interledger Protocol suite consists of several protocols and some related specifications, it can be difficult to understand the relationship between each of the protocols. This document attempts to provide an overview and mental model of how the components fit together. It is primarily intended for implementers of parts of the Interledger stack but may also be useful for others who wish to understand how Interledger works as a whole. A mental model is what a reader assumes something to be, reading some explanation. The mental model helps the reader predicting the behavior of what is explained even if the details are not given at that time.

Scope

The scope of this document is the explication of the relationship between the Interledger protocols, and the details for it are out of scope. Some references are given for it though. i.e. This document draws a whole, big picture of ILP.

Perspective

There are two diagrams, one is for the macro view that shows the main elements and its IP layer connections, another is for the micro view that shows protocols, the interaction between programs, and the other details.

The alphabets (A-C) and numbers (1-15), that are shown in the diagrams as red characters, are explained briefly in the Elements and Brief Explanations section.

Connections

Perspective of connections

Protocols and Details

This is an enlarged diagram of the above diagram, that shows mainly Node A, Node B, and Ledger B.

Perspective of connections

Elements and Brief Explanations

Connections

  • (A) BTP over WebSocket
    • Node to Node connection
    • In order to exchange payments, configuration and routing information, a node MUST have secure communication channels with its peers. The current implementation uses WebSockets for it.
  • (B) Ledger specific connection
    • Node to Ledger connection
    • A node needs the means to settle its payments. Therefore the node has a connection to a ledger. The type of connection varies depending on the ledger.
    • In the cases where blockchains are used for settlements, nodes MAY utilize payment channels because it is quick and inexpensive than issuing transactions every time the nodes want to settle.
  • (C) SPSP over HTTPS
    • Application to SPSP Server connection
    • To determine end-to-end payment information such as a shared secret, a destination address and so forth, an application connects to SPSP server over HTTPS.

Protocols

  • Link Layer Protocol
  • Core ILP Protocols
    • (2) ILP
      • Interledger Protocol V4 (ILPv4)
      • ILP is used for sending payment packets across multiple hops. Some other protocols, including those for node configuration and routing, are also built on top of ILP and use ILP packets to communicate that information between peers.
    • (3) ILP Address
  • Transport Layer Protocol
  • Application Layer Protocols
    • (5) Application
      • Applications MAY build their own protocols on top of STREAM, and put data of the protocols into the extensible data area of STREAM packets.
    • (6) SPSP
  • Connector to Connector Protocol
    • (7) DCP
    • (8) RBP
      • RBP is a protocol built on top of ILP, and is used for transferring routing information to build routing tables.
      • RBP data is put into the extensible data area of ILP packets.

Data Structure and Encoding

  • (9) ASN.1
    • ASN.1 Project
    • ASN.1 is used in order to specify data structure (order, type, and length).
    • Concrete packet structure of protocols is defined using ASN.1.
  • (10) Canonical OER
    • Notes on OER Encoding
    • ASN.1 doesn't specify its encoding rule. i.e. How packets are encoded in binary is defined separately.
    • Canonical OER is one of the specifications that define the encoding rule.

Functions

The functions shown below are just concepts, so their exact form may differ between implementations.

  • (11) Account Module
    • An account module manages connections to both a counterpart node and a ledger.
  • (12) Bilateral Ledger
  • (13) Routing Table Module
    • A routing table module aggregates incoming routing information and builds the best routes for prefixes.
  • (14) Configuration Module
  • (15) SPSP Server
    • The Simple Payment Setup Protocol (SPSP)
    • An SPSP server provides end-to-end payment information such as a shared secret, a destination address and so forth over an HTTPS connection. The SPSP server entity MAY be different from the payee node entity. i.e. The payee node may not serve as an SPSP server.