What is a recursive network?

Introduction to recursive networks.

The current networking paradigm

Every computer science class that deals with networks explains the 7-layer OSI model. Open Systems Interconnect (OSI) defines 7 layers, each providing an abstraction for a certain function that a network application may need.

From top to bottom, the layers provide (roughly) the following functions:

The application layer implements the details of the application protocol (such as HTTP), which specifies the operations and data that the application understands (requesting a web page).

The presentation layer provides independence of data representation, and may also perform encryption.

The session layer sets up and manages sessions (think of a session as a conversation or dialogue) between the applications.

The transport layer handles individual chunks of data (think of them as words in the conversation), and can ensure that there is end-to-end reliability (no words or phrases get lost).

The network layer forwards the packets across the network, it provides such things as addressing and congestion control.

The datalink layer encodes data into bits and moves them between hosts. It handles errors in the physical layer. It has two sub-layers: Media access control layer (MAC), which says when hosts can transmit on the medium, and logical link control (LLC) that deals with error handling and control of transmission rates.

Finally, the physical layer is responsible for translating the bits into a signal (e.g. laser pulses in a fibre) that is carried between endpoints.

This functional layering provides a logical order for the steps that data passes through between applications. Indeed, every existing (packet) network goes through these steps in roughly this order (however, some may be skipped). There can be some small variations on where the functions are implemented. For instance, TCP does congestion control, but is a Layer 4 protocol.

However, when looking at current networking solutions, things are not as simple as these 7 layers seem to indicate. Just consider this realistic scenario for a software developer working remotely. Usually it goes something like this: You connect to the company Virtual Private Network (VPN), setup an SSH tunnel over the development server to your virtual machine and then SSH into that virtual machine.

The use of VPNs and various tunneling technologies draw a picture where the functions of Layers 2, 3, and 4, and often layers 5 and 6 (encryption) are repeated a number of times in the actual functional path that data follows through the network stack.

Enter recursive networks.

The recursive network paradigm

The functional repetition in the network stack is discussed in detail in the book “Patterns in Network Architecture: A Return to Fundamentals”. From the observations in the book, a new architecture was proposed, called the “Recursive InterNetwork Architecture”, or RINA.

Ouroboros follows the recursive principles of RINA, but deviates quit a bit from its internal design. There are resources on the Internet explaining RINA, but here we will focus on its high level design and what is relevant for Ouroboros.

Let’s look at a simple scenario of an employee contacting an internet corporate server over a Layer 3 VPN from home. Let’s assume for simplicity that the corporate LAN is not behind a NAT firewall. All three networks perform (among some other things):

Addressing: The VPN hosts receive an IP address in the VPN, let’s say some 10.11.12.0/24 address. The host will also have a public IP address, for instance in the 20.128.0.0/16 range . Finally that host will have an Ethernet MAC address. Now the addresses differ in syntax and semantics, but for the purpose of moving data packets, they have the same function: identifying a node in a network.

Forwarding: Forwarding is the process of moving packets to a destination with intent: each forwarding action moves the data packet closer to its destination node with respect to some metric (distance function).

Network discovery: Ethernet switches learn where the endpoints are through MAC learning, remembering the incoming interface when it sees a new soure address; IP routers learn the network by exchanging informational packets about adjacency in a process called routing; and a VPN proxy server relays packets as the central hub of a network connected as a star between the VPN clients and the local area network (LAN) that is provides access to.

Congestion management: When there is a prolonged period where a node receives more traffic than can forward forward, for instance because there are incoming links with higher speeds than some outgoing link, or there is a lot of traffic between different endpoints towards the same destination, the endpoints experience congestion. Each network could handle this situation (but not all do: TCP does congestion control for IP networks, but Ethernet just drops traffic and lets the IP network deal with it. Congestion management for Ethernet never really took off).

Name resolution: In order not having to remember addresses of the hosts (which are in a format that make it easier for a machine to deal with), each network keeps a mapping of a name to an address. For IP networks (which includes the VPN in our example), this is done by the Domain Name System (DNS) service (or, alternatively, other services such as open root or namecoin). For Ethernet, the Address Resolution Protocol maps a higher layer name to a MAC (hardware) address.

Recursive networks take all these functions to be part of a network layer, and layers are mostly defined by their scope. The lowest layers span a link or the reach of some wireless technology. Higher layers span a LAN or the network of a corporation e.g. a subnetwork or an Autonomous System (AS). An even higher layer would be a global network, followed by a Virtual Private Network and on top a tunnel that supports the application. Each layer being the same in terms of functionality, but different in its choice of algorithm or implementation. Sometimes the function is just not implemented (there’s no need for routing in a tunnel!), but logically it could be there.

Last modified July 6, 2019