--- title: "Elements of a recursive network" author: "Dimitri Staessens" description: "what" date: 2019-07-11 #type: page draft: false --- This section describes the high-level concepts and building blocks are used to construct a decentralized [recursive network](/docs/what): layers and flows. (Ouroboros has two different kinds of layers, but we will dig into all the fine details in later posts). A __layer__ in a recursive network embodies all of the functionalities that are currently in layers 3 and 4 of the OSI model (along with some other functions). The difference is subtle and takes a while to get used to (not unlike the differences in the term *variable* in imperative versus functional programming languages). A recursive network layer handles requests for communication to some remote process and, as a result, it either provides a handle to a communication channel -- a __flow__ endpoint --, or it raises some error that no such flow could be provided. A layer in Ouroboros is built up from a bunch of (identical) programs that work together, called Inter-Process Communication (IPC) Processes (__IPCPs__). The name "IPCP" was first coined for a component of the [LINCS] (https://www.osti.gov/biblio/5542785-delta-protocol-specification-working-draft) hierarchical network architecture built at Lawrence Livermore National Laboratories and was taken over in the RINA architecture. These IPCPs implement the core functionalities (such as routing, a dictionary) and can be seen as small virtual routers for the recursive network.
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In the illustration, a small 5-node recursive network is shown. It consists of two hosts that connect via edge routers to a small core. There are 6 layers in this network, labelled __A__ to __F__. On the right-hand end-host, a server program __Y__ is running (think a mail server program), and the (mail) client __X__ establishes a flow to __Y__ over layer __F__ (only the endpoints are drawn to avoid cluttering the image). Now, how does the layer __F__ get the messages from __X__ to __Y__? There are 4 IPCPs (__F1__ to __F4__) in layer __F__, that work together to provide the flow between the applications __X__ and __Y__. And how does __F3__ get the info to __F4__? That is where the recursion comes in. A layer at some level (its __rank__), will use flows from another layer at a lower level. The rank of a layer is a local value. In the hosts, layer __F__ is at rank 1, just above layer __C__ or layer __E_. In the edge router, layer __F__ is at rank 2, because there is also layer __D__ in that router. So the flow between __X__ and __Y__ is supported by flows in layer __C__, __D__ and __E__, and the flows in layer __D__ are supported by flows in layers __A__ and __B__. Of course these dependencies can't go on forever. At the lowest level, layers __A__, __B__, __C__ and __E__ don't depend on a lower layer anymore, and are sometimes called 0-layers. They only implement the functions to provide flows, but internally, they are specifically tailored to a transmission technology or a legacy network technology. Ouroboros supports such layers over (local) shared memory, over the User Datagram Protocol, over Ethernet and a prototype that supports flows over an Ethernet FPGA device. This allows Ouroboros to integrate with existing networks at OSI layers 4, 2 and 1. If we then complete the picture above, when __X__ sends a packet to __Y__, it passes it to __F3__, which uses a flow to __F1__ that is implemented as a direct flow between __C2__ and __C1__. __F1__ then forwards the packet to __F2__ over a flow that is supported by layer __D__. This flow is implemented by two flows, one from __D2__ to __D1__, which is supported by layer A, and one from __D1__ to __D3__, which is supported by layer __B__. __F2__ will forward the packet to __F4__, using a flow provided by layer __E__, and __F4__ then delivers the packet to __Y__. So the packet moves along the following chain of IPCPs: __F3__ --> __C2__ --> __C1__ --> __F1__ --> __D2__ --> __A1__ --> __A2__ --> __D1__ --> __B1__ --> __B2__ --> __D3__ --> __F2__ --> __E1__ --> __E2__ --> __F4__. A recursive network has __dependencies__ between layers in the network, and between IPCPs in a __system__. To avoid problems, these dependencies should never contain cycles (so a layer I should not directly or indirectly depend on itself). The rank of a layer is defined as the maximum depth of the (directed acyclic) dependency graph. --- Changelog: 2019 07 11: Initial version.