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----
-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.
-
-<center> {{<figure class="w-200" src="/images/rec_netw.jpg">}} </center>
-
-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__.
-
-<center> {{<figure class="w-200" src="/images/dependencies.jpg">}} </center>
-
-A recursive network has __dependencies__ between layers in the
-network, and between IPCPs in a __system__. These dependencies can be
-represented as a directed acyclic graph (DAG). 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 (either locally or globally) as the maximum depth of this
-layer in the DAG.
-
-[Next: Creating layers](/docs/irmd/)
-
----
-Changelog:
-
-2019 07 11: Initial version.<br>
-2019 07 23: Added dependency graph figure