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diff --git a/content/en/docs/Concepts/problem_osi.md b/content/en/docs/Concepts/problem_osi.md index 845de5e..e86582c 100644 --- a/content/en/docs/Concepts/problem_osi.md +++ b/content/en/docs/Concepts/problem_osi.md @@ -5,19 +5,35 @@ author: "Dimitri Staessens" date: 2019-07-06 weight: 1 description: > - The current networking paradigm + The Internet, our big bungle of joy. --- +Every computer science class that deals with networks explains the +[7-layer OSI model](https://www.bmc.com/blogs/osi-model-7-layers/) and +the +[5-layer TCP model](https://subscription.packtpub.com/book/cloud_and_networking/9781789349863/1/ch01lvl1sec13/tcp-ip-layer-model). + +Both models have common origins in the International Networking +Working Group (INWG) in the seventies, and therefore have many +similarities. The TCP/IP model evolved from the implementation of the +early ARPANET in the '70's and '80's. The OSI model was the result of +a standardization effort in the International Standards Organization +(ISO), which ran well into the nineties. The OSI model had a number of +useful abstractions: services, interfaces and protocols, where the +TCP/IP model was more tightly coupled to the Internet Protocol. + +### A birds-eye view of the OSI model + {{<figure width="40%" src="/docs/concepts/aschenbrenner.png">}} -Every computer science class that deals with networks explains the -[7-layer OSI model](https://www.bmc.com/blogs/osi-model-7-layers/). Open Systems Interconnect (OSI) defines 7 layers, each providing an -abstraction for a certain *function* that a network application may -need. +abstraction for a certain *function*, or _service_ that a networked +application may need. The figure above shows probably +[the first draft](https://tnc15.wordpress.com/2015/06/17/locked-in-tour-europe/) +of the OSI model. From top to bottom, the layers provide (roughly) the following -functions: +services. The __application layer__ implements the details of the application protocol (such as HTTP), which specifies the operations and data that @@ -46,35 +62,64 @@ 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. +The benefit of the OSI model is that each of these layers has a +_service description_, and an _interface_ to access this service. The +details of the protocols inside the layer were of less importance, as +long as they got the job -- defined by the service description -- +done. + This functional layering provides a logical order for the steps that data passes through between applications. Indeed, existing (packet) -networks go through these steps in roughly this order (however, some -may be skipped). +networks go through these steps in roughly this order. + +### A birds-eye view of the TCP/IP model + +{{<figure width="25%" src="https://static.packt-cdn.com/products/9781789349863/graphics/6c40b664-c424-40e1-9c65-e43ebf17fbb4.png">}} + +The TCP/IP model came directly from the implementation of TCP/IP, so +instead of each layer corresponding to a service, each layer directly +corresponded to a (set of) protocol(s). IP was the unifying protocol, +not caring what was below at layer 1. The HOST-HOST protocols offered +a connection-oriented service (TCP) or a connectionless service (UDP) +to the application. The _TCP/IP model_ was retroactively made more +"OSI-like", turning into the 5-layer model, which views the top 3 +layers of OSI as an "application layer". + +### Some issues with these models However, when looking at current networking solutions in more depth, -things are not as simple as these 7 layers seem to indicate. Consider -a realistic scenario for a software developer working -remotely. Usually it goes something like this: he connects over the -Internet to the company __Virtual Private Network__ (VPN) and then -establishes an SSH __tunnel__ over the development server to a virtual -machine and then establishes another SSH connection into that virtual -machine. - -We are all familiar enough with this kind of technologies to take them -for granted. But what is really happnening here? Let's assume that the -Internet layers between the home of the developer and his office -aren't too complicated. The home network is IP over Wi-Fi, the office -network IP over Ethernet, and the telecom operater has a simple IP -over xDSL copper network (because in reality operator networks are -nothing like L3 over L2 over L1). Now, the VPN, such as openVPN, -creates a new network on top of IP, for instance a layer 2 network -over TAP interfaces supported by a TLS connection to the VPN server. - -Technologies such as VPNs, tunnels and some others (VLANs, -Multi-Protocol Label switching) seriously jumble around the layers in -this layered model. Now, by my book these counter-examples prove that -the 7-layered model is, to put it bluntly, wrong. That doesn't mean -it's useless, but from a purely scientific view, there has to be a -better model, one that actually fits implementations. - -Ouroboros is our answer towards a more complete model for computer networks.
\ No newline at end of file +things are not as simple as these 5/7 layers seem to +indicate. Consider, for instance, __Virtual Private Network__ (VPN) +technologies and SSH __tunnels__. We are all familiar enough with this +kind of technologies to take them for granted. But a VPN, such as +openVPN, creates a new network on top of IP, for instance a layer 2 +network over TAP interfaces supported by a Layer 4 (using, for +instance Transport Layer Security) connection to the VPN server. + +Technologies such as VPNs and various so-called _tunnels_ seriously +jumble around the layers in this layered model. For instance, +Multi-Protocol Label switching MPLS, typically sits in between Layer 2 +and IP, is categorized as Layer 2.5. + +Which protocol fits in which layer is also not clear-cut. The Border +Gateway Protocol (BGP) performs (inter-domain) routing. Routing is a +function that is usually associated with Layer 3. But BGP runs on top +of TCP, which is Layer 4. There is no real concensus of what layer BGP +is in, some say Layer 3, some (probably most) say Layer 4, because it +is using TCP, and some say it's application layer. But the concensus +does seem to be that the BGP conundrum doesn't matter. BGP works, and +the OSI and TCP/IP models are _just theoretical models_, not _rules_ +that are set in stone. + +### Are these issues _really_ a problem? + +Well, in my opinion: yes! And _big_ ones too! If there is no +universally valid theoretical model, if we have no clear definitions +of the fundamental concepts and no clearly defined set of rules that +unequivocally lay out what the _necessary and sufficient conditions +for networking_ are, then everybody is _engineering in the dark_, and +progress in developing computer networks is condemned to a sisyphean +effort of perpetual incremental fixes, its fate to remain a craft that +builds on tradition to keep the cobbling together of an ever-growing +bungle of technologies and protocols within the limits of +manageability.
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