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I've done searching here and on Wikipedia. What I've seen most mentioned is that I should look at old SMTP specifications as that protocol is the simplest one to look at and understand (HTTP is apparently very complex, I don't disagree). However, comments on those suggestions say looking at SMTP is the wrong way to go because the way it's written is outdated and inefficient.

I have also seen people suggest custom protocols to be written in some variant of C, or Java.

I would like to know where I could go for an example of a custom/simple protocol written for modern times, and what language is ideal for writing protocols. I assume a low-level language is best, and that's where my knowledge of such ends.

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    Are you aware that a link-layer protocol is what shovels bits between a physical layer (electric signals in a wire or photons in an optical cable etc) and a network-layer (such as IP)? In turn, SMTP and HTTP are examples of application-layer protocols wich work on top of network-layer and/or transport-layer protocols (such as TCP). So I suspect you have a terminology confusion here. Commented Jul 11, 2014 at 16:17
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    And if you want to implement a link-layer protocol, with popular contemporary operating systems, the only sensible way to implement one is to write a driver which, in most cases, means writing code in C making heavy use of OS kernel-specific APIs. So this kind of removes the question of applicable programming languages. Commented Jul 11, 2014 at 16:18
  • If you want an application-layer protocol, then what task is it supposed to solve? The idea of "just creating a new protocol" has little sense as there's no possibility to discuss possible approaches to implementing one, and their merits. Commented Jul 11, 2014 at 16:23
  • @kostix: In regards to your first question, yes I do understand that. As this is a new thing I am learning about, I asked my question weirdly. My apologies. In regards to your second question, does every OS vendor write wrappers for all the protocols? If so, that's interesting. In regards to your third question, I wanted to use something like foobar:// for a project I am working on. Any dot com would procedural generate a 3D model on my project. Commented Jul 11, 2014 at 20:36

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I realize that this is an old question, but I think I have some insight on this. I recently developed some embedded prototypes for a project I was working on and needed a way for these device to communicate at high speeds to my PC. USB was quite a headache, and frankly, so is writing a custom tcpip stack although I have at least done that before. In addition, the embedded device had no way to get an ip address from the user other than hardcoding it into the c/assembly. So I thought what if I could connect over ethernet and just avoid all the IP layer nonsense. I decided to add magnetics to the board and bitbang 10Mbps Ethernet/MAC frames across the channel. On the PC side, I used libPCap to read out the raw ethernet frames and process them in my application. Worked like a charm for creating a high speed connection between my embedded project, and my pc. Well except there is a catch. It worked when connected to my pc over a crossover cable or through a dummy switch, but if you tried to connect it to the rest of the LAN through a traditional router, the router would discard the packet. Why? I never found out for sure, but my best guess is that the EtherType field in an Ethernet Frame which is used to denote the protocol used in the upper layer did not match the one of the known types (i.e. IP, ARP, etc).

The bottom line is while you can make a custom link layer protocol and implement it application side with a library like libPCap, its not guaranteed to work over all routers since some routers will drop your packets if they don't recognize the protocol you are using. It will definitely not work over the internet since the IP layer is what decides routing on the internet.

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Ooh, thank you for sharing your experience! Looks like my idea won't work outside of my own network, but at least I have a better understanding as to why.

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