Question

I’ve almost got it — but I’m missing key points…..

I’m trying to figure out how one might built “layered” protocol stacks in Twisted. I can understand how one attached a single protocol and handles events with Defers, but with if I want the traditional OSI layers model? Assume I have a line-oriented protocol at the bottom on a TCP connector, and I want multiple line-oriented protocols stacked on top until I get to application space.

(Think of me wanted to implement the TCP/IP stack itself in twisted)

Answer 1

The OSI layering model is mostly irrelevant to Twisted (and, I would posit, mostly irrelevant to software in general). Looking at each layer at a time:

1. physical: obviously, Twisted is not an ethernet cable or a physical switch, so it can’t do this.
2. data link: in order for Twisted to run over a network interface, your operating system needs to be connected to a physical network. data-link protocols typically need to be implemented in hard real-time equipment, often in hardware, so Twisted isn’t suitable for that.
3. network: this layer, if it’s distinct from the “transport” layer, then it’s stuff like BGP and routers and whatnot which is happening distinct from your application.
4. transport: At this layer, we have two interfaces, IProtocol and ITransport. The transport delivers bytes from the transport to an IProtocol via dataReceived, and the application delivers bytes to the transport via ITransport.write(). (This relationship is then inverted on the other end of the wire.)
5. session: (this is implicitly part of the transport)
6. presentation: this is like, CSS stylesheets or something
7. application: obviously Twisted doesn’t do this part, you do it yourself.

In-protocol layering, however, is somewhat more ad-hoc. The usual idiom right now is to simply subclass Protocol and then delegate from dataReceived to a new method, specific to your layering idiom, such as lineReceived, then have the next layer up subclass that.

If you actually want a TCP implementation that uses Twisted, look here.

If you want to see a proposal for interfaces that will improve layering within Twisted itself, see here instead.