Here are some answer to questions about gensio.
Why did you create gensio?
Originally gensio was part of ser2net (https://sourceforge.net/projects/ser2net). I was rewriting ser2net to add in encryption and authentication, and I needed some sort of layered protocol handling to do this. I realized that I had a fairly general purpose library that I couldn't find anything else like (except maybe System V Streams? remember those?), so it made sense to split it out into it's own thing.
It then grew to add things on I needed for other reasons, or to provide proof of concept.
Why don't you have ssh support?
I would dearly love to have ssh support. However, I can't find a library that is really suitable. The openssh and libssh2 code only lets you have a single connection at a time and uses tons of global variables. Obviously not suitable. libssh might work, but it is not nicely layered where I could split it out and provide just a protocol layer without sockets underneath. It might be possible to integrate libssh as a base gensio, but I didn't like that very much. libwebsockets has a client implementation that might work, but it's only a client implementation. If anyone knows of a suitable library, I'd be happy to hear about it.
You wrote a mux implementation. Why didn't you use websockets?
That was my first inclination. However, websockets has no mechanism for flow control. I almost didn't believe it at first. The websocket description talks about the user implementing their own flow control. Take the hardest part of the problem and pass it to the user. The design of gensio is all about flow-control. If I used websockets, I'd have to write a flow-control implementation on top of it, and that would be non-standard anyway.
Why did you write gtlssh and gtlsshd? Isn't ssh good enough?
Well, ssh is fine, I use it all the time. gtlssh and gtlsshd were written more as a proof of concept to make sure I got all the pieces necessary to implement proper authentication with a reasonable amount of flexibility. With that said, they provide about everything that ssh can do except for X11 forwarding, which is kind of a pain. And they are only about 1500 LOC each. The only really annoying thing about them is that keys expire, since they are based on SSL. Which is probably a good thing, since keys need to be replaced every once in a while. gtlssh will warn you if you key expires in less than 30 days. If that's too much of a problem, make keys that expire in 100 years.
What's the deal with SCTP?
SCTP is a nice protocol for a number of reasons. It has multi-homing, meaning that both sides communicate all the IP addresses that they have, and communicate when they change, and use all of them. So you could have a connection stay up even if a server transitions over to new IP addresses. It has better protection agains SYN attacks. A few other things.
That said, the current implementations of SCTP (at least the one on Linux) leave something to be desired. There are performance issues and some unimplemented features.
Is IPv6 supported?
Of course. Full support. The documentation describes how to use it. The library tries to make unsurprising decisions, but everything can be overridden.