At present s2n operates directly on file descriptors. We have some applications that would prefer to handle the actual network I/O themselves and use "raw" buffers of bytes to interact with s2n.

There are two main possibilities here that we're considering:

1. Provide an enqueue/dequeue API - are these even possible in a non-blocking way?
2. Allow a caller to specify callbacks to be used in place of send()/recv() whenever a write or read is needed.