There's not too much, but we can give you pointers. The transport API is internal and we don't support out-of-tree custom transports. If you have a need for a custom transport, 1) I'd first double-check that there is a serious need, as creating a new transport is involved and 2) would suggest it to be developed in tight coordination with us. I will note that many times a custom transport is not necessary, but it depends on what your needs are.
What are you needing a custom transport for? Client-side and/or server-side?