Marshal/Unmarshal struct with bit fields and variable length fields

I'm working on a library for handling a wire protocol that sends specific messages where elements could be represented by bits in a byte, a fixed byte or word, or even a variable array of bytes. The methods in encoding/binary sounds great for my purpose except they only work with fixed length fields. It is important for my project that each unique message has a specific struct associated with it.

My first approach was to jimmy a struct tag and custom Marshal/Unmarshal methods that handles bit fields and slices via reflect. The prototype works, and could be generalized to a separate library, but I don't want to recreate the wheel.

Does anyone know of an existing library/pattern?

Protocol buffers and/or msgpack sounds a lot like what you're doing here, you may be able to use those off the shelf if you have control over the other consumer of the protocol.

Thanks, but my client is the consumer of the wire protocol so I don't get to define the serialization representation. Instead, it is parsing a series of bytes as a fixed structure describing fields that may be bit flags, bytes, words, or an array of those. 

You can write binaries using net/http.

Here is an example where I convert a struct to binary representation:

https://github.com/amitu/rtime/blob/master/src/rtime/boltdb.go#L267-L282

Let me know if you have questions or example is not clear.