There is no portable way to define structure alignment and padding in C. Your options are to make assumptions about how structs are aligned for a specific ABI, but which could be different on other ABIs or architectures resulting in broken code. Or to use compiler extensions to force packing which will make the alignment deterministic, but which could still blow up on other architectures if the alignment you used isn't valid on that architecture.

In some cases you could manually pad your structs to satisfy lowest common denominator architecture requirements, and use compiler extensions mark them as packed (which should be a noop on the architectures you've considered), if the protocol padding/alignment is compatible with those requirements. But the code still theoretically could break on ABIs you didn't consider.

Manually packing and unpacking bytes is the safest way to do things in production.