As a user of sftp(1) you don't have to care. You can even specify unsupported buffer sizes, because that sftp client does have all that complexity that I described above built-in. But if you need to interact with the protocol - well it's just a headache, plain and simple. (However, sftp(1) will write files non-sequentially - so you really can't just restart a transfer or you'll very likely end up with a corrupted file).