Streaming. What if the part(s) are being generated on the fly, and don't exist to have a length? Requiring that the sender specify the length up front require

Streaming. What if the part(s) are being generated on the fly, and don't exist to have a length? Requiring that the sender specify the length up front requires the length to be known. Maybe I'm (e.g.) tarring/zipping up a lot of data, and I don't want to hold it all in memory/write it to a temporary file first.