sfn is a simple and efficient protocol for transferring files over network. It's easy to use, and a full-featured client can be implemented in half an hour. Due to this simplicity, many implementations in different languages were created. Send yours.

L1 is the most basic one, L3 has checksum support, L4 has faster checksum support. L5 supports sending directories. Revisions are backward-compatible: every new revision includes support for existing opcodes all the way back to L1. See the full spec.