It is the original UUCP protocol, and is supported by all UUCP implementations. Many implementations of it are only able to support small window and packet sizes, specifically a window size of 3 and a packet size of 64 bytes, but the protocol itself can support up to a window size of 7 and a packet size of 4096 bytes. Each packet begins with a six byte header. Control packets consist only of the header.
Data packets contain additional data.
The k value is always 9 for a control packet. For a data packet, the k value indicates how much data follows the six byte header. Thus a k value of 1 means 32 data bytes and a k value of 8 means 4096 data bytes. The k value for a data packet must be between 1 and 8 inclusive. The checksum value is described below.