To reduce the costs of window acknowledgements, the protocol uses a large window and only requires an ack at the halfway point. Each packet starts with a six byte header, optionally followed by data bytes with a four byte checksum. The sequence numbers are independent for each side. The first packet sent by each side is always number 1. Each packet is numbered one greater than the previous packet, modulo 32.
Every packet has a local channel number and a remote channel number.
For all packets at least one channel number is zero. When a UUCP command is sent to the remote system, it is assigned a non-zero local channel number. All packets associated with that UUCP command sent by the local system are given the selected local channel number. All associated packets sent by the remote system are given the selected number as the remote channel number. This permits each UUCP command to be uniquely identified by the channel number on the originating system, and therefore each UUCP package can associate all file data and UUCP command responses with the appropriate command.