Linux Network Administrator's Guide, Second Edition

UUCP Low-Level Protocols

To negotiate session control and file transfers with the remote end, uucico uses a set of standardized messages. This is often referred to as the high-level protocol. During the initialization phase and the hangup phase these are simply sent across as strings. However, during the real transfer phase, an additional low-level protocol that is mostly transparent to the higher levels is employed. This protocol offers some added benefits, such as allowing error checks on data sent over unreliable links.

Protocol Overview

UUCP is used over different types of connections, such as serial lines, TCP, or sometimes even X.25; it is advantageous to transport UUCP within protocols designed specifically for the underlying network protocol. In addition, several implementations of UUCP have introduced different protocols that do roughly the same thing.

Protocols can be divided into two categories: streaming and packet protocols. Protocols of the streaming variety transfer a file as a whole, possibly computing a checksum over it. This is nearly free of overhead, but requires a reliable connection because any error will cause the whole file to be retransmitted. These protocols are commonly used over TCP connections but are not suitable for use over telephone lines. Although modern modems do quite a good job at error correction, they are not perfect, nor is there any error detection between your computer and the modem.

On the other hand, packet-oriented protocols split up ...