The Inner Workings of uucico

To understand why uucico needs to know certain things, a quick description of how it actually connects to a remote system might be in order here.

When you execute uucico -s system from the command line, it first has to connect physically. The actions taken depend on the type of connection to open-- e.g. when using telephone line, it has to find a modem, and dial out. Over TCP, it has to call gethostbyname(3) to convert the name to a network address, find out which port to open, and bind the address to the corresponding socket.

After this connection has been established, an authorization procedure has to be passed. It generally consists of the remote system asking for a login name, and possibly a password. This is commonly called the login chat. The authorization procedure is performed either by the usual getty/login suite, or-- on TCP sockets-- by uucico itself. If authorization succeeds, the remote end fires up uucico. The local copy of uucico which initiated the connection is referred to as master, the remote copy as slave.

Next follows the handshake phase: the master now sends its hostname, plus several flags. The slave checks this hostname for permission to log in, send and receive files, etc. The flags describe (among other things) the maximum grade of spool files to transfer. If enabled, a conversation count, or call sequence number check takes place here. With this feature, both sites maintain a count of successful connections, which are compared. If they do not match, the handshake fails. This is useful to protect yourself against impostors.

Finally, the two uucico's try to agree on a common transfer protocol. This protocol governs the way data is transferred, checked for consistency, and retransmitted in case of an error. There is a need for different protocols because of the differing types of connections supported. For example, telephone lines require a ``safe'' protocol which is pessimistic about errors, while TCP transmission is inherently reliable and can use a more efficient protocol that foregoes most extra error checking.

After the handshake is complete, the actual transmission phase begins. Both ends turn on the selected protocol driver. The drivers possibly perform a protocol-specific initialization sequence.

First, the master sends all files queued for the remote system whose spool grade is high enough. When it has finished, it informs the slave that it is done, and that the slave may now hang up. The slave now can either agree to hang up, or take over the conversation. This is a change of roles: now the remote system becomes master, and the local one becomes slave. The new master now sends its files. When done, both uucico's exchange termination messages, and close the connection.

We will not go into this in greater detail: please refer to either the sources or any good book on UUCP for this. There is also a really antique article floating around the net, written by David A.-Novitz, which gives a detailed description of the UUCP protocol. The Taylor UUCP FAQ also discusses some details of the way UUCP is implemented. It is posted to comp.mail.uucp regularly.