The automatic creation and disconnection of a networklink over an ISDN line is also very usefull when you are accessing the Internet through an ISP.  You have the advantage of a lower phonebill.

However most ISPs work with dynamic IP addresses and from that problems can arise.

Usually things will go well,  as long as you are websurfing,  because in that case there exists most of the time TCP links for a short period of time.

If you are using ftp and telnet,  it is possible that during a longer period of time you have an open TCP link,  without any networktraffic.  If at such a moment a timeout occurs and the link is disconnected,  your client will hang.

The reason for this is that when the networklink is reconnected,  you will be assigned another IP address and thus another defaultroute.  However the existing TCP link still relies on the existence of a route that does not exist any more.

Of course the best solution would be not to use dynamic IP addresses.

If you nevertheless are working with dynamic IP addresses,  you can alleviate this problem as much as possible by:

• not taking the timeout too short and,
• not leaving unused ftp and telnet links open for too long.

The configurationprogram conf-isdn-account takes this into account by setting the time-out to 5 minutes when dynamic IP addresses is choosen and 1 minute for fixed IP addresses.