How to deal with changing IP?

[phasemaster]

I am trying to do a file transfer between two DSL connections that takes multiple days. One DSL connection has the peculiarity that its IP address changes every night. This leads to a discontinuation of the file transfer for which I have found no clean solution yet (other than making the timeout ridiculously short). Am I overlooking something? Is there a best practice to deal with such a situation?

This sequence of events is as follows:

• Start file transfer A to B appending a file at B
• At night, the IP of A changes so that A will reconnect to B after a short time with a new IP; at this time, B shows two connections to A, one with the old IP address an one with the new IP address
• A will try to restart the file transfer, which fails (550 can't access file), because the old connection still has write access to the file
• after some failed tries (over a few seconds), A will give up initiating a transfer
• after the connection timeout, the old connection will terminate
• after the connection timeout, the new connection will terminate