Joe's FAQ on "FTP Problems" is the primary resource for debugging connectivity problems. Since he wrote that section, we have seen a small but steady number of users that have difficulty accessing the site after using a download accelerator or download manager program.  Here is an explanation of why you may have a problem after using these applications, and how you can avoid them.

Our servers have experienced significant problems with client use of download accelerators. There is a very dark side to these applications.  While they speed up downloads for the client, they severely tax the resources of the server by opening up many download connections in parallel.

We have had a number of occurrences where single individuals used up all the available resources of our server. The end result was a temporary denial of service to the rest of the science community.

We want to ensure the availability of the servers to the community. To do so, we instituted connection limits to the web and ftp servers. Automated filters at the project's firewall prevent massively parallel downloads attempted by download accelerator programs. Blocks are placed automatically, and in most cases are temporary.  In more extreme cases, a permanent block may be installed.

Ftp: Avoid parallel connections. Single threads preserve server bandwidth for other users.

Web: don't use page accelerators that attempt to download all items on a page in parallel.  Limit the number of concurrent connections you make.

Both:  Avoid running downloads on multiple systems.  If you have multiple systems behind a firewall, our server will most likely see that as one single IP address. Filtering rules will be applied against the single IP address, increasing your chances of being blocked.

To prevent exhaustion of the firewall resources, any packets received in excess of the firewall limits are thrown away rather than queued.

Paul