For simplicity, where the Operating System permits, fasp transfers to and from a server will use the same UDP port, for example UDP/33001. This is convenient because the firewall in front of the server only has to allow one UDP port. Some Operating Systems don't allow UDP port reuse at all (Windows) while others only allow it partially, for transfers under the same user account (Mac, FreeBSD). In this case each transfer will use its own UDP port, incrementally, starting at the base port UDP/33001 (by default).
On Linux servers, however, there is a limitation on how many concurrent transfers can be established in the same time. If a client starts many transfers at the same time, the port reuse mechanism will make it such that some of the later transfers will fail to establish the UDP connection.
This is not a problem for high concurrency servers where transfers are not started in rapid succession by the same client.
To avoid this problem, the Linux server can be configured to not reuse the UDP ports for fasp transfers.
Here is a sample aspera.conf file with the UDP port reuse option turned off:
<?xml version='1.0' encoding='UTF-8'?>
To verify the configuration, use "asuserdata":
/opt/aspera/bin/asuserdata -v -a | grep reuse