Here is what  I suspect (of course I could be wrong).  USRP uses UDP as its transport. 

If you are using DVSM, you set the mobile client to talk to AB on a single port.  This setting in AB (where TX and RX ports are the same) tells AB to use an enhanced protocol to talk to the client.  This enhancement is used to keep the NAT traversal path open to the client. The  trick is that AB  will send keep alive packets back to DVSM to tell the router/firewall that the NAT connection should be maintained.  As long as the keep alive packets are sent often enough the router will maintain the connection.

If you are using ASL as your USRP client to AB, you are using two ports  (TX and RX).  AB does not try to tell ASL to keep the  connection open (since ASL would not understand the keep alives).  So, if you are using ASL on a different machine than AB and there is a NAT between them, you will have problems.

Any of the above make sense?