I am currently in the process of troubleshooting a file transfer between the sender (which in this case is a z/OS main frame) and a receiver server across a WAN. The RTT between sender and server is 72ms. In reviewing the captures and traces I am seeing A LOT of DUP ACKs and FAST Retransmits. I am of the believe that the main frame is not optimized for WAN transfers as between the DUP ACKs and the reply FAST Retransmits are occurring in less than 0.0003 seconds. So my question is: Is it possible to configure a server to make it look like there is a network problem? I am thinking that there is a queuing or timeout setting that is being overlooked. I truly don't feel a main frame is not the right choice for a WAN transfer. As they are meant for data center transactions (IMO). asked 23 Sep '14, 17:13  EdJ edited 24 Sep '14, 06:07 |
One Answer:
A lot of DUPACKs are an indication that packets arrive out of order at the receiver. When the TCP stack receives the 3rd dupack it retransmits the missing segment withhout a delay, so the 300µs is why it is called a 'Fast' Retransmit' and this is happening on purpose. Not sure I understand the question. You want to make the zOS believe that there is a network problem and refrain from 'Fast Retransmitting' missing packets? If so, contact IBM support, they might have something in their back pocket ;-) Not sure if you really would want this though... "I am thinking that there is a queuing or timeout setting that is being overlooked." There is an old APAR PQ82943 (2004) that is fixing a similar problem but I assume you are at a current z/OS release so that fix should be implemented in your TCP stack already. As for Out-of-Order arrival, it is often due to a multipath-per-packet configuration somewhere along the path. This could start within z/OS sending packets out via different OSA cards. To avoid this make sure that you have MULTIPATH PERConnection in your IPCONFIG statement. " I truly don't feel a main frame is not the right choice for a WAN transfer. So if you feel that the mainframe IS the right choice I agree with you. They (mainframes) are meant for all types of workloads, including long latency connections. Regards Matthias answered 25 Sep '14, 23:19  mrEEde |
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To answer your question; very probably.
But maybe to help further:
What is the bandwidth of the smallest link between the sites? What throughput are you getting? What throughput were you expecting? If you produce an IO stat graph, do you see a constant level of load? Which file transfer mechanism is being used?
Best regards...Paul
Tracewranger... Cool.