• 3 replies
  • Latest Post - ‏2014-06-10T23:40:10Z by Trey
412 Posts

Pinned topic 'TCP Retransmission' messages

‏2014-06-09T10:20:21Z | networking smashyrahul

Hello experts,

We have configured ISAM at DataPower to make the authorization decision. ISAM connects to Authorization server.

The load balncer IP of authorization server has been configured at ISAM config file.Today, I had just looked at the packet capture and I could see that there are several 'TCP Retransmission' and 'TCP-Out-Of-order' errors in wireshark. ( during the transactions between DP and Authorization server)

However, we didnt face any issues as such, in authorization decisions. Are these errors normal? I am novice in networking and hence not sure much about it.



Updated on 2014-06-09T10:20:49Z at 2014-06-09T10:20:49Z by smashyrahul
  • smashyrahul
    412 Posts

    Re: 'TCP Retransmission' messages



  • Harley Stenzel
    Harley Stenzel
    184 Posts

    Re: 'TCP Retransmission' messages


    An occasional TCP retransmission is nothing to worry about.  In fact, TCP is designed to go too fast, suffer loss, which causes TCP to slow down so that it might go as fast as possible in the steady state.  But it will keep trying to go faster until it suffers loss, which causes it to back off.   Now, if we're talking about a lot of loss -- so much that overall throughput is affected -- then that's a different story.  The quickest way to tell the difference is to look at a Time Sequence Graph (TSG).  Wireshark and other tools can generate that for you from your packet capture.  You're looking for a relatively straight line up and to the right when the transfer is in steady state.  If you see that, then there is no need to worry about occasional TCP retransmission.

    Out of order is a little different.  On one hand, it shouldn't happen.  On the other hand, it shouldn't matter.  Overall, you can still use the TSG to help tell the difference.  Depending on the source, it might also be worthwhile to dig into the reasons for the out of order, but this is really outside the role of DataPower administration.

    A word of caution about relying on the TSG in this oversimplified way -- if it's flat then things are probably fine.  On the other hand, if it's not flat, it does not necessarily mean that there is a problem in tcp or below.  It could simply be an artifact of the way the application layer is sending data, which could be an artifact of the source of its own application layer data.

  • Trey
    227 Posts

    Re: 'TCP Retransmission' messages


    This fits with Harleys response Rahul, if you are worried about latency if the authorization actions then we can help through a pmr to dig into the latency of the transaction and see what is going on.  You said several frames but it was not clear if these were within a single connection with 12 packets exchanged or or if you had several packets out of a dozen connections which sent hundreds or more frames.

    These are typical in networks though, if you have latency issues then by all means we should dig into the behavior.  If you want to share any other details about the behavior please feel free.