Win7 Client and Win7 Server: TCP Retransmission followed by DUP ACK and TCP Retransmission

asked 2019-07-09 21:35:55 +0000

Vincent gravatar image

My Windows 7 Client Application can abruptly disconnect from the Windows 7 Server Application after several successful queries.

A successful query:

  1. Client to Server [PSH, ACK]
  2. Server to Client [PSH, ACK]
  3. Client to Server [ACK]

Error I see on the Server Side:

  1. Client to Server [PSH, ACK]
  2. Server to Client [PSH, ACK]
  3. Client to Server [TCP Retransmission] (of #1)
  4. Server to Client [TCP Dup ACK] (of #2)
  5. Server to Client [TCP Retransmission]
  6. Repeat step #5 until the client disconnects

Error I see on the Client Side:

  1. Client to Server [PSH, ACK]
  2. Client to Server [TCP Retransmission] (of #1)
  3. Server to Client [TCP Previous Segment not Captured] (referring to DUP ACK)
  4. Listen indefinitely for a response that doesn't come causing a hang.

My questions are:

  1. Why is the client missing the transmissions the server is sending?
  2. Would this be because of the server, client, or something in-between?


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Are you able to share (an anomimized version) of the client and server trace on a public file sharing service like OneDrive, Dropbox, etc? Analysis of text is so much harder and error-prone than analysis of pcap files.

SYN-bit gravatar imageSYN-bit ( 2019-07-10 19:16:39 +0000 )edit

One possible cause would be that only large packets get dropped by having tunneling somewhere between the client and the server without proper adjustments of the TCP MSS. Again, being able to look at the packets will help (please make sure the 3-way handshake is in the capture file)

SYN-bit gravatar imageSYN-bit ( 2019-07-10 19:18:20 +0000 )edit