Initiate precision packet captures to analyze zero window conditions

In TCP metrics, window size specifies the amount of data that a device can receive and process during a flow. When the window size is zero, transmissions are halted until the device signals that it has the space to receive data again.

Zero window conditions that last 1 or 2 seconds are not too unusual, especially during periods of heavy traffic. However, longer-lasting zero window conditions can indicate a more serious problem and cause performance issues.

You can create a dashboard or configure alert notifications to track zero window occurrences, but the cause can be hard to determine. For example, CPU, memory, and NIC usage might be normal, and you don't know if the issue is with the network, the servers, or the application. But you can always find the truth in the packet!

In this walkthrough, you will create a trigger that captures packets with zero window conditions on HTTP transactions. Then, you will download the captures so that you can upload the data to a packet analyzer to help you determine the state of the client and server on a flow when zero window conditions occurred.


Write the precision capture trigger

In the following steps, you will write a trigger that initiates a precision packet capture each time a zero window condition occurs on an HTTP transaction.

  1. Log in to the ExtraHop system through https://<extrahop-hostname-or-IP-address>.
  2. Click the System Settings icon and then click Triggers.
  3. Click Create.
  4. Specify the following trigger configuration settings:
    1. Type Zero Window PCAP into the Name field.
    2. In the Assignments field, type HTTP Servers, and then select HTTP Servers.
    3. From the Events list, select FLOW_TICK.
    4. Select the Enable debug log checkbox.
    5. Click Show Advanced Options and type 128 in the Bytes Per Packet to Capture field.
      Tip:The default value is 0. Keep this value to capture all the bytes in each packet.
  5. In the right pane, type the following code to initiate the packet capture when a zero window condition occurs:
    // Check to make sure that this is an HTTP transaction
    if ( Flow.l7proto !== 'HTTP' ){    
    //The packet capture name, which includes the client and server 
    //IP addresses and port numbers
    var pcapName = 'Zero Windows_'  
        + Flow.client.ipaddr + ':' + Flow.client.port  
        + '-'  
        + Flow.server.ipaddr + ':' + Flow.server.port;
    //Initiate packet capture each time a zero window occurs on 
    //the client or the server
    if ( Flow.zeroWnd1 > 0 || Flow.zeroWnd2 > 0 ) {  
        var opts = {  
            maxPackets: 30,        // Capture up to 30 packets 
            maxPacketsLookback: 15 // Capture up to 15 lookback packets 
        Flow.captureStart(pcapName, opts);
        //Show capture activity in debug log  
        debug('Start Zero PCAP: ' + pcapName);    
  6. Click Save.

View debug output in the debug log

In the following steps, you will view the trigger debug output to confirm that the trigger is running and capturing packets. After you assign the trigger to your data sources, the system runs the trigger when HTTP traffic occurs, and if any transactions contain a zero window, the system sends debug results to the debug log.

  1. Click the System Settings icon , and then click Triggers.
  2. Click the Zero Window PCAP trigger you just created.
  3. Click Edit Trigger Script.
  4. Click the Debug Log tab.
    The debug log displays results similar to the following figure:

Download and view packet captures

In the following steps, you will download packet captures.

  1. Log in to the ExtraHop system through https://<extrahop-hostname-or-IP-address>.
  2. From the top menu, click Records.
  3. Click View Records.
  4. From the Record Type drop-down list, select Packet Capture.
  5. After the records associated with your packet capture appear, click the Packets icon , and then click Download PCAP.
Last modified 2024-05-01