Overview… capturing packets
All packets arriving at a port are counted and analyzed if they contain test payloads. In addition, selected packets can be retained for closer inspection using the capture mechanism, which is accessed under its own “Capture” tab in the content area of the XenaManager.
Capturing is explicitly started and stopped, like traffic generation on the transmit side of the port:
Once started, the capture mechanism is based on three criteria: first an optional start trigger event needs to happen, then packets meeting a keep criteria are retained, until an optional stop trigger event happens:
Trigger events and keep criteria include detection of FCS-errored packets, a specified test payload TID value, or filter conditions.
When using a start trigger, capturing is automatically stopped if the hardware capture buffer runs full. When using only a stop trigger, the hardware capture buffer retains as many packets as possible up until the stop trigger event. The XenaManager provides a simple illustration of the start/stop trigger behavior.
Using the XenaManager the captured packets can be inspected using a horizontal packet viewer strip identical to the one used for building packet headers:
The header of each packet is inspected and decoded according to the protocols known to the XenaManager. The packets can also be exported to Wireshark for more comprehensive analysis.
You use the up and down arrows on the left to scroll the list of captured packets.
Each packet is tagged with the time it was captured relative to capture-start, and its actual length. The length is significant because only a portion of each packet may be retained in order to save space in the hardware capture buffer. If the packet contains a test payload then its latency is also calculated. You select which of these numbers to display using the drop-down box next to the scroll arrows.
The XenaManager also provides a more condensed graphical view of the length or spacing of the captured packets, as well as the latencies. Here is an example of a captured stream using the butterfly length distribution:
