Friday, 1 February 2013

A Tool for Measuring Forwarding Delay in Packet Captures

I have access to some pretty expensive test kit in work. One of its main purposes in life is to measure the latency of traffic streams passing through a network, which is a pretty useful feature. Occasionally, though, the figures produced can be hard to believe and it would be nice to be able to validate them independently. It would also sometimes be nice to be able to see down to the packet level how the delay varies over time.

Since the tester inserts a "unique" signature into each frame, it is possible to do the calculations by hand - simply take a packet capture of traffic entering and leaving the device (at minimum capture using 2 ports on the same box, preferably use one port with a two-source port mirror), then manually compare the timestamps of the packets pre- and post-routing.

Finding matching pairs of packets is pretty tedious, especially for large captures and particularly where high throughput rates mean that there may be thousands of other frames between a "before" and "after". The technique is sound, though, if you're patient.

For some work I was doing recently, I needed to do this on a grand scale. A multi-megabit stream did not appear to be queuing as expected and it was unclear why. Eventually that particular problem was traced, using Wireshark IO graphs, back to overly bursty traffic being offered into the device under test but it made me think it would be very nice to have a tool for doing this kind of verification and, actually, it would not be difficult to write one. So I wrote a tool, in case I needed it in a hurry later on. The impatient may just want to scroll down to "Obtaining the Tool".

How it Works

Packet headers are inevitably be changed by routing (and, in fact, encapsulation could be added or removed in the process) so packet payloads must be compared in order to find "before and after" pairs. The Spirent TestCenter tester includes a 20 byte "signature" in each generated packet, which is always at the very end of the payload. In practice it is not necessary to compare the entire, variable length, payload to pair up packets. Rather it is sufficient, and much faster, to compare the last 20 bytes of each packet for a matching signature.

The process implemented in the tool is to read each packet from the pcap file, storing the following details in a list entry:
  • Frame number
  • Arrival time
  • Signature
Each entry is then stored in a linked list, as in the following diagram:

Each packet read in adds another node of around 36 - 44 bytes in size. This is smaller than the original capture but can still be a considerable amount of memory when working with very large captures.

Once the complete list has been built, the next job is to identify the "before and after" pairs. This is done by considering each list entry in turn, then looking forward in the list for an entry with a matching signature. If such an entry is found, the frame numbers and timestamps of each frame are output along with the time delta between the two frames. Pretty simple, really.

Obtaining the Tool

The tool is available to download as C source code and as a Windows binary at

To build from source code simply extract the source,  change into the directory and type "make".

Using the Tool

Once the binary has been compiled or downloaded, simply run it with the name of the pcap file as its only parameter: For example:

lab@lab:~/Projects/fwding$ ./fwding input.cap
Arrival Frame Number, Arrival Time, Departure Frame Number, Departure Time, Forwarding Delay
1, 1359461693.826304, 2, 1359461693.826354, 0.000050
5, 1359461693.826418, 6, 1359461693.826468, 0.000050
7, 1359461693.826585, 8, 1359461693.826701, 0.000116
9, 1359461693.826818, 11, 1359461693.826946, 0.000128
10, 1359461693.826830, 12, 1359461693.826958, 0.000128
17, 1359461693.826999, 18, 1359461693.827014, 0.000015
21, 1359461693.827078, 22, 1359461693.827128, 0.000050
25, 1359461693.827192, 26, 1359461693.827242, 0.000050
27, 1359461693.827359, 29, 1359461693.827488, 0.000129

The output produced is standard CSV-formatted text.which can be piped or redirected to a file as necessary for manipulation by your favourite spreadsheet or command line tool. Timestamps are in seconds since Unix epoch. Delay is reported in seconds.

Note: The pairing-up mechanism is highly dependent on the test traffic containing unique data in the last 20 bytes of each frame. For tester traffic that's taken care of automatically but your mileage with "real" traffic will vary. I would expect that FTPing a compressed file or playing music / white noise over VOIP  should give relatively good entropy to your data if a tester is not available. For best results filter out non-test traffic beforehand - OSPF hellos and LACPDUs are very repetitive so will generate lots of false hits.

For example, if you want a quick-and-dirty graph of latency over arrival time using gnuplot, just pipe the output to file then use a command such as:

gnuplot> plot "all-ways.txt" using 2:5 with points pt 2

Alternatively, graph the latency by frame number:

gnuplot> plot "all-ways.txt" using 1:5 with points pt 2

Hopefully your output won't look like this - it is an intentionally odd example caused by sending very bursty traffic.


I always ask but it's never happened yet... if you try the tool out, please leave a comment. I'm interested in feedback, good or bad, and if it doesn't quite do what you want I may change it!

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