SLAC QBSS Measurements

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Effect of QBSS on QBSS, Best Effort and Priority TCP traffic

Measurement Methodology

To see the effect of various router QBSS configurations on simultaneous file copies made with QBSS, Best Effort (BE) and Priority TOS code points, we set up and used a QBSS test network. On the test network we installed and a machine called evagore copying files to and from pharlap. Evagore was a Sun Sparcv9 single processor running at 333Mhz with a 100Mbps Ethernet running Solaris 5.7. It was connected to the test network which had a 10Mbps connection to the SLAC public network and thence to pharlap. Pharlap was a Sun E4500 with 6*336 MHz processors, a 1Gbps Ethernet and running Solaris 5.8. We used bbcp to make the file copies. To eliminate disk I/O issues we wrote the file to /dev/null on pharlap, and to allow the copies to run for an extended period without requiring a large file, we read the data from /dev/zero.

Each measurement consisted of:

  1. starting a bbcp copy from evagore to pharlap (we also tested the reverse direction, i.e. initiating from pharlap a copy from pharlap to evagore, with similar results) with QBSS set;
  2. about 30 seconds later a bbcp copy was started with BE set;
  3. then about 30 seconds later again, a bbcp copy was started from bbcp with the priority code point; after about 30 seconds the priority code point file transfer was stopped;
  4. after another 30 seconds the BE copy was stopped;
  5. after 30 more seconds the QBSS copy was stopped and the measurement terminated.
We analyzed and graphed the data using Excel.


We initially set the QBSS to have 1% of the 10Mbps bandwidth, and the Priority traffic to have 70%. All bbcp copies used an 8KByte window and 1 stream, and reported the incremental throughout for each 3 second interval. The results are shown to the right. It is seen that the QBS traffic starts out using over 90% of the 10Mbps bandwwidth. When the BE copy starts, the BE gets about 87% of the 10 Mbps bandwidth and QBSS drops to about 5%. When the priority copy starts it gets about 70%, BE gets about 19% and QBSS about 1%. As the priority and later the BE copies are stopped the reverse happens. Each time things appear to happen very quickly with no long (we also made measuremenst with 1 second incremental throughput reports, that indicate the time to transition from one throughput state to another is < 1-2 seconds) learning process.

We repeated the measurement with 10 streams of BE copies, to represent 10 competing TCP users. The other parameters were the same as the previous measurement. In this case we also added up all the throughputs and plotted them. The behavior is almost identical to the single stream version. The total throughput stays pretty constant regardless of the competition. There are positive and negative spikes in the total throughput. This is probably since we were using three separate processes for the file copies, and they were not synchronized. Again the transitions occur quickly.

We changed the router percentages assigned to QBSS and Priority copies to 30% and 1% respectively and remeasured with 10 streams of BE copies. In this case we plotted the throughputs on a log scale to better show the utilization by the QBSS copy when both other copies were running. With this graphical representation, unfortunately it is tricky to spot the priority percentage utilization since it almost overlaps the QBSS throughput. It is seen that the QBSS stream backs off to 20KBytes/sec and 10KBytes/sec when the BE and priority copies are successively introduced. The BE throughput drops to about 66% when the priority traffic starts with about 30% utilization. In all cases the transitions occur quickly.

Effects on ping response time

We used the Cisco 6509/SUP2/MSFC2 based test bed with GE connected hosts and a 100Mbps bottleneck between the hosts. We used iperf to send bulk TCP traffic with a 128KByte window and 1 stream from antonia to pharlap with different TOS bit settings (using the iperf -S option with values of 32 for QBSS and 40 for Priority traffic). At the same time we used the Linux ping with the -Q option to set the TOS bits and default packet (56Bytes) and spacing (1 second). Each of the tests consisted of starting the iperf transfer and then running ping for 100 seconds (while iperf was still running). We noted down the min/avg/max standard deviation of the ping RTTs and the iperf throughput achieved. We also verified by using tcpdump on antonia that the iperf and ping applications were setting the TOS bits in the IP headers. Further study with tcpdump on pharlap of the TOS bits also identified that the TOS bits were cleared before they reached pharlap. Thus pharlap did not set any TOS bits in the ping echo resposne or the iperf acks. This clearing of the TOS bits may be since packets on 100Mbps links are not trusted so the resetting occurs at the 100Mbps interface on the 2nd 6509 in the path from antonia to pharlap. However, since the bulk of the traffic (over 100 to 1) is from antonia to pharlap we do not believe the lack of setting the TOS bits on the reverse path will have a major effect on the current results. The iperf throughputs achieved were about 93Mbits/s. The following results were achieved:
Revised August 9, 2001.
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