iPerf throughput is way under WAN bandwidth limit and the reason behind it
Article ID: 381259
Updated On:
Products
Issue/Introduction
When customer performs stress testing, iPerf is a common tool to validate the throughput performance between sites. However, customer very likely observes that throughput result is way under WAN bandwidth limit and raise case for it. This article explains why it is not a issue and the reason behind it. Refer this KB to customer if they raise ticket for similar issue.
root@A41-5Site-b1e1-client1:~# iperf -c 10.0.3.25 -p 8080
------------------------------------------------------------
Client connecting to 10.0.3.25, TCP port 8080
TCP window size: 45.0 KByte (default)
------------------------------------------------------------
[ 3] local 10.0.1.25 port 45496 connected with 10.0.3.25 port 8080
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 101 MBytes 83.5 Mbits/sec <<----Actual WAN bandwidth is 100Mbps on both edges
Environment
Supported VMware VeloCloud SD-WAN versions
Cause
It is because iPerf test result is application layer (L7) throughput. Consider normal Internet WAN, In iPerf, the specified/displayed speed rate should only be calculated based on the Data part, where can be simply understood as the length of the TCP/UDP Data section. The resulting packet will also include TCP/UDP headers, IP headers, and the second layer frame header. For TCP packets, an additional overhead of 66 bytes is added, consisting of 12 (TCP Option) + 20 (TCP header) + 20 (IP header) + 14 (data link layer header).
Now consider iPerf across SD-WAN overlay, there are additional 120 bytes overhead for VCMP headers. Total overheads would be 120+66=186 bytes. Application level transmission efficiency is:
As we know SD-WAN reserves 5% total bandwidth so the final efficiency is:
In short, as long as iPerf result reaches 83.33% of WAN bandwidth, we consider it expected. If iPerf result is lower than 50% of WAN bandwidth, there could be problems.
Resolution
From SD-WAN perspective, what customer can do to maximize overlay throughput are as below:
1. Validate the LAN port is a routed port taken over by DPDK to enhance performance and reduce CPU load.
2. Validate the LAN port MTU is set to 1500.
3. Validate there is a direct tunnel connection between SD-WAN edges with a tunnel MTU of 1500, low tunnel latency, and no packet loss.
4. Validate the load on SD-WAN edge is within a healthy range and avoid conducting tests when SD-WAN edge load is high.
5. Validate the WAN overlay bandwidth is correctly measured and configured on SD-WAN orchestrator.
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