# Simulate different network conditions ## In this learning path - [Introduction](https://learn.arm.com/learning-paths/servers-and-cloud-computing/microbenchmark-network-iperf3/) - [Set up Arm-based Linux systems for network performance testing with iPerf3](https://learn.arm.com/learning-paths/servers-and-cloud-computing/microbenchmark-network-iperf3/setup/) - [Microbenchmark the network connection](https://learn.arm.com/learning-paths/servers-and-cloud-computing/microbenchmark-network-iperf3/basic-microbenchmarking/) - [Simulate different network conditions](https://learn.arm.com/learning-paths/servers-and-cloud-computing/microbenchmark-network-iperf3/simulating-network-conditions/) - [Tune kernel parameters](https://learn.arm.com/learning-paths/servers-and-cloud-computing/microbenchmark-network-iperf3/tuning/) - [Next Steps](https://learn.arm.com/learning-paths/servers-and-cloud-computing/microbenchmark-network-iperf3/_next-steps/) You can simulate latency and packet loss to test how your application performs under adverse network conditions. This is especially useful when evaluating the impact of congestion, jitter, or unreliable connections in distributed systems. ## Add delay to the TCP connection The Linux `tc` (traffic control) lets you manipulate network interface behavior such as delay, loss, or reordering. First, on the client system, identify the name of your network interface: ```bash ip addr show ``` The output below shows that the `ens5` network interface device (NIC) is the device that you want to manipulate. ``` __output__ 1: lo: mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host noprefixroute valid_lft forever preferred_lft forever 2: ens5: mtu 9001 qdisc mq state UP group default qlen 1000 link/ether 0a:92:1b:a9:63:29 brd ff:ff:ff:ff:ff:ff inet 10.248.213.97/26 metric 100 brd 10.248.213.127 scope global dynamic ens5 valid_lft 1984sec preferred_lft 1984sec inet6 fe80::892:1bff:fea9:6329/64 scope link valid_lft forever preferred_lft forever ``` Run the following command on the client system to add an emulated delay of 10ms on `ens5`: ```bash sudo tc qdisc add dev ens5 root netem delay 10ms ``` Rerun the basic TCP test as before on the client: ```bash iperf3 -c SERVER -v ``` ``` __output__ [ 5] local 10.248.213.97 port 43170 connected to 10.248.213.104 port 5201 Starting Test: protocol: TCP, 1 streams, 131072 byte blocks, omitting 0 seconds, 10 second test, tos 0 [ ID] Interval Transfer Bitrate Retr Cwnd [ 5] 0.00-1.00 sec 282 MBytes 2.36 Gbits/sec 0 8.02 MBytes [ 5] 1.00-2.00 sec 302 MBytes 2.53 Gbits/sec 0 8.02 MBytes [ 5] 2.00-3.00 sec 301 MBytes 2.52 Gbits/sec 0 8.02 MBytes [ 5] 3.00-4.00 sec 302 MBytes 2.54 Gbits/sec 0 8.02 MBytes [ 5] 4.00-5.00 sec 302 MBytes 2.53 Gbits/sec 0 8.02 MBytes [ 5] 5.00-6.00 sec 304 MBytes 2.55 Gbits/sec 0 8.02 MBytes [ 5] 6.00-7.00 sec 302 MBytes 2.53 Gbits/sec 0 8.02 MBytes [ 5] 7.00-8.00 sec 303 MBytes 2.54 Gbits/sec 0 8.02 MBytes [ 5] 8.00-9.00 sec 303 MBytes 2.54 Gbits/sec 0 8.02 MBytes [ 5] 9.00-10.00 sec 301 MBytes 2.53 Gbits/sec 0 8.02 MBytes - - - - - - - - - - - - - - - - - - - - - - - - - Test Complete. Summary Results: [ ID] Interval Transfer Bitrate Retr [ 5] 0.00-10.00 sec 2.93 GBytes 2.52 Gbits/sec 0 sender [ 5] 0.00-10.01 sec 2.93 GBytes 2.52 Gbits/sec receiver CPU Utilization: local/sender 3.4% (0.0%u/3.4%s), remote/receiver 11.0% (0.4%u/10.7%s) snd_tcp_congestion cubic rcv_tcp_congestion cubic iperf Done. ``` ## Observations - The `Cwnd` size has grown larger to compensate for the longer response time. - The bitrate has dropped from ~4.9 to ~2.3 `Gbit/sec` - demonstrating how even modest latency impacts throughput. ## Simulate packet loss To test the resiliency of a distributed application, you can add a simulated packet loss of 1%. As opposed to a 10ms delay, this will result in no acknowledgment being received for 1% of packets. Given TCP is a lossless protocol, a retry must be sent. Run these commands on the client system. The first removes the delay configuration, and the second command introduces a 1% packet loss: ```bash sudo tc qdisc del dev ens5 root sudo tc qdisc add dev ens5 root netem loss 1% ``` Now rerunning the basic TCP test, and you will see an increased number of retries (`Retr`) and a corresponding drop in bitrate: ```bash iperf3 -c SERVER -v ``` The output is now: ``` __output__ Test Complete. Summary Results: [ ID] Interval Transfer Bitrate Retr [ 5] 0.00-10.00 sec 4.41 GBytes 3.78 Gbits/sec 5030 sender [ 5] 0.00-10.00 sec 4.40 GBytes 3.78 Gbits/sec receiver ``` ## Explore further with tc The tc tool can simulate: - Variable latency and jitter - Packet duplication or reordering - Bandwidth throttling For advanced options, refer to the [tc man page](https://man7.org/linux/man-pages/man8/tc.8.html).