iperf3 3.5 TCP option -n not working

Author: paagutiCreated Jul 6, 2018Updated Jun 5, 2026

Labelsbugbug:test-data

Context

Version of iperf3: iperf3.5
Hardware: amd64 laptop with Intel i5 CPU,
Operating system (and distribution, if any): Running on QEMU based VMs Host OS: Ubuntu 16.04.4LTS Guest OS: Alpine Linux 3.7+testing
Other relevant information (for example, non-default compilers, libraries, cross-compiling, etc.):

Please fill out one of the "Bug Report" or "Enhancement Request" sections, as appropriate.

Expected Behavior iperf3 -c -n 1.8M should produce a data transaction with 1.8M bytes
Actual Behavior Dumping from the PCAP file and filtering the FIN/ACK packet:

       time       sport      dport          seq              ack      flags
     0.19117      56562       5201               38          2904690  FA

Expecting something around 1887436 and not 2904690 in the ack number

Steps to Reproduce VM1 as server, iperf3 -s ,capture packets with tshark VM2 as client, iperf3 -c <IP_VM1> -n 1.8M Analyse TCP SEQ and ACK numbers
Possible Solution Yet N/A

Source: esnet/iperf