Local network speed between devices

Question

I have Raspberry Pi 3 B+ with IP 192.168.0.100 and Rock Pi 4 B with IP 192.168.0.200. Both in the same network connected to TP-Link 1043N V5 router. Both Pis have 1Gb Ethernet. However I am not able to use the whole bandwith even if it is not used elsewhere (only SSH on both to view tests).

What I get is:

Rock Pi receives data from Raspberry at 33.6MB/s and Raspberry receives data from Rock at 27.8MB/s.

I've made scripts to test that, both run either ways. Receive:

nc -l -p 3333 | pv -W > /dev/null

Send:

pv -B 1M /dev/zero | nc [other device's IP addr] 3333

What could be the problem here to achieve 1Gbps or 125MB/s?

Answer 1

The Raspberry Pi organization publishes a summary of the 3B+ here. If you look under the "Specifications" tab, you'll find the answer to your question:

• Gigabit Ethernet over USB 2.0 (maximum throughput 300 Mbps)

There has been a lot written about this since the 3B+ was released. In addition to the link provided in one of the comments to your question, this website has compiled some benchmarks, and you may find some of the information useful in comparison testing your 3B+. In a similar question here on SE, it's been pointed out that using SSH may slow things even further, suggests a "typical real-world" speeds and notes other demands on the USB may further reduce Ethernet throughput.