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I have a VPN in which (mainly two) clients communicate with each other. I realized that the throughput between two of the clients is extremely low ((2) -> (1) in the picture below) in one direction and later found out that this problem also occurs without a VPN (direct connection between two routers attached to the internet).

The following picture should help understanding my infrastructure:

Network speeds

where Ref is a reference to determine the maximum speed for each node (that where two speed test providers in my case).

(1) is a Raspberry Pi using latest Raspbian.

The throughput of the Raspberry Pi (1) is about 20 MBit/s in local network in either direction (wireless).

Throughput of (2) is also fine from any computer on the internet in any direction.

The combination described in the picture is the only combination known to me in which the bottleneck appears.

Latency is really stable between both nodes but I realized when pinging from (1) to (2) that icmp_seq sometimes increases by more than one. Is that an indicator for packet loss?

PING 192.168.4.1 (192.168.4.1) 56(84) bytes of data.
64 bytes from 192.168.4.1: icmp_seq=2 ttl=128 time=58.10 ms
64 bytes from 192.168.4.1: icmp_seq=4 ttl=128 time=48.8 ms
64 bytes from 192.168.4.1: icmp_seq=6 ttl=128 time=71.5 ms
64 bytes from 192.168.4.1: icmp_seq=7 ttl=128 time=66.3 ms
64 bytes from 192.168.4.1: icmp_seq=9 ttl=128 time=58.4 ms
64 bytes from 192.168.4.1: icmp_seq=12 ttl=128 time=53.9 ms

Please let me know which information you need - for now, I have no idea where else to search for the problem.

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    What loss stats does 'ping' show once stopped? Are both (1) and (2) on the same ISP? If not, how does the direct path from one to another (traceroute) compare to paths taken to (ref)?
    – grawity_u1686
    Commented Mar 21, 2020 at 17:35
  • Indeed - I got an average packet loss of around 35%. ISPs differ on both nodes. traceroute reveals that the route to (ref) uses three hops while the route to (2) uses six hops. But that can't result in a performance difference, right? When packets arrive, they arrive with very low latency...
    – Ercksen
    Commented Mar 21, 2020 at 19:21
  • What I forgot to mention is that the problem occurs sporadically. In the first weeks, everything worked fine. But in the last weeks the problem occured more and more often. Maybee a result of COVID-19 quarantine? That would be the most simple solution but that really does not seem to be likely to me as other routes are unaffected.
    – Ercksen
    Commented Mar 21, 2020 at 19:28
  • No, it's actually very likely because they're different routes.
    – grawity_u1686
    Commented Mar 21, 2020 at 19:32
  • Is there a way to find out which hop is dropping the packets?
    – Ercksen
    Commented Mar 21, 2020 at 20:06

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