Re: Asymmetric forwarded TCP TX collapse over one GRE path on FreeBSD 14.3-RELEASE-p8 (PF independent, offloads disabled)

The problem is that I've tried everything I know/found on the internet/was recommended by Codex and GPT, and I (and LLVMs) have run out of ideas :(

> 25 февр. 2026 г., в 10:55, Ivan K. <[email protected]> написал(а):
> 
> I don't quite understand your question.
> iperf3 shows 200-300 Mbit through VPS0 and 100 kbit through VPS2 at the same time when I run them from my laptop. 
> If I run them from the router, the speeds are good on both tunnels.
> 
>> 25 февр. 2026 г., в 01:12, Mark Saad <[email protected]> написал(а):
>> 
>> 
>> 
>> On Tue, Feb 24, 2026 at 3:35 PM Ivan K. <[email protected] <mailto:[email protected]>> wrote:
>>> Yes, I wrote about this in my original letter. It is an Intel Gigabit server network card. I disabled everything possible on it with the command:
>>> 
>>> ifconfig igb0 -mextpg -vlanhwtag -vlanhwfilter -vlanhwtso -vlanhwcsum -rxcsum -txcsum -rxcsum6 -txcsum6 -tso -lro
>>> 
>>> The options that remained are: VLAN_MTU, JUMBO_MTU, WOL_MAGIC, HWSTATS
>>> 
>>>> 24 февр. 2026 г., в 23:23, Mark Saad <[email protected] <mailto:[email protected]>> написал(а):
>>>> 
>>>> 
>>>> 
>>>> On Tue, Feb 24, 2026 at 3:02 PM Ivan K. <[email protected] <mailto:[email protected]>> wrote:
>>>>> Each real life - 1500, gre 1472.
>>>>> I checked this. I tried setting mtu 1200 on the client device. I have mss clamping at 1400 in pf.
>>>>> If it's mtu, it doesn't explain why the identical tunnel on the same machine functions normally.
>>>>> 
>>>>> 
>>>>>> 24 февр. 2026 г., в 21:10, Mark Saad <[email protected] <mailto:[email protected]>> написал(а):
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> On Tue, Feb 24, 2026 at 12:19 PM Ivan K. <[email protected] <mailto:[email protected]>> wrote:
>>>>>>> Hello, everyone. The text below was written by me and “polished” by a neural network, which I have been using to research this problem for three days now.
>>>>>>> 
>>>>>>> I am investigating a reproducible GRE issue and would appreciate advice on further debugging.
>>>>>>> 
>>>>>>> Versions / hosts
>>>>>>> - Router: FreeBSD 14.3-RELEASE-p8
>>>>>>> - VPS0: FreeBSD 14.3-RELEASE-p8
>>>>>>> - VPS2: FreeBSD 14.3-RELEASE-p8
>>>>>>> 
>>>>>>> Topology
>>>>>>> - Router has WAN on VLAN interface `onp` (parent `igb0`) and LAN on `igb0`.
>>>>>>> - Two GRE tunnels from the same router:
>>>>>>>   - GRE to VPS0: healthy
>>>>>>>   - GRE to VPS2: problematic
>>>>>>> - There is also an ng tunnel to VPS2 (works better than bad GRE, but not full speed).
>>>>>>> - Clients behind router tested via both Wi-Fi and wired Ethernet (same problem).
>>>>>>> - Also tested traffic from a VM on the router: VM path does not reproduce the severe collapse.
>>>>>>> 
>>>>>>> Core symptom (asymmetric)
>>>>>>> - Only TX direction is affected (client uploads through GRE->VPS2).
>>>>>>> - RX/download direction is not affected.
>>>>>>> - Through GRE->VPS2, TCP upload collapses to about ~100 Kbit/s per flow.
>>>>>>> - 5 parallel TCP flows produce roughly 5 x ~100 Kbit/s.
>>>>>>> - iperf3 TCP shows many retransmissions and very small cwnd (~2.7 KB).
>>>>>>> - Same client via GRE->VPS0 is fast (normal high throughput).
>>>>>>> - Traffic generated locally on router through GRE->VPS2 is also fast.
>>>>>>> 
>>>>>>> Firewall / policy checks
>>>>>>> - PF disabled completely (`pfctl -d`) for tests: problem remains.
>>>>>>> - Therefore this is not PF rule/policy dependent in current testing.
>>>>>>> - Also checked for ipfw/dummynet:
>>>>>>>   - `kldstat | egrep 'ipfw|dummynet|pf'` shows only `pf.ko`.
>>>>>>>   - `ipfw show` returns “Protocol not available”.
>>>>>>>   - `net.inet.ip.fw.enable` OID not present.
>>>>>>> 
>>>>>>> Offload / NIC checks performed
>>>>>>> - Explicitly disabled offload features (multiple attempts), including:
>>>>>>>   - `-lro`
>>>>>>>   - `-tso` / tso-related toggles
>>>>>>>   - `-rxcsum` / `-txcsum` / v6 checksum toggles
>>>>>>>   - VLAN hardware offload related toggles where supported
>>>>>>> - No improvement.
>>>>>>> - Current interface snapshot during testing:
>>>>>>>   - `ifconfig igb0` shows options: `VLAN_MTU,JUMBO_MTU,WOL_MAGIC,HWSTATS` (no LRO/TSO flags active)
>>>>>>>   - `ifconfig onp` shows `options=0`
>>>>>>> - Reboots of router and VPS2 done after changes: no improvement.
>>>>>>> - Also tested with minimal manual GRE setup (without tunnel-management scripts, only one GRE up): no improvement.
>>>>>>> 
>>>>>>> Queue/counters/sysctl checks
>>>>>>> - `netstat -Q` shows no queue drops (`QDrops=0`).
>>>>>>> - `netstat -iW -I ...` during tests shows no obvious interface error growth.
>>>>>>> - `sysctl net.inet.ip.fastforwarding` does not exist on this release.
>>>>>>> 
>>>>>>> UDP vs TCP behavior
>>>>>>> - UDP over problematic GRE path can run at high throughput (similar order from client and from router), i.e., no analogous dramatic collapse.
>>>>>>> - TCP over same path collapses hard in TX direction from forwarded clients.
>>>>>>> 
>>>>>>> Packet capture findings
>>>>>>> - Collected captures at multiple points (router and VPS side).
>>>>>>> - In bad runs, client-side TCP packets are seen, but many segments are retransmitted repeatedly.
>>>>>>> - Comparing capture points suggests segments often appear on VPS GRE side only much later / after retransmission, consistent with loss on the GRE path for this forwarded TCP case.
>>>>>>> - In control runs (local router-generated traffic), packet timing/count alignment is normal.
>>>>>>> 
>>>>>>> What makes this puzzling
>>>>>>> - Same router, same OS version, same general config:
>>>>>>>   - GRE to VPS0 works well.
>>>>>>>   - GRE to VPS2 fails for forwarded TCP TX only.
>>>>>>> - PF disabled does not change behavior.
>>>>>>> - Offloads disabled does not change behavior.
>>>>>>> - No obvious drops in standard queue/interface counters.
>>>>>>> 
>>>>>>> Request
>>>>>>> Any suggestions on what to inspect next for FreeBSD 14.3-p8 in this scenario:
>>>>>>> - netisr / forwarding path diagnostics
>>>>>>> - GRE-specific kernel instrumentation
>>>>>>> - known igb/VLAN/GRE forwarding edge cases
>>>>>>> - any tunables or debug knobs that could explain “forwarded TCP TX collapse only on one GRE peer/path”
>>>>>>> 
>>>>>>> Thanks.
>>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> Ivan
>>>>>>  Just a guess but it sounds like you may have a MTU issue . What is the MTU settings on each of the real interfaces and the gre tunnels ?
>>>>>> --
>>>>>> mark saad | [email protected] <mailto:[email protected]>
>>>>> 
>>>> 
>>>> Ivan
>>>>   What sort of network cards are you using? This is another long shot but do you have anything that does GRE offload? Some of the intel
>>>> and mellanox cards support some sort of checksum offloads. 
>>>> 
>>>> --
>>>> mark saad | [email protected] <mailto:[email protected]>
>>> 
>> 
>> Ivan
>>   So back to how you are testing. Do you have iperf / iperf3 blasting traffic from router to target VPS0 and VPS2 at the same time ?
>> 
>> 
>> --
>> mark saad | [email protected] <mailto:[email protected]>
>